Gryphon Alpha FPSO - Experience Gained During Moorings Replacement and Hook-Up

摘要

In a North Sea storm during February 2011 the Gryphon Alpha FPSO broke four of her 10 mooring linesand moved partially off station, causing damage to subsea assets. Heading control was re-establishedusing the FPSO’s thrusters and in due course the moorings which had parted were re-connected back tothe Tentech turret. Following the incident, the FPSO was taken to dry dock for life extension work andto allow replacement of the damaged subsea infrastructure and moorings. The original mooring systemwas recovered for forensic inspection.This paper describes the measures which were put in place so that the Gryphon mooring system couldbe replaced and the FPSO re-connected in accordance with a schedule which would not delay theinstallation of the subsea infrastructure or first oil. The intention is that this paper should be a usefulreference for others engaged in similar work. It was decided to replace the original moorings, apart fromthe recovered Stevpris anchors that were refurbished. Since the size of the chain into the gypsy wheelfairleads at the turret was fixed, the mooring designers had to optimise their design to meet latest coderequirements. This resulted in a 100 m long tri-parallel chain configuration to reduce FPSO mooring loadsand excursions, but which also caused some handling issues. High specification Anchor Handling Vessels(AHVs) were needed for the prelay operation, which had after deck chain handling gantries to manipulatesafely the heavy chain, delta plates and associated shackles.A challenging design and procurement exercise was needed to make sure that all the mooringcomponents were delivered on time. The initial mooring sections were pre-laid in February/March of2012, including anchor proof tensioning, so as to suit the subsea schedule. The mooring lengths had to beadjusted offshore by chain cutting, depending on the final resting place of the chain ends after tensioning.This was to ensure each tri-chain end was correctly located so that the system stiffness would be uniformand the moorings properly balanced. The pre-deployed mooring lines were each buoyed off subsea forlater ROV recovery.The FPSO moorings hook-up took place in September 2012 using the AHVs previously used for thepre-lay. Due to the presence of three Mid Water Arches (MWAs) within the FPSO swing circle, fourAHVs were used to hold Gryphon in position during the reconnection operation. The AHVs werepositioned two forward and two aft about the FPSO. Adjustment of AHV tow lines enabled the FPSO to be held within just 5 m of the Field Turret Centre (FTC) during the entire reconnection operation. TheFPSO also had access to a 3D computation and visualization package which provided real time in-situposition monitoring. This ensured that no mooring chain to subsea assets contact occurred despite theunusually tight installation tolerances.