Analysis Of Deep Sea Moor And Cable Structures
Authors: Webster R.L. , McCreight W.R. 
Published: October 2008

A computer aided analysis procedure for static and dynamic behavior of deep sea moors is presented. A fully coupled system is employed in which the mooring lines are represented by finite elements and the ship and mooring buoys(if any) are modeled as six degree of freedom rigid bodies. The effects of wind, currents and wave-induced drift forces are treated in the frequency domain. Results of a number of numerical experiments are discussed. Some of the simplifying assumptions made in moored ship analyses are examined for validity.

INTRODUCTION

The mooring of surface vessels in deep waters is an important problem in commercial and military applications. Deep sea exploration, drilling, mining, rescue, surveillance and construction are examples which may demand accurate and reliable restraint of a vessel at a deep water site. A typical deep water mooring may involve multiple lines with lengths measured in thousands of feet. When these are considered in conjunction with the equipment deployed from or associated with the moored vessel, the losses resulting from a mooring failure can be significant. Consequently, there is much interest in design and analysis methods applicable to deep water moors.

The physical complexity of a deep water mooring led to the early abandonment of some of the more traditional models for cable structures. Usually it was intractibility or cost which led to the rejection of a technique.1 Some of the early models of moored ship responses made no attempt to model the mooring lines beyond a simple linear spring behavior with no inertial effects.2,3 Their emphasis was primarily on the response of the vessel. Then secondary calculations were made for the mooring lines. These were usually limited to estimates of maximum tensions at the point of attachment to the vessel.

In the early 70's discrete element models for cable behavior were the subject of renewed interest. Following the pioneering work of Walton and Polachek,4 these efforts pursued cable dynamic behavior using the lumped parameter technique.5,6 Such procedures offered versatility that was needed but appeared to have little impact on the moored vessel problem. One reason this may have been so is that the lumped parameter technique follows mostly intuitive developments and does not naturally lead to either a stiffness or flexibility matrix. With the introduction of formal finite element techniques into the cable analysis realm7,8, the stage was set for detailed modeling of mooring lines with a variety of solution procedures.

Parallel to the developments in cable dynamics, although independent of them, the modeling of surface vessel behavior was underway. By the early 70's relatively detailed strip theory models of ship behavior were available.9,10 In the same period the importance of the wave induced drift forces on the vessels was being defined.3,11,12 Earlier developments produced models of non-slender bodies typical of buoys.13,14 More general models applicable to platforms have also been developed.15 Typically these methods transform the equations of motion for the body (assumed to be rigid) into the frequency domain and restrict the motions to be small.