Abstract

This paper presents a discussion of the effect of including mooring line dynamics and riser friction on Spar response. Data from the Neptune Spar revealed that the heave motion of the vessel was considerably less than predicted by an uncoupled analysis1. It was believed that the primary cause of the reduced heave was damping forces such as friction between the risers and the supporting guides and mooring line dynamic drag that were unaccounted for. A new analysis capability was subsequently developed to simultaneously predict the dynamic response of the vessel, mooring lines, and risers. Results of the coupled analysis reveal that mooring line dynamics and riser friction have significant effect on the Spar heave response. In this paper, comparison of coupled analysis results to Neptune Spar motions during Hurricane Georges will be presented. As a result of reduction in heave response, the draft of the Spar can be reduced. A coupled analysis of a shorter draft Spar is presented. Results for matthieu's instability problem, effect of coupling in different water depths and comparison of coupled response of classic versus truss Spar configurations are also presented. Coupled response of Spar and a second vessel moored together with chains is presented to demostrate the multiple vessel simulation capability of the coupled analysis program.

Introduction

A characteristic feature of moored offshore structures such as a Spar or a semi-submersible platform is their slow oscillatory motion that occurs at resonant frequencies, well beyond the frequency range of the wave spectrum. Since the damping of such structures is low at resonant periods, correct estimation of damping is important in predicting the motions, maximum offsets and extreme mooring loads. Generally, response of Spar platforms is predicted conservatively by excluding the damping from mooring lines and risers.

Damping from risers on the Spar platform occurs from Coulomb friction at the riser guides and keel as well as from the hydrodynamic viscous effects. The risers exert a normal force at the keel guide and other air can guide locations. As the Spar pitches or offsets laterally, the riser induced normal reaction increases. As the Spar heaves vertically, a friction force is developed on the guides, which is proportional to normal reaction from the risers and depends on the coefficient of friction. If the Spar vertical motion is small enough, the static friction will prevent the Spar from moving further. When the Spar motions are larger, the kinetic friction opposes the motion and thus produces damping. In addition to the damping, coupling forces between the riser and the Spar arise in both surge/sway motions as well as pitch/roll motions. The buoyancy force of the riser air cans provide additional restoring moments that affect the pitch/roll motions. The riser lateral reaction at the keel and other guide locations affects the surge/sway motions. Current drag on risers, if significant, produces additional lateral reaction at the keel which can affect both surge/sway and pitch/roll motions.

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