Deepwater Mooring Line Dynamics With Emphasis On Seabed Interference Effects
Authors: Thomas D.O. , Hearn G.E. 
Published: October 2008

This paper presents the formulation of a mooring line dynamics model through the use of the lumped mass method. Important aspects of the formulation and solution using finite difference schemes are highlighted and a step by step solution procedure is indicated. A model to account for both friction and suction effects as well as the lifting and grounding of nodes is discussed in some detail. Results are presented which illustrate the seabed interference effects upon the total dynamic solution. The implications which these results have for the nodal lifting/grounding model are further discussed.

Introduction

For many years the inclusion of mooring line effects in the analysis of the motions of moored floating structures has been carried out through the use of quasi-static methods, In this approach, as the floater moves under the action of wind, waves and current, the mooring line is assumed to take up a new static position; therefore the static tension at the top of the mooring line is calculated for each new fairlead position. The assumption made to justify this approach is that the motion of the floater is slow enough to make any induced dynamic effects negligible. However, there are two problems wit h this approach. Firstly, having made this assumption, it is not then possible to demonstrate its validity - a full dynamic analysis would be needed for this. Secondly, any mot ions induced at wave frequency are ignored. The amplitude of the wave frequency motions are certainly much smaller than the horizontal root mean square excursions of the floater, but these cannot be neglected until their effects can be quantified. Ideally, therefore, a dynamic model should be developed. With the advent of moorings in very deep water the exclusion of dynamic effects becomes harder to justify. This has been recognised by the American Petroleum Institute (API)(1,2) whose guidelines recommend that dynamic analysis methods be used in preference to quasi-static design tools.

Further justification for the development of a dynamic mooring capability arises when studying the coupled floater/ mooring line system. Usually the analysis of either of these components is carried out in isolation from the other. This means that for the analysis of mooring lines, the endpoint displacement is already assumed, implying that the mooring line has no influence upon the motions of the floater. Conversely, when determining the motions of a moored floater, the effects of the mooring line are incorporated as a static modification to the hydrostatic restoration matrix. Although various attempts have been made to couple the analysis of the two components3,4,5,6, all have resulted in a simplification of one or both of the separate analyses.

Given these clear requirements for the development of a dynamic mooring line model, a choice must then be made regarding the theoretical approach to be adopted and the degree of simplification required. An excellent survey of the analysis of mooring lines can be found in reference (7), and although somewhat dated, this survey contains explanations of the principal methods for analysing this class of problem.