DP assisted and Passive Mooring for FPSO's

ABSTRACT For the permanent positioning of an FPSO in the field, internal turret type SPM systems, based on fully passive anchoring, and combinations of such anchoring systems with Dynamic Positioning are being used. The paper presents the specific characteristics of each of the two positioning concepts in relation to field specific conditions and aplicable legislation. Strong and weak points for the two concepts will be highlighted, on basis ofresults from computer simulations and model tests. Additionally the paper will describe the state-of-the-art tools, used to evaluate the alternative positioning concepts. 1.INTRODUCTION In the early seventies, the oil companies requested the design and supply of permanent tanker moorings for application in offshore fields. At first, the tankers were designed to serve as storage and offloading units (example: Ashtart Field, Tunisia), but only a few years later the firstfloating production tanker was installed (the Castellon FPSO in Spain). For the storage units the (low-pressure) fluid transfer between the seabed piping and the vessel could take placevia the existing underwater hose strings. Therefore the mooring systems used resembled the well-known CALM - buoy type terminals, with that respect that the permanently moored tanker was hooked-up to the buoy via a rigid arm instead of by a mooring hawser. The mooring system described is known as a Single Buoy Storage (SBS) system. However for FPSO vessels mooring systems were developed based on steel structural columns for connecting the production tanker to the seabed. These columns could support hard piping for fluid transfer, as high pressure flexible risers for live crude were not yet developed to a sufficient level of confidence. For the mooring system design oil companies relied on the established mooring contractors, who developed the so-called SALS and SALMRA systems. All mooring systems mentioned are completely passivepositioning systems, of the single point mooring type, with the natural weathervaning centre at or in front of the vessel bow. Reliable swivels were developed, so that the flow paths coukt bridge the weathervaning bearing arrangement. Over the years, the reliability of high-pressure flexible risers increased and the technology of catenary moorings was further developed, with the use of composite (combinedchain and steel wire ropes) lines. That allowed the TURRET type single point mooring system to become the most economic and reliable (redundant) mooring for an FPSO. The turret mooring is widely used, in many variants, depending on the environmental criteria: attached to bow or stern, above water; internally integrated with the vessel bowor even disconnectable, in typhoon area's. See Ref. 9. Recent examples of this technology comprise the internal turret for the Alba Floating Storage System, disconnectable Internal turrets for various fields in the South China Sea, offshore China, for operators ACT, JHN and Phillips Petroleum, and the FPSO for Shell Expro for application in the Central North Sea, which is presently underconstruction. Since these systems have historically evolved from single point mooring technology, they are entirely passive (no thruster assistance required) and naturally weathervaning.