The Impact of Vortex-Induced Motions on Mooring System Design for Spar-based Installations

Abstract

Abstract This paper presents a study on the impact of Vortex-Induced Motions (VIM) of a spar on its mooring system design. The unique response features of a spar under environmental conditions of wind, waves and current are first reviewed. Especially, the VIM of a spar under loop/eddy currents in the Gulf of Mexico is addressed through sensitivity studies for its effects on mooring line strength and fatigue damage. Procedures and guidelines for mooring analyses of a spar due to VIM are then outlined. As alternative mooring components for deepwater applications, the advantages and disadvantages of mooring systems using spiral strand wire rope versus polyester rope are investigated in the study matrix with water depths ranging from 2000 ft. to 6000 ft. The safety factors on line strength and fatigue damages are summarized for investigating the impact of VIM due to loop/eddy currents from 2 knots up to 5 knots. Considerations to improve mooring design for spar VIM are also discussed. The analytical procedures, guidelines and findings derived from the study can greatly enhance our understanding of mooring responses of a spar under the effects of VIM due to loop/eddy currents prevalent in the Gulf of Mexico. It is hoped that this paper can assist engineers/designers to design a safe, reliable and cost-effective mooring system for spar-based installations. Introduction In recent deepwater oil and gas production developments in the Gulf of Mexico, a deep draft floater or "spar" has become a popular choice. A classic spar has a continuous cylindrical hull. A truss spar usually consists of a cylindrical hard tank to provide buoyancy for supporting the weight of deck loads on its top, and a soft tank at its bottom to provide permanent ballast for keeping the center of gravity lower than the center of buoyancy. These two tanks are connected by a truss structure. Under strong currents with deep depth penetration through the water column, these cylindrical tanks or hulls under strong currents are susceptible to VIM. In the Gulf, hurricane induced wind, current and wave conditions used to govern mooring system designs for floaters including spars. Recent industry experience and investigation indicate, however, VIM may dominate the mooring design under certain circumstances, and therefore the impact of VIM on mooring system design must be investigated. Design parameters, which will determine the VIM amplitude of a spar, and their impacts on a wide range of different mooring systems are identified and discussed. Environmental Criteria Wind, current and wave conditions induced by both events of hurricane and loop/eddy current in the Gulf are to be considered in the mooring design of a spar. In the past hurricane conditions typically governed the design of spar mooring systems for the Gulf. However, because of availability of VIM field measurement data and advanced model testing technique, recently VIM of a spar under the strong loop/eddy currents was found to be a major design issue, even when a spar is equipped with strakes to mitigate these motions.