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7874 |
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Wednesday, September 13, 2023, Brüssel 2:30 PM Offshore applications |
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Fatigue long term assessment for root cause analysis in turret moored FPSO bilge keel crack findings |
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Bernardo Xavier* / Proper Marine, Brazil Guilherme Leal / Proper Marine, Brazil Uwe Aschemeier/ Subsea Global Solutions, USA |
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A Floating Production Storage and Offloading also known as FPSO from converter tanker vessels became widely popular among field Operators the last 30 years due to its versatility employed on remote of deep ocean production facilities in the O&G industry. This alternative was able to provide further use of ageing tanker vessels in the end of service life and adapted to the growing world energy demands. However from a design perspective, it imposes great challenges in order to maintain hull integrity throughout the FPSO life cycle duration as these traditional tanker vessels were initially designed to operate for 20 to 25 years which covers mainly the first life cycle. During tanker conversion to FPSO, many steel plates are replaced and reinforcement structures are also added to provide adequate strength capacity for the FPSO loading regime (e.g., turret, riser balcony, helideck, flare, topside modules, etc.). On the other hand, a great part of the existing hull structure is refurbished, repainted and set forth during FPSO operation. In this case, naval architects and structural engineers often verify hull integrity checking strength capacity due to plate corrosion reduction in the new expected life cycle and fatigue damage. One of these items which is upgraded to comply with FPSO operational standards is the bilge keel. A bilge keel is one of a pair of longitudinal plates that, like fins, project from the sides of the vessel running parallel to the center line intended for roll damping and contribuites to stabilize the motions of the FPSO when its subjected to long swell periods. This offers better conditions for offloading to shuttle tankers, crane, supply vessel back load operations, diving campaigns and not to mention improves crew comfort on board by reducing motion sickness. In most of these cases of FPSO conversion from tanker vessels, the original bilge keel is enlarged to increase roll damping and meet up with the FPSO requirements. On the other hand, FPSO bilge keels are reinforced in order to withstand the increase of hydrodynamic loads on the plate profile and is considered a key aspect for hull integrity management during the life cycle of the FPSO. Based on the above, a case study is presented of a tanker vessel built in late 70s and converted to a turret moored FPSO in early 2000 design for additional 20 years of service presented innumerous cracks in the welded steel keel structure on both sides of the Unit identified during Class Society regular hull underwater inspections around 2010. Since then, a detailed keel integrity management and retrofit program has been developed providing engineering background analysis and root cause investigation. In addition, an intense underwater diving repair and reinforcement campaign has also been executed by inspecting and adding new welds in almost 400 meters length of bilge keel never done before in the O&G industry. |