ABSTRACT

The Girassol field is located in the Gulf of Guinea, in about 1,400 metres of water offshore Angola. The Girassol development scheme includes an FPSO vessel and an export buoy, with 39 subsea wells and associated flowlines. The connection between the seabed flowlines and the FPSO is made by means of three riser towers. The floating facilities (FPSO, export buoy and riser towers) are held m position by suction anchors, and the subsea manifolds are supported on suction-installed caissons The soil conditions at the Girassol site consist of highly plastic and normally consolidated clays. All the suction foundations and anchors were successfully installed in March-July 2001. The paper presents an overview of the geotechnical design analyses and of the installation procedures, together with the Installation behaviour of the FPSO and buoy anchors. The comparison between the observed results and the pre-installation predictions shows that the conventional approach was over-conservative in the highly plastic Guassol clays and should be improved Such an improved approach is proposed and validated on the data recorded during the installation of the Girassol FPSO and buoy anchors.

INTRODUCTION

Discovered in 1996, the Girassol field is located in about 1,400 metres of water, in block 17 at 150 kilometres off the coast of Angola The Girassol project is a development operated by TotalFinaElf, under a production sharing agreement awarded by Sonangol to a contractor group including TotalFinaElf, ExxonMobil, BP, Statoil and Norsk Hydro First oil was produced m December 2001, marking the start of ultra-deepwater field developments in West Afiica. With 39 wells spread across the field over an area of about 45 sq km, Girassol is one of the biggest subsea developments undertaken in this kind of water depth. The selected development scheme is based on a subsea production system, associated with an FPSO unit and an export buoy [Serceau and Pelleau (2002)l. The umbilical and flowlines system, which links the FPSO to the subsea components, comprises three riser bundle towers and bundled flowlines (Fig. 1).

Suction caissons, used as main foundation and anchoring system at the Guassol field, were applied for the riser towers (Fig 2), the subsea wells manifolds (Fig 3) and the FPSO (Fig 4) and export buoy (Fig 5) Similarly to other deepwater fields, the suction caisson/anchor concept was selected due to the following perceived advantages [Colliat (2002)l:

  • fixed location on seabed, which is important in congested subsea developments,

  • simple installation procedure, with no need for proof-load testing at the site; and

  • reliable design methods, providing confidence in long term ultimate holding capacity

The installation operations at the site took place between March and July 2001, from the crane barge "Seaway Polaris" and the vessel "Seaway Eagle". The contractors, in charge of both the design studies and installation operations, were AMG (Alto Mar Girassol) and MPG (Mar Profundo Girassol), two Stolt Offshore - Bouygues Offshore consortia, and SBM (Single Buoy Moorings).

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