OTC FPSO INTEGRITY: JIP on FPSO Fatigue Loads Stephen Bultema (Bultema Marine), Henk van den Boom (MARIN), Max Krekel (Bluewater)

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1 OTC FPSO INTEGRITY: JIP on FPSO Fatigue Loads Stephen Bultema (Bultema Marine), Henk van den Boom (MARIN), Max Krekel (Bluewater) Copyright 2000, Offshore Technology Conference This paper was prepared for presentation at the 2000 Offshore Technology Conference held in Houston, Texas, 1 4 May This paper was selected for presentation by the 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 or its officers. 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. Abstract The hull forms used for FPSOs and trading tankers look very similar. So similar in fact that it may appear that one could use the same design and design methods for both vessels. Both vessels will be subject to extreme and fatigue loading and both will have a similar design life. But each will have different design requirements and therefore different designs. The trading tanker spends most of its life moving oil from one location to another, in either one of two draft conditions, ballast or laden. They are dry-docked at regular intervals for maintenance, inspection and if necessary repair. FPSOs are stationary, i.e. they have zero forward speed, the vessel s draft is continuously changing, the maintenance and inspection is done in situ, and they are not usually dry-docked. These apparently subtle differences, see Table 1, in design requirements are not so subtle with respect to the load differences, both short term and long term for the two designs. This difference is the driver for the FPSO Integrity Joint Industry Project. The JIP was established to obtain fatigue load information on an FPSO through full-scale measurements. The main goals were as following. Develop of a computational model for fatigue loading Evaluate current practice. Have the Classification Societies conduct a comparative study, and Recommend a fatigue method for use in design of FPSOs. The work in this JIP is being further enhanced through a sister JIP FPSO Fatigue Capacity being administered by DnV. By combining the results of the two JIPs, the industry is gaining a better knowledge of the fatigue loads and fatigue resistance on FPSOs. Predictive methods are being developed to calculate the fatigue lifetime of FPSOs and this is leading toward more reliable systems. Introduction In the upstream oil industry it can be generally said that all ideas are bred, birthed and reared from within. In order to maintain this axiom it often means that nothing gets implemented in the upstream sector unless they have thoroughly analyzed and tested it and meticulously prepared proper standards and specifications. The idea of using an oil trading tanker as a floating production, storage and offtake unit is one solution that breaks the rule. It is a crossbreed, a marriage between upstream and downstream, a child of unsuspecting parents. It arrived in time to feed a hunger in the industry to expand operations into the seven seas. It looked positively prefect; it was a solution waiting on a problem. In the enthusiasm to apply the solution not all areas of design and design methods were assessed. The industry has now reached a stage where they are asking the question whether or not shipping standards are applicable to FPSO designs. The design rules for trading tankers developed through time by trial and failure. In the past, the rules did not recognize fatigue was not a primary issue in ship structure design. In many FPSO cases this is not suitable. In addition, the practices and requirements for trading tankers are not consistent with the approaches used for design in upstream production systems what was needed was a design solution specifically for FPSOs. Background The first tanker based floating production and storage facility was installed in As this new technology caught on the industry began to add, on average, two FPSOs per year for the next 15 years. Most of these were converted trading tankers. A large drop in oil prices had caused a collapse in the tanker market and this resulted in a significant surplus of vessels. Large numbers of recently built trading tankers were available and at prices for close to scrap value. The upstream oil industry took advantage and made use of an inexpensive solution for expanding their market using these cast off trading tankers for new field development. In these early years conversions of trading tankers were the norm and it was nearly 10 years before the first purpose built

2 2 STEPHEN BULTEMA (BULTEMA MARINE), HENK VAN DEN BOOM (MARIN), MAX KREKEL (BLUEWATER) OTC FPSO hull was introduced. This trend was slow to change, by the early 1990s, of the 28 vessels put into service as an FPSO only a hand full of these was specially designed and built for this purpose. As the decade of the 90s came to a close this trend has reversed. The aging tanker fleet along with increases in the cost of conversions and low cost new construction shifted the industry preference from conversion into new builds. Before the trend to new build the industry was focused on the problem of how to quickly and inexpensively modify existing vessels to suit the loading conditions they would see in service as an FPSO. The answer was usually to add more steel. In most locations throughout the world where FPSOs were employed this was a cost effective solution. Often times the field life was short enough or metocean conditions were benign. Not so for the North Sea. The North Sea oil rush is 20 years old now and the focus has shifted from the giant fields with large reserves to smaller more marginal discoveries. The smaller fields required lower capital investment and the FPSO is a good candidate. But the North Sea has very aggressive wave conditions and is not as well suited for the converted tanker solution. Although there are a few very successful conversions more and more the cost and schedule advantages are disappearing. The recent trend is to use the very old tankers in more benign environments, such as offshore Brazil and West Africa. These tankers were designed and built in the 70s and because of MARPOL 13G will soon to be prohibited for use in trade market. The problem with assessment of fatigue life in these cases is that fatigue damage endured during trading needs to be added to the damage expected during FPSO service, a nearly impossible task. An aging tanker fleet, 13F and 13G, and the experience in the North Sea are pushing the industry to look toward using a new build vessel. The question is, how to design a vessel that would only be used as an FPSO and not to transport cargo. Until recently, fatigue was not specifically addressed in the ship classification rules. In harsh environments, these standard procedures have been seen to not be conservative, and fatigue has been identified as a major mode of failure for FPSOs. The FPSO INTEGRITY Joint Industry Project (JIP) was initiated in 1996 to develop to address fatigue loading on FPSOs and to begin to establish an industry standard procedure for the assessment of fatigue on FPSOs. Four major tasks were chosen to achieve the projects goal: 1. Structural monitoring of FPSO Glas Dowr operated by Bluewater for Amerada Hess on the Dauntless and Durward field in the North Sea. 2. Analysis of the measured data to derive the fatigue loads on the vessel. 3. Development of a fatigue load assessment method. 4. Compare existing fatigue analysis methods in use by the Classification Societies. The structural monitoring campaign on board Glas Dowr is presented in reference [1] and the comparative study conducted by five classification societies is reported in reference [2]. Organization of the JIP The JIP formally started on September 1, It originated as a 3-year program supported 20 companies and by the Thermie Programme of the European Community. It now has 25 participants and has been extended 1 year, Table2. At its start the project was of the largest JIPs ever put together. The participants include: 5 classification societies; American Bureau of Shipping, Bureau Veritas, Det Norske Veritas, Germanischer Lloyd, Lloyd s Register. 8 oil companies; Amerada Hess, Chevron, Conoco, Petrobras, Shell, Statoil, Texaco and one asking not to be identified herein. 6 shipyards; Astano, Daewoo Heavy Industries, Hyundai Heavy Industries, Kawasaki Heavy Industries, Mitsubishi Heavy Industries, Samsung Heavy Industries. 2 vessel owner/operators; Bluewater and Maersk Contractors. 2 engineering companies, Nevesbu and Marin, and the U.K. Health & Safety Executive, and as well as by the Dutch Ministry of Economic Affairs. See Figure 5. The project is significant not only for its size but also because of the diversity of its members and because of the support it is receiving from the Class Societies. The project is structured so that each participating company is represented in the Project Steering Group (PSG). The PSG is responsible for supervising the project and they meet twice yearly. Each of the four major tasks in the JIP is lead by one of the participants and each has a separate work package. Bluewater was responsible for all operational activities on board Glas Dowr. Marin s Trials and Monitoring group conducted the measurements on board Glas Dowr as well as the analysis and reporting of the measured data. Nevesbu and Marin developed a computational model for fatigue loads on arbitrary FPSOs under arbitrary conditions. Bureau Veritas lead the activities of the Comparative Study. The successes of the project are due to the strong cooperation and input from all stakeholders. Together, by sharing knowledge and working together in a co-operative spirit, the results, which have been achieved, could not have otherwise been obtained nor applied throughout the industry. FPSO GLAS DOWR The Glas Dowr is the fifth FPSO owned and operated by Bluewater, and the second operating on the UKCS. The main particulars are presented in Table 3, the general arrangement is shown in Figure 4.

3 OTC FPSO INTEGRITY: JIP ON FPSO FATIGUE LOADS 3 The vessel was used to produce the Durward and Dauntless fields, in UK blocks 21/11 and 21/16, on behalf of Amerada Hess Limited from August 1997 to August The fields are located some 150 nm east of Aberdeen in a water depth of 75 meters, Figure 3. The Glas Dowr was converted from a new-built double hull Aframax Size tanker built by Namura shipyard, to their standard design, Figure 1. The vessel was originally classed under ABS, as an oil tanker but never actually used for transporting oil. The ship was delivered to Bluewater in March The vessel sailed under its own power to Harland & Wolf where the conversion work was conducted. The mooring system was installed, the propulsion plant decommissioned, all steel work modifications were made, and the coating system was upgraded. During this time, the preparatory work for the structural monitoring system was done. Deck-, hull-, and bulkhead penetrations were fitted and the cable conduit pipes were installed in the ballast tanks. The consoles for the wave radar s were fabricated and installed. In December 1996 she was towed to Teesside, were the topsides facilities were lifted on, and final outfitting took place. At Teesside, the structural monitoring system was installed and precommissioned under supervision of Marin s Trial & Monitoring Group. The tow-out to field was in July 1997, and first oil was achieved on August 8, Glas Dowr is moored via an internal Turret Mooring System (TMS) located in the forward cargo tanks. The mooring system is chain/wire rope and the nine anchor legs are grouped in threes, spaced apart. Each leg is terminated at the seafloor by a 31 tonne Stevpris anchor. Well fluids from both fields flow via flexible risers to the turntable manifold. In all eight risers are suspended below the turret to for production, water injection, gas-lift, and subsea control. The swivel stack comprises three identical modules for production, test, and water injection with an in-line swivel for gas-lift. The facilities include a crude oil separation unit, water injection plant, a lift gas compression module, electrical power generation, and utility systems. Stabilized crude oil is stored in the vessels cargo tanks, and off loaded on a regular basis to shuttle tankers to shore. The FPSO is classed under Lloyd s Register. The vessel is double hull construction. The material used for the hull envelope is grade AH32. The intermediate floor that halves the frame spacing in the lower parts of the hull allows the use of relatively light longitudinals in the double bottom and lower regions. This keeps the cross section of the hull girder relatively balanced. Limited modifications were needed for fatigue life extension. The Glas Dowr midship section is shown in Figure 2. FPSO JIP Weeks & FPSO Research Forum The FPSO Integrity JIP has been conducted in conjunction with the FPSO Fatigue Capacity JIP initiated in 1997 by Det Norske Veritas [3]. The Integrity JIP is looking into fatigue loads and the Capacity JIP is focusing on the construction, material response and resistance to fatigue loads. It has been very fortunate that many of the companies involved in Integrity also joined the Capacity JIP. Because of the close relationship between the two topics the members decided that the Steering and Working Group meetings of both JIPs would be held at the same locations and in the same week. This has become the so-called FPSO JIP Week. Such weeks were held in Houston (May 98), Ulsan (October 98), Oslo (March 99), Hamburg (October 99) and Berkley Ca. (March 00). In combination with the two JIPs is the FPSO Research Forum. The Forum meets on the Wednesday sandwiched between the DnV JIP and the MARIN JIP. The Forum is always well attended and open to the industry. It is quite successful with 40 companies from more than a dozen nations involved in an ongoing dialog. It is aiming at a long-term co-operation between all parties involved in FPSO developments. The Forum shares experiences, identifies technology demands, directs new developments and initiates and co-ordinates Joint Industry Projects. Besides the oil companies, vessel operators, yards, also research institutes, classification societies and national authorities such as the UK Health & Safety Executive and the Norwegian Petroleum Directorate participate actively in the Forum. The FPSO Research Forum is proving herself as a permanent platform and is likely to continue her gatherings in combination with the meetings of new JIPs in the field of floating production. References: [1] Boom, H.J.J. van den, M. Krekel and P. Aalberts, FPSO Integrity; Structural Monitoring Glas Dowr OTC 12143, May 2000 [2] Francois, M., et al, FPSO Integrity; Comparative Study Fatigue Load Prediction OTC 12144, May 2000 [3] FPSO Fatigue Capacity JIP Lotsberg, I. OTC 12145, May 2000

4 4 STEPHEN BULTEMA (BULTEMA MARINE), HENK VAN DEN BOOM (MARIN), MAX KREKEL (BLUEWATER) OTC Table 1 Comparison Service Trading tanker World wide, and not known before hand. Weather from all directions, and will avoid storms. Forward speed FPSO Site specific, and most often, permanently moored. Weather predominantly over the bow. No speed Loading Ballast / Laden, tanks are either full or empty Continuously varying draft, tanks operated while slack Inspection Pre-determined scheme of surveys, increasing with vessel age. Regular dry docking, enabling repairs Extended surveys before contract. Once on location, no drydocking for extended periods. Only limited possibilities for repair & maintenance. Table 2: JIP Organization Participants Steering G roup MARIN Monitoring & Analysis Marin Installation & Operation Bluewater Model & Software Development Marin Comparative Study Bureau Veritas Shipyards Nevesbu ABS, Lloyd's, GL, DNV

5 OTC FPSO INTEGRITY: JIP ON FPSO FATIGUE LOADS 5 Table 3 Main Dimensions & Capacities Length between perpendiculars (m) Breadth moulded (m) Depth moulded (m) Draft moulded, design (m) Displacement design draft (te) 121,500. Cargo tanks 98% full (m 3 ) 105,100. Produced water 98% full (m 3 ) 7,500. Ballast 100% full (m 3 ) 39,600. Internal Turret Mooring System 3 by 3 chain/wire (m) Stevpris anchors (te) 31. Water depth (m) 75. Production System Fluid capacity, total (bpd) 75,000. ~ 3,000 Max. Crude production (bopd) 60,000. Reid Vapour Pressure (psia) 10.0 BS&W (%) < 0.5% Max. Produced water (bwpd) 65,000 Oil content (ppm) < 40 Max. Gas (produced + lift) (MMscfd) 24.5 Max. Capacity (bwpd) 100,000. Pressure at wellhead (psia) 3,800. Flowrate (MMscfd) 4.0 ~ 20.0 Pressure at wellhead (psia) 1,000 ~ 1,800

6 6 STEPHEN BULTEMA (BULTEMA MARINE), HENK VAN DEN BOOM (MARIN), MAX KREKEL (BLUEWATER) OTC Figure 1: Before and After

7 OTC FPSO INTEGRITY: JIP ON FPSO FATIGUE LOADS 7 Figure 2: Midship Section

8 8 STEPHEN BULTEMA (BULTEMA MARINE), HENK VAN DEN BOOM (MARIN), MAX KREKEL (BLUEWATER) OTC Figure 3: Dauntless & Durward

9 OTC FPSO INTEGRITY: JIP ON FPSO FATIGUE LOADS 9 Figure 4: Glas Dow

10 10 STEPHEN BULTEMA (BULTEMA MARINE), HENK VAN DEN BOOM (MARIN), MAX KREKEL (BLUEWATER) OTC Figure 5: JIP Participants