This paper deals with a simplified, linear approach for analyzing the nonlinear creep effect of a composite steel-concretestructure by the use of any standard linear finite element program. The Heidrun concrete platform represents a novel conceptin floating offshore platforms. In this challenging design, the affixing of the topside modules to the main support beamscauses significant, secondary shear forces in the footings due to creep deformations. Creep effects in offshore structures arenormally accounted for in linear analyses by modifying stiffness and loads. As a part of the design verification a separateverification study has been performed to see if there is reason for confidence in such a linear approach of including creepeffects. Two alternative linear approaches are compared with a more comprehensive and accurate nonlinear method foranalyzing creep effects. For the specific platform problem studied, the main conclusion is that one of the two linear methodsis suitable for calculating creep forces transferred to the steel topside modules and may be used in design analyses.
The Heidrun platform is the first tension leg platform (TLP) with a hull structure made of lightweight aggregate concrete (Fig. 1). During construction, the lower part of the hull was cast in adrydock. Then this part was set afloat and the upper part of thehull was slip-formed. The two main support beams (MSBs) wereplaced on the hull structure from a barge. Then the topside moduleswere mounted on the MSBs, and the platform was transportedto the field location and installed with tethers anchored at theseabed. The Heidrun platform is installed at a water depth of 345 m. The height of the hull structure is approximately 110 m and itsweight approximately 166000 tonnes