ABSTRACT

This paper describes the results and the future objectives of a program which SEDCO, Inc. has undertaken to continually measure and analyze the offshore environmental conditions and to compare these conditions with the motions, anchor line tension and stresses on several offshore drilling units. The program has given a good comparison between theoretical design and field measurement. The most significant result to date is the correlation of wave conditions with the resulting stresses in the frame of the SEDCO 135 structure.

INTRODUCTION

The strength and stability of offshore structures are of vital concern to the offshore petroleum industry and have become increasingly crucial as the industry has progressed into the more open, severe and deeper water of the ocean. Over the past thirty years, the drilling site has moved from the Calichi Road of West Texas to 1500 feet of water offshore. As offshore experience increases, the drilling sites have moved into areas where the ocean environmental conditions are extremely severe. Engineers and managers have been required to make decisions based on the limited technology available to keep moving always further into the ocean. Today the drilling contractor is operating 200 miles offshore where the rig must survive waves of 100 foot height, winds of 120 miles per hour and ice forming on the rig. For the more usual conditions, the rig and people are expected to drill hole when 40% of the time the waves are higher than 10 feet and the winds are blowing 30 miles per hour and extreme storm conditions can be expected at anytime. The decisions made by engineers are directly related to the safety of human life, to corporate profit and industry success. SEDCO has approximately &200 million invested in offshore mobile drilling rigs similar to the SEDCO 135 vessel shown in Figure1. the company owns and operates 40% of the world';s semi-submersible rigs, and has selected the semi-submersible rig as the best offshore drilling tool for the reason of reduced vessel motion. Reduced motion permits continued drilling in the most severe weather conditions. Figurate 2 illustrate the typical heave motions of a SEDCO 135 vessel as compared with a ships-shape vessel. Heave motion inversely related to the amount of hole drilled per time anchored on location and Figure 3 illustrates the downtime experienced by the SEDCO semi-submersible rigs while drilling off Eastern and Western Canada. It was in these areas that the most severe conditions reported by oil companies and drilling contractors around the world have been experienced. In Western Canada, for example, two 100 year storms, as predicated by A. H. Glenn, occurred in five month period of time. In one case, the vessel experienced ? and remained on location during ? an extreme wave height of 95 feet. It has become standard practice to continue drilling in 35 foot wave conditions with winds of 60 miles per hour. Figure 4 illustrates the severity of the weather for various areas of the world. Note that Eastern and Western Canada and The North Sea are the most severe ocean wave areas where offshore exploration is active.

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