New Foundation Systems for the Snorre Development

Abstract

ABSTRACT The Snorre field is located in more than 300 m water depth with difficult foundation conditions. The field installations include a Tension Leg Platform (TLP) and a Subsea Production System (SPS), to date the deepest North Sea installations. The foundation systems selected for both the TLP and SPS are new. The TLP is anchored by 4 Concrete Foundation Templates (CFTs). Each CFT comprises a 720 m2 tricell concrete structure of 20 m height. The CFTs, each weighing 5660 tonnes, were successfully installed in June 1991. The CFTs reached target penetration at 12 m by applying underpressure in the individual cell compartments. An extensive series of model tests were undertaken to verify the behaviour of the CFTs in the design phase. The SPS Template is 48 m long, 31 m wide and (including the 3 m skirts) 19mtall. The Template comprises 20 well slots. Up to 10 wells will be operating at one time. The Template weighs 2400 tonnes in air and will be installed in April 1992, 6 km NE of the Snorre TLP. Each end of the structure contains 3 water tight skirt compartments. The skirt compartments are designed to give adequate support to the structure, and secondly, the skirts will be used to level the Template by pumping water into the skirt compartments. Newcost-effective foundation systems foranchoring offloating structures have been proven by the installation of the Snorre CFTs. The foundation system selected for the Snorre SPS represents a simple and cost effective method for foundation and levelling of subsea production units. INTRODUCTION Large suction anchors in form of Concrete Foundation Templates (CFTs) were developed and utilized forthe first time for anchoring of the Snorre TLP (Fig. 1). This novel concept proved to be a very cost efficient and safe foundation system, well suited for soft clay seabed conditions. The Snorre CFTs (Fig.2) were successfully installed within planned cost, time and position in June 1991. The TLP will be installed in May 1992. The foundation system of the Snorre Subsea Production System (SPS) (Fig.3) is based on a principle similar to the CFTs with watertight skirt compartments which provide the structural support and additionally, act as a levelling system during installation. The foundation systems have been subject to extensive design, verification and test programmes. The Snorre field development has introduced these new foundation concepts for offshore application demonstrating that attractive altematives to piled foundations exist. This paper describes two new type of foundation systems for offshore structures which opens for a wide range of applications. The main topics highlighted are design principles, results from model tests and obseNed behaviour during offshore installation. TLP FOUNDATIONS Description The Snorre TLP is anchored by 4 Concrete Foundation Templates (CFTs). An artist impression of a CFT is shown in Figure 2. The geometry is shown in Figure 6. Each CFT consists of 3 individual skirt compartments, each with a diameter of 17 m and covers a total area of 720 m2" The skirt wall thickness is 0.35 m.