FPSO Life Extensions Antoine Le Cotty, Andrew Newport, Andrew Peden SBM Offshore

Transcription

1 FPSO Life Extensions Antoine Le Cotty, Andrew Newport, Andrew Peden SBM Offshore 24, Avenue de Fontvieille, P.O. Box 199 MC98007 MONACO CEDEX, MONACO ABSTRACT Since the start of the FPSO lease market, it has been common practice for FPSO leases to be extended at the end of the initial contract period. Depending upon the initial design basis, these extensions have often resulted in revised service lives that go beyond the original design lives of the FPSO s. This trend continues, and a substantial portion of the 150 FPSO s in service are expected to receive lease extensions. A similar situation exists for FSO s. In order for an FPSO to remain in service beyond its initial design life, a number of issues must be considered: The integrity of the unit, particularly with respect to fatigue and corrosion of the hull, topsides and mooring system; the impact of changing performance requirements, regulatory requirements and classification rules. This paper will examine these key technical issues, drawing particularly on the experience of life extension projects within the SBM Offshore fleet of FPSO s. The main life extension activities will be identified, and possible methods of increasing the operating life of a unit will be proposed and discussed. This paper also aims to give feedback on life extension issues and solutions, identifying the key factors that need to be addressed for the successful life extension of FPSO s. DOT 2013 ID 54 FPSO Life Extensions Page 1

2 1.0 Introduction The requirement to extend the operating life of an FPSO or FSO beyond its initial design life can occur for a number of reasons: longer viable production of the reservoir than envisaged during initial appraisal; tie-back of new reservoirs that extend the need for the facility; or late arrival of a replacement facility (or cancelation in case of poor reservoir performance). Life extensions can be required for both new building and converted units, and the issues involved are similar in both cases. However to date there are only six new-built FPSO s that are operating beyond their initial 20 year design life, the oldest (Petrojarl I) being built in All six of these units have been relocated at some time, and life extension work may have been performed as part of the relocation. The large majority of life extension projects concern conversions. At present there are 100 converted FPSO s either in service or on standby, 55 of which are operated under lease contracts. This paper will address the issues associated with life extension of these converted units, with a focus on FPSO s operated under lease. The same technical issues are relevant whether the unit is leased or operated directly by an oil company, but projects for leased units often have less flexibility in schedule, as the operator may be dependent upon the lease extension being signed before commencement of work, which can then impact the project execution approach. A sub-category of life extension projects comprise FPSO relocations, where life extension work is usually performed at quayside or in a shipyard dock. 2.0 Issues to be considered The conversion of a tanker to an FPSO is based upon a specific design life. In the case of leased units, the design life may be identical to the duration of the lease contract, or the owner of the FPSO may elect to use a longer design life in order to provide flexibility for lease extensions or rapid relocations after completion of the contract. Following lessons learned from contracts in the 1980 s and early 1990 s, SBM uses a minimum design life of 15 years for all conversions, but this is not consistent throughout the industry. However, it should be noted that the industry trend is towards longer lease periods, at least for complex units, with contracts now commonly placed for 20 year leases, with a corresponding design life for the most complex units extending up to 25 years. As an indication of this trend, the average lease length for the present SBM fleet of FPSO s is 15 years. The impact of extending the operating life of an FPSO life is primarily an issue of asset integrity. Asset integrity addresses all FPSO systems, but there is a particular focus on performance of the structural components as these are designed for a specific life with regard to fatigue and expected corrosion. The integrity of the hull is clearly of paramount importance, also due to its function as a platform to support the onboard systems and contain the stored oil. The hull should be assessed taking into account several factors, including the tanker trading history, the recent FPSO life and DOT 2013 ID 54 FPSO Life Extensions Page 2

3 the future period of FPSO operations. Hull assessment is generally based on a 3D finite element model of the complete cargo block. Other structural parts, i.e. the mooring systems and the topsides structures, should also be assessed to ensure there are no future failures of structural elements. An FPSO life extension may therefore require steel works spread throughout the unit, including in tanks, which can be a construction challenge with respect to the location of the existing topsides modules. It should be noted that there is considerable relevant experience of structural integrity issues from FSO life extensions on projects, many of which have seen long service. Two such case histories are given in later sections of this paper. The condition and future performance of equipment should be assessed based on operational experience on the field and their maintenance under the planned program. Consultation with the equipment vendors will often be necessary to determine the required refurbishment scope for the new life time, including review of any equipment which may become obsolete during the future period, for which the availability of spares may then not be guaranteed. In addition to the available documented history of the unit, essential information on the condition of the unit should be collected through targeted surveys on structures, marine systems and topsides production systems as discussed in sections below. Although the main issue to be considered in assessing the scope of a life extension project is usually asset integrity, two further issues generally require consideration. The first of these is whether there are any required changes in topsides requirements for the future production of the aging reservoir or to accommodate production from well tie-backs. In the rare cases that this is required, the main issues associated with these changes are available deck space and availability of the required power and utilities. The second potential issue is regulatory compliance, and specifically whether the unit must be upgraded to comply with regulations enacted since the FPSO was installed on-station. The issues of changing performance requirements and regulatory compliance are discussed in later sections of this paper. It should also be noted that relocation projects often include the requirements for life extension, and can include the requirement for major topside upgrades [1]. 3.0 Asset Integrity The integrity of the unit at the time of a life extension project is partially dependent on how the unit has been operated against the terms of the lease contract. The integrity arrangements determined at the contract stage have significant impacts on life extension projects. DOT 2013 ID 54 FPSO Life Extensions Page 3

4 3.1 Onboard Surveys The quality and accuracy of the extension options are heavily dependent on the data gathered in service. Onboard surveys first originated as class compliance exercises, however, as specific failure modes and problems associated with FPSO operations emerged, these surveys later evolved into more focused inspections. Current practice encompasses the output from the design phase, class rule requirements and operation experience, however it is recognized that this alone is rarely adequate as input to a life extension exercise. The realization that inspection data is needed as input to possible life extensions has influenced the adoption of enhanced gauging, better record keeping with database approaches and more scientific measurements of corrosion profiles over the life of the unit Survey types The survey methods utilized during the life of the vessel, covering the hull structure, piping systems & pressure vessels, fall into two basic categories; 1) thickness measurements and 2) visual inspections. There are a variety of techniques for assessing thickness available and the method selected will be driven by location and the type of item being assessed Thickness measurements Initially the approach to the collection of degradation of wall thicknesses were driven by class compliance and establishing extent of required repairs. As the realization of the importance of life extension has evolved the approach to thickness measurements has been driven towards proper bench marking. This effect has resulted in more extensive gauging of hull structure during the conversion to establish the baseline and enhanced gauging in service to facilitate assessments with corrosion coupons, discussed later, being required to accurately determine internal structural degradation rates. In terms of piping and pressure vessel systems there is a shift to risk based inspection techniques with full baseline surveys during conversion Visual inspections Visual inspections have always been subjective to some degree, as personal perspectives change between inspectors and as a result, a lack of consistency can be an issue. Historically coatings and plate condition were assessed by a simple good, fair & poor categorizations. The need to improve this has been widely recognized within the industry and coating manufacturers and classification societies have developed more effective scoring systems. The development of technology has also played a significant role in allowing the transfer of digital images. Visual inspections are now much more stringently monitored than previously, and third party verification has become common practice. An effect of these approaches to onboard surveys is the emergence of centralized databases which facilitate and provide more robust interrogation techniques and enhanced confidence in DOT 2013 ID 54 FPSO Life Extensions Page 4

5 the level of knowledge with respect to the corrosion performance of the structure & piping systems. This has been coupled with more rigorous assessment of the validity of the data prior to entry into the database as the awareness of the long term consequences emerges Underwater Surveys Underwater surveys are taking place to review the condition of hull or mooring components, covering the hull structure, sea chests, the rudder and propeller, and mooring chain inspections. Those components need some cleaning operations in order to be able to assess the condition of steel, coating or welding. Cleaning can take place with high pressure water jetting, allowing to inspect visually or gauge the steel components. Hull Structure after cleaning Mooring chain before and after cleaning 3.2 Condition assessment The information available from condition assessments is used as an input to any life extension program. Such information is typically available as a result of onboard surveys as mentioned above, though is subject to post-processing as part of the assessment. The condition assessments listed below are of particular relevance: Corrosion Assessment Corrosion monitoring by means of coupons and post-processing of results are covered by a number of industry standards [2][3], and examples of their use are given below: Piping, Vessel and Valve corrosion Electrical Resistance (ER) probes or corrosion coupons are used to monitor corrosion of piping, vessels and valves based on a detailed inspection programme prepared following a risk analysis of the anticipated corrosion. The risk analysis includes design and operational data, material data and fluid characteristics. DOT 2013 ID 54 FPSO Life Extensions Page 5

6 The corrosion monitoring and post processing starts at a frequency allowing verification of the corrosive nature of the environment (typically 6 month intervals) which can then be lengthened if longer intervals are deemed adequate Hull coupons Example of new and corroded coupons Similarly to the monitoring of piping and vessels corrosion, racks of coupons have been introduced into cargo tanks allowing retrieval of individual coupons to monitor the corrosion at regular intervals during the operational life. This supplements the standard Class Society requirements for hull gauging. Furthermore, such coupons provide valuable information on the distribution of corrosion depending on the location within the tank and the associated environment (crude, vapour space of the tank, inert gas or hydrocarbon gas atmosphere) Hull Gaugings Typical rack of coupons installed in Cargo Oil tanks Whether corrosion is monitored by means of coupons or not, hull structure must gauged at regular intervals in accordance with Classification Society Rules. These gaugings can then be entered into a Hull Maintenance Programs allowing trending of corrosion rates and anticipation of possible corrosion issues. Such gauging campaigns are complementary to visual inspection. DOT 2013 ID 54 FPSO Life Extensions Page 6

7 Hull Maintenance program - Screenshot Whether for Piping, Vessels, of Hull structure, the monitoring of corrosion allows confirmation that the initial design assumptions are valid and development of possible corrective actions if required Coating condition assessment The assessment of coating condition for life extensions to date has been focused on internal tank coatings. External coatings are covered by the general fabric and maintenance program and providing this has been maintained the provision for this is relatively straight forward in terms of life extensions. There may also be the need to campaign the topsides where an asset has been winding down to the end of contract where extension prospects were not realised. Tank coatings however can be more problematic as the task of recoating ballast and cargo tanks offshore are significant exercises running into many months per tank. The life extensions undertaken so far and vessel redeployments has shown that vessel operation as FSO/FPSO is favourable to cargo tank coatings. The figures below show the typical condition experience for bottom coatings within cargo tanks after 10 years operation. This is currently put down to the high through put of oil and associate low residency periods. That is not to say issues do not occur, but those that have are generally found to revolve around isolated pitting corrosion. For this reason it is important either on a periodic basis throughout the vessels inspection regime and/or as part of the life extension work that tanks are fully cleaned and inspected for these problems. DOT 2013 ID 54 FPSO Life Extensions Page 7

8 Cargo tank Before cleaning Cargo tank After cleaning Localized pitting Ballast tanks also seem to fair well as FSO/FPSO in SBM s experience with large scale coating programs consistently limited to coating touch up campaigns. Whilst it may not be true for all cases SBM s experience is that through life internal coating condition assessments will tend towards relatively easy to achieve maintenance programs within the context of life extensions Fatigue assessment The fatigue life of a unit (Hull, Topsides, and Mooring) is determined during the initial design stage with its appropriate safety factors. However, where units are required to remain on station longer than initially anticipated, further fatigue assessment is required and would include: an assessment of environmental conditions, a fatigue analysis, providing input to the surveys described earlier, a monitoring plan for the remaining operational life. In case where elements fail to meet the extended operational life, a repair/upgrade solution supported by fatigue analysis can be implemented. Many scenarios are then possible dependent upon several parameters: Access to the structural elements while the unit is in operation Location of the elements, e.g. whether on the outer shell or on the inner structure of the unit, or above deck Type of proposed repair. Repairs should preferably be by means of adding material rather than cropping and renew to simplify the work logistics However, an alternative solution can be to combining an initial inspection of the details concerned to detect the start of any deficiency (cracks, coating breakdown revealing high stress areas) with an enhanced survey program of those details. This assessment should be made on a case by case basis, considering the criticality of the details, the targeted lifetime increase, and the ease of inspection. In some cases, there are clear benefits to implement an enhanced survey program rather than major preventive modifications which may result in quality issues and for which the associated welding executed offshore is subject to residual stress. DOT 2013 ID 54 FPSO Life Extensions Page 8

9 Rotating Equipment Condition Monitoring Condition Monitoring is used to provide awareness on the equipment running condition on an operating unit and to allow detailed preparation of the maintenance tasks. The target is to ensure reliable production and cost-effective operations throughout the life of an installation. A number of condition monitoring tools are available to assist in assessing and optimizing assets integrity in operation. In particular, rotating equipment condition monitoring comes with recording and trending facilities for several parameters on the installed machinery including bearing vibrations, casing/skid vibrations, operating temperatures, lube oil temperatures, lube oil conditions and equipment performances Lube Oil Analysis The sampling and testing of lube oils is also very effective in determining mechanical wear of internal machinery components such as bearings, piston rings, etc. Lube oil analysis techniques detect metallic particles in the samples that can be indicative of wear, as well as the degradation of oil properties. It can also determine if contaminants such as water are present Life Extension of Rotating Machinery As noted above, the monitoring of rotating equipment is planned as part of initial design activities by specifying the instrumentation to be implemented on the equipment. The combination of time based maintenance, condition based maintenance, and corrective maintenance promotes the long lasting operation of equipment. The available information, supplemented with specific onboard surveys, allows determination of the equipment condition, and determination of the action needed for a given life extension duration. 4.0 Changes in Performance Requirements Based on SBM s experience of life extension projects associated with the extension of a lease, it is rare for process performance requirements to change as part of the project. However, there are cases when topside upgrades have been made in advance of the lease extension, and where the enhanced performance has been a factor in the decision to extend the lease. Such upgrades have included enhanced water injection, upgrades to chemical injection capabilities, and process debottlenecking. The main issues associated with these changes have been available deck space and availability of the required power and utilities. It should also be noted that relocation projects often include major topside upgrades in addition to the requirements for life extension [1], but such projects are not considered in detail here. DOT 2013 ID 54 FPSO Life Extensions Page 9

10 5.0 Regulatory Compliance In recent years there has been a considerable effort amongst regulatory bodies to develop specific FPSO Rules that differ from those for tankers or ships, which take account of the specifics of FPSO design and operation. Consequently, there will often be a gap between the rules in force at the time of the initial FPSO construction/conversion, and those in force at the time of the life extension: Feedback from units in operation has been included in the new FPSO Rules which generally results in more analysis being required. In tandem, there has been continuous developments in analytical methods and increases in computing capacities, Applicability of International Rules are continuously clarified for FPSO, eg Marpol Annex I for oil pollution, Environmental issues are given increased attention, Permanent mooring systems are covered more widely by the Rules and rather than relying solely on the practices of the relevant contractors. For a life extension project, one of two cases generally applies: i. The FPSO unit stays connected in the field It is generally accepted that new Rules for FPSOs are not applicable to life extension projects if the FPSO stays connected on station, eg no requirement for partitioning of tanks, facilities/retention tanks for treatment of effluents, side collision protection, etc. It is however a requirement to confirm and document that the extended design life is met, particularly for the fatigue aspects. The use of original fatigue calculations is acceptable if they demonstrate the required service life is within the conversion design life initial predicted, in which case no new analysis would be required. In the case that a new fatigue analysis is required, they shall generally be performed under the Rules in force at the time of the life extension project. This has a practical aspect in that may be difficult to carry out assessment under the former Rules with analytical software that have also evolved to meet the requirements of the new Rules, the older version not being maintained. ii. The FPSO unit is disconnected with life extension works executed in a Yard This case applies when either the FPSO unit is relocated to another field, or when the extent of the works is so large that there is benefit for a disconnection and refurbishment in a Yard. In cases encountered so far, the FPSO was required to be upgraded to be compliant with the Rules of the new Contract date. The observed practice is that the above principles are applied for Class Rules and International/Coastal Regulations. DOT 2013 ID 54 FPSO Life Extensions Page 10

11 6.0 Project Execution Strategy A major factor in ensuring the success of life extensions projects is the project execution strategy. If the life extension scope of work is to be carried out on station, bed space availability and environmental constraints may favour an enhanced maintenance route where the required system upgrades are implemented over an extended period. On other units, where the life extension scope is significant, a shutdown and use of an accommodation barge may be the approach taken with large numbers of personnel being involved and a shorter duration achieved. Alternatively the solution may be a hybrid of these two basic options. For an optimum project solution, it is important that both of these options are developed in parallel to ensure the most efficient solution is selected. The execution strategy selected will influence the scopes that can be achieved, and the associated methodology and complexity of operation. The cost models will require continuous modifications during the review process, and it is essential to remain open minded during this process as the preferable solution can change quickly as more survey data becomes available. In determining the execution strategy it is important to account for factors such as statutory inspections and class cycle due date. Availability of materials and work force in the area of operation are also a consideration. 7.0 Case Histories The following sections give details of two life extension projects within the SBM fleet. The examples include a recent life extension performed on station (LPG FSO K KOSSA II), and an older case (FSO DOMY) performed in a ship repair yard. 7.1 LPG FSO N KOSSA II In 1996 the 4 year old LPG tanker (Inger Maresk) was converted into the FSO N Kossa II and deployed offshore Congo. The initial FSO design life was 15 years, thus allowing operation until 2011.The life extension project considered an additional 10 years of operation, until LPG FSO N Kossa II DOT 2013 ID 54 FPSO Life Extensions Page 11

12 This life extension project was performed without any fundamental modification of the field characteristics or project requirements. Attention was therefore focused on condition assessment on the various FSO components, a check of their suitability for the new lifetime, and determination of any resulting preventive repair or inspection program. The following components were reviewed as part of the project: Hull structure Turret structure Helideck structure Underwater parts Rotating Equipment Piping systems Electrical and Instrumentation systems Offshore Surveys The available documentation from the operation of the unit was analyzed by the life extension Project Team. This exercise was conducted independently of the unit operating team to ensure an objective assessment of the unit condition. Based on the available information, the requirement for additional surveys was determined. The additional surveys were conducted in several campaigns, each based on a clear scope of work providing guidance for the survey teams. When surveys required Non Destructive Examination (NDE), certified third parties were used to conduct the work. There is considerable added value in the engineering team that prepared the survey scope of work and who will perform the analyses participate in those surveys, ensuring strict compliance with the survey programs and developing a good level of awareness of the unit condition. The following surveys were completed: General Survey for Accommodation, Engine Room, Deck Turret survey with NDE General Underwater survey Mooring system survey with Underwater Inspection Hull Structure survey Electrical and instrumentation Survey The hull structure survey and turret survey are described below as examples of the work undertaken. DOT 2013 ID 54 FPSO Life Extensions Page 12

13 Hull Survey The survey included close-up visual inspection, a gauging campaign, and pitting maps as necessary. The close-up visual survey examined the general condition of all area, identifying possible coating breakdown and areas prone to corrosion and structural failure. Since the unit is in operation and continuously monitored, identification of new unknown issues was not expected. It was nevertheless beneficial to perform surveys and closely inspect areas identified by the engineering team as potential issues for the future lifetime. A typical graphic identifying details for close-up inspection is shown below: Turret Survey Details given specific attention during close-up surveys The turret is a critical component of the unit, and the structure and all main components were surveyed. These surveys included: An inspection of the turret chamber, including chain table, swivel support structure, coating condition, weld condition, and condition of drain pipes. Inspection and condition assessment of the chains and chains connectors, with thickness measurements on the chain hawse plating to determine the presence of any wear. An inspection of turret access means An inspection of turret fixed parts, including riser and umbilical terminations, ESD valve condition, and pig catcher condition. An inspection of the rotating part, including the slewing bearing. For this component, grease sampling were taken and sent onshore for analysis. In addition, a review was made of all past turret survey reports. DOT 2013 ID 54 FPSO Life Extensions Page 13

14 Chain hawse Slewing Bearing Review of Hull Structure Strength A global finite element model was available from the original conversion engineering and this allowed the hull strength to be determined once the model was modified to include corrosion values. Hull strength analysis was started early in the initial phase of the life extension engineering, in order to obtain the sensitivity of the hull structure to future corrosion, irrespective of the actual condition of the hull structure (that was unknown at that time). In parallel, a survey scope was prepared which included both close-up visual inspections as well as thickness gauging, as reported above. Areas sensitive to buckling when performing the strength analysis were considered in 2 ways: The actual corrosion of those areas was compared with the values assumed in the structural analysis in order to validate or modify the assumptions used in the model. The areas sensitive to buckling are identified in future inspection programs, even if their condition appeared adequate at the time of the analysis. Overall, due to the very limited corrosion of the unit, no areas required buckling reinforcement. However, it should be noted that in general the addition of such reinforcement is a likely requirement in life extension projects. However, a number of areas were identified for inclusion in the operational inspection plan, to ensure that they are included in future surveys and that the intended lifetime is actually achieved. Ultimately, it is important to consider that achieving a lifetime is not only dependent upon good initial design, but a combination of sound design with appropriate assumptions, well targeted surveys, and preventive maintenance Review of Hull Structure Fatigue A hull structural analysis was performed to determine areas where remaining fatigue life could be insufficient for the extended duration. This assessment included a hydrodynamic analysis on the N Kossa field considering 4 representative loading conditions. The past fatigue damage DOT 2013 ID 54 FPSO Life Extensions Page 14

15 incurred as a LPG Carrier and as an FGSO have been calculated, in addition to the future expected fatigue damaged during the extended period on station. As a conservative approach, fatigue damage was calculated assuming net thickness for the last 15 years of operations. The fatigue analysis identified a limited number of details requiring attention, and two such details using different solutions are reported below: Web frames Lower bracket flange The fatigue life of detail number 1 below was less than required (probably due to the flange thickness not tapering towards the snipped end). By contrast, detail number 2 had a future fatigue life of over 100 years. Lower bracket flange Based on this result, several considerations were made: The thickness gaugings revealed a unit in very good conditions with no specific corrosion issue in the area of the details concerned. The fatigue analysis performed with net thickness was therefore conservative. The cargo hold space were these details are present are inerted and consequently substantial future corrosion is not expected. No coating breakdown was visible around the details at the time of the life extension surveys. An additional calculation without corrosion revealed that the future life would be longer than the required value. The fatigue life was therefore considered to be generally met, but without a substantial safety factor for this area. Fatigue analyses are based on a probability of failure of 2.5% (mean S/N curve 2 x Standard Deviation). In the event that fatigue cracking did occur, it would be unlikely to affect more than a few details out of 220. Based on the above considerations, the following approach was adopted: A proposed repair was defined for the case that a crack developed during the future life. Based on the low likelihood of occurrence and the limited number of brackets involved, no immediate modification of the detail was proposed. DOT 2013 ID 54 FPSO Life Extensions Page 15

16 An inspection program was defined including close-up survey of those details. Should any accelerated coating breakdown or sign of cracking appear, the proposed repair would be implemented and possibly extended to other area still in good condition Tank dome deck hatch corner connection to deck plating The fatigue analysis has revealed similar issues on the tank dome detail number 1 below, with a fatigue life less than required. Tank dome detail In this case, the selection approach was different, due to the consequences of a crack initiation in the area which could affect the main deck, and that repair by means of an insert plate would then need to be executed offshore. Consequently, a preventative structural detail improvement was made and implemented after approval by Class. Repair detail In summary, hull structural analysis, taking account of the past operating history and future operational life, is the starting point for defining the life extension scope of work. However, the analytical results must be treated in a pragmatic manner, allowing definition of preventive solutions that maximize added value, and considering monitoring to avoid repair solutions that may generate additional issues. DOT 2013 ID 54 FPSO Life Extensions Page 16

17 7.2 FSO XV DOMY The FSO XV Domy was converted from the trading tanker SS Nielstor in 1992 in order to operate offshore Nigeria. The Domy ended operations in 2003, and was then laid-up in Labuan Offshore East Malaysia. FSO XV - DOMY In 2005, the Domy was brought to a Singaporean yard for hull refurbishment and a life extension program in preparation for a future project, and in 2006 to 2007 the hull was then converted into the FPSO Espirito Santo for operation offshore Brazil. The main steps of this life extension program followed the same basic methodology adopted for the FGSO N Kossa II, though the following paragraphs will focus on aspects specific to the FSO Domy. In general, the hull was a well-built VLCC with good structural details and no overlap connections. The 2003 conditions of the unit was generally good, with the exception of the uncoated wing tanks 3 aft which had been used to store seawater for a prolonged period. Starting from the list of known defects, an extended structural analysis was conducted combined with a visual inspection and a campaign of NDE and extended UTM throughout the vessel. The known defects were: Severe corrosion in wing tank 3 aft (Port and Starboard), due to sea water loading when using the uncoated tanks for ballast. Cracks at bottom longitudinal penetration through the transverse bulkhead between centre tank 2 and centre tank 3. The fatigue analysis confirmed high damage ratio in these locations, which were solved by the addition of backing brackets: DOT 2013 ID 54 FPSO Life Extensions Page 17

18 Steel cropped in way of penetration Repair by addition of backing bracket The structural analysis revealed a few additional fatigue sensitive details which were then subject to close visual inspection and NDE. These details includes connections between horizontal girder and webframe, Slots in way of bottom longitudinals, and a number penetrations of side shell longitudinals in way of webframes. Connection of horizontal girder number 38 and webframe Side Shell Longitudinal penetration in way of webframe The table below summarises the reinforcement and repairs conducted following the structural analysis results: DOT 2013 ID 54 FPSO Life Extensions Page 18

19 LOCATION CAUSE ACTION TAKEN Bottom longi's iwo transverse bulkhead centre tanks Fatigue Additional brackets & intermediate flat bars Bottom longi's iwo transverse bulkhead wing tanks Fatigue Additional brackets & intermediate flat bars Side Shell longi's iwo transverse bulkhead Fatigue Additional brackets Cracks iwo stringer 38 and struts Corrosion buckling Replaced to as-built values buckling stiffener added Longitudinal bulkhead Buckling Intermediate stiffener added Longitudinal bulkhead wing tanks 1 Sloshing Brackets added to reduce the unsupported span In addition to the structural analysis and subsequent repairs, the UTM gauging campaign identified areas for steel renewal. Significant renewals were required on the main deck, as shown in the adjacent figure. Renewal of main deck plate started Wing tank 3 Aft Much of wing tank 3 Aft was subject to general corrosion, and it was therefore decided to, cut and remove the existing tanks, and insert newly fabricated tanks, as shown below. This approach removed the risk of missing defects in such a larger generally corroded structure. The total block weight for one side was 980t and was lifted using a floating crane. New tank under construction Preparation works in the hull Lifting and installation DOT 2013 ID 54 FPSO Life Extensions Page 19

20 8.0 Discussion The industry has shown a recent trend towards longer lease contracts for converted FPSO s, at least for the more complex units, with many new lease durations ranging from 15 to 20 years. However, many earlier lease contracts were shorter, and these continue to be subject to extensions, sometimes requiring units to stay in operation beyond their original design life. Extending the operating life of a unit beyond it s design life is primarily an issue of asset integrity, and the age related mechanisms of fatigue and corrosion require particular attention. However, experience of life extension projects, including the example of the FSO N Kossa II reported in this paper, has demonstrated that considerable extensions in life can be achieved through a combination of detail modifications and enhanced inspection programs. The analysis of asset condition requires accurate and comprehensive survey information, often obtained from a combination of in-service surveys. These are supplemented by specific project surveys based on a scope derived from the initial structural survey and a thorough review of the in-service survey records. Translation of structural analysis results into a life extension scope of work requires a pragmatic approach if the work is to be completed on station, accounting for the risk and consequence of failure. When the probability and consequence of failure are low, enhanced inspection with a remedial plan in case of future failure can be considered as an alternative to immediate modification of structural details. In contrast, the case history of the FSO XV Domy detailed the life extension of a unit that had completed its contract, and was being prepared for use in a new project. In this situation the survey requirements, analysis and methodology are essentially the same as for a life extension project to be undertaken on station. However, the availability of yard facilities focused the life extension scope on modifications rather than future in-service enhanced inspections. A further difference was the need to modify the hull to satisfy the requirement of regulatory compliance to the current legislation. Life extensions of this type can allow the continued supply of single hull vessels to projects when such a hull configuration is advantageous. 9.0 References [1] George, B., Le Cotty, A., and Newport, A., The Challenges and Opportunities of FPSO Relocations, Proc. Offshore Technology Conference, OTC-23122, May [2] ASTM G4 95 Standard Guide for Conducting Corrosion Coupon tests in Field Applications [3] ASTM G1 90 Standard Practice for Preparing, Cleaning, and Evaluating Corrosion test Specimens DOT 2013 ID 54 FPSO Life Extensions Page 20