A Method of Estimating Line Tensions and Motions of a Semi-Submersible Based on Empirical Data and Model Basin Results
Authors: Pangalila F.V.A. , Martin John P. 
Published: October 2008

For a semisubmersible platform, a comparison is made between results obtained at sea and those predicted from model basin tests. The different techniques used for model testing are discussed. Diagrams indicating cable tensions vs surge motion are shown for different chain weights, number of chains, water depths and pretensions. Based on this information, the authors have concluded that heavier chain is most effective for shallow water use while relatively light weight chain is most satisfactory for use in deeper water.

INTRODUCTION

The semisubmersible, because of its superior characteristics with regard to motions, is becoming increasingly important to the offshore oil industry.1 Compared to conventional ship configurations the semi submersible is a rather new type of displacement vessel. Therefore, prior to construction, model tests are usually conducted to determine the behavior of the rig. The purpose of this paper is to determine whether the results obtained from these tests actually reflect the behavior of the full size rig at sea. The authors are grateful to one of the companies presently operating a large number of semisubmersible platforms for supporting them with data from model basin and full scale experiments.

MODEL BASIN TESTING TECHNIQUES

In order to predict accurately the motions and mooring forces of the rig in a seaway, it is necessary to realistically reproduce the pattern of a confused sea in the model basin. However, waves at sea usually have a very irregular pattern that does not appear to follow any physical law. This irregular surface is caused by waves of various heights and lengths occurring simultaneously. Because of the different length and speed of each wave, its energy is continuously being either increased or decreased by the other waves present.

It is impossible to reproduce this wave pattern exactly in a model basin since, in order to do so, it would be necessary to generate many waves simultaneously. To accomplish this, the model basin would have to have a large number of wave generators working independently of each other. Although some model testing facilities have multiple wave generators, most have only one, and certainly none have the ability to reproduce accurately an actual sea state. To compensate for this short coming, the technique adopted by the model basin is to generate the shorter, low speed waves first and the longer, high speed waves later in the program. Using this method the waves will arrive at the test location at the same time.

Because of these inaccuracies, many rig owners and operators prefer to conduct tests in regular waves rather than the imperfect irregular waves. Regular waves consist of a series of waves of the same height and length repeated continuously. The assumption is that, by using an energy technique developed by the communications industry, it will be possible to predict the behavior of the platform in confused seas if the reaction of the rig to each regular wave that is contained in the confused sea is known.2 In this technique the mathematical model of the confused sea is defined as an energy spectrum.