Joint Industry Project: Deepwater Fixed Steel Tower For North Sea
Authors: Fu S.L. , Anderson J. , Bouquet A.G. , Gudmestad O.T. , Piermattei E.J. 
Published: October 2008

The Gulf Deepwater Fixed Platform Joint Industry Project design concept for severe environmental conditions is presented after five years of research. The participants include: Britoil, Norske Shell, Statoil and the project managers, Gulf Oil. The base case Deepwater Fixed Platform (DWFP) shown on Figure 1, is a four legged, self-floating steel tower in a water death of 365m, with a deck load of 22,700 tonnes. Two alternative deck load and water depth variations investigated are: a light deck load of 12,500 tonnes in 457m, and a heavy deck load of 60,000 tonnes in 365m. The investigations produced feasible structures and confirmed simplified design methods. The structure weights determined in the project agreed with projections based on existing structures in severe environments.

INTRODUCTION

The four-legged advanced self-floating steel tower for deepwater and harsh environmental conditions uses temporary auxiliary flotation for floatout and sea transportation. The structure is upended by controlled flooding of legs and flotation tanks. The temporary flotation raft is removed with the tower in the vertical orientation prior to docking with a subsea template and/or setting on bottom. After ballasting the structure, one piece pile installation begins using a semi submersible crane vessel (SSCV) equipped with underwater hammers. The exterior piles of the group at each leg are stabbed into guide cones atop the pile sleeves, laid back into an open guide, and lowered to the seabed. The underwater hammer is stabbed into the pile, laid back and lowered to rest on an internal driving shoe. The interior piles of the group are installed using a lengthened hammer and a pair of fixed guides located outboard of the structure. Although many engineering aspects of the self-floating deepwater steel structure were investigated in the Joint Industry Project, this paper will focus on the in-place storm and fatigue conditions; and the comparison of member strength using API RP 2A and the Norwegian Petroleum Directorate Rules Proposal for steel member design, foundation configuration and construction methods. Results in terms of structural steel weight requirements are contained in the CONCLUSION.

DEVELOPMENT OF CONFIGURATION

The underlying principle behind the configuration of the DWFP is that of wave transparency. In order to achieve a feasible structure the storm wave load on the tower must be substantially reduced compared with a traditional North Sea design extended to deep water. Concurrently, the structure configuration required o achieve this fundamental aim must be capable of performing other operational requirements adequately.

The use of the advanced self-floater concept eliminates the need for large diameter flotation legs penetrating the waterline. Adequate structure buoyancy for floatout from the graving dock and transportation to the field is provided at the top of the structure by a removable temporary auxiliary flotation tank or raft. Without the large flotation legs, wave loads under storm and fatigue conditions are significantly reduced.

The use of large plan bay spacings reduces the number of members. As a result, the number of nodes in the tower is reduced, further reducing the inspection and maintenance costs of the structure during operation.