The Application and Extension of Ship Motion Theory to Prediction of Motions and Loads of Various Types of Offshore Platforms
Authors: Kaplan Paul , Sargent Theodore P. 
Published: October 2008

An existing ship motion theory and computer program (published by a U. S. Government agency and used by classification societies in the U. S. and England) is used to predict all motions and loads of offshore platforms, such as drillships and barge forms. Extensions are illustrated to account for the effects of moorings; application to a catamaran hull; and evaluating slowly varying-drift forces. Computed results are compared with model test data to illustrate prediction capability. Only limited efforts are required to adapt the program for ships to similar problems for offshore platforms.

INTRODUCTION

In the design of ships, it is necessary to determine the various ship motions and structural loads that will be experienced by the ship during its operation in waves. In addition to model tests, the use of mathematical predictions via computer simulation is often used, in view of the efficiency of such an approach when covering a large range of possible conditions. In addition, the evaluation of ship motions and loads is also carried out by classification societies in order to determine the ship responses for both safety and insurance aspects, e.g. see [1] for a description of this analytical approach.

The basic element in the prediction of ship motion and loads in waves is the determination of the ship response operator, representing the frequency response characteristics in different regular waves for various headings and speeds. A particular theoretical development and computer program based upon the theory are given in [2] and [3], which are part of the basic tools used in classification society evaluations in the U. S. (ABS) and the U. K. (Lloyds Register). Similarly, when evaluating the expected performance characteristics of a proposed network of data buoy systems, the responsible agency also makes use of a computer simulation tool for such efforts, as represented in [4]. The work in [4] is essentially an extension and/or generalization of the procedures used in [2], dependent upon the particular configurations being analyzed.

All of the work described in [2]-[4] is presently available in open published literature, and can be used for application to ship and buoy design. However, there are also a number of applications and/or extensions of this work that can be made which have great utility for a large class of vessels used in the offshore industry. The particular type of vessels; the required efforts for extension of the available theories and computer programs; and the results obtained for some of these examples are provided in the present paper, thereby indicating to the industry the possible use of already developed tools for specialized assistance in design and prediction efforts associated with different types of offshore vehicles.

GENERAL METHOD OF ANALYSIS

A description of the basic analysis provided in [2] is given here, together with an indication of the different extensions that have been applied for application to offshore technology problems.