The development of the tension leg platform (TLP) for the Hutton Field in the North Sea represents the first application of this deepwater concept. The use of vertical tension legs as the mooring system is the most novel part of the TLP design. One of the unique features of the tension legs is the use of 92.5-mm (3.64-inch) thick tubulars forged from 3 1/2 percent Ni-Cr-Mo-V high strength steel (795 MFa (115.3 ksi) minimum yield strength) as mooring elements. Because of the importance of these elements to the survival of the TLP, the chemical composition and the steelmaking procedure were critically examined and optimized. In addition, extensive evaluation of the corrosion fatigue behavior of the steel was undertaken. This paper discusses the basis for the steel selection and specification, the results of the rigorous corrosion fatigue test program, and the assessment of the variation of properties along the length and through the thickness of full-size components. These results illustrate the suitability of the proposed high-strength steel for the mooring system application and establish confidence in both steelmaking and quality control procedures.
The tension leg platform (TLP) for the Hutton Field in the North Sea will be the first floating oil drilling and production platform which is designed to be moored by vertical tension legs for the full duration of the field's productive life. 1 The TLP, shown in Figure 1, is a compliant structure which allows significant lateral movements in response to environmental loads. The vertical movements (heave, pitch, and roll) are restrained by the tension legs which are maintained in tension under all loading and environmental conditions. The tension is maintained by ensuring that the TLP buoyancy is in excess of its operating weight under all environmental conditions. The Hutton TLP is a permanent installation for the 20 year life of the field. It supports all the necessary equipment and facilities for drilling and oil production, at a peak rate of 110,000 BPD, as well as provides accommodation and life support systems for up to 239 people.
The Hutton TLP mooring system consists of 16 tension legs (4 at each corner). Each leg (Figure 2) is designed to withstand very high cyclic tension loading which could vary from almost 0 to about 2400 tonnes (5.3 × 106 pounds) for extreme design environmental conditions. 2 Detailed description of the mooring system is given by Tetlow and Leece. 2 One of the important features of the tension leg system is the use of high-strength, thick-wall tubulars that are connected together by rotary shouldered screwed couplings as mooring elements. During the 20year design life, the mooring elements will be subjected to more than 108 load cycles in seawater environment, thus making corrosion fatigue an important factor in their design.
Because of the importance of the mooring elements to the survival of the TLP, the chemical composition and the steelmaking process were critically examined and optimized to ensure the highest possible strength level commensurate with adequate fracture toughness and minimum sensitivity to both hydrogen and temper embrittlement.
