Analytical models for mooring chain-soil interaction

For most catenary mooting systems, a quite long part of the line is resting on the bottom, even in extreme situations. Hence, a good design of the anc hor requires an accurate prediction of the loading from the anchorline, inc luding any soil-chain interaction This requires that the longitudinal chain -soil characteristics are well described for different soil conditions and that efficient computational models exist. This is particularly important i n fatigue design of e.g., suction anchors. Design against fatigue is in man y cases critical for these type of anchors. A problem is the lack of design tools handling the soil-chain interaction correctly. Traditionally, the so il effects are neglected completely in a fatigue design process. A conseque nce in many cases is an expensive overdesign. There exist chain-soil models to be applied together with FEM programs. These are too impractical to use in efficient fatigue design which requires the load assessment due to many seastates. They also suffer from the lack of reliable soil parameters. The paper addresses analytical models of the tension and longitudinal displace ment along the part of the line resting on the bottom, assuming the tension at ''touch-down'' to be known. Hence, only longitudinal chain-soil interac tion is considered. The models are based on recently established soil param eters for longitudinal chain-soil interaction. Comparison with FEM computat ions shows that the analytical models are performing very well in the predi ction of cyclic loads. They are therefore a major improvement to the existi ng design procedures for anchor design.