Multiple Riser Configurations
Top-Tensioned Risers
Hybrid Riser System
RISERS – ANALYSIS & DESIGN Risers:- A short Outlook
Conduits to transfer materials from the seafloor to production and drilling facilities atop the water's surface
Transfer materials from the facility to the seafloor, subsea risers are a type of pipeline developed for this type of vertical transportation.
Serving as production or import/export vehicles, risers are the connection between the subsea field developments and production and drilling facilities.
Similar to pipelines or flow lines, risers transport produced hydrocarbons, as well as production materials, such as injection fluids, control fluids and gas lift.
Insulated to withstand seafloor temperatures, temperatures, risers can be either rigid or flexible.
RISERS – ANALYSIS & DESIGN Risers:- A short Outlook
Conduits to transfer materials from the seafloor to production and drilling facilities atop the water's surface
Transfer materials from the facility to the seafloor, subsea risers are a type of pipeline developed for this type of vertical transportation.
Serving as production or import/export vehicles, risers are the connection between the subsea field developments and production and drilling facilities.
Similar to pipelines or flow lines, risers transport produced hydrocarbons, as well as production materials, such as injection fluids, control fluids and gas lift.
Insulated to withstand seafloor temperatures, temperatures, risers can be either rigid or flexible.
RISERS – ANALYSIS & DESIGN Risers:- A short Outlook
Attached Risers – Used on Fixed Platforms, Concrete gravity Structures & Compliant Towers.
Steel Catenary Risers – Connect Seafloor facilities to production facilities above as well as two floating production platforms.
Top Tensioned Risers – Completely vertical systems that terminate directly below the facility.
Flexible & Hybrid Risers – Ideal for floating facilities as vertical & horizontal movement take place.
Single Line Offset Risers – Relatively new, consists of a vertical steel riser section connected by a jumper to the production vessel.
Drilling Risers – Connect the subsea BOP stack at the bottom to the rig at the top, and transport the drilling fluid to surface.
RISERS – ANALYSIS & DESIGN Introduction to Riser Systems Attached risers are deployed on fixed platforms, compliant towers and concrete gravity structures. Attached risers are clamped to the side of the fixed facilities, connecting the seabed to the production facility above. Usually fabricated in sections, the riser section closest to the seafloor is joined with a flow line or export pipeline, and clamped to the side of the facility. The next sections rise up the side of the facility, until the top riser section is joined with the processing equipment atop the facility. Pull tube risers (used on fixed structures ) are pipelines or flow lines that are threaded up the center of the facility. For pull tube risers, a pull tube with a diameter wider than the riser is preinstalled on the facility. Then, a wire rope is attached to a pipeline or flow line on the seafloor. The line is then pulled through the pull tube to the topsides, bringing the pipe along with it. Steel catenary risers use this curve theory, as well. Used to connect the seafloor to production facilities above, as well as connect two floating production platforms, steel catenary risers are common on TLPs, FPSOs and spars, as well as fixed structures, compliant towers and gravity structures. While this curved riser can withstand some motion, excessive movement can cause problems.
RISERS – ANALYSIS & DESIGN Introduction to Riser Systems Used on TLPs and spars, top-tensioned risers are a completely vertical riser system that terminates directly below the facility. Although moored, these floating facilities are able to move laterally with the wind and waves. Because the rigid risers are also fixed to the seafloor, vertical displacement occurs between the top of the riser and its connection point on the facility. There are two solutions for this issue. A motion compensator can be included in the toptensioning riser system that keeps constant tension on the riser by expanding and contracting with the movements of the facility. Also, buoyancy cans, can be deployed around the outside of the riser to keep it afloat. Then the top of the rigid vertical top-tensioned riser is connected to the facility by flexible pipe, which is better able to accommodate the movements of the facility. Riser towers were built to lift the risers the considerable height to reach the FPSO on the water's surface. Ideal for ultra-deep water environments, this riser design incorporates a steel column tower that reaches almost to the surface of the water, and this tower is topped with a massive buoyancy tank. The risers are located inside the tower, spanning the distance from the seafloor to the top of the tower and the buoyancy tanks. The buoyancy of the tanks keeps the risers tensioned in place. Flexible risers are then connected to the vertical risers and ultimately to the facility above.
RISERS – ANALYSIS & DESIGN Introduction to Riser Systems A hybrid that can accommodate a number of different situations, flexible risers can withstand both vertical and horizontal movement, making them ideal for use with floating facilities. This flexible pipe was originally used to connect production equipment aboard a floating facility to production and export risers, but now it is found as a primary riser solution as well. There are a number of configurations for flexible risers, including the steep S and lazy S that utilize anchored buoyancy modules, as well as the steep wave and lazy wave that incorporates buoyancy modules.
Hybrid Riser System
RISERS – ANALYSIS & DESIGN Introduction to Riser Systems While production and import/export risers transfer hydrocarbons and production materials during the production phase of development; drilling risers transfer mud to the surface during drilling activities. Connected to the subsea BOP stack at the bottom and the rig at the top, drilling risers temporarily connect the wellbore to the surface to ensure drilling fluids to not leak into the water.
RISERS – ANALYSIS & DESIGN Objectives
Impart an understanding in the function, technology, design of riser systems.
Introduce tools and methodologies required for global riser analysis.
Review basic pipe mechanics and summarize standard fatigue analysis methods.
Describe ocean environment, wave models, and vessel motions.
Provide understanding of riser response and vortex induced vibration.
RISERS – ANALYSIS & DESIGN Design Requirements and Considerations Riser configuration design shall be performed according to the production requirement and site-specified. Static analysis shall be carried out to determine the configuration. The following basis can be taken into account while determining the riser configuration:
Global behavior and geometry Cross sectional properties Means of support Material costs Structural integrity, rigidity and continuity
The riser system must be arranged so that the external loading is kept within acceptable limits with regard to:
Tension Bending Torsion Compression Interference
RISERS – ANALYSIS & DESIGN Design Requirements and Considerations - Layout
The first step in riser design is to develop to address the pipeline approaches towards the platform in close co-operation with the Client. Next for each individual pipeline a routing for the riser through the jacket bracing has to be established. The riser must be routed close enough to the main jacket legs or bracing members such that it can be supported at regular intervals. Routing the riser alongside a main leg allows supports to be spaced at almost any desired interval. Large diameter risers sometimes only need to be supported at plan bracing elevations. Smaller diameter risers may have to be housed in (or outside) a caisson in order to provide adequate support. In developing a layout all bends in the riser should be bent at a five diameters bend radius or more, to permit pigging operations.
RISERS – ANALYSIS & DESIGN Design Requirements and Considerations – Anchor Point
The ideal riser design incorporated a long straight run of pipe down the platform face, the dead weight of the pipe is hung in tension on an anchor flange above waterline. Above and below this anchor flange, riser supports are then designed to allow the riser to lengthen axially due to the pressure and temperature increase caused by the hot fluid flow. Movement downwards form the anchor point is then restrained only by the bending stiffness of the horizontal run at the jacket base. A carefully planned layout can considerably reduce the bending stresses caused by expansion and ensure that the riser is never in nett axial compression.
RISERS – ANALYSIS & DESIGN Design Requirements and Considerations - Protection
The protection of the risers in the splash zone is particularly important as the risers must be protected against possible boat impact. Either the riser must be protected in the splash zone or routed behind a jacket structural member. Risers should never be supported from a member susceptible to ship impact damage. Routing must also bear in mind that risers will be inspected annually and they should thus not be routed too far inside the structure such that diver access will be difficult. Following the piper Alpha disaster it is prudent to route the risers as far away from the living quarters as possible. This is to prevent the consequence of jet fires.
RISERS – ANALYSIS & DESIGN Design Requirements and Considerations – Connection to Pipeline Exit from the jacket structure at seabed level will be dependent upon subsea pipeline routing. It should also be borne in mind that the riser, when pre-installed, must have adequate clearance form the (substructure) transportation barge. The riser termination should have sufficient clearance from the jacket and other risers for a hyperbaric chamber, if welded to the pipeline. There will be a limited number of suitable locations for routing the riser through the jacket structure and it is important that the topsides layout engineers are made aware of these restrictions. Inspection Aspects
In order to facilitate easy riser inspection it is recommended to leave 1m clearance between the riser and its nearest obstruction.
Intelligent Pigging
In the selection of the internal or external diameter due consideration should be given to intelligently pigging. As a consequence it is recommended to maintain constant internal diameter and change the external diameter in accordance with the required wall thickness.
RISERS – ANALYSIS & DESIGN Riser System Components Riser joints
A riser joint is constructed of seamless pipe with mechanical connectors welded on the ends. Kill/choke lines are attached to the riser by extended flanges of the connector. The riser can be run in a manner similar to drill pipes by stabbing one stalk at a time into the string and tightening the connector.
Flexible joints
Flexible joints allow limited angular motion of the riser. In some cases, these flexible joints may be a series of ball joints. Pressure compensated flexible joints should be used to decrease the torque required to deflect the joint. The forces acting on the joint push the inner ball against the outer casing, causing the joint to bind. To decrease the required torque hydraulic fluid is injected to spread apart and lubricate the moving parts. With the large area involved, relatively small pressure are required.
RISERS – ANALYSIS & DESIGN Riser System Components Slip joints
A slip joint comprises two concentric cylinders or barrels that telescope. The outer barrel is attached to the marine riser, and the riser is held in tension by wire ropes from the outer barrel to the tensioner.
Buoyancy modules
Buoyancy modules can be attached to the riser to decrease the tension required at the surface. These modules may be thin-walled air cans or fabricated syntactic foam modules that are strapped to the riser. These buoyancy modules require careful design and the material for their construction needs to be selected appropriately so as to ensure that they have a long-term resistance to water absorption.
RISERS – ANALYSIS & DESIGN Riser System Components (Auxiliary Components) End Fittings
The end fittings provide the important function of ensuring that the riser loads (in tension, bending and torsion) are satisfactorily resisted whilst ensuring that a comprehensive sealing system is attached both radially and axially. The adequacy of terminations must be determined through careful detailed design, prototype as well as through in-service experience.
Bending stiffener
This is normally located at the bottom and top connections. The purpose is to provide additional resistance to over-bending of the riser at critical points (such as the ends of the riser, where the stiffness is increased to infinity).
RISERS – ANALYSIS & DESIGN Design Codes
API 16Q for drilling riser
API 2RD for production riser attached to floating systems
API 17B for flexible pipes
ISO 13628-5 for steel tube umbilical
API 17A: Design and operation of subsea production systems
API 17B: Flexible pipe
API 17C: Through flow line systems
API 17D: Subsea wellhead and Christmas tree equipment
API 17E: Subsea production control umbilical
API 17G: Design and operation of completion/work over riser systems
API 17I: Installation of subsea umbilical
API 17J: Un-bonded flexible pipe
API 17K: Bonded flexible pipe
RISERS – ANALYSIS & DESIGN References: Petroleum and natural gas industries ISO 13628 (Design and operation of subsea production systems) Part 1:General requirements and recommendations Part 2: Un-bonded flexible pipe systems for subsea and marine applications Part 3: Through flow line (TFL) systems Part 4: Subsea wellhead and tree equipment Part 5: Subsea umbilical Part 6: Subsea production control systems Part 7: Completion/work over riser systems Part 8: Remotely Operated Vehicle (ROV) interfaces on subsea production systems Part 9: Remotely Operated Tool (ROT) intervention systems Part 10: Specification for bonded flexible pipe Part 11: Flexible pipe systems for subsea and marine applications
ISO 14723 - (Pipeline transportation systems)
Subsea pipeline valves
ISO 13624 - (Drilling and production equipment)
Part 1: Design and operation of marine drilling riser equipment
RISERS – ANALYSIS & DESIGN Loadings: A comprehensive analysis of risers and attachments require the structural checking for many different loading conditions. These conditions can be categorized as follows
Functional loads (risers)
Transportation and installation loads
Environmental loads
RISERS – ANALYSIS & DESIGN Loadings: Functional Loads on Riser
Weights - For riser assessment the consequences of the following weights will have to be determined: self weight, contents (water), buoyancy and marine growth corrosion protection, flanges and supports. Internal pressure - The internal design pressure should be equal to the pressure specified in IP6** of 1.5 times the maximum working pressure. The maximum working pressure is to be used when combined with the extreme environmental condition. It should be noted that hydrostatic testing in the fabrication yard with the riser filled with water can be the limiting equivalent static stress check for the riser. External hydrostatic pressure - The external pressure will only be of importance for thin-walled, empty risers (or gas-risers) in deeper water. Thermal expansion - Through proper layout (see prev. slides) the effects of temperature differentials between the internal riser contents and exterior can be minimized. Subsea Movements - The loads imposed by the pipeline on the bottom end of the riser due to pipeline expansion or pipeline scour or permanent platform displacement are to be incorporated. Topsides interaction - The relative displacement of the piping system on the deck may have an effect on the riser. Slug Loading - This condition will occur when pigging the pipeline. ** Institute of Petroleum Model Code of Safe Practice, Part 6 Pipeline Safety Code
RISERS – ANALYSIS & DESIGN Loadings: Transportation & Installation Loads Transportation Roll, heave, pitch, yaw and sway accelerations during transportation can be predicted by linear motion theory. Predicted accelerations need to be resolved from space-centered axes into body-centered axes. Beam seas, head seas, and quartering seas should be considered although seas will generally produce the highest accelerations. Maximum resolved body forces are unlikely to exceed 0.75g. Parts of the structure (e.g. bottle sections, buoyancy tubes) may enter the water during transportation. Here it should be noted that linear theory will over predict accelerations but that model test data will give more realistic predictions. Furthermore risers and attachments may be subjected to wave slam which should be properly addressed. Launch and Upending Launch of a jacket off a barge can cause slam of the order of 5-10m/s as the top of the jacket enters the water. The maximum loading on an attachment will depend on the direction of impact. Maximum hydrodynamic loads should also be considered. Upending of selffloating tower structure may include 45° roll (as Magnus) which will cause significant slam loadings over the full height of the structure.
RISERS – ANALYSIS & DESIGN Loadings: Transportation & Installation Loads (Cont..) Pile Driving Vibrations In the past there have been various occurrences of damage to a platform as a result of pile driving. This damage could be on pile sleeves, anodes, grout ports etc. It should be recognized that high capacity piles require some 5000 blows per pile and that local acceleration due to driving can be as high as 100g. The trend to apply vertical piles will remedy the severity of these accelerations. The subject of fatigue of anode supports due to pile driving has to be addressed. In order to reduce damage to supports and attachments the following recommendation are made:
Apply larger diameter pipes and/or doubler plates for anode supports Use welding rather than bolting Employ half round pipes for grout distribution ring Arrange the routing of attachments away from the pile sleeves Check recent project design briefs on this subject
RISERS – ANALYSIS & DESIGN Loadings: Environmental Loads The design should include the following design checks. Check against the wave loads for the design winter storm. Water particle velocities can be extracted for wave grid runs for the structure. Because attachments are small, drag forces should predominate. The drag coefficient used in platform design (CD = 0.7) is significantly smaller then measured in a laboratory environment. Its justification is that it leads to realistic total platform leading and most platform members are governed more by total rather than local loading. For risers, conductors and caissons this is different and local loading is governing. Therefore it is good engineering practice to design these components using CD = 1.0 Increased water particle velocity allowance should be made for increased water particle velocity due to proximity of member to which attached. As a simple rule of thumb, twice the free stream velocity may be used at any point within half a tubular diameter. A more accurate relation is v = vo (1 + r²/a²) where r is the distance from the center and a the radius of this tubular.
Loadings: Environmental Loads (Contd…)
Checks against fatigue should apply a simple deterministic fatigue analysis in accordance with codes; in it waves will be grouped into a limited number of height classes, and water particle velocities calculated using the appropriate wave theory. For pre-installed risers and those risers early in the operational life of a platform the fatigue analysis can be incorporated in the substructure fatigue analysis. In order to allow connection of attachments to structural steel it is recommended that all primary structural members are sized to satisfy class F2 along the entire length in combination with a stress concentration factor equal to 1.0. Checks for slam loading where applicable, using the theory developed by Ridley which accounts for the natural frequency of the member. It is noted that wave slamming is only of relevance for horizontal sections of a riser in the splash zone or for risers during transportation.
RISERS – ANALYSIS & DESIGN Loadings: Environmental Loads (Contd…)
Check on (riser) pipes for vortex shedding in steady current. This is an important design consideration because it defines the distances between (riser) pipe supports. As a simple rule of thumb, pipe supports no further apart than 40 diameters will generally suffice, both for vortex shedding and static wave load. Imposed deflections by the jacket during storm sea condition. It is anticipated that this effect is negligible in most cases. An exception must be made when a platform will be installed on a soft foundation.
RISERS – ANALYSIS & DESIGN Parts of Riser:-
RISERS – ANALYSIS & DESIGN Parts of Riser:-
RISERS – ANALYSIS & DESIGN Parts of Riser:-
Risers connected to Mudmat
RISERS – ANALYSIS & DESIGN Parts of Riser:-
Risers connected at intermediate level with guide clamps
RISERS – ANALYSIS & DESIGN Parts of Riser:-
Risers connected at Top with Hanger clamps
RISERS – ANALYSIS & DESIGN Design Philosophy and Considerations The design philosophy adopted in this chapter is to apply proved technical advances in order to conduct safe and cost-effective design of marine risers. The design of a marine riser system will require consideration of a number of factors in relation to its functional suitability and long term integrity. Considerations should be given to:
Consistence with laws, acts and regulations; Riser integrity: reliability, safety and risk; Riser functional requirements; Riser operational requirements; Riser structural design criteria; Materials; Installation requirements; Fabrication requirements; Inspection and maintenance; Costs.
RISERS – ANALYSIS & DESIGN RISER STRUCTURAL ANALYSIS PROCEDURES COMPUTER SOFTWARE Various analysis tools are available for riser design, examples of these are:
General purpose finite element programs: ABAQUS, ANSYS, etc;
Riser Analysis Tools: Flexcom, Orcaflex, Riflex, etc;
Riser VIV Analysis Tools: Shear?, VIVA, VIV ANA, CFD based programs;
Coupled motion analysis programs: HARP, etc;
Riser Installation Analysis Tools: OFFPIPE, Orcaflex, Pipelay, etc.
RISERS – ANALYSIS & DESIGN
RISERS – ANALYSIS & DESIGN RISER STRUCTURAL ANALYSIS PROCEDURES GENERAL The design of the risers shall be based on the Institute of Petroleum Pipeline Safety Code (Latest Edition) which is Part 6 of IP Model Code of Safe Practice, and IP6 Supplements including revisions. (Other codes shall also be applicable as per project / client standards. With the exception of the hoop stress check, all calculations are to be based on nominal wall thickness. The hoop stress is to be checked using minimum thickness defined as 12½% less than nominal thickness for diameters less than or equal to 18" and 5% less than nominal for diameters greater than or equal to 20". All stress criteria are to be satisfied for all points on the riser.
RISERS – ANALYSIS & DESIGN RISER STRUCTURAL ANALYSIS PROCEDURES Riser analysis tools are special purpose programs for analyses of flexible risers, catenary risers, top tensioned risers and other slender structures, such as mooring lines and pipelines. The most important features for the finite element modeling are listed below: Beam or bar element based on small strain theory. Description of non-linear material properties. Unlimited rotation and translation in 3D space. Stiffness contribution from material properties as well as geometric stiffness. Allowing varying cross-sectional properties.
RISERS – ANALYSIS & DESIGN RISER STRUCTURAL ANALYSIS PROCEDURES Typical analyses are for instance: Strength analysis; Fatigue Analysis; VIV Analysis; Interference Analysis. The results from the finite element analysis are listed below: Nodal point co-ordinates; Curvature at nodal points; Axial forces, bending moment, shear forces and torsion.
RISERS – ANALYSIS & DESIGN RISER STRUCTURAL ANALYSIS PROCEDURES Time domain analysis and frequency domain analysis
The purpose of the analysis is to determine the influence of support vessel motion and direct wave induced loads on the system. The results from the frequency domain analysis are the systems Eigen frequencies and eigenvectors. The results from the time-domain analysis are time series of a selected limited response parameters, such as stress, strain and bending moment.
RISERS – ANALYSIS & DESIGN RISER STRUCTURAL ANALYSIS PROCEDURES The results from the above analyses are stored in separate files for subsequent post processing, such as plots or calculation. Some of the more interesting output is listed below: Plots System geometry; Force variation along lines; Pipe wall forces; Geometry during variation of parameters; Response time series; Vessel motion transfer function; Animation of the dynamic behavior of the complete system including support vessel and exciting waves. Tables Support forces; Pipe wall forces; Velocities and accelerations from wave and vessel motion time series;
RISERS – ANALYSIS & DESIGN RISER STRUCTURAL ANALYSIS PROCEDURE – Hoop Stress The maximum hoop stress ( h) (IP-6 Section 4.2.2) should not exceed 60% of the specified minimum yield stress ( y), and is calculated by:
ℎ = ≤ 0.60 Where P ro tmin
= Design Pressure = Outside Radius = Minimum Wall Thickness
The minimum wall thickness is equal to the nominal thickness minus the thickness tolerance and the thickness corrosion. The hoop stress may be increased to 0.90 y for the hydrostatic pressure condition.
RISERS – ANALYSIS & DESIGN RISER STRUCTURAL ANALYSIS PROCEDURE – Expansion Stress The expansion stresses ( e) due to pressure and temperature (IP-6 Section 4.2.5.7) are calculated according to the formula below and should not exceed 72% of specified minimum yield stress:
=
. 2 . 2
2
1 ≤ 0.072
Where MA MB MC ii io Z
= = = = = =
Torque In-plane Bending Moment Out of plane Bending Moment In-plane intensification factor Out of plane intensification factor Section Modulus
All moments are due to the pressure and temperature expansion of the riser. The displacements and forces at the riser to spool piece flange shall be taken into account.
RISERS – ANALYSIS & DESIGN RISER STRUCTURAL ANALYSIS PROCEDURE – Longitudinal Stress The longitudinal stresses ( L) due to the combined effect of weight ( w) and internal pressure (p) (IP-6 Section 4.2.5.7) should not exceed 54% of the specified minimum yield stress and shall be calculated as follows:
=
(+) +
1
Where Fa = Axial Force A = Pipe metal Cross sectional Area
3
= ≤ 0.54 .2 = (2 − 2)
2
Where ro = Out of plane Bending Moment ri = In-plane intensification factor
RISERS – ANALYSIS & DESIGN RISER STRUCTURAL ANALYSIS PROCEDURE STRESSES DUE TO SURGE Contractor shall consider in his design the results of a surge analysis to be advised by Client. SLUG LOADINGS Contractor shall consider the effects of slug induced loadings in the design of the risers. Loadings and design procedures, including fatigue, shall be agreed with Client.
RISERS – ANALYSIS & DESIGN RISER STRUCTURAL ANALYSIS PROCEDURE – Allowable Stresses The following environmental load stress check not specified in the IP code must also be satisfied. This is a check that the riser will not fail under the maximum operating plus extreme environmental loads. The intention is not to exceed the Von Mises equivalent stress criterion. The total stress ( T) defined below shall be considered:
= ± ≤ 0.9 Where
L
S
= Longitudinal Stress = Max. Direct Stress due to 100yr Storm waves and currents
RISERS – ANALYSIS & DESIGN RISER STRUCTURAL ANALYSIS PROCEDURE- Allowable Stress Von Mises equivalent stress (eq) is calculated according to the following formula for all points on the riser, and should not exceed 90% of specified minimum yield stress:
= √ − 2 − ℎ 2 ℎ − 2 32 ≤ 0.9 Where
h
p
T
= Pressure Hoop Stress = Internal Pressure = Total Longitudinal Stress due to weight, pressure, temperature and wave loading = Shear stress due to torque in the riser
RISERS – ANALYSIS & DESIGN RISER STRUCTURAL ANALYSIS PROCEDURE It is noted that the allowable Von Mises stresses in the DNV pipeline & riser code are somewhat different; a full set allowable stresses is given in the table below.
STRESS/YIELD STRESS
HYDRO TEST
OPERATING
SURVIVAL
0.90
0.60
0.60
Longitudinal Stress
~NA~
0.54
~NA~
Tensile Stress
~NA~
0.90
0.90
0.90
0.60
0.80
Hoop Stress
Von Mises (DNV)
RISERS – ANALYSIS & DESIGN RISER STRUCTURAL ANALYSIS PROCEDURE – Fatigue Design The ability of the risers to withstand cyclic loading shall be considered. The Department of Energy F2 curve / DNV / API and Miner's rule shall be applied to assess the cumulative damage, including from the following sources:
Service cycles of internal pressure and temperature Wave loading Vortex induced vibrations Slug induced loadings
In addition a stress concentration factor (SCF) for single sided closure welds of 1.4 may have to be considered. The fatigue life thus calculated shall at all points be in excess of three times the design life of the jacket. It is common practice to carry out the detailed fatigue analysis on pre-installed risers or those risers which are installed at the beginning of the platform life as part of the substructure fatigue analysis.
RISERS – ANALYSIS & DESIGN RISER STRUCTURAL ANALYSIS PROCEDURE – Fatigue Design Service Cycles Fifty shutdown and start-up cycles each year shall be assumed when assessing the fatigue due to cyclic internal pressure and temperature loading. Note that pressure hoop stress may exhibit the largest direct stress range over most of the riser, rather than longitudinal stresses.
Wave Loading A deterministic analysis shall be used to assess fatigue. The stress may either be drawn from a dynamic analysis or a static analysis combined with dynamic amplification factors.
Vortex Induced Vibrations Based on DnV
RISERS – ANALYSIS & DESIGN FATIGUE ANALYSIS OF ANODE SUPPORTS Anode supports near the pile guides need to be designed against the consequences of environmental loading and pile driving vibrations. The first subject of environmental loading is addressed in Slides 23~25 . It is anticipated, however, that even when full marine growth and velocity enhancement together with a Cd=1.0 are used that this loading will not be governing for the anode supports. For a design of the anode support against fatigue attention should be given to accelerations, dynamic amplification, number of blows, attenuation, SCF and SN-curve which will subsequently be addressed.
RISERS – ANALYSIS & DESIGN FATIGUE ANALYSIS OF ANODE SUPPORTS ACCELERATIONS AS A RESULT OF DRIVING As indicated in slide 24, the accelerations due to pile driving can be as high as 100g. It is anticipated, however, that this high value is to be associated with the driving of inclined piles. Basic accelerations due to the driving of vertical piles can be taken as:
Zone Zone Zone Zone Zone
1: 2: 3: 4: 5:
Pile sleeves 50g Parts of structure adjacent to pile sleeves 25g All other areas of a pile cluster 10g Bottle leg/leg up to next plan level above 10g Remainder of the structure 0
DYNAMIC AMPLIFICATION Dynamic amplification can only be assessed accurately if the acceleration versus time history is specified or calculated using a timedomain analysis of the pile driving process. In the absence of a detailed analysis a uniform dynamic amplification factor (DAF) should be applied of: 1.5 < DAF < 2.0 .
RISERS – ANALYSIS & DESIGN FATIGUE ANALYSIS OF ANODE SUPPORTS NUMBER OF BLOWS The number of blows is to be taken from the pile drivability report; it will be of the order of 5000. In using this number in a simple and straight forward manner an additional conservatism will be incorporated because it assumes that target pile penetration will be reached using continuous heavy driving. ATTENUATION Each blow will lead to vibrations of the anode the amplitude of which will be reduced in time due to damping. Using a damping coefficient ksi = 0.02 the amplitude reduction for each full cycle will be reflected in a factor r given by: r = exp(-2ð ksi) = 0.88
RISERS – ANALYSIS & DESIGN FATIGUE ANALYSIS OF ANODE SUPPORTS STRESS CONCENTRATION FACTOR AND SN CURVE Since the thickness of the anode attachment is small in comparison with the thickness of the component to which it is attached it is common practice to adopt SCF = 1.0
RISER CLAMPS – ANALYSIS & DESIGN
RISER CLAMPS – ANALYSIS & DESIGN GENERAL DESIGN REQUIREMENTS Design Methods Clamps should be designed generally to the methods outlined in this Specification. Adequate elastic analysis should be performed to show that permissible stresses in bolts, clamp components, and jacket supporting members are not exceeded. Where ultimate criteria is adopted the load factors recommended in this specification should be used. Load Combinations The following load combinations should be checked so that permissible stresses or allowable load capacities are not exceeded under any of the following load combinations: a) Xp (Xp = Design Pre-tensioning Load) b) Xp+XF (XF = Functional loads such as dead loads, temperature loads) c) Xp+XF+XE (XE = Extreme environmental loads) NOTE: i) The load factors recommended in Section 2.0 (2), (3), (4) should be applied to the above combinations. ii) The stub and support should be designed using normal elastic design. A factor of 0.75 load combination (c) can be used for design of these components.
RISER CLAMPS – ANALYSIS & DESIGN GENERAL DESIGN REQUIREMENTS Slippage Friction clamps should be designed to have adequate safety factors against slippage. Safety factors recommended in Section 2.0 of this specification should be used. Fatigue The maximum permissible stress range in the stud bolts should be calculated and the fatigue life determined.
The reference life method of fatigue analysis enables the relatively simple calculation of fatigue life given that certain parameters pertaining to the Bass Strait wave envelopes are known and do not vary. An arbitrary stress range is input into the formula which has a unique reference fatigue life. A stress range of 690 MPa has been chosen as the reference.
RISER CLAMPS – ANALYSIS & DESIGN DESIGN PARAMETERS The following design parameters should be used for a standardized design of long bolted clamps. 1) Ultimate coefficient of friction (steel to steel/inorganic zinc silicate coating) = 0.25 2) Ultimate load safety factor on frictional resistance = 1.5 3) Ultimate load factor for punching of stub on top plate = 1.5 4) Load factor for checking and destressing of bolts= 1.1 to 1.21* * Recommended actual "induced pretension" should include a 10% increase on the design bolt pretension to allow for inaccuracies in bolt tensioning equipment and a further 10% increase for bolt relaxation if bolts are tightened one at a time. The recommended "induced bolt pretension" should not exceed 0.72Fy (396 MPa).
RISER CLAMPS – ANALYSIS & DESIGN RISER STRUCTURAL ANALYSIS PROCEDURES Elastic Design of clamp components Under the load combinations given in 1.2.2 with design to working stress methods in accordance with AS 3990, API RP2A or special provisions of this specification, whichever is appropriate.
Stud bolts shall be grade ASTM A193 Grade B7M (UTS = 690 MPa, Fy = 550 MPa).
Design bolt pretension is not to exceed 0.6Fy (330 MPa) The clamp components may be fabricated using either mild or high strength steel. The use of high strength steel will allow significant weight reduction to be achieved over mild steel however the susceptibility to fatigue will be increased.
RISER CLAMPS – ANALYSIS & DESIGN CHOOSING THE MOST SUITABLE CLAMP CONFIGURATIONS
RISER CLAMPS – ANALYSIS & DESIGN DESIGN LOADINGS Types of Forces on Clamps Design loadings on clamps are fundamentally environmental loadings arising from wave and current forces. Other types of loadings can affect clamp design in some applications. The following forces on clamps should be considered: Dead load Environmental loads wave current wind Pipeline forces expansion and contraction other forms of pipeline movement Platform movement Impact forces from vessels All other forces considered relevant.
RISER CLAMPS – ANALYSIS & DESIGN DESIGN PROCEDURES FOR LONG BOLTED CLAMPS
RISER CLAMPS – ANALYSIS & DESIGN DESIGN PROCEDURES FOR LONG BOLTED CLAMPS- Contd ….
A long bolted clamp using only four bolts and very stiff clamping shells has been adopted as standard and the formulae quoted have been developed on this basis. The stiff shells are considered necessary to eliminate the "pincer" effect of clamp shells that are relatively flexible and to ensure that the brace being clamped is deformed to fit the shape of the shell to ensure good frictional contact.
The use of more than 4 bolts can easily be accommodated as the formulae have been presented giving the required total clamping force. However in the majority of cases it is considered 4 bolts will be sufficient to produce the clamping force required.
RISER CLAMPS – ANALYSIS & DESIGN LOADS
The design loads shall be taken as the worst combination. Clamp Configuration - The most suitable configuration of clamp and support stub should be selected taking into consideration: the degree of restraint required amount of adjustment required ease of installation See Figure on next slide for typical clamp configurations. Component Loads - for design purposes, the loads acting on the clamp configuration are to be resolved about the major axes of each component. Refer to Figure (this slide) for details of force components.
RISER CLAMPS – ANALYSIS & DESIGN TYPICAL CLAMP CONFIGURATIONS
RISER CLAMPS – ANALYSIS & DESIGN RISER STRUCTURAL ANALYSIS PROCEDURES Max Allowable Bolt Pretension The clamped member hoop stress is limited by two criteria: a) The general requirement of AS 3990 that compressive hoop stresses be limited to 0.6Fy. b) Von Mises criteria for combined stresses assuming a 1.1 factor of safety for yield and using the maximum axial stresses expected in the member. Tension will be the critical axial stress.
Either a) or b) above may govern the allowable bolt tension, although b) will usually only be critical when the clamped member is in tension exceeding 110 MPa.
RISER CLAMPS – ANALYSIS & DESIGN Required Bolt Force Frictional Resistance, Radial Contact Pressure, and Bolt Force The frictional resistance of a long bolted clamp is a function of the radial contact pressure between the clamp and jacket tubular, which is a function of the bolt pretension force. A uniform radial pressure distribution along the clamp contact surface, resulting from bolt pre-tensioning is assumed. The relationship between contact pressure and bolt force is given below (refer next slide) The bolt design procedure will be divided into two basic parts: Frictional Resistance - Bolt force required to prevent clamp sliding; Radial Contact Pressure - Bolt force required to ensure that contact pressure is maintained at all points along the clamp. Design Bolt Force The design criteria discussed above will yield two bolt forces, one to prevent sliding and one to prevent loss of contact pressure at any location along the clamp. The design bolt force shall be taken as the larger of the two.
RISER CLAMPS – ANALYSIS & DESIGN RADIAL CONTACT PRESSURE FROM A GIVEN BOLT FORCE
RISER CLAMPS – ANALYSIS & DESIGN REQUIRED BOLT FORCE TO DEVELOP FRICTIONAL RESISTANCE Frictional Resistance There can be six components of external load acting on the clamp (see slide 61). However, only four of these components affect the required frictional resistance. These are: Torsion Transverse shear (combined with torsion) Longitudinal shear Pull-off force The above loads are used to calculate the bolt force required to prevent clamp sliding. The pull-off force is included here since it causes a net reduction in contact pressure, thus reducing the frictional resistance. The safety factor (SF) to be used in these calculations is 1.5. In addition to the above, axial tension in the clamped member will reduce the member diameter thus reducing the radial contact pressure and the frictional resistance. This effect shall be considered in the calculation of the bolt force required to prevent clamp sliding.
RISER CLAMPS – ANALYSIS & DESIGN TORSION + TRANSVERSE SHEAR
This is a conservative formulation which does not account for the contribution of frictional resistance from the bottom half of the clamp.
RISER CLAMPS – ANALYSIS & DESIGN LONGITUDINAL SHEAR
RISER CLAMPS – ANALYSIS & DESIGN PULL OFF FORCE This load will cause a net reduction in contact pressure along the entire length of the clamp, thus reducing the fictional resistance. The total bolt force must be increased to counter-act this load. Call this bolt force F3, F3= SF ⋅ P
RISER CLAMPS – ANALYSIS & DESIGN CALCULATING EFFECT OF JACKET MEMBER AXIAL STRESS
Total Total Bolt Force to Prevent Clamp Sliding To determine the total bolt force required to prevent clamp sliding, the individual components calculated above shall be combined as follows:
RISER CLAMPS – ANALYSIS & DESIGN Required Bolt Force to Prevent Loss of Contact Pressure Radial Contact Pressure In addition to preventing sliding failure as discussed above, another combination of loads shall be used to ensure that positive contact pressure is maintained locally. If contact pressure is maintained along the clamp, the initial bolt force will not change as a result of externally applied clamp loads. This is to prevent the continuous cycling of loads (stresses), thus enhancing the fatigue performance of the bolts. The loads that may affect the contact pressure are: Longitudinal moment Transverse moment Transverse shear Pull-off force
Note that while the first three components cause local changes in radial contact pressure, they do not cause a net contact pressure reduction, and thus, do not affect frictional resistance. Also note that the pull-off force affects both the contact pressure and frictional resistance. Again, axial tension in the jacket member shall be considered. The above load components will be factored, converted to equivalent bolt forces, and summed. Clamp sliding is a function of load and resistance, loss of contact pressure is only a function of loads. Clamp sliding represents "failure" whereas the consequences of loss of contact pressure near one or two bolts are minimal. If loss of contact pressure occurs, any additional applied load would increase the bolt load(s), but failure would not occur. As a result, a safety factor of 1.2 is used for loss of contact pressure calculations calculations It should also be mentioned that by using this criterion, loss of contact pressure would be a rare event and thus would not have a significant effect on bolt fatigue. A simple statics approach will be used to determine the required bolt force. To do this, assumptions will be made. A four-bolt clamp is assumed.
RISER CLAMPS – ANALYSIS & DESIGN LONGITUDINAL MOMENT
RISER CLAMPS – ANALYSIS & DESIGN TRANSVERSE MOMENT
RISER CLAMPS – ANALYSIS & DESIGN TRANSVERSE SHEAR
RISER CLAMPS – ANALYSIS & DESIGN Pull-Off Refer Slide No: 69
Effect of Jacket Member Axial Stress Refer Slide No: 70
Total Bolt Force to Prevent Loss of Contact Pressure Fc = F5 + F6 + F7 + F8 + F9
Critical Bolt Forces The critical bolt force should be the maximum of Fs and Fc (Slide 66~74 ).
RISER CLAMPS – ANALYSIS & DESIGN Check Jacket Tubular The clamped jacket member stress shall be checked for the combination of the following: Design stresses in member prior to clamping; Stresses caused by bolt pre-tensioning; Stresses caused by external loads on the clamp.
Check Local Buckling of Tubular
In addition to comparing the bolt pretension to the maximum allowable member hoop stress, local buckling of the clamped member shall be checked. The section between the clamp halves shall be checked for column buckling and the eccentricity of the load due to the curved section should be considered
RISER CLAMPS – ANALYSIS & DESIGN Top Plate of the Clamp Using a configuration of gusset plates that allow only a small clearance around the bolts, the critical top plate size is determined by punching shear forces and moments obtained from the connecting stub. Ultimate punching shear force if given by:
RISER CLAMPS – ANALYSIS & DESIGN Design of Clamp Components The design of the clamp members themselves should comply with the requirements of AS 3990 and/or API RP2A. a) Gusset Plates - The limiting width to thickness ratio shall be 6. Clearance to bolts shall be small (10mm) to allow bolt forces to be taken in direct bearing. b) Side Plates - Side Plates shall be designed as stocky struts. Particular attention should be paid to the weld along the base of the side plate, with the critical weld area immediately behind the gusset plates and weld failure plane at 45o to axis of clamp. c) Wrap Plate - This shall be the same thickness as the side plate, and it is recommended that minimum thickness be 20mm. It is recommended that all welds, except those fixing the side plates to the shell, be full strength butt welds. (refer next slide)
RISER CLAMPS – ANALYSIS & DESIGN Clamp Layout & Details
RISER CLAMPS – ANALYSIS & DESIGN Clamp Strength
The strength of the clamp shells in bending about both principal axes should be checked. For bending about the y-y axis of the clamp, it is assumed that the vertical component of loading on the tubular member follows a sine curve. The properties of the clamp shells should be determined, and the bending stresses evaluated for both across the width of the clamp and in a longitudinal direction.
RISER CLAMPS – ANALYSIS & DESIGN Installation Stresses during Re-tensioning
During the installation process the tubular brace will be deformed to take up the shape of the clamp shells provided that the bolt force is sufficiently high. Otherwise, the clamp may only achieve point contact eg. at a weld bead, and the shell should therefore be checked for either the bolt force or the force required to deform the member (MF), whichever is the lesser.
RISER CLAMPS – ANALYSIS & DESIGN Installation Stresses during Re-tensioning – Contd…
This should be less than the allowable bending moment across the width of each shell. NOTE In most cases the tubular will deform at relatively low bolt loads and the critical bending moments on the shell will be due to the full bolt load and the contact pressure.
RISER CLAMPS – ANALYSIS & DESIGN Typical Hinge Clamp Details
RISER CLAMPS – ANALYSIS & DESIGN
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RISER CLAMPS – ANALYSIS & DESIGN References : 1. Hollow Section Joints - Jaap Wardenier 2. Cojac User Manual - E.P.R. 3. Report on Design of Fortescue Repair Clamps - Wimpey Offshore 4. Sea load Computer Program – Theoretical Manuals 5. Strand7 Computer Program – Theoretical Manuals 6. Derivation of Fatigue Life for bolts - WGP-BC-01
RISER CLAMPS – ANALYSIS & DESIGN A TYPICAL ANCHOR CLAMP
