Abstract:

Far East Livingston Shipbuilding (FELS) is presently completing the construction of the Smedvig Production Unit SPU 380, which will be operated as FPSO for Esso BALDER Field Offshore Norway.

In good cooperation with FELS and ND&A Inc. of Houston an extensive model test program was carried out for approval and optimization of the DP assisted mooring system. The main aspects were:

  • Investigate the performance of the mooring in two water depths, i.e. 250 m and 70 m.

  • optimization of DP control for the three azimuthing thrusters.

  • Measurement of motions and wave induced loads at e.g the bilge keels, keel and deckhouse front.

  • Determination of limit sea state for turning the vessel around against the weather.

The tests were carried out in the Wave and Current Basin of MARIN, using a closed loop DP control system to steer the thrusters.

The paper presents the findings with respect to the effect of DP control strategy on mooring loads and present selected results of wave induced loads on bilge keels and deck house.

Introduction

In 1994, the SPU 380 construction was started on speculation. Therefore the model test investigation of its mooring covered a range of water depths. The main dimensions of the vessel are given in Table 1. II is an FPSO concept with a drag chain turret located at about 30 m forward of the midships, see Fig. 1. Nowadays, turret mooring concepts are considered the most economicalSPM systems, since the reliability of high pressure flexible risers has been proved and the technology of catenary moorings has developed to high standards of refinement with the use of composite mooring arrangements. Basically, two mooring concepts are in the market: passive moored and DP assisted moored F(P)SO'S. The passive systems stem from a past of single point moorings withfully rotatable fluid swivels e.g on the Tazerka (SALS, 1982) and Central North Sea (1996) FPSO'S, The DP assisted concepts were developments from drillship technology through e.g. the Petrojarl vessel in 1986 [o the Gryphon A FPSO in 1993.

In DP assisted mooring on a drag chain turret, as for the SPU 380, jumper hoses are used instead of a continuous multistack fluid swivel. The presence of the DP system allows to turn the vessel in moderate sea conditions to uncoil the jumper hoses. The deckhouse is placed at the bow, upwind of the process area and turret area. Due 10 the heading control that can be exerted by the thrusters, the permanent mooring on a turret located relatively far aft the bow is feasible, see e.g Ref. [1].

For the SPU 380 the mooring consisted basically of a 92 mm (3.75'), 10 leg chain system, connected to winches and running out through fairleads below the keel of the turret.

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