Effect of soil on mooring system dynamics

Current mooring line analysis methods do not consider the anchor-chain-soil interaction effects. The Navy`s efforts to gain a better understanding of the static dynamic behavior of the drag embedded anchoring system indicate that in soft soils, such as clay and silt, the current analysis methods may significantly overpredict the mooring line forces. This paper describes numerical tools developed considering the anchor-chain-soil interaction effects. Centrifuge tests have also been conducted in order to verify and calibrate the numerical tools. Examples of calibration comparisons and mooring line analyses are also presented. Two finite element models for the anchor-chain-soil interaction have been developed. One method implicitly models the anchor and the embedded portion of the mooring line by lumping their effects at one generalized element at the sea floor surface. The other method explicitly models the local soil resistance to the anchor and along the embedded mooring line. The basic components in both methods include a nonlinear spring and two dashpots. For the centrifuge testing, significant energy absorption behavior of the embedded mooring line have been found. Mooring line forces therefore can be reduced when energy absorption behavior is properly modeled using either of the two proposed approaches. Consideration of the anchor-chain-soil interactionsmore » in the analysis and design of a mooring system subjected to environmental loads is expected to lead to an economical design of the mooring system.« less