GUIDELINE ON UWILD OFFSHORE TECHNICAL GUIDANCE OTG-08, MARCH 2014
DNV GL Headquarters, Veritasveien 1, P.O.Box 300, 1322 Høvik, Norway. Tel: +47 67 57 99 00. www.DNV GL.com [Legal information]
OTG-08 UWILD guidance May 2014 rev 1.0.docx
TABLE OF CONTENT Guideline on UWILD ..................................................................................................................... 1 Offshore Technical Guidance OTG-08, March 2014 ........................................................................... 1 Table of Content .......................................................................................................................... 2 1
2
3
4
Introduction......................................................................................................................... 4 1.1
Introduction ................................................................................................................. 4
1.2
Objective ..................................................................................................................... 4
1.3
Scope .......................................................................................................................... 4
1.4
Structure ..................................................................................................................... 4
1.5
References ................................................................................................................... 4
1.6
Abbreviations ............................................................................................................... 5
Background ......................................................................................................................... 6 2.1
Regulatory Basis ........................................................................................................... 6
2.2
Position DNV GL............................................................................................................6
2.3
Risk assessment ........................................................................................................... 6
Approach............................................................................................................................. 8 3.1
General........................................................................................................................ 8
3.2
In-service Inspection plan (IIP)....................................................................................... 8
3.3
Flag Authorities............................................................................................................. 8
3.4
Survey Reporting .......................................................................................................... 8
3.5
Conditions.................................................................................................................... 9
Inspection scope ................................................................................................................ 11 4.1
General...................................................................................................................... 11
4.2
Applicable for all MOU .................................................................................................. 11
4.3
Column-stabilized units................................................................................................ 12
4.4
Ship-shaped units ....................................................................................................... 13
4.5
Self-elevating units/Jack-ups........................................................................................ 14
4.5.1
Introduction ........................................................................................................... 14
4.5.2
Separate surveys .................................................................................................... 14
4.5.3
Hull/Deckbox .......................................................................................................... 15
4.5.4
Leg and Spudcan ..................................................................................................... 15 Page 2 of 17
4.5.5
Long-term operation at location ................................................................................ 16
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1 INTRODUCTION 1.1 Introduction Statutory and class regulations requires two bottom surveys in a 5 year period to inspect integrity and water tightness of the hull. So called ‘Under water inspections in lieu of drydock’ (UWILD) survey may replace the traditional drydocking related to these bottom surveys.
1.2 Objective The objective of this guideline is to clarify to owners and Flag State Administrations the conditions for which DNV GL to accept and support in-water bottom survey of Mobile Offshore Units (MOUs) as an alternative to drydocking.
1.3 Scope This guideline summarizes the scope expected to be covered by the UWILD for different types of MOUs, including column stabilized, self-elevating and ship-shaped units. The guideline is for the units covered by DNV GL Offshore Classification, including but not limited to drilling units, FPSO’s, FSO and LNG production and storage units. The guideline is line with the relevant unified requirements of the International Association of Class Societies (IACS).
1.4 Structure Section 2 describes the background of the UWILD approach; the regulatory basis, the position of DNV GL and a risk assessment for acceptance of the approach for the described units. Section 3 outlines the approach of DNV GLs UWILD based on four specific elements/conditions. Section 4 describes the UWILD inspection scope, both generic as applicable for all hull types and as specific for successively ship-shaped, column stabilized and self-elevating units. For the last are specific considerations discussed, taking into account their specific fixed/floating nature and operation profile. The document is completed with example of a UWILD planning document in Appendix A.
1.5 References This guidance is based on t he international regulations, standards and DNV-GL Offshore Service Specifications/ Rules as listed in Table 1.
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Table 1 References Reference1
Title
IMO MODU Code
IMO Code for the Construction and Equipment of Mobile Offshore Drilling Units
IMO SOLAS
IMO International Convention for the Safety of Life at Sea
IACS UR Z15
International Association of Classification Societies’ Unified Requirement Z15, Survey of Mobile Offshore Drilling Units
DNVGL-OSS-101
Rules for Classification of Offshore Drilling and Support Units
DNVGL-OSS-102
Rules for Classification of Floating Production, Storage and Loading Units
DNVGL-OSS-103
Rules for Classification of Floating LNG/LPG Production, Storage and Loading Units
DNVGL-OSS-104
Rules for Classification of Self-Elevating Units
1.6 Abbreviations The following abbreviations are used in the document.
Table 2 Abbreviations Abbreviation
Description
CSU
Colum Stabilized Unit
IACS
International Association of Class Societies
IIP
In-service Inspection Plan
MOU
Mobile Offshore Unit
ROV
Remotely Operated Vehicle
SEU
Self-elevating unit
SSU
Ship-shaped unit
UWILD
Under Water inspection In Lieu of Drydocking
1
It is mentioned that the referred DNV GL service documents are subject to re-numbering as part of the DNV GL integration Page 5 of 17
2 BACKGROUND 2.1 Regulatory Basis Bottom surveys are governed by the following regulatory basis:
According to IACS UR Z15, 4.1.1 and IMO MODU Code 1989 and 2009, 1.6.1.5, MOUs shall be subjected two drydock surveys during any five-year period whereof the intervals between any two such surveys should not exceed 36 months;
Further according to IACS UR Z15, 4.2 and Appendix A and IMO MODU Code 1989 and 2009, 1.6.2.5, an Administration may allow Underwater Inspections in Lieu of a Drydock survey (UWILD), often called in-water bottom survey or bottom-survey afloat, provided that they are satisfied that such an inspection is equivalent to a drydock survey;
The IMO SOLAS Ch. I, Reg. 10(a)(v) requires two bottom surveys in a 5 year period with the interval period not exceeding 36 months.
The DNV GL Offshore Service Specifications governing the classification of MOUs require two bottom surveys in the course of 5 years, and one of these is to be done together with the Renewal Survey.
It is underlined that this regulatory basis implies acceptance by the Flag State Administration for any UWILD approach as an alt ernative to drydocking for the required bottom surveys.
2.2 Position DNV GL As a Recognized Organization (RO), DNV GL acts on behalf of the Flag State Administration in carrying out inspections and surveys related to the statutory certification as, among others, governed by SOLAS and MODU code as referenced above. DNV GL have long term experience with such arrangements and consider an equivalent solution to be fully acceptable for MOUs provided that specific criteria with regard to the condition of the unit and the alternative inspection arrangements are met, ref. Section 3 and 4 of this guideline.
2.3 Risk assessment The main reason to consider UWILD for Mobile Offshore Units compared to trading ships is their exposure to a lower risk profile related to damages to the hull;
Main operations are in open sea and fixed locations, minimizing risk of grounding and fouling of propellers and thrusters
By nature of design corrosion protection systems including anodes and / or impressed current systems are installed to cover periods exceeding 5 years / design life. The system performance of which may be monitored by measurement equipment / ROV surveys.
Design life of Production units stationed permanently at location in most cases exceeds the five year certification period. Page 6 of 17
MOUs normally operate in open waters with good visibility, making bottom survey by ROV/divers well suitable
In addition to the lower specific risk profile as listed above, DNV GL Classification implies a tight, controlled and transparent follow up on the conditions of the unit under class. In accordance with DNV GL procedures, In-service Inspection Plans (IIPs) are established for all MOUs which specify the complete inspection scope according to class rules. Subsequently, the control of the structural integrity of the MOUs is ensured through regular class surveys according to the IIP’s.
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3 APPROACH 3.1 General The approach of DNV GL to UWILD is based on the following elements: 1.
Inspection of the MOU following the In-Service Inspection Plan
2.
Acceptance of flag authorities for the UWILD approach
3.
Survey report clearly stating the accreditation criteria of the bottom survey
4.
The fulfillment of a set of specific conditions
The elements are further described in the following sub-sections. The requirements for the actual execution of the UWILD scope are given in Section 4.
3.2 In-service Inspection plan (IIP) The scope of work for inspection of MOUs is covered by the In-service Inspection Program (IIP) which will contain the structural items to be surveyed to satisfy the requirements of main class including watertight and structural integrity. Requirements to this can be found in DNVGL-OSS-101, OSS-102, OSS–103 or OSS-104. The IIP constitutes the formal basis for surveying structural items under main class and shall be completed to the satisfaction of attending surveyor before periodical survey can be credited.
3.3 Flag Authorities DNV GL Offshore Classification will address relevant flag authorities for their comment and general acceptance to the practice outlined in this guideline. As stated in Table 3, a request shall be made to the Administration when UWILD survey is intended to be carried on an MOU as an alternative to inspection in drydock. Note:
MODU Code 1979 : The bottom survey is not required for units covered by the 1979 IMO MODU Code.
Norway – NMA: NMA do not specify any requirement for intermediate bottom survey.
3.4 Survey Reporting In connection with the survey reporting, the criteria on which the bottom survey has been credited shall be clearly stated, referencing the acceptance from flag authorities (as applicable) and statement from owner/master as specified in Table 3. For non-completed scope for spudcan/mat of jack-ups, please refer to Section 4.5.4. Page 8 of 17
3.5 Conditions The conditions to be fulfilled for DNV GL to grant acceptance for UWILD are listed in Table 3.
Table 3 General survey scope for bottom survey of MOUs Condition
Description
Planning of in-
water survey
The owner shall submit plans and procedures for survey of the under-water hull to DNV GL in advance of the survey and make these available on board.
Plans should include drawings or forms for identifying the areas to be surveyed, the extent of underwater cleaning, NDT locations and methods, and provision for reporting of any damages or deteriorations found.
See Appendix A of this document for example of survey plan table of content.
Plans should be in place for how to handle the discovery of cracks and damages which may require repair at yard or in dock.
The owner should consider the implications of the choice of inspection method with regard to the inherent limitations and possible need for supplementary inspection
In case of consecutive UWILD surveys the history of prior surveyed areas shall be available in the plan.
Request to
Administration
DNV GL shall forward an application from the MOU owner/operator to the Administration (flag state), seeking exemption from the drydocking requirements.
The application shall contain the owner’s survey plans and procedures reviewed and accepted by DNV GL that the alternative arrangement will provide equivalent results to drydocking.
It is assumed that the criteria set forth in the plans are covered by the owner when preparing for the in-water survey.
Diving / ROV
company
The diving company shall be certified as a Service Supplier. Pictorial/video equipment as described for approved diving companies shall be used to ensure that the surveyor is satisfied with the information relayed to him.
ROV1 companies need not be approved as a service supplier, however the standard of inspection should be equivalent to that achieved by a diver.
Diving/ROV surveys are to be witnessed and the results to be to the satisfaction of the attending surveyor.
1
ROV means Remote Operated Vehicle, carrying camera/video equipment used for inspection purposes
as an alternative to divers.
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Condition
Description
Site conditions
and visibility
The site must have sufficient water depth and clearance to all sides to allow divers to safely survey the underwater hull.
Weather conditions at site including wave height and sea current affecting vessel motions must be sufficiently low to allow safe inspection. If not so – the vessel needs to be taken to a sheltered area where weather conditions are acceptable.
Water visibility must be sufficient to allow divers or ROV photo/video to get clear pictures/recordings to the satisfaction of the surveyor.
Degree of marine growth should be limited to make a proper general visual survey possible. Necessary cleaning is required if this is not the case.
MOU Draught
Floaters shall be at minimum light draft during survey. If the vessel cannot deviate from operational draft during survey, all parts of the hull below the operational water line shall be covered by the bottom survey.
Condition of the
unit
The owner must give a written confirmation that the MOU has no record of damage from dropped object, collision or grounding since the last bottom survey.
There should be no current recording of substantial corrosion, abnormal deterioration or damage for the underwater parts or areas subject to UWILD or bottom surveys.
Units to comply with class requirements and documented through class survey work according to the IIP.
Scope of the in-water survey to cover all relevant items of the underwater hull and equipment
Reporting
For units where the FUI > 1 special consideration will be given to an in-water survey
Upon completion of the survey, an inspection report including pictures/video recordings shall be produced by the inspection company and presented to DNV GL for review. Reporting should include measurements of cathodic protection system performance at renewal survey.
In case of severe finding /
In case of findings or failure to complete in-water survey properly, drydocking may still be required
incomplete survey
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4 INSPECTION SCOPE 4.1 General The UWILD survey shall provide an adequate assurance that t he unit’s bottom is in a satisfactory condition and will remain so until next scheduled bottom survey. Each survey shall involve a general visual inspection of the unit’s bottom. The following sub-sections summarize the expected areas covered by t he alternative survey method as applicable for all MOUs (Section 4.2) and as specific for the different structure types. For completeness it is noted that other surveys and inspections may be carried out at the same time as the UWILD. Examples are surveys for the Position Mooring equipment, the survey of the production system, Cathodic Protection measurement etc. Additional inspections that may be included with the UWILD scope could be:
In water inspection of risers
In water inspection and dimensional control of mooring chains and wires
4.2 Applicable for all MOU Table 4 lists the scope of inspection as applicable for all MOUs.
Table 4 General survey scope for bottom survey of MOUs Areas covered
Inspection approach
Cleaning
In order to carry out a satisfactory inspection, the areas to be inspected must be sufficiently clean to permit meaningful examination and photograph/video by diver/ROV.
Overall / spot cleaning may be required to the satisfaction of the attending surveyor.
Areas where marine growth, etc. may have to be removed are around and inside sea chests and sea valves, on rudder nozzles with connections, guard plates for propeller shafts and thrusters.
Hull structure
A representative selection of the underwater hull, as defined in the approved plan, shall be covered by the survey.
The selection should include sufficient zones/spots to cover the hull from stern to bow, including side and bottom plating.
Areas susceptible to damage from supply vessels, mooring equipment, dropped objects etc. are to be inspected.
If an in-water approach is accepted for consecutive bottom surveys, the survey selection should aim at covering different zones/spots for each survey.
The general condition of the unit, actual age, surface protection and history are important factors to consider when deciding the actual survey coverage.
The inspection shall cover reported suspect areas from past surveys.
The extent of inspection should be sufficient to identify damage to the hull structure, hull Page 11 of 17
Areas covered
Inspection approach
coating and general condition of critical welds.
General requirements to NDT of critical connections are covered by the IIP, whereas additional NDT may be required for critical connections as part of the bottom survey if requested by the Surveyor.
Seawater inlets
Sea suction openings are to be confirmed clear.
(sea chests and
Sea inlets and valves are to be blanked off and dismantled for inspection during renewal
strainers), Sea
surveys (once in five year period)
valves
Valves to be checked for tightness
Other openings
To be visually examined either from outside or inside.
below water line
Inspection to ensure that valves are functional and can be fully closed with no leakage.
Thrusters
Survey the thruster for damage and visible oil leaks. Survey the connection to the hull.
Corrosion
Anodes
protection
Visual inspection of a representative amount of galvanic (sacrificial) anodes.
Potential measurements (cathodic protection readings), when required at renewal survey, shall be performed with a bathycorrometer. Equipment should be tested and calibrated before use.
Impressed current system
Initial reading before cleaning to assess effect of installed system.
Then to be cleaned and re-inspected.
The surveyor should request readings from the permanent monitoring system and attach a representative copy to the survey report.
4.3 Column-stabilized units The bottom survey shall cover the submerged parts of the semi-submersible hull. More details for the inspection scope specific for CSUs are provided in Table 5.
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Table 5 Specific additional scope for Column-stabilised units Areas covered
Inspection approach
Hull structure
For general scope, see Table 2
Pontoon bottom plate areas
Pontoon side plate areas (both
sides)
All connections to be covered by survey, i.e. foundations fairleads, anchor rack to hull.
For cathodic protection system specific attention
Pontoon top plate
should be paid to blind spots / poor coverage due
Columns below transit water line
to brace arrangement / layout.
1
Bracings
Propeller
Visual inspection of propeller blade, propeller boss, stern tube.
Check for damage, corrosion, erosion/cavitation, oil leakage, cracks etc.
Rudder
Attachments of rudder (if applicable) to be cleaned and examined.
Visual inspection of welding, rudder blades, attachments, paint, bearing
Units permanently on location may be exempted from measuring the clearance of the rudder bearing.
4.4 Ship-shaped units Bottom survey shall cover the submerged parts of the ship-shaped hull. More details for the inspection scope specific for SSUs are provided in Table 6.
Table 6 Specific scope for ship-shaped units Areas covered
Inspection approach
Appendages, bilge keel etc
Ends of bilge keels to hull to be cleaned and examined
General inspection of bilge keels.
1
Areas above transit water line including bracings and bracing connections which are accessible for dry surveys and covered by regular class inspections are not subject to inspection as part of bottom survey. These structural items are covered by class scope and are included in the IIP. Page 13 of 17
Areas covered
Inspection approach
Propeller
Visual inspection of propeller blade, propeller boss, stern tube.
Check for damage, corrosion, erosion/cavitation, oil leakage, cracks etc.
Rudder
Attachments of rudder (if applicable) to be cleaned and examined.
Visual inspection of welding, rudder blades, attachments, paint, bearing
Units permanently on location may be exempted from measuring the clearance of the rudder bearing.
Moonpools and turrets
The extent of the survey in the moonpool area should be clearly identified showing any areas that are inaccessible for survey
4.5 Self-elevating units/Jack-ups 4.5.1 Introduction This section starts by introducing the two separate surveys as DNV GL has defined to cover the bottom survey. Subsequently the scope of each of these is detailed further. Conclusively, the section defines how to proceed for units for a period more than 5 years on location.
4.5.2 Separate surveys Different from floating units, the hull (deckbox) of a jack-up structure is elevated above sea and fully accessible for inspection during operation. At the same time the legs are available when the jack-up is in transit mode with legs fully elevated. With due regard of this specific situation, DNV-GL has organized the bottom survey in two parts (ref OSS-104, Pt. 7, Ch. 3, Sec. 2.3): 1) Bottom survey; covering external hull/deckbox 2) Spudcan-leg survey; covering spudcans and the underwater areas of legs, together with their connections The separation gives more control in planning and execution of the survey, at the same time provides a clear overview of the unit’s (inspection) status. It is underlined that these surveys taken together complement the statutory bottom survey as defined by the different regulations as defined in 2.1.
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In a typical situation on location, the separate bottom survey can be completed while the spudcan/mat is partly or fully obscured below mud line. In this situation, the separate bottom survey can be completed (and credited) where the spudcan-leg survey remains pending.
With acceptance from the Flag, drydock in line with Section 3.3 and the referred OSS-104 paragraph, the spudcan-leg surveys shall postponed till the next rig move1,2.
4.5.3 Hull/Deckbox The deck box bottom and side surfaces are normally subject to general visual inspection when the jackup is in elevated mode. Alternatively the bottom survey may be carried out dry during transit if the jackup is placed on a barge or in water by divers when in floating mode, ref. requirements given in Table 7. The following areas of the structure and the following scope shall be carried out during the survey:
Table 7 Normal Bottom survey of self-elevating units Areas covered
Inspection Approach
Visual inspection of hull, external
Visual inspection of the deck box from boat or adjacent platform.
plate areas
Alternative inspection by divers when in floating/transit mode.
Corrosion protection (CP)
Visual check of the anodes, since the upper hull is usually out of the water the CP system is usually inoperative.
Sea valves
These can normally be opened from inside while the rig is in operation.
Legs down to the splash zone
Full closing of valves is to be checked.
Visual survey from the hull.
4.5.4 Leg and Spudcan Leg and spudcan become available for inspection when the unit is in transit mode and is therefore normally inspected separately from the hull when lifting the legs. Table 8 specifies the scope of this survey.
1
The next due date taken from the date when the survey was due
2
For all clearness, it is noted that the interval of the spudcan-leg survey is equal to the traditional bottom survey, i.e. two times in any five (5) year period, with an interval not exceeding three (3) years between examinations. Page 15 of 17
Table 8 Normal Survey Scope Leg and Spud Can Survey Areas covered
Inspection Approach
Legs
General visual inspection of legs d uring jacking operation.
Connections between vertical chords and braces/diagonals are checked together with racks and rack plate welding.
Spudcans
Around the pin holes in cylindrical legs
Leg-well structure
Jacking gear support structure
ROV/diver inspection of the underside of the spudcan to look for gross deformation.
Visual inspection around piping penetrations
Visual inspection of the spudcan from the outside, alternative check of spudcan bottom from the inside, if access from outside is not possible
Legs – lower part
Spudcan and mat internal structure in way of leg support
The lower part of the leg, i.e. up to level 4 (in way of the
Leg / spudcan connection
lower/upper guides in transit) and leg / spudcan connection is to be visually surveyed.
Corrosion protection
Visual inspection of galvanic (sacrificial) anodes above sea level when in transit condition
Visual inspection of a representative amount of galvanic (sacrificial) anodes by diver or ROV when in other than transit condition
Alternative - potential measurements to evaluate efficiency of the cathodic protection system when in elevated mode.
To be taken at different levels and sides of the leg to confirm sufficient coverage.
4.5.5 Long-term operation at location Self-elevating units planning for long term operations on location beyond 5 years shall be designed as permanently installed self-elevating units for the site specific environmental and soil conditions in accordance to DNVGL-OSS-104, Pt. 7, Ch. 6. A separate approval process for UWILD applies for this condition.
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APPENDIX A
EXAMPLE OF PLAN DOCUMENT - TABLE OF CONTENT
The following figure represents a table of content of a plan document to be used for UWILD planning.
1. Introduction 1.1. Main Project/Vessel Particulars
Vessel Name:
Vessel Type:
Flag:
Class:
Vessel Owner / Operator:
Field:
Field Operator:
Vessel Location
Vessel Summer Draft
Water Depth:
Project Launch Date:
2. Survey Procedure & Scope 2.1. General Short description of scope, DNV GL’ s role/supervision, use of vendors 2.2. Planning 2.3. Hull Conditions Damage/leaks/corrosion/coating 2.4. Weather/sea condition limitations 2.5. Cleaning methods hull/sea-chest 2.6. Use of Support vessel/Divers/ROV/CCTV etc. 2.7. Diving/ROV company 2.8. Items to be inspected 2.9. NDT methods to be used if required 2.10.
Recording of survey and findings
2.11.
Approval of vendors to be used
3. Project Team Responsibilities 4. General 4.1. Client /OIM/etc. Description of equipment/tools to be used Procedures to be used to ensure safety during inspection Risk assessment Project Procedures & Reference Documents - o0o -
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