Dynamic Response in Anchor Lines: Numerical Simulations Compared With Field Measurements
Authors: Ormberg Harald , Fylling I.J. , Mørch Morten 
Published: October 2008

The present paper deals with the problem of dynamic loads in anchor lines for spread mooring systems. A time-domain analysis is used for computing responses to irregular excitation. Results show good correspondence with field measurements on the SSV Henrik Ibsen. Results are presented as motion-to-tension transfer functions. It is pointed out that the difference between commonly used quasistatic analysis and the dynamic behaviour becomes increasingly important with increasing water depth. Possible alternatives for design procedures are briefly discussed. Using time-domain al1Blysis to compute responses to irregular motions is a straightforward way to obtain linearized transfer functions. In spite of the nonlinear character of the problem, these may prove very useful in a design procedure.

INTRODUCTION

The dynamic effects in mooring lines caused by wave induced motions of the moored vessel have traditionally been overlooked or taken care of by safety factors in practical design and operation of mooring systems. The anchor lines are usually modelled as nonlinear springs in such analyses, i.e. the cable force is determined by the terminal point position only. This model, often referred to as quasi-static analysis, is in most cases sufficient for calculation of the vessel motions. It is, however, also applied for determination of the mooring line design load. According to current practice this load is not to exceed a specified portion of the line breaking strength. The margin between this design load and the nominal breaking strength shall cover uncertainty of load assessment, dynamic effects and uncertainty in breaking strength of the line. Results obtained during the past five years at the Norwegian Institute of Technology and The Ship Research Institute of Norway show that the difference in line loads obtained by the quasi-static procedure and loads found by analyzing dynamic line response increases with increasing water depth /1,2/. This means that the current practice gives decreasing safety with increasing water depth. For large water depths, e.g. 300 m and beyond, the quasi-static procedure to obtain the line design load is most uncertain and may give misleading results.

As the oil activities progress towards deeper water, the current design practice should be reviewed taking the dynamic effects into account. To meet this challenge, several computer programs for analyzing dynamic cable responses have been developed. In our search for a new design method we have used the finite element program STOCCA /3/ to determine the dynamic cable forces. The program has previously been verified against model tests of a submerged line with high elastic stiffness and mainly geometric compliance. The correspondence was found to be good, also for dynamic tension of the same order of magnitude as the static tension level /4/. The present paper gives results and comparison with full scale measurements of a more linear system where the elastic deformation is important, also for static behaviour. As a part of a full scale offshore measurement program /5/, platform motions and the anchor line tensions were measured.