PETROLEUM OPEN LEARNING
Subsea Equipment & Tools Part of the Oil & Gas Subsea Technology Series
Subsea Equipment & Tools
MATERIAL TECHNOLOGIES, SUBSEA SYSTEMS & FLOW ASSURANCE - VERSION 1.2 Designed, Produced and Published by OPITO Ltd. OPITO Ltd - Petroleum Open Learning Minerva House Bruntland Road, Portlethen Aberdeen AB12 4QL Tel: +44 (0)1224 787800
© OPITO 2014 All rights reserved. No part of this publication may be reproduced, stored in a retrieval or information storage system, transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise without the prior permission in writing of the publishers.
With thanks to
and to Hugh Grant for his subsea expertise
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Contents
Subsea Equipment & Tools
Section 1. Topside Facilities For Different Water Depths......................................... 7 Section 2. Oil Industry Engineering Standards....................................................... 13 Section 3. Wellhead & Hanger System.................................................................. 16 Section 4. Workover Systems................................................................................ 20 Section 5. Conventional Xmas Tree....................................................................... 25 Section 6. Spool or Horizontal Xmas Tree............................................................. 33 Section 7. Well Intervention System....................................................................... 36 Section 8. Protective Structures............................................................................. 39 Section 9. Subsea Field Layout.............................................................................. 42 Section10. Case Study........................................................................................... 47 Check yourself answers......................................................................................... 51
4
Visual Cues
Training targets for you to achieve by the end of the Unit
Check yourself answers to let you see if you have been thinking along the right lines
Test yourself questions to see how much you understand
Summaries for you to recap on the major steps in your progress
Training Targets When you have completed this Unit you will be able to: • List the reasons for drilling subsea wells • Describe a Surface and Deepwater platform • Describe both types of subsea Xmas trees • List the tree Safety and Protective structures • List places that need extra vigilance for personal safety • Describe the items in the oil flow that can cause production problems
Tick the box when you have met each target 5
Introduction:
Why do we need subsea wells? A surface Production Platform is simpler and less expensive to install and it will do the drilling, producing, processing and exporting from there. Unfortunately the demand for oil and gas cannot be satisfied from the volume produced by the land or shallow water reservoirs therefore exploration has to look further afield. By following the places where sands have been washed from rivers out into the oceans gradually the water depths will increase but that is where new discoveries will be made. This module will take you through the different stages of technology and how subsea wells are taken to the production condition after the drilling operations are completed. Various options are available firstly that subsea Xmas trees will produce up to a Surface Platform or secondly a deepwater structure will be used or the entire system will be located on the seabed. The first option is only possible if there is a conveniently available Platform, then the others can be located in any suitable area within the constraints of the water depth. This is currently at 3,000 meters. Similar Xmas trees can be used at any depth but will need to be adjusted for water depth (pressure) and have added aids to help with the installation. 6
The seabed installation can function in very remote places and can be controlled from a shore terminal as far as 200 kilometres away. All that is necessary are a Manifold to collect production from all the subsea Xmas trees and Flowline to the shore terminal. Two types of Xmas tree are in common use, a conventional vertical bore tree and a Spool or horizontal bore tree. Both are equally up to the job but some well conditions may favour one over the other. In certain areas of the world well conditions will create a need for a technical development. These can be fluid, wax, gas Hydrogen Sulphide/ Carbon Dioxide or water related from within the well or they can be problems coming from frozen gas leaking from the immediate seabed around the Xmas tree. Many countries have medium sized reservoirs that will only produce for 10 -12 years. Systems to economically produce these are readily available. Much of this equipment can be recovered, reconditioned and used on another project of a similar size. When you have completed this module you will have a good grasp of how a subsea system is installed and what is involved.
Section 1 - Topside Facilities For Different Water Depths Topside facilities suitable for different water depths and weather conditions. Types of Xmas Tree... The Fixed platform, is a rigid structure, using land well technology and is installed on the ocean floor in up to 500m of water. It becomes the base for directionally drilling a number of development wells, and ultimately the base for the produced oil or gas to flow or be pumped from, to an offshore producing terminal or a shore-based production facility.
Tension Leg Platform / Deep Draft Caisson Vessel (TLP / DDCV) or a combination of any of these three. Each method brings its own advantages and disadvantages and will be discussed later.
Figure 1... Fixed Platform
Depending on the extent of the discovered field, it may take a number of these fixed platforms to fully develop it. The number of wells drilled and completed from any one fixed platform varies considerably. Fixed platforms often become the control point for subsea developments and provide the Xmas tree control, processing and export facilities. Field developments have moved to greater water depths where fixed platforms cannot be used. These developments can use Subsea trees, Floating production facilities or Dry Completion Units 7
Figure 2... Jacket Platform
Jacket Type Platform... Jacket type platform structures have been the most proven and safe way of developing offshore fields, but economics and increasing water depths are directing thoughts to other alternatives. Concrete structures were developed as an accepted means of developing a field, particularly in the Norwegian sector of the North Sea. These concrete structures also house storage tanks for the produced oil. This helps to solve the problem of long subsea pipelines necessary to get the oil to a shore based production and processing facility.
Historically, offshore production technology had adhered to the platform method of development, using the cluster well approach, because of the tried and true nature of the equipment used for drilling and production. The economics of large reserve exploitation to offset high capital cost has been preferred to the essentially new technology of equivalent techniques. Fixed Platforms are economically feasible for installations in water depths up to about 500m.
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Figure 4... Spar Platform
Figure 3... Tension Leg Platform Tension Leg Platforms (TLP’s) offer another alternative to conventional platforms for the development of deep water fields. Because of the success attained by with the worlds first TLP, more operators have made use of these designs. In this concept, a platform closely resembling a semisubmersible is floated out to location and tethered to the ocean floor over a multi-well template and each corner is tethered by single or multiple tendons attached to large anchoring pads on the ocean floor.
complete success and with the small field depleted the facilities have now been removed. A Spar or DDCV (Deep Draft Caisson Vessel) is a deep water floating drilling and production system. Rather than using steel tendons to fix the structure over subsea wellheads the Spar is moored on site in the same manner of a conventional floating drilling rig. The structure (approximately 110’ / 33m diameter x 750’ / 235m) resembles a long cylindrical steel column with the drilling and production equipment mounted on top and operate in water depths to 3000 meters and beyond.
The object of positively tethering the floating platform in this manner is to eliminate the vertical motion of the platform and also to provide a large degree of resistance of lateral motion. This means that a series of wells can be drilled and completed as if from a fixed platform.
The steel column is ballasted into the water several hundred feet which make it less vulnerable to the effects of sea conditions. A square moon pool through the centre of the structure provides the access to the subsea wellheads and subsequently installed production risers. These are drilled through a template so as to accurately space out the wells to ensure there is space for drilling equipment especially the blowout preventer to be fitted.
The design of these tension leg platforms is such that the lateral movement is reduced to a minimum and the TLP can be used in water depths of between 300-1500m. The illustration (Figure 3) shows the large topsides the tendons running down to the seabed anchors, the template with the production risers and the export flow lines.
Production processing and controls are situated in the control room at the surface with two way communication provided by an umbilical made up of hydraulic hoses, power and telephone lines.
The UK North Sea was chosen for the first of these installations because of the harsh environment but with water that was not too deep. The trial was a 9
Compliant Towers
Figure 5... Compliant Tower
A third method of deep water field development is the Compliant Tower. The Compliant Tower resembles a conventional platform in that it is set and supported at the ocean floor and is used in depths ranging from 450-900 meters. The wide base design of a conventional platform which gives it its stability is not used. The support structure of a Compliant Tower remains a consistent perimeter dimension its entire length. The compliant tower is less expensive than a conventional platform simply because less material and effort is used in the construction. Current and tide flows through the structure thereby minimizing vortex shedding. As offshore production moves into deeper water, the reliance on remotely controlled vehicles (ROV) becomes greater and the suppliers of these vehicles have responded. ROV’s are available with multiple colour cameras, tools and manipulator arms that open and close valves, cut cables, make up cables, rotate handles and a do a complete range of functions and have allowed new technology to move forward.
Subsea Production Systems
Subsea Completions
Subsea production systems (using a subsea tree installed on a subsea wellhead drilled from a floating drilling vessel) gained initial acceptance as adjuncts to platform production operations, enabling exploitation of areas at the periphery of a field out of reach of the deviated wells. Submerged Production System (SPS) was an early experimental alternative to the fixed platform.
Subsea completion is a broad subject that includes subsea production systems and subsea wells tied back to surface facilities. Platform Tiebacks address the adaptation of subsea wellheads (drilled by a semi-submersible drilling rig) for conventional or production platform completion at the surface. This practice is desirable because of time savings made possible by pre-drilling wells while the fixed platform is being built.
A three well prototype was built and tested in a large shallow tank. The system was covered by only one meter of water so as to accurately assess the total underwater situation.
All subsea completions are arranged to have at least two and in cases of very high pressure three barriers in place between the reservoir and the environment. The Subsea Xmas Tree is a very strong component that incorporates high integrity valves and other devices configured to comply with Oilfield, Safety and Environmental standards.
The design of the SPS system was a large tubular support structure or template that was preassembled on land and taken to its offshore location on a supply boat or on the deck of the drilling rig that would install it. The support structure was then placed on the sea floor and the wells drilled through it putting in place a Subsea Wellheads System that will support the lengths of steel casing used to line the well. As the well is drilled these are cemented in place to give support and also seal off the space between the casing and the drilled hole. This work is done from a floating drilling vessel when the water depth is greater than 350 meters.
The purpose of any Xmas Tree is to give safe control of fluids and gas from the well and provide two barriers between the reservoir and the ocean with the object of a no pollution scenario during production operations. The basic control valve components of a Xmas Tree are configured to achieve this. The control valves are referred to (from bottom to top) as Lower Master Valve, Upper Master Valve, and Crown Valve and off to the side is the Wing Valve. The two tree formats shown below have different advantages and disadvantages.
The SPS was part of the world’s original seabed development and even in these early years (1960) was pioneering many new ideas that are common place today. 10
All the systems have a Tubing Hanger that supports and seals the production tubing that will convey all the production from the well to the sea floor Xmas Tree.
The tubing hanger is equipped with a wireline plug profile above the side outlet. Once in place, this plug serves the same purpose as the swab valve in a conventional tree.
The Conventional Tree system has the tubing hanger installed in the wellhead below the Xmas Tree prior to the Xmas tree being installed. The BOP is removed and the Xmas Tree installed after the tubing hanger has been locked in place and tested as part of the completion installation. Later, intervention well work such as slickline, e-line or coiled tubing operations can be carried out by removing the tree cap and connecting to the Xmas Tree with either a Lower Riser Package (LRP) or Subsea Intervention Lubricator (SIL).
There are no vertical bore valves on a horizontal tree compared to 3 vertical bore valves on the conventional tree (lower master valve, upper master valve and swab valve). As with the conventional tree, intervention well work such as slickline, e-line or coiled tubing operations can be carried out by removing a debris cap and connecting to the Xmas Tree with a Subsea Intervention Lubricator (SIL). The two crown plugs are then removed using slickline to gain access to the wellbore.
The shallow-water conventional tree system is usually deployed on guidelines from an anchored semi-submersible drilling rig, typically utilizing a flowbase configuration so that the tree can be retrieved without disturbing the flowlines. Because the tubing hanger is installed within the wellhead system, in the event of a heavy (tubing retrieval) workover, the tree needs to be recovered.
There are two major advantages for a horizontal tree as compared to conventional. Firstly, since the tubing hanger is run after the tree is in place, a workover requiring the tubing to be pulled can be achieved without recovering the tree. Secondly, the tree and tubing hanger do not require a purpose built completion riser system. The tree can be run on drillpipe or connected to the BOP stack on marine riser.
The Horizontal Tree system has some major differences. The Xmas Tree is installed on the wellhead prior to the completion and tubing hanger being installed. The tubing hanger is then locked in place and tested inside the Xmas Tree treehead. The tubing hanger has a side port which lines up with the horizontal production outlet within the tree body.
ROV based flowline and umbilical connections can made directly to the tree assembly. The system can be supplied with a flowbase if required, however most applications feature flowline and umbilical connections made directly to the tree assembly eliminating this item.
Horizontal Tree
Conventional Tree
Tree Cap Crown Plugs Tubing Hanger
Master Valve Block
Treehead
Tree Connector
Tree Connector
Tubing Hanger Wellhead Wellhead
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Figure 6... Subsea Tree Cutaway Section
Tree Cap
Test Yourself
1
What system would you choose for a water depth of (a) 150-300 meters? (b) 750-1,000 meters (c) 3,000 meters? You will find the answer to Test Yourself 1 on page 51 12
Section 2 - Oil Industry Engineering Standards Information about the engineering standards needed for sound design, safety and environmental soundness for sub-sea production equipment and their component parts to gain International acceptability.
• American Petroleum Institute (API) • N ation Association of Corrosion Engineers (NACE) • A merican Association of Mechanical Engineers (ASME) • National Aerospace Society (NAS)
The Oil and Gas Industry requires the assistance of certain standards for the quality and performance of every item no matter how big or small it may be.
API 17 D. Lays down the requirements for material, testing and certifications for subsea equipment in general whether it is for well drilling or production purposes.
The importance of knowledge and familiarity of these standards is very important because they will let us check and understand the complete specification that must be clearly marked on all the equipment that is about to be installed on the seabed.
NACE MR-01-10. Specifies special materials heat treatments and other recommendations for use in a sour environment.
Standards for materials and pressure, temperature and reliability have been developed over a period of some 60 years these apply to the manufacture of steels and other materials and processes, the physical, functional and pressure testing of every Wellhead and Xmas tree assembly and its component parts to be installed they are:
ASME V111. Provides dimensional information of flanges, nuts and bolts and seals to be used in various service conditions of pressure and temperature. NAS 1638. Specifies permissible contamination of all control system fluids in levels rated 1-8.
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The information provided is in tabulated form and it is easy to follow as can be seen here: TABLE 1 - Material Requirements Minimum Material Requirements Materials Class
Body, Bonnet, End and Outlet Connections
Pressure Controlling Parts Stems and Madrel Hangers
AA - General Service BB - General Service CC - General Service a DD - Sour Service a EE - Sour Service a FF - Sour Service a HH - Sour Service
Carbon or low alloy steel Carbon or low alloy steel Stainless Steel b Carbon or low alloy steel b Carbon or low alloy steel b Stainless Steel b CRA’s
Carbon or low alloy steel Stainless Steel Stainless Steel b Carbon or low alloy steel b Stainless Steel b Stainless Steel b CRA’s
a
b
As defined by NACE Standard MR 0175 In compliance with NACE Standard MR0175 Ref. API 6A, 17th Edition, Table 4.3 - Materials Requirements
TABLE 2 - Temperature Ratings Temperature Ratings Operating Range ºF Temperature Classification
Min.
Max.
Min.
Max.
K
-75
to
180
-60
to
82
L
-50
to
180
-46
to
82
P
-20
to
180
-29
to
82
R
API 17D
ºC
ROOM TEMPERATURE
ROOM TEMPERATURE
S
0
to
150
-18
to
66
T
0
to
180
-18
to
82
U
0
to
250
-18
to
121
V
35
to
250
2
to
121
X
0
to
350
-18
to
180
Y
0
to
650
-18
to
345
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TABLE 3 - Particle contamination coding system NAS 1638 - ISO 4406 Maximum Number of Particles / 100ml Class
5-15
15-25
25-50
50-100
>100
00
125
22
4
1
0
0
250
44
8
2
0
1
500
89
16
3
1
2
1000
178
32
6
1
3
2000
356
63
11
2
4
4000
712
126
22
4
5
8000
1425
253
45
8
6
16000
2850
506
90
16
7
32000
5700
1012
180
32
8
84000
11400
2025
360
64
9
128000
22800
4050
720
128
10
256000
45600
8100
1440
256
11
512000
91200
16200
2880
512
12
102400
182400
32400
5760
1024
(Oil Industry standard for Hydraulic fluid cleanliness is Class 6) Note: Never use anything but the latest for any information you require
Test Yourself
2
Using this information refer to the tables to determine what the specification of a Xmas tree would be for the following actual data collected from the drill testing operations. What does the engineering standard API 17 D relate to? You will find the answer to Test Yourself 2 on page 51
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Section 3 - Wellhead & Hanger Systems After a discovery has been made, a consortium of companies will buy into the development plan. The very high capital outlay and infrastructure costs will usually be shared. The participant with the biggest share will normally become the operator and the field will be named after them. When crude oil is produced it is a mixture of water, oil and gas. Testing of the crude oil will be done at the discovery stage to determine the ratio of each.
Environmental impact will be as required by the country that owns the block where the reservoir is located and will play a major part in the type of Xmas tree design that will be used and how it will be serviced or developed over its life span. Running tools will require a certain amount of maintenance, refurbishment and protection between uses and longer term protection between developments. Many companies prefer to take tools from the equipment manufacturer on a rental basis so as to avoid any responsibility for their serviceability.
Reservoirs can be sweet or sour if there is a portion of Hydrogen Sulphide (H2S) present. This presence will dictate the metallurgy that must be applied to the tubing hanger, Xmas tree and any other areas where the production fluids are in contact.
Many running tools will arrive on the rig housed in long shipping baskets and will be lifted from a boat onto the rig deck. Always beware of these items swinging around and or lifting eyes breaking or basket contents being insecure. Take care of every ones safety and especially your own. This applies to onshore handling and loading also.
Sour crude contains a cocktail of gasses that are poisonous to man and destroy steel if not specially treated. The pressure of the reservoir is very important since that will drive the oil up to the Xmas tree and onward to the processing and export facilities. The length and diameter of the tubing and flow lines will contain a weight of crude and the underground pressure has to deliver this in sufficient quantity over a long period to make the project profitable. This factor will determine the production bore for the tubing hanger and Xmas tree so you can see where all the relevant information is coming from.
Vulnerable areas would be:
• Slings • Lifting eyes • Equipment clamps • Damage to baskets
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The Wellhead
This housing will be rated at least:-
The Wellhead is the first component to be run and is rated to suit the Pressure, Temperature, and Bending needs.
250 Centigrade of continuous temperature capability
10,000 psi for pressure containment
Two types of top profile exist for Wellheads:
Up to 7 million lbs (500 tons) of bending load resistance
(1) Mandrel (2) Hub
High strength internal load shoulder
The profile choice will be determined by the connection on the bottom of the Blowout Preventer on the particular Rig that is doing the work. The mandrel profile is the most commonly used because of the greater number of rigs that have a BOP that is configured to them. The work done by the wellhead system will minimize difficulties that could arise during the long production period of the well.
Corrosion resistant area to house and lock the tubing hanger into. A special taper sealing area to fit a large bore very strong metal seal ring that will be placed between the wellhead and the Blowout Preventer or the Xmas tree. The steel used will be in compliance with NACE recommendations in case of sour service. The design of the wellhead system will allow all the bending forces generated by the BOP stack and later the Xmas tree or and remedial work over the life of the well to be dissipated into the area below the seabed.
The strata through which drilling is done is far from perfect and by lining all the sections with a steel liner or Casing and filling the annulus between them with cement they were sealed each one inside the other to make a very strong and stiff reinforced column that is suspended and sealed inside the wellhead. This creats an assembly that would share bending loads and dissipate them into the strata below. The main component part of the wellhead system is the High Pressure Housing.
Wellheads rated at 15,000psi are also available and will be used in more arduous and harsher environments.
VX Seal Area
Running Tool Cam Profile
VT Seal Area
Tubing Hanger/ Lockdown Profile
Seal Wickers Ratch Lockring
Duel Socket Interface
High Strength Insert Ring on 15k System Only
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Figure 10... Mandrel High Pressure
The High Pressure Wellhead (Mandrel Profile)
The High Pressure Wellhead
in the water parts” will be crucial because of the bending forces being applied to these areas and the rigs ability to keep them in tension. The long Riser connecting the Blowout Preventer to the Drilling Rig can be the controlling factor in our choice of Rig to do the work.
(Mandrel Profile)
The areas marked are where the main seal will be positioned for each casing hanger. The ribbed profile at the top is what the BOP will lock onto the uppermost inner taper is the main high pressure seal gasket between the BOP and the Xmas tree.
The drilling Riser forms a conduit that will bring drilling fluids (mud) and rock debris from the drill-bit to the surface where a series of cleaning systems will remove all unwanted materials from the mud and this will be reconditioned for re-use.
The seals between the high pressure wellhead and the casing hanger body are metallic encapsulated elastomeric type. This works by squeezing a rubber element which then deflects a metal lip onto the wellhead and hanger surfaces and pre-loads it there for the life of the well. These seals are also rated at 10,000 psi and 250C.
The Riser is made of lightweight steel tubing with a high tensile strength. The weight of the riser whilst in operation is very high and if allowed to sag it would flex and break from metal fatigue. The Rig has a special Riser tensioning system that will hold the entire system up in tension this will overcome that problem.
Mandrel High Pressure Normally three casing hangers will be landed and sealed into the high pressure housing:
Another force that acts on our Riser is Vortex Shedding, this produces large amounts of vibration as the current or tide flows past.
• One for 13.3/8” casing. • One for 9.5/8” casing.
Where the water is very deep, >2000 meters or more, it can be artificially lightened by adding buoyancy modules.
• Should it be needed an extra 7” one can be accommodated.
The modules are strapped around the riser body, which will effectively reduce the weight–in-water to give the entire Riser an overall reduction this may allow the use of a lesser rig that would be less expensive.
The casings used during the drilling phase are selected for their ability to resist external pressure from the strata (collapse), internal pressure (burst) and tensile strength to carry the total weight that will be suspended by the casing hanger inside the wellhead.
Now that the drilling work is finished a period of time will elapse until the well is finished off or completed as a production oil or gas well or maybe a well for injecting water into the reservoir to replace the production that has been removed.
The limit of the weight carrying ability of that area will determine the length of a particular casing string. Pressure test loads that are applied at the time of installation are a substantial part of that equation.
The water injection process is done under pressure and will sweep the oil towards a production well and help to slow down the declining pressure within the reservoir that is responsible for pushing the production up to th e seabed and beyond.
Bending Force Bending force is generated by the pressure applied to everything that is in the water between the seabed and the surface of the sea. We have tide that ebbs and flows twice every twenty four hours and various strengths of current that can run indifferent directions but usually at the same level. The result of this condition acting on the total area of the BOP stack and the drilling riser transfers the load to the wellhead and this in turn will distribute these down into the first 800 to 1,000 meters of strata below the seabed. The graphic below shows the approximate areas involved.
A drilling rig will now run a production flow-base this component will lock onto the wellhead and orient the position of a connection that will be connected to the export pipeline.
The Tubing Hanger This is a very important component which has to be strong enough to support the weight of the tubing that is attached to its lower end by a special premium thread.
The bending and tensile strength capacity of “all
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made in its bore for special plugs to be run on a wire and locked and sealed in place. This will prevent any pressure from leaking past this point. These plugs are removed when it’s time to bring the well on stream
It suspends and seals the removable tubing string inside the wellhead. This may require removal and replacement during the lifetime of the well. The tubing hanger is a one piece component that is required to seal off inside the wellhead. Provision is
Rig Tension
Rig Buoyancy capacity 4,800 tons Ocean Surface
Oceanic Forces B.O.P. stack 200 tons surface area 75sq/mtr per
Xmas tree 40 tons sq/mtr per side Max Bending Point
Wellhead
Test Yourself
3
(a) Name one of the wellhead types? (b) What is the bending load of the wellhead? (c) How many casing hangers can be fitted into the wellhead? (d) Where does the Riser fit? You will find the answer to Test Yourself 3 on page 51 19
Figure 11... Bending Force
Riser 400 tons 1,800 sq/mtr
Section 4 - Workover Systems A Production Flowbase is a fabricated unit that is fitted with valves and connections to facilitate the connection of subsea flowlines and umbilicals.
stored on reels below the rigs drilling deck. The lines are numbered 1, 2, 3 and 4, and the rig is oriented until due North is in line with the rotary table in the centre of the drill floor and guideline No1. The guide lines are connected to four Guide Posts, that are part of the flowbase, and four remotely unlocking Post Tops so that the guide lines can be recovered when required.
If a Production Flowbase is installed, the flowlines and controls umbilical can be connected either before or after the Xmas Tree is installed. Also, in the event that the Xmas Tree needs to be recovered sometime in the future, because they are connected to the flowbase there is no requirement to disconnect the flowlines and umbilicals.
To release these post tops and the guide lines at the seabed, a 100 kilo cylindrical weight called a Go Devil, is attached to each guide line and dropped down the line until it stops on the post top unlock trigger, which once depressed, releases the guide line. The line, along with the Go Devil and post top, is then recovered to surface. The guide lines also have an independent tensioning system.
The Production Flowbase is installed on the wellhead prior to the Xmas Tree being installed. This assembly will be fitted over and locked on to the wellhead. It is orientated to the correct position to align the flowline connections on the flowbase with the subsea flowline jumpers to enable easy make-up.
The BOP is installed after installing the production flowbase and locked to the wellhead. Drilling operations can now commence.
The Production Flowbase is run on a guide line system. This is a series of four 25mm diameter wire ropes or guide lines that will extend from the rig to the seabed and give the means of guiding the flowbase through the water. The guide lines are
After drilling operations are completed and the casing / liner has been installed, the well is ready to be ‘completed’ in readiness for production. This
20
involves running production tubing and various downhole flow control devices into the well. One of these flow control devices is a special fail safe closed hydraulic safety valve. This is called a Surface Controlled Sub-Surface Safety Valve (SCSSSV).
areas. Numerous access penetrations or couplers can be incorporated into the hanger. They will carry hydraulic fluid, chemicals, and pressure/ temperature sensors. On a conventional system, only half of the coupler is installed in the hanger face, the other half will be part of the underside of Xmas tree to be run later hence the need for accurate radial alignment.
The SCSSSV acts as a failsafe to prevent the uncontrolled release of reservoir fluids in the event of a worst case scenario surface disaster. It is almost always installed as a vital component in the completion. It is controlled hydraulically from the surface, meaning it is opened using a hydraulic connection linked directly to a well control panel. When hydraulic pressure is applied down a control line, the hydraulic pressure forces a sleeve within the valve to slide downwards. This movement compresses a large spring and pushes the flapper downwards to open the valve. When hydraulic pressure is removed, the spring pushes the sleeve back up and causes the flapper to shut. In this way, it is failsafe and will isolate the wellbore in the event of a loss of the wellhead.
At this point the control of all subsea functions is carried out by the Installation and Workover Control System (IWOCS). The IWOCS controls and monitors the deployment, the operation and the retrieval of subsea production equipment such as Tubing Hangers, Landing Strings and Xmas Trees. The IWOCS system consists of several major pieces of equipment: • Hydraulic Power Unit (HPU) is the primary system component of the IWOCS. It provides the hydraulic power to operate the various running tools and subsea tree valves. The integrated local control panel provides control of tree functions, tree running tool (TRT) / workover umbilical reel and the tubing hanger running tool (THRT) umbilical reel.
Connected to the top of the production tubing is the Tubing Hanger. It is set in the tree or wellhead and suspends the production tubing. The hanger is a critical component made from special alloy or stainless steel to give a well lifetime guarantee free from corrosion. It provides porting to allow the communication of hydraulic, electric and other downhole functions, as well as chemical injection. It also serves to seal-in the annulus and production
Figure 13... HPU • Workover / TRT Reel and Umbilical is a reeled, multi-way electro hydraulic umbilical providing power and communication connections to control TRT and tree functions. A multi-way stab plate terminates the subsea umbilical hydraulic hoses.
Figure 12... IWOCS System
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• Umbilical Sheaves assist deployment of the workover / TRT and THRT control umbilicals and are sized to ensure that an umbilical cannot be bent beyond recommended minimum bend radius.
Figure 16... Umbilical Sheave • Remote ESD Station consists of push buttons and indicators to initiate a single level shutdown of the IWOCS and a remote shutdown of the subsea production system.
Figure 14... TRT Umbilical • THRT Reel and Umbilical is a reeled, multi-way hydraulic umbilical providing communication connections to control THRT functions. Individual hydraulic couplers terminate the subsea umbilical hydraulic hoses, while wet-mateable connectors terminate the electrical cables downhole pressure & temperature monitoring.
The IWOCS will control locking the tubing hanger. A small port that is normally open will be closed off when the hanger is in its fully landed position and by seeing pressure build up in the corresponding umbilical line confirms this. The lockdown wedges are activated and the seal hanger is tested. In the case of a convention tree system, once the hanger has been locked and tested, isolation plugs are set in the tubing hanger bores as barriers prior to removing the landing string and BOP. Once these items have been recovered the Xmas tree is installed and tested followed by the recovery of the tubing hanger plugs. The well is now ready to be perforated prior to being put on production.
Figure 15... THRT Umbilical • Hydraulic Deck Jumpers connects the WHPU to the THRT and workover reels. The jumpers consists of a thermoplastic hydraulic bundle terminated at each end with stab plates.
22
Figure 17... Workover System Figure 18... Installation & Workover Tooling
Installation and Workover Tooling
23
Depending on the field design the Permanent Guide Base will be replaced by a Production Guide Base and positioned two meters above the seabed and where hydrates are present a large diverter plate or (mud mat) will be installed at the actual seabed level. The other assemblies are standard items of tree running equipment.
Figure 19 is a typical example of a conventional subsea tree. You can see the corner post receptacles and the large front ROV interface plate. The bright coloured tabs at each of the ROV intervention points are positioned to be directly in shot for the video camera. This information is transmitted back up to the surface control room and recorded for record purposes. The tabs are made of a luminescent plastic and will be covered in tape to preserve there qualities. This tape is removed by the installation supervisor immediately prior to the Xmas tree deployment.
Figure 19... Conventional Tree
Test Yourself
4
(a) What gets connected to the Flowbase? (b) What opens the SCSSV? (c) What is the function of the Tubing Hanger (d) What is the IWOCS function? You will find the answer to Test Yourself 4 on page 51
24
Section 5 - Conventional Xmas Tree separate pieces of equipment not to be mistaken as the same piece. The Xmas tree is installed on top of the wellhead. A wellhead is used without a Xmas tree during drilling operations when the drilling BOP is installed on the wellhead.
The conventional tree design that has been our subject comprises of a number of well proven parts that are assembled together to make one unit. Most user companies have there own requirement over and above the API standards although they are mainly on overall configuration or a need for an extra valve or some additional chemical or control lines.
The primary function of a Xmas tree is to control the flow of well fluids and gas out of the well. A Xmas tree may also be used to control the injection of gas or water into a non-producing well in order to enhance production rates of oil from other wells.
The main component is the valve block. This is preferred to be made from one piece to minimize joints as they could be a potential leak point.
When the well and facilities are ready to produce and receive oil or gas, tree valves are opened and the formation fluids are allowed to flow through a flow line to the production facilities on the host platform or FPSO.
The preferred API 6A design for production of oil or gas is the gate valve. A number of these will be machined into the block body and arranged for physical operation by ROV and some others will be automated by hydraulic actuators with their operation integrated into the overall control system. The arrangement shown below is fairly typical of this design in up to 300 meter depths.
A tree often provides numerous additional functions including chemical injection points, well intervention means, pressure relief means, monitoring points (such as pressure, temperature, corrosion, erosion, sand detection, flow rate, valve and choke position feedback), and connection points for devices such as down hole pressure and temperature transducers (DHPT). On producing wells, chemicals may be injected to prevent production problems such as hydrates or scale.
The Master and Wing valves are red and the Swab valve is yellow. The smaller valves on the right are for monitoring and control of the area below the tubing hanger called the annulus.
The control system attached to the Xmas tree, controls the downhole safety valve (SCSSSV) while the tree acts as an attachment and conduit means of the hydraulic control line to the downhole safety valve.
Xmas trees are used on both surface and subsea wells. It is common to identify the type of tree as either “subsea tree” or “surface tree”. In this section we are referring to what is known as a conventional subsea tree. Note that a Xmas tree and wellhead are
A ‘Conventional Xmas Tree’ generally refers to a
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dual bore tree which has two vertical bores running through the tree. The bores are typically 5” x 2” ID but can be built to specific customer requirements. The larger 5” bore is the production bore through which the produced formation fluids flow. The smaller 2” bore is a conduit to the annulus and is used for access to the annulus with the likes of gas injection. A crossover line which is isolated with valves, connects both the production bore and annulus bore to facilitate circulation / communication between the bores.
when flowing, thus preserving the master valves for positive shut off for emergency or maintenance purposes. Hydraulic operated wing valves are usually built to be fail safe closed, meaning they require active hydraulic pressure to stay open. This feature means that if control fluid fails the well will automatically shut itself in without operator action. The valve at the top of the vertical bores is called the swab valve and lies in the path used for well interventions such as wireline and coiled tubing operations. For such operations, a lubricator is rigged up onto the top of the tree and the wire or coil is lowered through the lubricator, past the swab valve and into the well. There is also an annulus swab valve to enable access to the annulus bore of the tubing hanger to facilitate plug setting in the tubing hanger.
Tree complexity has increased over the last few decades. They are frequently manufactured from blocks of steel containing multiple valves rather than being assembled from individual flanged components. This is especially true in subsea applications where the resemblance to Xmas trees no longer exists given the frame and support systems into which the main valve block is integrated.
Subsea trees may range in size and weight from a few tons to approximately 70 tons for high pressure, deepwater (>3000 feet) guidelineless applications. Subsea trees contain many additional valves and accessories compared to Surface trees. Typically a subsea tree would have a choke (permits control of flow), a floline connection interface (hub, flange or other connection), subsea control interface (direct hydraulic, electro hydraulic, or electric) and sensors for gathering data such as pressure, temperature, sand flow, erosion and flow measurement.
Subsea Xmas trees have a large variety of valve configurations and combinations of hydraulic actuated valves. Examples are identified in API Specifications 6A and 17D. The conventional tree has vertical bore valves. The two lower valves on each bore are called the master valves (upper and lower respectively). Master valves are normally in the fully open position and are never usually opened or closed when the well is flowing (except in an emergency) to prevent erosion of the valve sealing surfaces. The valves are hydraulically actuated, allowing a means of remotely shutting in the well in the event of emergency.
When a dual bore subsea Xmas tree is connected to a subsea wellhead it must interface with the tubing hanger previously installed in the wellhead. The tubing hanger and tree must be correctly orientated so that the production and annulus bores are properly aligned with each other.
An actuated wing valve is located on the horizontal flow path and is normally used to shut in the well
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TREE HUB
TREE CAP
ASV
PRODUCTION CHOKE ACTUATOR
HYDRAULIC STAB CONNECTOR
PSV
XOV
PT PT PUMV
AMV TEST PORT
CI CIV
PLMV
AWV
SCSSV LINE
PWV
MECHANICAL OVERIDE
WELLHEAD CONNECTOR
TUBING HANGER
ASV - Annulus Swab Valve AWV - Annulus Wing Valve PUMV - Production Upper Master Valve PWV - Production Wing Valve
AMV - Annulus Master Valve PSV - Production Swab Valve PLMV - Production Lower Master Valve XOV - X-over Valve
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API 6A Gate Valve The API 6A Gate Valve is the industry standard and can be fitted with a variety of internal parts that suit different types of produced medium.
Figure 23... Gate Valve
Crude oil as produced will have been analysed at the time the well was flow tested, this will have determined the proportions of water, oil and gas or Oil to Gas ratio. When the oil ratio is high say 90% or more with the balance mainly water with a little gas and no presence of Hydrogen Sophie( (H2S)) this will be an OIL WELL. The parts or trim of this valve will be as listed in table 1.1 (page 14) If the well is 95% gas with 2% water and 3% Carbon Dioxide (CO2) it will be a Gas well. The parts for this valve will be as listed in table 1.
Valve Actuators
If we have approximately equal parts of water, oil and gas this well will probable be left unfinished until later when it can be used as a Water Injection well so as to replace some of the oil that has been removed or to sweep oil towards another producer.
Valve Actuators are very specialized single acting hydraulic cylinders that are fitted to valve bonnets so they can be remotely functioned. Water based or oil fluid can be used as the power supply but where we are operating in an oceanic environment, water based is normally the preferred option.
Production gate valves vary in bore size and pressure/temperature rating for subsea production. They are usually 5” bore for oil and 6” bore for gas. With a temperature rating of -20/+250 (API. P-U) Pressure ratings of 20 and 30,000 psi with temperatures of up to 450oC are available for other oilfield purposes.
This fluid is a mixture of 80% water, 19% glycol and 1% biocide and lubricating agents. Certain companies prefer to use non foaming hydraulic oil and this undoubtedly offers considerably more lubricating qualities but since the units will rarely be functioned during there operational life lubrication is not an issue. Operating pressure for an actuator will vary depending on two things:
1) The operating pressure of the well. 2) The depth of the water. (1.3 lbs/mtr)
Inside the actuator are a piston and a large spring, because a spring is a highly stressed component we must make sure it is not allowed to become corroded. In order to do this, the spring housing will be filled with control fluid. When fluid is pumped in, the piston is displaced down, the spring gets depressed and its fluid is displaced into a holding chamber that is open to sea pressure at one end. This is the holding position for an open valve.
Figure 21... API Gate Valve
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The API requirement for Xmas tree valves is fail safe closed (FSC). In the event of a loss of input fluid from a malfunction, feed line failure or a system command the large spring assisted by sea pressure acting on the Sea Chest will push the valve closed and fill the spring housing with fluid. This is called fail safe closed.
During the life of a field, possibly as long as 20 years, the only routine maintenance of the system will be fluid filtration and the examination of any particles of metal, rubber or plastics since they will be our first indicator of wear or erosion taking place somewhere in the equipment. By identifying the particles we will be aware of something happening and this could avert a costly shut-in of the well.
Before fluid can be filled into a system it should be taken from sealed drums that contain pre-filtered fluid that are in compliance with specification NAS.1638 CLASS 6.
The actuator is attached to the valve body by the valve bonnet and is function tested as a complete unit.
When the system is completely charged it needs to be flushed. This flushing process will circulate the fluid aggressively around the system so as to dislodge any small microscopic particles from the actuators, the piping and any other part of the swept area.
These two parts are the interface and have to be supplied by the valve manufacturer and will allow any make of actuator to be fitted. The actuator is subject to the same trim material requirements and falls under the same API classification as the valve.
Fluid cleanliness is the key to system reliability and at the manufacturing stage a fluid sampling system will be used to make checks by taking small amounts of fluid and checking them for contamination by comparing the sample with a known standard under a microscope.
The external surfaces will be finished in high tech high visibility epoxy paints so as to ease relocation in low or no light conditions. All the actuators on a subsea tree are arranged to failsafe closed. Three forces will combine to do this;-
When compliance with NAS.1638 CLASS 6 is achieved the fitter, QC inspector and often a third party representative will sign the system off as fit for purpose.
The internal spring force. The ocean pressure on the sea chest. The flowing pressure of the well acting on the unbalanced stem (one end in one end out of the valve).
During the operating life of the system the fluid will slowly be circulated back to the control vessel or platform where it will be conditioned back to the NAS.1638 CLASS 6 standard.
The flowing pressure is by far the greatest and will automatically push the stem upwards and closes the valve.
System operating pressures will normally be from 3-5,000 psi in most areas of the world but can be as high as 20,000 psi so as to function the SCSSSV in a deep water high pressure well.
Figure 24... Actuator
During the Xmas tree installation the Xmas tree functions are controlled by the IWOCS system on the drilling rig. Once the tree is installed and operations on the drilling have been completed, the IWOCS system is recovered severing hydraulic control to the tree. The Tree Cap is then installed. The tree cap is fitted with a number of hydraulic couplers which mate up with the hydraulic lines on the Xmas tree, providing a hydraulic link between the tree functions and the Xmas tree control pod. This enables the Xmas tree to be functioned by the host platform. Conventional layout trees are made to be used in a variety of locations and water depths of up to 2,000 meters. 29
Figure 25... Adjustable Choke
Beyond this depth a different design will be used that we will see later. The guideline-less system for water up to 3500 meters uses the same parts but the are guided together by moving the drilling rig with the equipment hanging in the water until a funnel–up captures a sleeve down then orientation guides present the parts correctly to be locked together. When this activity is going on, our eyes are the cameras and lights onboard the ROV supporting equipment. With the exception of the cap, the tree will probably remain on the seabed for all its production life of twenty years or more.
deployed by a support vessel thereby avoiding the need for a drilling rig.
Within the frame work of the tree, we also have two other parts that play a major role during that period.
Control Pod
These are the: Choke and the Control Pod
The Control Pod is other item on the Xmas tree this is the unit that will receive electronic signals and translate them into hydraulic commands then send that to the requisite function. The unit is completely self contained and will function in all water depths simply by adjusting the operating pressure of the hydraulic section for valve actuators and the nitrogen atmosphere inside the casing is increased
The difference with them is we have the ability to remotely, with the ROV remove parts that have worn beyond an acceptable condition of have suffered some component failures.
Choke A Choke is bolted into the flow system downstream of the wing valve and as its name suggests it is a device that will regulate flow from the well. This device is a made very heavily with thick sections so as to withstand high pressure and erosion from flowing well fluids.
Figure 26... Control Module Internals
They are also fitted with an aperture that will be variable so we can make adjustments up or down on the flow rate from the Xmas tree the choke rating is quoted in controlled velocity (C.V.) There is a high rate of erosion of the choke insert, it is made from a very wear resistant man made alloy called Tungsten Carbide. The wear is usually higher at the start up and early years of high flow rate where particles are still breaking loose from the reservoir. The flow through the choke is supersonic and therefore it has a high decibel rating but because we are in some depth of water this is not an issue. When the insert is worn to where the necessary adjustment cannot be achieved it can be removed and replaced on the seabed remotely using an ROV 30
to prevent collapse from sea pressure.
Tree Cap
Hermetically sealed inside the outer brightly coloured can is the electronic package this is normally a solid state system that is designed to be long life and reliable. It also contains fluid accumulators that hold 210% for re total swept volume of all the valve actuators and the SCSSV. This volume is sufficient to give two complete cycles of the entire system. As high pressure fluid is used it will be replenished from the platform down the umbilical line.
Guide Posts
Xmas Tree
On the left is the removable orange upper pod sitting inside its angled catchment funnel with the “V” orientation slot at the front. On the right shows the electronics section and the orange actuator operating fluid storage accumulators. The umbilical line has a finite pressure rating, should we need a higher pressure fluid it may be impractical because of flow rate or physical size just make things bigger.
Production Guide Base
Drilling Guide Base
Figure 28... Tree Assembly with cap
We must then use an Intensifier to boost the pressure the intensifier works by applying umbilical pressure to a piston of a given diameter which will in turn boost the pressure at it’s outlet.
At this point a large and very strong plate will be used and this is populated with one half of the fluid couplers with the other half being on the underside of the control pod.
All the Xmas Tree pipework terminates in the section under the control pod and will be subjected to the same cleaning and flushing procedures.
The lower half will be fitted with a floating guide sleeve to reduce the positional criticality of the couplers. The two large nuts that will hold the upper and lower halves together and prevent separation of the couplings can be seen on the upper right and lower left of the plate. The area of the tree where this unit is located is well protected by bumper bars to ensure its safety.
Figure 27... Subsea Control Module
Cathodic Protection is part of the Xmas tree design and 99% pure zinc anodes will be welded to the framework to ensure a perfect connection. The amount of zinc needed is calculated using the temperature and salinity of the water and the estimated life of the immersion period.
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Figure 29... Hydraulic Intensifier
Figure 30... Control Module Mounting Base
Remember... The function of the Flowbase is to provide the export line orientation and isolation vale to shut off the flow.
Test Yourself
5
(a) Where is the master valve in a conventional tree, in a horizontal or vertical bore? (b) What is the function of the tree cap? (c) What is the function of the choke? (d) When is a hydraulic intensfier used in the control system?
You will find the answer to Test Yourself 5 on page 51
32
Section 6 - Spool or Horizontal Xmas Tree A wireline retrievable plug (tubing hanger crown plug) fits in the vertical passage of the tubing hanger above the horizontal passage. This is the primary pressure barrier.
One type of Xmas tree assembly, particularly used offshore, is known as a Horizontal tree. The tree mounts on top of the wellhead housing. The tree has a vertical bore and a horizontal or lateral production flow outlet. No valves are in the vertical bore of the wellhead / tree system. The completion is run after the tree is installed, and the completion can be pulled with the tree in place and flowline connections undisturbed.
An Internal Tree Cap (ITC) fits above the tubing hanger in the bore of the tree. The ITC is run and retrieved through the BOP stack using the tubing hanger tools. The cap seals and locks into the tree using the same mechanism as the tubing hanger.
The top of the tree is designed so that the BOP can be landed and locked on top of the tree. Eliminating the need to remove the tree to pull production tubing saves rig time especially for completions that require frequent retrieval of downhole equipment.
The tree cap has a vertical passage within which a second retrievable plug (ITC crown plug) fits creating a second barrier to the environment. The ITC plug is metal sealing with an elastomer backup..
Eliminating the need for an expensive, dualbore workover and completion riser system cuts equipment costs.
A tubing annulus between the tubing and the casing communicates to a lower annulus port formed in the tree. This port leads through an annulus passage to an upper annulus port which extends into the bore of the tree above the tubing hanger seals. One or more valves are used to open and close the tubing annulus. The upper tubing annulus port communicates with a void that is located between the tubing hanger crown plug and the seal of the internal tree cap.
A tubing hanger lands in the bore of the tree and is secured to a string of production tubing extending through the casing hangers and into the well. The tubing hanger has a lateral flow passage that aligns with the lateral passage of the horizontal tree. the tubing hanger has a concentric bore and side production outlet. During installation, the hanger orients, lands and locks to the bore of the tree body. At the bottom of the hanger, an orientation sleeve with a helix engages a key in the tree, to positively align the hanger outlet with the tree body outlet.
A debris cap covers and protects the tree top, and is non-pressure retaining. The cap is mechanically weight set and is run and retrieved with drillpipe. When landed, the cap 33
latches to the external profile on to the top of the tree body. The debris cap is filled with corrosion inhibiter to protect the ITC sealing areas.
and workover activities can be performed under full BOP control using a standard drilling riser and blowout preventer, which connects to an 18-3/4” profile on top of the tree body. Because tubing is installed after the tree is in place and the BOP connected, there is uninterrupted BOP protection from the moment the well is perforated. With the conventional tree system equipment, the tubing must be installed before removing the BOP and installing the tree.
The tree is run using an 18-3/4” Cam Actuated Running Tool (CART) which is connected to the tree upper mandrel. The CART is run on a landing string (typically tubing) and hydraulic control is provided by connecting the Installation and Workover Control System (IWOCS). Once the tree has been landed and locked to the wellhead the CART is released and recovered to surface. The Horizontal tree can be argued to be safer than ordinary trees. All completion Master Valve
Tree Cap Internal Tree Cap Tubing Hanger
Tree Block
SSSCV Wellhead Flowline
Figure 31 ...Horizontal (Spool) Tree General
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HORIZONTAL XTREE 51/2” TUBING WITH 4” PRODUCTION CHOKE & 2” GAS LIFT CHOKE INTERNAL TREE CAP
DEBRIS CAP CROWN PLUGS DHPTT ANNULUS PRESSURE TEMP. TRANSDUCER
1/2” TUBING HANGER VENT 1/2” DOWNHOLE CJ CHECK VALVE 1/2” SSCV LINE PROD, PRESS/TEMP TRANSDUCER
2” GASLIFT CHOKE
1/2” CHEMICAL INJ CHECK VALVE 4” PRODUCTION CHOKE
2” GASLIFT CHOKE
AAV PMV
AMV
PMV AMV
XOV
DUMMY TUBING HANGER
1/2” VX GASKET TEST
WELLHEAD 18 3/4”
SPLIT TUBING HANGER (OPTION) PRODUCTION
ANNULUS GAS LIFT
DOWNHOLE CHEM INJ 5 1/2” TUBING
DHPTT LINE 13 3/8” CASING
SCSSV TUBING
20” CASING
10 3/4” CASING
30” CONDUCTOR
AAV - Annulus Access Valve AWV - Annulus Wing Valve PWV - Production Wing Valve
AMV - Annulus Master Valve PMV - Production Master Valve XOV - X-over Valve
Test Yourself
6
(a) What are the advantages of running a Horizontal Tree as opposed to a Conventional Tree? (b) Where are the crown plugs installed? (c) How is the Horizontal Tree installed? You will find the answer to Test Yourself 6 on page 51 35
Section 7 - Well Intervention System Subsea Landing String
allows the string to be unlatched and re-latched as conditions require. Once unlatched, the upper section of the landing string is recovered leaving the lower section containing the dual ball valves in the BOP. The BOP blind / shear rams can be closed above the remaining SSTT section. In the event of hydraulic control line failure the latch can be unlatched manually by rotation.
When running or performing an Intervention on a well with a Horizontal Xmas Tree, the landing string system provides a means of shutting in the well and disconnecting in the case of an emergency. It is a critical component in the safety of personnel, the well and the rig. The landing string provides well control functions and disconnection capabilities during well installation, workover, intervention and well test operations on wells with horizontal subsea Xmas trees. The system provides the dual barriers required during intervention operations and while running the horizontal Xmas tree.
Retainer Valve The Retainer Valve is sits above the SSTT just above the BOP Shear Rams and is designed, in the event of a disconnect, to isolate the landing string contents and vent trapped pressure from between the Retainer Valve and the SSTT to the marine riser. The retainer valve closes prior to unlatching the subsea landing string from the xmas tree, thus preve http://cdn.inskinmedia.com/CreativeStore/ ps/BV4ZBI_1381940861/assets/left_watch.png nting any hydrocarbons in the landing string from escaping into the environment.
The landing string system consists of the following components: Sub Sea Test Tree (SSTT) The SSTT acts as the main safety barrier towards the well and is positioned on the bottom of the landing string assy. It is fitted with two fail safe close ball valves, each able to isolate & contain maximum well bore pressure from below. These valves can be configured to cut Coil Tubing and Wireline while still maintaining sealing integrity. It is designed to sit within the drilling BOP allowing the BOP rams to be closed around the SSTT providing a barrier to the production bore / annulus.
As the Landing String is bled, the Retainer Valve allows riser fluid to enter, displacing hydrocarbons from within the string. Lubricator Valve The Lubricator valve is located at the upper end of the landing string at a predetermined depth below the rig floor. This enables long intervention toolstrings (wireline or coiled tubing) to be made up or recovered from a live well. The Lubricator valve therefore allows the Landing String to be used in place of very long wireline lubricator assemblies on surface.
The SSTT has a pump through ability for well control in the event that hydraulic control pressure is lost. The SSTT is fitted with a disconnect facility in case of emergency. The hydraulic latch mechanism 36
Annular Ram Retainer Valve Shear Sub
Shear Rams Subsea Test Tree Integral Slick Joint Fixed/Variable Pipe Rams
Tubing Hanger Running Tool Adaptor Sub
Tubing Hangar Running Tool
Figure 32... Subsea Landing String
37
Figure 33... SSTT & Retainer Valve
Test Yourself
7
(a) What is the function of the landing string?
You will find the answer to Test Yourself 7 on page 51
38
Section 8 - Protective Structures Horizontal or Spool Tree Protective Structures
occasions the ROV is unable to carry out work before cutting away large sections of net snagged on the tree framework.
More and more effort is going into the design the making of the subsea well-site as environmental and fishing friendly as is practical.
No shelter is gained for small pipe-work during deployment and some smaller items can get dislodged or damaged during passage through the splash zone.
The Open Structure Tree Frame is designed to withstand a substantial pull without causing any damage to the Xmas tree components and offers a reduced snag profile to meet the requirements for fishing nets.
The onus falls on the driller and his crew to get below the surface as expediently as possible to avoid wave damage Certain areas will be sensitive to ocean floor obstructions so the more natural the object they can be the better.
Even although the regulations preclude fishing in the immediate area of a subsea facility on many
Figure 34... Subsea Tree Deployment
Figure 35... Horizontal Tree
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The Totally Enclosed Xmas Tree
They must resist the pull of fisherman’s equipment such as Otter boards and nets.
The Totally Enclosed Xmas Tree is within a plastic casing, this is installed in two parts:-
The over-trawlability of the tree and its structure must stand a 50 ton shock pull without any distortion that could cause a mal-function to take place.
• The lower skirt section is a seabed level and prevents shifting sand from entering the well-site.
One International operator has enclosed the trees completely by using the clam shell approach.
• The upper unit is fitted with a hinge and can be opened to give open access for the ROV and closed for a streamlined shape. Apertures are fitted with plastic gratings to exclude larger fish.
Angled plates from the seabed up and opening upper sections expose the tree should any work be required and when it is all closed-up the tree is fully streamlined.
The protective structures that are built onto or surround a conventional Xmas Tree have to have considerable strength.
To save weight and maintain strength new composite material are being developed for this type of structure.
Figure 36... Xmas Tree under testing
Figure 37... Overtrawable Protection Structure
Figure 38... Overtrawable Protection Structure
40
Figure 39... Xmas Tree with two Control Pods
Test Yourself
8
(a) What force are the structures designed to withstand? (b) Why do we use a completely enclosed version? You will find the answer to Test Yourself 8 on page 51
41
Section 9 - Subsea Field Layout A field layout to produce a small reservoir with an economical production life of 30 years. FIELD CHARACTERISTICS • Gas Production: 20MSm3 / sd (223 kg/s) • Reservoir Pressure: 270 bar, 2280m • Reservoir Temperature: 91ºC • Expected life: 30 years • CO2 Re-injection @210 bar: 2600 Tun/sd, 0.03 kg/s
Subsea System
Figure 40... Field Layout
• 3 x 4 Slot Production Template/Manifold • 1 CO2 Injection Well • Subsea Control System • Workover System
Field Location • Norwegian Sea • 330m Water Depth • Transportation to Melkoya • Pipeline 160km • Electro Hyd Fibre Optic Umbilical • 9 Production Subsea Xmas Trees • 1 Injection Subsea Xmas Trees • 1 Workover Riser System
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Figure 41... Snohvit field Schematic
Templates
Drilling and completion work is now complete and you can see the four Xmas trees are in place.
As its name suggests, this is a component that has a number of well centres accurately incorporated into its structure.
With the manifold and control pod connected the protective structure is fitted with angled corner posts to comply with industry standards.
It is quite normal for 6-8 wells to be clustered together in a closely confined space and this means we can possible operate all of them from one control pod and umbilical line making a saving on the capital budget of a substantial amount of money. Because this is a one piece unit, the flow-line, umbilical lines and piping are all built-in. Many connections will automatically make-up together when the Xmas tree is landed in its allocated position.
This type of template will be fitted with Xmas trees especially designed for re-injecting carbon dioxide that is a bi-product of some other process like exhaust gasses from a gas burning power station.
Figure 42... Manifold Well Slots
When mixed with water this gas becomes very corrosive so all wetted parts will be to API HH Trim. This technology is new and is likely to become
The large cylinders are the suction feet these units weight will sink it into the sand, water will be allowed to escape from the top of each cylinder and it will sink-in. we will achieve it level to half of one degree using this method. Guide posts can be seen protruding from the four corners of each drilling guide base and a depth indicator can be seen marked on the cylinder on the right. When landed, all posts marked No1 will face due North. 43
widespread as reservoirs become depleted of oil we can use them for the long term shortage of all types of greenhouse gasses. A lot of development will have to take place before a system like this and the associated pumping systems will see it in common use. This is an example of a CO2 Xmas tree the format is horizontal or Spool design and will fit into and connect up to the facilities provided on the template. Because of the nature of this trees service conditions and the restrictive access to two sides all the intervention or adjustment points are carefully worked out. Top access will be used for choke and control pod removal or replacement. Here the tree is on the rig finger deck being checked to ensure all the systems are operating before deployment. Figure 44... Field Layout
The ROV intervention docking buckets are see with there function marked on tape used to cover the luminescent signs because they we be impaired if exposed to direct sunlight.
the choke, control pod and the panels that make up the over-trawl protective structure. On the left of the template are two flow lines the top one is a 16” oil export lin going to the shore terminal for onward shipment by tanker and the lower smaller one is bringing CO2 from shore for injection back into a zone that does not connect to the oil reservoir.
The tape is removed just before deployment. What do the signs mean? PMV...............................Production master valve. MIV.2...................... Methanol injection valve No2 XOV.............................................Cross over valve
To the right are tooling packages that will be required from time to time is a pigs unit (Propelled, Information, Gathering System) this will be pushed along the 16” line by the oil flow and check the line for corrosion, cracks and other defects storing the information as it goes.
This illustration shows the IWOCS system as it is on the rig floor with its riser and other tooling and control umbilical reels. Below it is the well control package, Xmas tree and support structures. Above the template are the remove/replace tools for
Figure 40 shows the gas injection Xmas tree in the test pit with the safety equipment stacked and locked to its top. The pit will be filled with water and pressure tested with Nitrogen gas to its maximum working pressure and held for 60 minutes. No bubbles are allowed over that test period. The size of the stacked-up system is evident as seen here in the test pit prior to it being filled with water in preparation for nitrogen gas testing. At this stage the assembly would weigh approx 120 tons.
Figure 43... Production Equipment Installed
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Figure 45... Subsea Equipment Components
Remember... ROV Intervention Docking Bucket label meanings: PMV.............. Production Master Valve MIV.2...Methanol Injection Valve No. 2 XOV...........................Cross Over Valve Figure 46... Gas test in water
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Test Yourself
9
(a) How is the structure held in position? (b) What is a template? (c) How are the produced fluids transported to the terminal? You will find the answer to Test Yourself 9 on page 51
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Section 10 - Case Study This is a case study of a modern subsea development where the water depth is 1200m and beyond the capacity of guidelines. The production is oil that has high paraffin wax content. The oil temperature at the seabed and through the Xmas tree is moderate so it is necessary to insulate the tree from the cold seabed temperature of +2 degrees Centigrade.
This field will produce about 120,000 barrels per day, and will be connected to a ship shape vessel called a Floating Production, Storage and Off-loading Unit. The (FPSO) is anchored in position and will process the produced fluids and transfer them to an export tanker.
Hydrates seen as gas bubbles are also present in the immediate vicinity.
A total of 16 trees will be installed and the field life is estimated to be 22 to 25 years.
The initial installation and any field life interventions need to be provided for and incorporated into the tree design in such a way that work can be done using specialized vessels instead of a full blown drilling rig.
At some point during that period some maintenance or repair work will be needed for example:Corrosion or erosion in the production tubing, Damage or wear in the SCSSV, Wear of the choke insert or its adjusting mechanism.
The functioning of a production subsea tree systems and valves is considerably less than a platform surface tree this will save on normal wear and tear so that should a problem develop below the tree. If it is of horizontal or spool configuration it can be reworked without the removal of the tree.
Production Xmas tree valves will have been specified to have a 25 year operating life without any maintenance hence the need for strict manufacturing controls on the assemble of valves and actuators the long term lubricants used and their ability to function in ever increasing hostile conditions.
In this study, after a tree shut down the wax content is of some concern, it is necessary to slow down the cooling rate and keep the tree warm this will keep the oil from thickening and ultimately solidifying and making the tree non-functional.
Because of the number of trees in the field it is more economical for the operator to buy their own tools and have them maintained and tested prior to each time they want to use them.
Tree insulation is crush resistant epoxy foam that is applied to all the areas that normally carry the production fluids areas like the tree block, the choke and the flow line connection.
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Field conditions are:
The equipment and the running tool stack-up are shown here.
Field:............................... West Africa
Lower most is the Tubing Spool, it provides the connector to lock on to the wellhead and a mandrel that mimics the wellhead with the internal bore to land, seal and lock the Tubing hanger in place. The export line also is attached here.
Water Depth:.................. 1200 metres Tree Type: ...................... 5” x 2” Conventional Pressure:........................ 5,000 psi
Above that is the Xmas Tree and that is locked on to the tubing spool (the white vertical pipe between the two is the insulated flow line downstream of the choke and where the export flow line will be connected.
Temp:.............................. API Class U (-18C to 121C) Weight (Tree): ................ 31 Tons Weight(TH Spool):......... 25 Tons
The two components above the tree are running tools The Lower Riser Package (LRP) and the Emergency Disconnect Package (EDP) these two are normally kept together however if an emergency arises when the tree is locked to the seabed and the drilling rig is still attached on the surface we can close the lower and release the upper to allow the rig to deal with the surface emergency.
Trim: ............................... Prod H-H, Annulus E-E Valve:.............................. API 6a Gate valves Actuator Type:............... Hydraulic Overrides: ..................... ROV Rotary W’Head Connector:....... MD-H4 Choke:............................ KI Stepping Type
Figure 48... Hydrate build up
Control Pod: .................. OS Multiplex Type Wellhead System: ......... Heavy duty High capacity Flowline Connector:...... 5-1/8”
Hydrates are gas molecules encased in an ice shell and where the seabed is soft they will rise to the seabed. The ice melts and we then have frozen free gas, as this gas rises towards the surface it can get trapped by our Xmas tree or other parts. When this happens it can cause a malfunction by freezing moving parts as it forms into ice blocks. In this case we are interested in preserving production heat rather than allowing the tree to freeze so as long as we have the well flowing everything will be OK. When hydrates are know to be present the drilling rig will set a Mud-mat at seabed level this is a steel plate that is larger than any equipment the will be used or fitted above it. Any hydrates will then have to migrate out towards the plate edges then continue upwards outside the equipment in a harmless fashion as the picture below shows. This is about 3 meters above the seabed.
Figure 47... Equipment Stack-up
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Figure 51... Tree Thermal Insulation
Production Heat must be conserved at all costs and this is why the areas that contain vital valves and other parts have to be protected from the cold ocean by a high strength and high efficiency epoxy foam. The seabed conditions in this case at 1200 meters will be 2 degrees Centigrade and 1580. phi. The window of time available to us to remedy any problem or take action using chemicals to prevent the solidification of the wax content of the oil is small only a few hours. The temperature slices shown here illustrate how the core temperature drops in a time frame of 12 hours. It can be seen how all the heat has been lost and a serious situation would exist. Although this has never happened it illustrates the planning and equipment design development and testing that has to be done to let planning a recovery take place.
The Epoxy Foam Insulation as applied to the Xmas tree block is 75 mm thick. This makes the assembly of actuators and other components very difficult as can be seen here. The vulnerable part of the tree is the valve cavity area that contains the sliding gates. Solidified wax filling this will prevent the reopening of the cooled valve that was closed in the hot condition the actuators are not powerful enough to overcome this condition. The insulation will delay the point where it is impossible for the valves to operate to somewhere in the region of 57 hours but this may not be sufficient to save a well and that would be a multi-million loss in any currency.
Figure 49... Tree thermal profile (producing)
Before the Xmas tree can be approved for service. To prove the strength of the casing it must be proof tested using water at twice the operating pressure for 5,000psi (10,000 test) and 1.5 times for higher rated Xmas trees this is necessary since the casing has no second barrier between the reservoir and the ocean. A second functional test for valves, actuators and choke is done with nitrogen gas at the full rated pressure, and is held for one hour with no bubbles permissible.
Remember... We use a floating production system with off-take facilities to take the oil to the customer
Figure 50... Tree thermal profile (cooling)
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Figure 52... Tree deployment
Figure 53... Bonga FPSO
Here the Xmas tree is seen being lowered in to the water by the drilling rig by its running tool this will be hydraulically released after the tree is locked in place and pressure tested.
take the contents and deliver them to a customer in any part of the world. All FPSO’s are custom built and fitted for their principal use but can be rebuilt for a second life at a later date. This one has a capacity of 400,000 barrels ( x 42 = 1.68 million gallons.)
All that remains now is to connect the flow-line and the control umbilical. In this case these will run from a collecting, processing and exporting vessel called a Floating Production Ship with Off-take facilities (FPSO).
Rising vertically up the ships side we can see the risers that bring the high pressure oil from the seabed. They are rigid at this stage but because we are in deep water the vessel is held on station by a series of thruster that are controlled by computer that are taking constant information from a number of satellites.
An FPSO is a complex vessel that accepts the crude oil directly from the wells and separates it out into oil gas and water. The oil will be stored within a double skinned hull the gas will be used to power all the on board systems and the water spun clean in hydro-cyclone until it contains less than 40 parts per million (PPM) in this condition it is cleaner than the ocean so it can be put in there.
Below the hull, the risers will be flexible to allow for the slight movement of the vessel and in the event of a runaway they will drop and seal automatically to prevent damage or spillage.
When the FPSO’s tanks are full a large tanker will moor behind it and using a hose connection will off-
Test Yourself
10
(a) What is the water depth? (b) What is the problem we are addressing? (c) How do we overcome this problem? (d) Are there any other seabed hazards? (e) How do we take the oil to the customer? You will find the answer to Test Yourself 10 on page 51 50
Check Yourself...
All the answers to the Test Yourself questions.
Check Yourself 1
Check Yourself 6
(a) Fixed Platform (b) Tension Leg Platform (c) Subsea Development
(a) 1) Since the tubing hanger is run after the tree is in place, a workover requiring the tubing to be pulled can be achieved without recovering the tree. 2) The tree and tubing hanger do not require a purpose built completion riser system as the tree can be run on drillpipe. (b) One plug fits in the tubing hanger above the horizontal passage. The second plug fits in the Internal Tree Cap. (c) Using a Cam Actuated Running Tool (CART) run on a landing string.
Check Yourself 2 (a) The requirements for material, testing and certifications for subsea equipment in general
Check Yourself 3 (a) Mandrel or Hub (b) 7 Million pounds (c) 3 Casing hangers (d) Fits onto top of Blowout Preventer (BOP)
Check Yourself 7 (a) The landing string provides well control functions and disconnection capabilities during well installation, workover, intervention and well test operations on wells with horizontal subsea Xmas trees.
Check Yourself 4 (a) Flowline and Umbilical (b) Hydraulic pressure (c) It suspends and seals the removable tubing string inside the wellhead (d) The IWOCS controls and monitors the deployment, the operation and the retrieval of subsea production equipment such as Tubing Hangers, Landing Strings and Xmas Trees
Check Yourself 8 (a) 50 tons snag load (b) To protect subsea production equipment and fishing nets and gear
Check Yourself 9 (a) Suction legs (b) A component that has a number of well centres accurately incorporated into its structure. (c) By pipeline
Check Yourself 5 (a) A verticle bore (b) It provides a hydraulic link between the tree functions and the Xmas tree control pod. This enables the Xmas tree to be functioned by the host platform. (c) It is a device that regulates the flow from the well. (d) To boost the pressure from the umbilical.
Check Yourself 10 (a) 1700 metres (b) Wax (c) Insulation (d) Hydrates (e) Floating production system with off-take facilities (FPSO) 51
Subsea Equipment & Tools Summary...
Having studied the capital drilling and production equipment and tools you should have a grasp of how these building blocks go together. It is important to remember that every single part of the subsea system has to be able to perform for the full field life without maintenance if necessary other than the removal of external debris such fishing equipment or seabed damage. Similar equipment is used for most subsea developments but will vary for increased pressure or temperature. Oil Companies such as Shell and BP will apply some of their own standards above and beyond those of the industry (API) ones. This will ensure that they take into account any specialities they feel is necessary they just cost extra.
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