Albacora Leste Field - P-50 FPSO Mooring System

This article, written by Technology Editor Dennis Denney, contains highlights of paper OTC 18243, "Albacora Leste Field Development - FPSO P-50 Mooring- System Concept and Installation," by F.E.N. Brandao, C.C.D. Henriques, J.B. de Araujo, O.C.G. Ferreira, and C. dos Santos Amaral, Petrobras, prepared for the 2006 Offshore Technology Conference, Houston, 1–4 May. The mooring system for the P-50 floating production, storage, and offloading (FPSO) vessel is an 18-line, partially compliant spread-mooring system, with 10 lines to the bow and eight lines to the stern. The mooring lines comprise polyester ropes and steel chain in a semitaut configuration. This project was the heaviest synthetic-line mooring system designed for a floating production unit. Introduction Since the late 1990s, free-fall piles, "torpedoes," have been used offshore Brazil. Initially, torpedoes were used to moor flexible risers to the sea bottom, then later to moor semisubmersible drilling rigs. In April 2001, the classification group Bureau Veritas certified a design called T-43 (a 43-tonne torpedo pile). These torpedoes were used to moor a deepwater drilling rig and the floating storage unit Avaré, a former 30,000-DWT tanker, installed in the Santos basin offshore Brazil. The mooring analyses of the P-50 FPSO showed that the required anchor-holding power would be approximately 1000 tonnes and the use of a differentiated compliance anchoring system with polyester ropes would require the use of anchors with high vertical-holding capacity, such as vertical-loaded anchors (VLAs) or suction piles. VLAs had been used to moor semisubmersible floating production units, such as P-27, P-36, and P-40 in the Campos basin. However, for the holding capacity required to moor the P-50 (a 270,000-DWT tanker), three or four anchor-handling vessels (AHVs) with sufficient bollard pull would be required to embed the anchors in the soil. Experience with the P-43 installations showed that the required installation time could be much shorter with torpedo piles than with VLAs. In October 2000, development of a torpedo that could meet P-50 requirements began and resulted in the T-98 design (a 98-tonne torpedo). P-50 Mooring System P-50, a former very large crude carrier converted into an FPSO, operates in 1240 m water depth in the Albacora Leste field, Campos basin, Brazil. The heading of the unit is 208° from true north, and the mooring pattern is shown in Fig. 1. The seabed has a maximum slope of 4°, and the deepest anchor was set at a water depth of 1384 m. Because flexible risers are connected to the FPSO, the mooring-system configuration was designed not to exceed the maximum horizontal offset and rotation limits around the vertical axis in the intact and damaged (one line broken) conditions. On the basis of the riser specifications and operating experience, the horizontal offsets were set not to exceed 10% of the water depth in the intact condition and 15% in the damaged condition. Each mooring line consists of studless chains and polyester ropes, designed for 25-year life.