Toal, Fred J. (Maersk Oil North Sea UK Ltd.) | Martin, John G. (Maersk Oil North Sea UK Ltd.) | Brown, Martin G. (GL Noble Denton Aberdeen) | Lindsay, Ian M. (GL Noble Denton Aberdeen) | Sinclair, Robert (GL Noble Denton Aberdeen)
In a North Sea storm during February 2011 the Gryphon Alpha FPSO broke four of her 10 mooring lines and moved partially off station, causing damage to subsea assets. Heading control was re-established using the FPSO's thrusters and in due course the moorings which had parted were re-connected back to the Tentech turret. Following the incident, the FPSO was taken to dry dock for life extension work and to allow replacement of the damaged subsea infrastructure and moorings. The original mooring system was recovered for forensic inspection.
This paper describes the measures which were put in place so that the Gryphon mooring system could be replaced and the FPSO re-connected in accordance with a schedule which would not delay the installation of the subsea infrastructure or first oil. The intention is that this paper should be a useful reference for others engaged in similar work. It was decided to replace the original moorings, apart from the recovered Stevpris anchors that were refurbished. Since the size of the chain into the gypsy wheel fairleads at the turret was fixed, the mooring designers had to optimise their design to meet latest code requirements. This resulted in a 100 m long tri-parallel chain configuration to reduce FPSO mooring loads and 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 manipulate safely the heavy chain, delta plates and associated shackles.
A challenging design and procurement exercise was needed to make sure that all the mooring components were delivered on time. The initial mooring sections were pre-laid in February/March of 2012, including anchor proof tensioning, so as to suit the subsea schedule. The mooring lengths had to be adjusted 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 uniform and the moorings properly balanced. The pre-deployed mooring lines were each buoyed off subsea for later ROV recovery.
The FPSO moorings hook-up took place in September 2012 using the AHVs previously used for the pre-lay. Due to the presence of three Mid Water Arches (MWAs) within the FPSO swing circle, four AHVs were used to hold Gryphon in position during the reconnection operation. The AHVs were positioned 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. The FPSO also had access to a 3D computation and visualization package which provided real time in-situ position monitoring. This ensured that no mooring chain to subsea assets contact occurred despite the unusually tight installation tolerances.