Effect of Soil on Mooring System Dynamics
Authors: Wung C.C. , Litton R.W. , Mitwally H.M. , Bang S. , Taylor R.J. 
Published: October 2008

Current mooring line analysis methods do not consider the anchor-chain-soil interaction effects. The Navy's efforts togain a better understanding of the static and dynamic behavior of the drag embedded anchoring system indicate that in soft soils, such as clay and silt, the current analysis methods may significantly over predict 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 numeric took. Examples of calibration comparisons and mooring line analyses are also presented.

Two finite element model 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 seaf/or surface. The other method exp/icit/y models the local soil resistance to the anchor and along the embedded mooring line. The basic components inboth 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 mode/ed using either of thetwo proposed approaches. Consideration of the anchor- chain-soil interactions in the analysis and design of a ooring system subjected to environmental loads is expected to lead to an economical design of the mooringsystem.

INTRODUCTION

Drag embedment anchors provide the primary means of anchoring most large offshore platforms. These anchors are sized to satisfy loads determined with the assumption of a fixed mooring line at the seafloor surface. Currently thereare no rational methods that account for the energy absorbing characteristics of the anchor and the embedded portion of the mooring system. The same is true for plate anchors or pile anchors, which are also embedded in the seafloor thus all moorings are designed to very conservative standards. Consideration of the anchor-chain-soil interactions during the analysis of a mooring system subjected to significant environmental loads will therefore lead to a more economical design.

Recently, the US Navy initiated an effort to study the static and dynamic analysis of mooring lines. Preliminary analysis results conducted by the Navy indicate that the resulting mooring line forces considering some form of soil-structure interaction are less than those evaluated assuming fixed endconditions. This research points out the importance of a better understanding and modeling capability of the static and dynamic behavior of the drag embedded anchoring system. The objective of the Navy research was to develop numerical tools for the static and dynamic analysis of drag embedded anchoring systems, including the evaluation of the initial configuration of the embedded portion of the mooring line. Centrifuge tests were also conducted to calibrate and verify the numerical tools and to gain a better understanding of the behavior of various embedded mooring systems. This paper presents the results of the research currently underway to develop numerical tools for the static and dynamic analysis of drag embedded anchor systems, Two new approaches are presented in which the static and dynamic anchor-cable-soil interaction is modeled.