Moorings Replacement and Hookup for a Damaged North Sea FPSO

Abstract

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 25322, ’Gryphon Alpha FPSO: Experience Gained During Moorings Replacement and Hookup,’ by Fred J. Toal and John G. Martin, Maersk Oil, and Martin G. Brown, Ian M. Lindsay, and Robert Sinclair, GL Noble Denton, prepared for the 2014 Offshore Technology Conference, Houston, 5-8 May. The paper has not been peer reviewed. In a North Sea storm during February 2011, the Gryphon Alpha floating production, storage, and offloading (FPSO) unit broke four of its 10 mooring lines and moved partially off station, resulting in damage to subsea assets. Following the incident, the FPSO was taken to dry dock for replacement of the damaged subsea infrastructure and moorings. This paper describes the measures put in place so that the mooring system could be replaced and the FPSO reconnected on an efficient schedule. The FPSO In September of 1993, the Gryphon Alpha became the first purpose-built longterm- moored FPSO to be installed in the North Sea. The unit is located 175 miles northeast of Aberdeen in a water depth of 112 m. Gryphon has a somewhat unusual layout in that the turret is located forward of midships (Fig. 1). Five fully azimuthing thrusters—two forward and three aft—are used to provide heading control. Each mooring line has its own windlass and azimuth swiveling-gypsy-wheel assemblies provided at the base of the turret. It is therefore relatively easy to adjust mooring-line tensions and lengths compared with other turret designs. A drag chain is provided for fluid transfer from the wells. This means rotation of the FPSO is limited to 270°, either clockwise or counterclockwise from the vessel’s neutral heading of 225°. During the original installation, the FPSO was provided with 10 all-chain mooring lines consisting of 84-mm studded chain attached to 35-t drag-embedment anchors. Background to the Event Gryphon sustained damage in a severe storm on 4 February 2011 when one mooring line failed and the vessel lost heading control, causing it to turn partially beam-on to the weather, resulting in failure of a further three mooring lines. The FPSO’s partial movement off station caused considerable damage to the subsea architecture. Because of the length of time required to reinstate the subsea equipment, it was decided that the FPSO should be removed from the field for shipyard work that would involve inspections; repairs; and upgrades of the structure, vessel equipment, and process equipment.