Wind-Tunnel Tests on a Model of a Semisubmersible Platform and Comparison of the Results With Full-Scale Data
Authors: Boonstra H. , Leynse C. 
Published: October 2008

This paper presents an analysis of a model investigation on a semi-submersible platform in a wind tunnel. Both forces and overturning moments on the part above water and the part below water are investigated. Furthermore full-scale anchor line force observations as reported earlier are re-analyzed and the results are compared to those of theoretical calculations and of the model tests.

Introduction

In recent years several papers have been published on the subject of wind forces and wind overturn moments on mobile offshore platforms3,4,5.

Knowledge of the forces exerted by wind on semi submersible platforms is necessary for at least two purposes, viz.The judgment of the hydrostatic stability and determination of allowable deck load at different draughts and under various conditions.The design of an adequate mooning system or a dynamic positioning system in order to keep the platform on location during drilling operations.

The total wind force on a semi-submersible platform is generally obtained by sidling the forces as calculated on individual items (such as columns, deck beams, deck houses and derrick). Drag coefficients for various geometrical shapes are prescribed by classification societies. However, mutual interactions, shielding effects am lift forces generally are not accounted for1 or are accounted for in a limited form2. As an alternative, wind forces and overturning moments can be obtained directly from tests on a scale model of the platform in a wind tunnel.

It has been demonstrated by various authors (e.g. 3, 4) that wind forces obtained from calculations are conservative when compared to results of model tests.

This, however, seems to be not necessarily the case for the overturning moments, as vertical forces on the part above water and, in case of a free floating platform, on the part below water can easily lope underestimated in the calculations. In the present paper, results of wind-tunnel tests on a model of the semi submersible platform "West Venture", theoretical calculations and results of full scale anchor line forces are compared and analyzed.

Semi-Submersible Platform West Venture

Two views of the platform are shown in fig. 1. The unit is owned by Smudging Drilling Co. in Stavanger and has been operating in the troth Sea since 1973, when it was built by Companies Franchise Enterprises Metallurgies in Le Havre. The platform was designed by Marcon in The Netherlands. It is basically a five-column stabilized unit of approximately 23,400 t displacement in operating conditions. The mooring equipment consists of ten 3 inch (7.6 an) anchor chains each 1100 m long, 30,000 Ibm (13,600 kg) anchors, and windlasses with a chain-stopper capacity of 5000 kN. The 10 anchor lines are laid out in a symmetrical pattern (Le., the angle between two adjacent lines is 36°). This seminary and the fact that the theoretically predicted wind forces on the platform are about equal in longitudinal and transverse directions (difference smaller than 12%) simplified the analysis of the full scale forces from anchor line tensions.