Developments in Watertight Integrity on Floating Offshore Installations Joseph H. Rousseau, J. Andrew Breuer / ABS

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1 OTC Developments in Watertight Integrity on Floating Offshore Installations Joseph H. Rousseau, J. Andrew Breuer / ABS Copyright 2007, Offshore Technology Conference This paper was prepared for presentation at the 2007 Offshore Technology Conference held in Houston, Texas, U.S.A., 30 April 3 May This paper was selected for presentation by an OTC Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Offshore Technology Conference, its officers, or members. Papers presented at OTC are subject to publication review by Sponsor Society Committees of the Offshore Technology Conference. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Offshore Technology Conference is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, OTC, P.O. Box , Richardson, TX , U.S.A., fax Abstract The hurricane seasons in 2005 and 2004 have highlighted the need for a better understanding of flooding hazards and the associated requirements for internal and external watertight integrity on mobile offshore drilling units and floating production installations. The American Bureau of Shipping has made significant changes regarding subdivision and watertight integrity to the ABS MODU Rules since they were first published in 1968, and is in the process of studying further changes for both MODUs and production installations. The relevant requirements as of 2006 are discussed, along with proposed modifications based on lessons learned in the Classification of the latest generation of MODUs, spars, tension leg platforms, and semisubmersible production installations including observations from the recent hurricane seasons. The proposed changes are aimed at limiting water ingress and preventing progressive flooding, and they address issues such as controls and alarms for watertight doors and hatches, protection of external ventilation openings, locations and types of valve controls, and measures for ensuring watertightness of penetrations for cables and ventilation. Introduction The Rules and Guides of the American Bureau of Shipping (ABS) have been applied in the design and construction of many offshore drilling units and production installations, particularly in the United States Gulf of Mexico. In addition to their use in Classification of these facilities, they are specifically referenced by, or share common background and requirements with, the United States Code of Federal Regulations 1 (CFR) as the relevant design and construction standard to be applied in association with registering an installation under the United States flag or when applying for a Certificate of Inspection (COI) to operate on the U.S. Outer Continental Shelf (OCS). The principal documents from the standpoint of hull structure and stability are the Rules for Building and Classing Mobile Offshore Drilling Units 2,3 (also known as the MODU Rules), the Rules for Building and Classing Steel Vessels 4 (SVR), and the Guide for Building and Classing Floating Production Installations 5 (FPI Guide). There are numerous additional Rules, Guides, and Guidance Notes which are referenced therein. The requirements for stability, ballasting, and watertight and weathertight integrity have been taken mainly from the MODU Rules to date. Stability Criteria The first stability criteria for Mobile Offshore Drilling Units (MODUs) were published in the ABS MODU Rules in This was a significant step as stability requirements for ships had typically been contained in statutory regulations rather than Classification Rules. The MODU stability criteria have evolved to the present day while retaining the separation between specific requirements for column stabilized, self elevating units, and surface units due to the differing stability characteristics of each type. The column stabilized unit is usually a semisubmersible but may also refer to a submersible (bottom sitting) unit in its afloat mode. Self elevating unit stability is for the jackup unit while in transit. The surface unit requirements are applicable to ships and barges. There are two conditions for which stability calculations were to be presented: intact and damage. Intact Stability For the intact case, an assumed environmental effect equivalent to a 100 knot wind was applied in the 1968 Developments in Watertight Integrity on Floating Offshore Installations 61

2 MODU Rules. Stability criteria from this point onward have used wind as the representation of environmental force since it is easily calculated and applied. Under the influence of the 100-knot wind, a minimum excess righting energy had to be demonstrated over the wind heeling effect; this was expressed as an area ratio under the righting and heeling moment curves. The measure of external watertight integrity was the non-immersion of any unprotected opening within the range of the righting energy required to meet the excess energy requirement. This was referred to as the down flooding point at which water could enter the hull envelope which was providing buoyancy and stability. For units operating in areas with demonstrably benign environment, intact stability could be calculated for restricted service at wind speeds as low as 50 knots. It was the responsibility of the owner to operate the unit only in such areas. In the next edition of the ABS MODU Rules, published in 1973, the wind moment applied for intact stability in unrestricted service was modified so that a reduced speed of 70 knots was adopted for transit conditions and for other normal operations (such as drilling) while afloat. Units had to be capable of changing mode to withstand a 100-knot condition for storm survival condition. Appropriate guidance on changing modes was to be included in the unit s operations manual. Restricted service conditions were still recognized. (It is important to note that the US Coast Guard has never accepted such restricted service conditions, requiring full compliance with the 70 and 100 knots wind speeds.) These intact stability criteria from 1973 remain in effect to the present day. The Inter-Governmental Maritime Consultative Organization (IMCO, later International Maritime Organization or IMO) stability requirements for drilling units were first laid out in a Resolution establishing the Code for Construction and Operation of Mobile Offshore Drilling Units 6 in This document is now referred to as the 1979 IMO MODU Code, and the intact stability criteria therein are the same as those in the ABS MODU Rules and the US Code of Federal Regulations. From an external integrity standpoint, it is important to note that intact stability is an expression of an intermittent phenomenon, so that the unit or installation is presumed to incline under the effect of the environment and then return upright when that effect is removed. This has implications for the types of closures that can be considered to eliminate down flooding as will be seen later. Damage Stability: Collision The 1968 MODU Rules were relatively vague in their damage stability criteria: a single compartment was to be assumed flooded, and calculations had to demonstrate to the satisfaction of the Class Committee that the unit has sufficient reserve buoyancy and stability to survive considering the adverse effects of wind and sea conditions. There was no specification of the manner in which this sufficiency was to be submitted, but the Rules did note that a post-damage condition with positive stability and where all openings (i.e., down flooding points) were above the waterline with the application of a 50-knot wind would be considered acceptable as an example. In 1973 the criteria were re-stated to provide a more detailed set of requirements for designers. The nature of the assumed damage was specified to be a collision or grounding (i.e. side or bottom damage), and the acceptance criterion was indicated to be the 1968 suggestion of maintaining all openings above the waterline after damage with a 50-knot wind. This is the same set of criteria as the 1979 IMO MODU Code, and in the Code of Federal Regulations. Unlike intact stability, damage stability involves a static inclination or and/or immersion due to the loss of buoyancy (or addition of weight). Damage Stability: Remote Flooding Following the 1982 loss of the semisubmersible Ocean Ranger which was drilling the Hibernia field off the east coast of Canada, major revisions were made to the ABS Rules in the areas of stability and ballasting. Ballasting system changes were introduced in the 1985 Rules; stability criteria were extensively modified in Now in addition to damage resulting from collision, it became necessary to assume flooding of compartments at or below the waterline if they were: Adjacent to the sea; Contained pumps for the handling of water ballast; or, Contained sea water-cooled machinery. These cases came to be known as remote flooding to separate them from collision damage situations. Remote flooding cases applied to all afloat conditions; in the storm survival case this permitted the deletion of collision damage criteria since it addressed survival of possible flooding scenarios while recognizing that there would be no risk of collision damage when no workboats were nearby in a storm. An additional requirement was added for column stabilized units, to specify that unprotected down flooding openings were to be located well above the final collision damage waterline (7 degrees of inclination and 4 meters of vertical clearance) to prevent water ingress during intermittent immersion of 62 Developments in Watertight Integrity on Floating Offshore Installations

3 such openings while the unit heaved or rolled. There was also a minimum excess righting energy specified so that the righting moment was at least double the overturning moment at some point within the presumed 7 degree range of rolling after damage. These criteria are effectively the same as those contained in the IMO s revision to the MODU Code 7 in The US CFR was never modified to adopt the remote flooding concept, retaining collision damage as the only presumed risk of flooding. Flooding from the Deck on Jackups Originally the 1973 ABS MODU Rules contained a provision for damage to include compartments which could be flooded from the deck, as in the case where tank vents or ventilators were carried away by shifting deck cargo However, this damage scenario was dropped from consideration by ABS in 1976 based on harmonization with the ongoing development of the International Association of Classification Societies (IACS) Unified Requirements. 8 Damage from the deck was not agreed within IACS to be a likely scenario in the IACS planning. The requirement was deleted from the Rules for the next release in The most recent updates to stability requirements in the ABS MODU Rules were made in 2004 and 2005, as the result of a study of jackup losses spurred by the issuance in 2001 of a Safety Notice 9 from the UK HSE questioning the adequacy of the criteria in effect at the time. The research found that jackup casualties were in many cases caused by flooding from the deck, as anticipated in the 1973 MODU Rules but withdrawn. The 2004 ABS Rules therefore included an interim measure setting a maximum compartment size regardless of location on the unit, as a function of the reserve buoyancy of the unit at its maximum draft. This was superseded in 2005 by the provision of a requirement for a range of stability after damage to any single compartment. Floating Offshore Installations The MODU stability criteria have been applied to floating production installations with the same topological features as their MODU equivalent, so that for example semisubmersible units are expected to meet the column-stabilized unit stability criteria. Direct references to applicable sections of the MODU Rules were included in the original ABS FPI Guide for such installations. The main difference applied to the production facilities has been the adoption of sitespecific wind, using the 100-year return, 1-minute mean wind speed instead of the MODU Rules value of 100 knots. In the US Gulf of Mexico, however, the US Coast Guard has required that all installations meet the 70- and 100-knot wind speeds regardless of the site specific data. It has become necessary in recent years to add specific requirements for certain different types of installations which are not similar to MODUs as well as to consider the transit conditions that often have different configuration from the final as-installed condition. Stability of Novel Units The stability criteria for MODUs are of limited applicability in the evaluation of the stability of installations such as spars and tension leg platforms (TLPs). The ABS FPI Guide was first published in 2000 without criteria for such platforms; subsequent revision in 2004 introduced some requirements for these installations and these are at present being revised to incorporate lessons learned. Spars Spars were initially treated as column stabilized units under the MODU Rules for stability purposes. However, the spar concept has stability characteristics that rely on the relative position of the centers of gravity and buoyancy rather than on the typical waterplanedependent form stability of floating units. The FPI Guide criteria introduced in 2004 therefore sought to simplify the intact stability evaluation to a static formulation which is based on the similarity of the spar response to the equation for a pendulum. However subsequent application of the criteria has indicated that this may be an oversimplification and that its dissimilarity to the conventional flag and coastal state requirements does not streamline the design and review process since the traditional methods must be checked also. As a result, the criteria will be reverted to the righting energy vs. heeling energy in an upcoming update, although a limitation on the angle of heel for such evaluation will be included to prevent calculations which assume excessive inclination in order to satisfy the criteria. It should be noted that loading conditions in service usually have inclination limits from global performance and from mooring and riser considerations which are more restrictive than the results of the stability calculations. In the damage case, the criteria include a margin to submergence of any openings which may cause flooding, similar to that for column stabilized units. As for MODUs, the damage is assumed to arise from collision or remote flooding. However, unlike the MODU Rules the collision case includes damage to a vertical bulkhead, resulting in flooding of two compartments. Typical spar compartmentation usually includes an annular collision belt above and below the waterline to limit the amount of flooding which will occur from such damage. Tension Leg Platforms The in-place stability of a TLP is dependent on the pre-tension of the tendons, and the platform is constrained in heave, roll and pitch. Therefore the Developments in Watertight Integrity on Floating Offshore Installations 63

4 stability of the TLP is actually expressed as the result of the global performance calculations done in accordance with the American Petroleum Institute (API) Recommended Practice RP2T. 10 The ABS FPI Guide makes reference to this standard in lieu of applying customary stability criteria based on inclination angles and righting energy. However its principles are analogous in terms of evaluating the effect of environmental forces and possibility of flooding. The TLP portions of the FPI Guide are being further developed to incorporate some of the lessons learned to date, such as addressing ambiguities in watertight integrity requirements (external and internal). Since the Guide makes direct reference to RP 2T, the current revision efforts to that Recommended Practice will necessarily be applied when that document is released. Transportation and Installation One important consideration for production installations, whether they be ship-shaped, semisubmersible, spar, or TLP, is the need to perform a transit to a specific site and installation at that site. Since the production installation is typically not designed to be operated in multiple locations over its life, the transit and installation conditions are often not a subject for design optimization and are often carried out in a different configuration than the as-installed unit. For example, a spar is towed on its side to the production site and then upended; obviously the stability characteristics are completely different in the tow and operating conditions. Tension leg platforms may undergo a wet tow to site and therefore behave as a surface or column stabilized unit. Even the column stabilized unit may have different stability characteristics due to extra hull openings for mooring and riser installation, and different loading patterns from those that will ever be experienced in operation. An FPSO may similarly have different loading conditions for its voyage to the operating site. There is therefore an additional set of calculations that must be performed, with different behavior and often with different subdivision and external openings present. Even the ballasting operation can be more complicated as temporary tanks are used; this can lead to different flooding scenarios and even unforeseen problems due to free surface effect at intermediate filling levels. Regardless of the particular situation being evaluated, however, the conventional approach to stability evaluation remains valid: the goal is to ensure that there is sufficient righting energy along with adequate freeboard to any down flooding points. The same applies to seldom-used but nonetheless critical conditions for tank inspections; it is important to consider the effects of a storm or of damage when the installation is not in a typical operating condition. At the current time, ABS is working on more definite specification of the considerations and criteria to be applied in these evaluations. These will be included in the FPI Guide at a later date. External Integrity As noted earlier, one of the most important parts of ensuring adequate stability involves providing external watertight and weathertight integrity so that the hull boundary remains effective in providing buoyant force and righting energy. This is most often expressed as the location of the down flooding points into the hull. There are generally two types of down flooding points assumed in the calculation of stability: unprotected openings, and weathertight openings. Openings which may be considered watertight may be ignored as down flooding points, but the types of these are limited. In addition to stability considerations, external openings are subject to the IMO International Convention on Load Lines 11 which specifies such parameters as heights and closure appliances. However this Convention is of limited applicability for installations that differ from commercial ships upon which the Convention was based. In addition, it is not enforceable on installations which are not undertaking international voyages and which often do not have a flag of registry. Unprotected Openings The most common unprotected opening is the ventilator, since provision of air to crew and to combustion machinery is necessary for operations. The possibility exists that in certain conditions, however, some of the unprotected openings may be closed such as during the preparation for severe storm or for the duration of the tow and installation phases when the hull is unmanned and not in an operational condition. The other common type of unprotected opening is the hawse pipe to the chain locker. This is often the first down flooding point to reach immersion since it is typically at the outer periphery of a MODU or production installation. Unprotected openings are important in both intact and damage stability, since water can enter the hull even during intermittent immersion of the opening. Weathertight Closures Providing weathertight closures on openings into the buoyant envelope removes them from consideration in intact stability because they are assumed to be effective in preventing the ingress of water during intermittent immersion. 64 Developments in Watertight Integrity on Floating Offshore Installations

5 Installations may therefore have closing devices affixed to the chain locker openings that will prevent unrestricted inflow of water into the locker when the hawse pipe is briefly submerged. As noted above, ventilator closures may provide the same benefit. There are two facts to remember regarding such closures, however: they must be engaged to be effective, and they will not prevent water ingress if they remain submerged. In order that engagement is assured, a closure must either be automatically closing (like a ball or float check closure on a tank vent pipe) or must be specifically closed as part of a procedure such activating a screw-down ventilator closure during storm preparation. Since they serve such a vital role in maintaining the external boundaries, it is important that closures are periodically inspected and are maintained in proper working condition. Weathertight closures are generally visually inspected and chalk tested for adequate sealing; some may be tested with a hose stream. Watertight Closures When it is possible for an opening to be submerged for long periods, as in the case of openings below the final damage waterline, it is necessary to provide positive closure and maximum degree of confidence of the effectiveness of the closing means in preventing entry of water when subject to the same pressure head of water as the surrounding structure. In general, this involves bolted manholes or positive closing valves which are as effective as the surrounding boundary. These openings are therefore excluded from the list of down flooding points in all analyses of stability. Penetrations in the shell for wire rope have been accepted based on a dual pinch valve assembly, which fails in the closed position and can be tested with applied pressure. In addition to such testing during construction of the unit, proper inspection and maintenance is also critical to ensure that the valve materials are not worn and rendered ineffective. Other watertight closures may be prototype tested to the required head in a test rig, such as watertight doors placed on a pressurized test compartment. Testing a closure after installation is done via hose test with a minimum pressure of 2.1 bar (2.1 kgf/cm 2, 30 psi), or air testing of a compartment may also be accepted with prior approval. Depending on arrangement and type of closure, chalk testing or calculations may also be accepted in lieu of testing for some closures. Ventilation closures are specifically excluded from consideration as watertight in the ABS Rules, due to the typically large size of ventilation openings and the concern over the provision of a truly watertight seal to the appropriate pressure head. A similar situation exists for chain pipe closures, since it is difficult to ensure that a device will effectively seal around the chain links. Existing solutions such as inflatable bladders which close around the chain will allow some water to leak through under a pressure head. Mechanical devices which engage at a specific position on the chain link can be more effective but require that the chain be in exactly the right position and orientation for engagement. This is more easily attained for production installations that do not require frequent chain operations due to drilling operations being performed directly from the facility or from a MODU that requires the facility to offset. Regardless of the type of chain pipe closure, it is important to carry out visual inspection and functional testing at regular intervals. Clear instruction on operation and maintenance also needs to be provided to operating personnel; it is advisable to highlight the reason for the importance of these closures as well. Penetrations for Access and Piping/Ventilation No less important than the ability to keep water outside of the buoyant envelope is the ability to limit the extent to which it can progress in the event that a damage has occurred. The subdivision of a floating installation is the means by which the final inclination or parallel sinkage is limited, which in turn helps keep the down flooding openings above the waterline. It is necessary to penetrate subdivision bulkheads and decks in order to provide access to spaces for operational purposes and for inspection. In addition, there must be penetrations for the various electrical cable, piping, and ventilation of compartments. First ABS MODU Rules: 1968 Other than forbidding penetration of a collision bulkhead, the original 1968 MODU Rules did not have any specific guidance on doors and hatches through subdivision boundaries; the assumption was that the structure would be equivalent to the surrounding bulkhead or deck, but there were no requirements for design, testing, or operation of the doors or hatches themselves. There was also no specific rule regarding penetration for electrical cable, but units were subject to the requirements of the Steel Vessel Rules. Pipe, valves, and fittings for the piping system were required to be of metal construction, and any remote poweroperated valves had to have a manual control unless they failed closed. The 1968 Rules included requirements for control and safety features such that there was more than one means of deballasting submerged units and that there were features for stability control during submersion and Developments in Watertight Integrity on Floating Offshore Installations 65

6 raising of submersible units. For towing or semisubmerged conditions, there had to be control against accidental flooding through valves including the possibility of having to blank off certain unnecessary systems. Rule Refinement: 1973 to 1980 In 1973, Rules were added for watertight doors and depended on the type of unit in consideration and the usage profile of the door: Self elevating units could have individually dogged hinged doors for inspection purposes or infrequent use while afloat, and quick acting hinged doors for more frequent access; in both cases the doors were not permitted in compartments not subject to penetration damage and were to be kept closed except while in use. If a door accessed a compartment in the collision zone or was normally open while afloat, it was to be of the sliding type with local control plus remote operation above the main deck. Surface units could not have individually dogged doors below the bulkhead deck. Quick acting doors were permitted but only above the load line draft and in centerline compartments which were normally manned. All other doors were to be of the sliding type, with local control plus remote operation above the main deck. Doors on column stabilized units were to be specially considered. Additional provisions were included in the 1973 MODU Rules for the possible effects of tank vents and overflows on damage stability to reflect the possibility of progressive flooding through such lines. Tank vents and overflows terminating above the final waterline after damage but subject to intermittent immersion were to be fitted with automatic closures. The 1980 MODU Rules included new provisions for penetrations of watertight bulkheads and flats, including the necessity to consider the prevention of progressive flooding. This might be assumed to occur through pipes and ducts serving multiple compartments, pipes and ducts near the side shell and hence within the damage zone, or non-watertight ducts which would collapse under pressure of water. Closures in such systems were to be remotely operated from the weather deck, pump room, or other manned space and indicators were required at all the control stations. Check valves and spring or gravity-actuated automatic valves were specifically excluded as effective watertight closures. The division between unit types was removed from consideration for internal and external openings, so that for all MODUs: Internal openings used for access while afloat were to be remotely and locally controlled, unless normally closed and fitted with open/closed indicators at the control station. Internal and external openings to be kept closed while afloat could only be opened if this was noted in the log book. External openings used for access were to be above the final damage waterline except for bolted manholes and for small hatches with open/closed indicators at the control station. Small hatches had to be quick acting and could not be emergency escape routes. In all cases, the closures had to have strength, tightness, and means for securing which made them equivalent to the surrounding structure in terms of the pressure head they could withstand. This requirement has remained the same to the present day. The 1980 Rules also incorporated new requirements for piping systems and valves. The design and layout of the systems had to take into account the damage stability criteria and assumed extent of penetration into the hull. Sea water inlet and discharge valves were for the first time required to have remote operation from outside the space, and a secondary means of local or remote control had to be provided. On self-elevating and surface units, it was considered acceptable, however, to install bilge alarms in the relevant space if the valves were readily accessible. The bilge system requirements were upgraded in 1980 to require permanent systems and multiple pumps in certain spaces. Finally, all tank vents and overflows were required to have automatic closures, not just the ones which might be subject to intermittent immersion. Further Systems Development: 1985 to 1991 In the years after the loss of the Ocean Ranger, significant changes were implemented to the bilge and ballast system Rules including the requirements for ballast system control. The bilge system requirements were expanded and clarified in Compartments which would eventually (in 1991) be subject to the new remote flooding stability criteria were required to have means of sounding and drainage, or a stability penalty was to be applied. Means of sounding had to be provided for all tanks and compartments adjacent to the sea, and had to include a manual sounding pipe whenever the primary measurement was by electronic tank-level indicating apparatus. Chain lockers were specifically listed as needing means for draining water and removing mud and debris. 66 Developments in Watertight Integrity on Floating Offshore Installations

7 The largest changes in 1985 were in the area of ballast systems, where the requirements were increased to a level which became the most comprehensive in the offshore industry. They addressed the pump capacity and performance, redundancy, and control and communication. The 1988 Rules saw the addition of testing requirements for hatches and watertight doors. The wording related to prevention of progressive flooding via ventilation and internal drain systems was also clarified. There was little change in watertight integrity in the 1991 Rules, as the focus was on updating the stability criteria to match the remote flooding hazards identified in the systems requirements. The addition of an option of air testing in lieu of hose testing for watertightness was the only notable modification. The Modern Rules: 1994 to 2007 The year 1994 brought a complete reformatting of the ABS MODU Rules. The pace of Rule development increased as ABS investment in offshore technology paid dividends and as companion initiatives within IACS for vessels and MODUs were incorporated. Rule Change Notices were issued on a regular basis, with new editions of the complete Rule books released at intervals. Notable changes to the Rules from the standpoint of watertight integrity include: the first specification of a requirement that penetrations for electrical cable in a watertight boundary must also be watertight (1994); exclusion of ventilation closures from being considered watertight (2003); progressive development of the provisions for remote and secondary control of valves; periodic clarification of requirements for penetrations in the side shell. The first item above is significant, as the increasing size and particularly depth of semisubmersibles has led to multi-cable transits (MCTs) being installed at locations which would see high pressure on one side of the bulkhead in a flooding case. Guide for Floating Production Installations: 2004 As mentioned previously, the ABS Guide for Building and Classing Floating Production Installations references the Steel Vessel and MODU Rules for shipshaped and column stabilized units, and the spar and TLP stability criteria were introduced in However, the expectations for watertight integrity of spars and TLPs are not specified in the Guide; the MODU Rules have been applied as the relevant standard to date. One important feature that has been included for spars is the means of access to spaces within the hard tank for inspection. Since some designs do not have access shafts from which each void or tank can be entered, it is necessary to open manholes in multiple levels to access the lower spaces. This was a concern for stability purposes since it is essentially a multiple compartment damage case if there should be a collision or other flooding incident. As indicated earlier, the stability of the spar must include such a case as part of the analysis, which can place draft restrictions on the inspection condition. The FPI Guide therefore details the conditions which must be met to dispense with the multiple compartment flooding analysis, including: Warnings on the access opening covers; Instructions and warnings in the installation s operations manual; System locks to prevent ballasting during inspection; Ability to pump the compartment without opening the access opening cover. Current Initiatives for Change The MODU Rules and FPI Guide are evaluated for revision as part of the ongoing Rule development process which seeks continual improvement based on findings by ABS in the course of design review. The feedback from clients and regulatory bodies are equally important in the process as they highlight their own findings from design and construction as well as providing lessons from operations. This allows the Rules and Guides to change and incorporate new technology or new approaches that were not considered, as well as identify any need to address a lack of clarity in the language of the requirements. There are several areas for improvement in the Rules and Guides that are under active development. Ongoing changes in the MODU Rules are centered around readability and clarity: a project begun in 2003 to reformat the Rules will reach its first major milestone in 2007 with presentation to the ABS Special Committee on MODUs for release in Beyond this, however, there are additional initiatives in the Rules and the FPI Guide that will address the lessons learned in Classification of spars, TLPs, and production semisubmersibles over the past decade. The FPI Guide will be modified to incorporate requirements for watertight integrity that until now have remained implied by reference to the MODU Rules. It has become apparent that this is an area for improvement since the Load Line Convention, which sets many of the closure and height requirements on MODUs, is not applied to floating production installations. Developments in Watertight Integrity on Floating Offshore Installations 67

8 There will in future be clarification in both the MODU Rules and FPI Guide of the delineation of acceptable closures for external openings depending on weathertight vs. watertight assumptions in the stability analysis. The stability requirements themselves will also continue to be examined for consistency and clarity. Risk-based approaches will be utilized where appropriate to provide maximum flexibility to deal with the continuing technological advances and innovative concepts in platform design. Some additional areas under consideration include: Updates to incorporate internal subdivision issues regarding access, including required closure mechanisms on watertight doors and hatches. Review of installation procedures for MCTs to ensure that the appropriate steps are consistently followed during construction to keep the asinstalled configuration in accordance with the design approval and prototype testing; Requirement to be able to test MCTs to the rated pressure after installation for better assurance that they will hold watertight; Review of interlock logic in systems where such interlocks are important parts of the safety of the system; Clarification and consolidation of valve position indication requirements; Consideration of operability during maintenance, since unlike MODUs there is no opportunity for drydocking and maintenance other than offshore; Evaluation of evacuation modes, including isolation valves with an independent control system, to prevent migration of water between different areas; Inclusion of a requirement for Hazard and Operability (HAZOP) and Failure Modes and Effect Analysis (FMEA) for safety aspects of the ballast and bilge systems including inlets and overboard discharges, and interconnections between the bilge and ballast systems; Increased scrutiny of hydraulic control systems for doors and valves including review of system functionality and use of HAZOP/FMEA, plus greater focus on function and operability during surveys. All of the above will be the subject of discussion at upcoming meetings internal to ABS and involving industry representatives as well. One goal of these efforts will be the release of the FPI Guide as the FPI Rules within the next few years. Conclusion The years since 1968 have seen many changes to the criteria for ensuring stability and watertight/weathertight integrity of mobile offshore drilling units and floating offshore installations. For the latter platforms, much knowledge has been gained within the past decade through review of innovations during construction and by evaluation of experience in installation and operation. At its core, the afloat stability of the floating offshore installation is a function of: Adequate buoyancy and stability of the hull form; Preventing water from ingress into the buoyant body; Limiting the movement of any water which does manage to enter the buoyant body. Keeping pace with technology and innovation in platform design is a necessary role for the Classification society in order to ensure that the above conditions are met. The lessons learned from the many floating offshore facility projects in which ABS has been involved over the recent years, including the valuable input from the designers and operators of such installations, will continue to be incorporated in the Rules and Guides with a view to ensuring that these documents provide a comprehensive standard for promoting the security of life, property, and the environment. Acknowledgements The authors would like to thank the members of the ABS Special Committee on Mobile Offshore Drilling Units for their ongoing commitment to the Rule development process, and for their valuable feedback over the years on the ABS Rules. Thanks also go to Technip and to BP for their work with ABS on suggesting improvements and providing comment on the FPI Guide. References 1. United States Code of Federal Regulations, Chapter 46 Shipping, Paragraph Rules for Building and Classing Offshore Mobile Drilling Units, American Bureau of Shipping, 1968 and Rules for Building and Classing Offshore Mobile Drilling Units, American Bureau of Shipping, 1980, 1985, 1988, 1991, , Rules for Building and Classing Steel Vessels, American Bureau of Shipping, Guide for Building and Classing Floating Production Installations, American Bureau of Shipping, 2000 and Code for the Construction and Equipment of Mobile Offshore Drilling Units, IMO Resolution A414(XI), International Maritime Organization, Code for the Construction and Equipment of Mobile Offshore Drilling Units, IMO Resolution A649(16), International Maritime Organization, Requirements Concerning Mobile Offshore Drilling Units, International Association of Classification Societies, Developments in Watertight Integrity on Floating Offshore Installations

9 9. United Kingdom Health and Safety Executive: Jack-up (self-elevating) installations: floating damage stability survivability criterion, Safety Notice 2/2001, September RP 2T, Recommended Practice for Planning, Designing, and Constructing Tension Leg Platforms, 2 nd Edition, API, International Convention on Load Lines, International Maritime Organization, 1966 Developments in Watertight Integrity on Floating Offshore Installations 69