Analysis of Full-Scale Wind Forces on a Semisubmersible Platform Using Operator's Data
Authors: Boonstra H. 
Published: December 2007

Measurements of chain tension and wind velocity aboard a semisubmersible drilling platform have been analyzed, resulting in an empirical relation between wind velocity and wind force. A comparison of the empirical and theoretically calculated forces shows that the actual wind force is appreciably smaller.

Introduction

Knowledge of the forces exerted by wind on submersible platforms is necessary for at least two purposes. purposes. 1. The hydrostatic stability is judged by comparing the stabilizing moment with the wind overturning moment, plotted on the basis of the angle of heel. Classification societies such as the American Bureau of Shipping and Det norske Veritas (DNV) require that the area under the righting curve is at least 30% more than the area under the overturning curve, up to the downflooding angle. A clear description of the background of the current stability requirements and their shortcomings is given by Numata et al.2. The mooring system of dynamic positioning of a semisubmersible platform is designed to withstand the more or less static forces of wind, current, and wave drift. The static offset resulting from these forces has to be minimized, whereas a catenary mooring system should be sufficiently flexible to allow for dynamic motions in waves.The total wind force on a semisubmersible platform is generally obtained by summing the forces as platform is generally obtained by summing 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 and are based on results of well-documented wind-tunnel tests. However, mutual interaction and shielding effects are not accounted for or are accounted for in a limited form, and lift forces generally are not accounted for.In the literature, no evidence can be found that this method results in an accurate estimate of the wind force. Comparison of calculated forces with wind-tunnel tests on a small-scale model generally reveal that the experimentally obtained forces are substantially lower than the theoretical forces.This paper presents results of an analysis of anchor-line forces and wind-velocity measurements. An empirical relation is established between the resultant horizontal component of the force and wind velocity. The basis for the analysis is the daily anchor-tension and weather report of the semisubmersible drilling platform West Venture. platform West Venture. Semisubmersible Platform West Venture

Two views of the platform are shown in Fig. 1. The unit is owned by Smedvig Drilling Co. in Stavanger and has been operating in the North Sea since 1973, when it was built by Compagnie Francaise D'Entreprises Metaliques 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-in. (7.6-cm) anchor chains each 3608 ft (1100 m) long, 30,000-lbm (13 608-kg) anchors, and windlasses with a chain-stopper capacity of 500 t.

The 10 anchor lines are laid out in a symmetrical pattern (i.e., the angle between two adjacent lines is pattern (i.e., the angle between two adjacent lines is 36 degrees).

JPT

P. 771