Concrete Hulls For Tension Leg Platforms

ABSTRACT This paper describes the main features of a concrete hull TLP concept developed for the Heidrun Field. The hydrodynamic response and the methods adopted to optimise the hull dimensions are first discussed, as well as the mooring system and the hull mechanical outfitting. The construction methods and possible transportation and installation scenarios are presented. The subjects of inspection, maintenance and repair are also briefly addressed. Finally the advantages of the concrete hull TLP concept are summarised, including it's adaptability to a large range of design requirement, its insensitivity to moderate design changes, and its low cost and short construction time. This paper shows how the concrete hull TLP can offer a very cost efficient solution for the development of deep water fields. INTRODUCTION The Tension Leg Platform (TLP) is a compliant offshore structure that offers great advantages for drilling and production operations, particularly in deep waters. The concept is now well known, given almost four years of successful operation of the Hutton TLP in the U.K. Sector of the North Sea. Efforts are now being made to improve the capabilities of such platforms, especially with regard to increased payload, more efficient methods of construction, simpler mooring systems and foundations. These efforts are leading to cost effective alternatives which will make the TLP concept a front runner for many offshore field developments. One such alternative is the concrete hull TLP, which is the subject of this paper. By choosing concrete as the building material for the TLP hull it is possible to design the platform for topside loads of 50.000 t or more. Also, concrete offers many advantages, such as a relative insensitivity to fatigue and corrosion, minimum maintenance requirements and small loads on the mooring system and foundations. The construction period required for concrete is significantly shorter than for steel, and the associated costs are also expected to be lower. It is felt that the possibility of using concrete as a building material for the TLP hull brings a new dimension to the TLP concept, and makes it even more attractive in the development of large deep water offshore fields. The present paper describes the main features of a concrete hull TLP conceptual design developed for the Heidrun Field, in the Haltenbanken area of the Norwegian sector of the North Sea. This concept has been adopted for the development of the Heidrun Field by Conoco Norway Inc. and its Co-licensees. A description of the concrete hull TLP is given, including the design basis and requirements, the platform dimensions and configuration, the hydrodynamic response, the hull structure, the mooring system, the mechanical outfitting, construction, transportation, installation and operation. Finally, the advantages of the concrete hull are discussed, and the most salient features of this concept are summarised.