Analytical Models for Mooring Chain-Soil Interaction

Abstract

For most catenary mooring systems, a quite long part of the line is resting on the bottom, even in extreme situations. Hence, a good design of the anchor requires an accurate prediction of the loading from the anchorline, including 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 in fatigue design of e.g., suction anchors. Design against fatigue is in many cases critical for these type of anchors. A problem is the lack of design tools handling the soil-chain interaction correctly. Traditionally, the soil effects are neglected completely in a fatigue design process. A consequence 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 displacement along the part of the line resting on the bottom, assuming the tension at “touch-down” to be known. Hence, only longitudinal chain-soil interaction is considered. The models are based on recently established soil parameters for longitudinal chain-soil interaction. Comparison with FEM computations shows that the analytical models are performing very well in the prediction of cyclic loads. They are therefore a major improvement to the existing design procedures for anchor design.