Abstract

Self-elevating drilling platform is one of the types of offshore drilling units. In this paper, we describe briefly the problems regarding motion and shipping water on deck of platforms in waves. The causes of capsizing of a damaged platform are investigated by experiment, in which the ocean environments are simulated in the towing tank, and various influences of wave height, motions of the platform, damaged tank and shipping water on capsizing have been platform, damaged tank and shipping water on capsizing have been analysed.

The suggestion and conclusions on the seakeeping and the capsizing of platforms presented in the paper can be used in the design of platforms and operations under tow. platforms and operations under tow

Introduction

In recent years, platform of self-elevating type is used for many kinds of offshore works. The reason this type of platform is widely used is due to the excellent conditions provided by such rigs for drilling works.

Even though the intact platform has sufficient stability, many accidents occurred. In the period from 1915 to 1980, 137 rig accidents were recorded, of which 96 mishaps were jackups. The majority of the mishaps is caused by jackup movement. For example, the capsizing of the Ocean Express and Bohai No. 2, which were self-elevating type platforms, were two well-known cases occurring in different parts of the world with heavy losses of lives. In order to insure these units to be operated safely and effectively, their seakeeping characteristics and capsizing are subjects of great concern.

Though a large number of such platforms are under construction, many unknown problems still remain to be solved due to their short history in comparison with the conventional ships. Because the seakeeping and capsizing of platforms involve complicated hydrodynamic and mechanics problems, a standard calculation method has not yet been definitely established. The series of tests of two model platforms with mat and without mat have been carried out in 110m × 6m × 3m towing tank of the ship Hydrodynamics Laboratory of Shanghai Jiaotong University in 1982.

The outline of the model 1, which is a four leg mat type jackup with rectangular upper hull and a mat, is shown in Figure 1, the model 2, which is one without mat, is shown in Figure 2. The main purposes of these tests are to investigate the dynamical behaviour of platforms with low freeboard.

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