The use of steel castings as major structural elements of the Hutton tension leg platform represents a relatively novel concept. In order to ensure that these castings would provide adequate service, an extensive testing program was undertaken to assess variations in chemical composition and mechanical properties of prototype castings. In addition, a rigorous acceptance procedure for production castings was developed. The results of these programs showed that steel castings possessed adequate strength and toughness. Also, a welding procedure was developed which consistently produced sound weld repair and satisfactory joints between cast and plate steel.
The structural integrity of an offshore platform is predominantly dependent upon the performance of its welded joints. The presence of high stress concentrations and unavoidable weld defects in conventionally fabricated joints can result particularly for deepwater structures, in low fatigue life.
One solution to this problem is provided by increasing the size of these joints, which results in increased fabrication complexity and cost. The other solution, that has recently been proposed, is to substitute cast steel joints for welded joints. 1- 3 This solution was made possible by recent improvements in steel processing, micro alloying, and casting design. The use of cast steel joints offers several potential advantages such as lower stress concentrations by forming a smooth transition at intersections, simpler fabrication, improved fatigue life, reduced weight, and lower total cost. The application of structural steel castings for an offshore structure in the Hutton Tension Leg Platform (TLP),4 to the authors' knowledge, is the first use on a production platform
The Hutton TLP (Figure 1), which is being developed for the Hutton Field in the North Sea, represents a pioneering application of this new deepwater platform concept. The TLP is a floating platform fixed by vertical tension legs to piled foundation templates on the seabed. s The overall buoyancy for the TLP is provided by the hull of the floating vessel. The hull structure consists of cylindrical columns, rectangular pontoons, and the column-pontoon nodes, as shown in Figure 2. Detailed finite-element analysis and acrylic model testing had demonstrated potential fatigue problems in the column-pontoon nodes at the intersection of the pontoon corners and the column due to the presence of severe stress concentrations. Two alternative solutions were considered to resolve this problem, the first solution using fabricated details (Figure 3) and the second using a steel casting (Figure 4). The casting solution was selected because it offered three distinct advantages. These are:
Improved Fatigue Life: The m1n1mum calculated fatigue life for the cast solution showed a tenfold increase over the minimum fatigue life of the fabrication solution.
Weight Saving: The cast solution was much lighter than the fabricated solution.
Simplicity of Fabrication: The fabricated solution would require the welding of steel plates up to 95 mm thick, which would require post weld heat treatment (PWHT), while the cast solution required the welding of plates less than 50 mm thick at the boundary of the castings, therefore, making PWHT unnecessary.
