Abstract

For the past 4t years the" semi-submersible rig, Sea Quest, has been operated by' British Petroleum in the rough waters of the North Sea and has been subjected to these arduous conditions for a longer period than any other barge. In such circumstances full consideration must be given to the fatigue life of the structural members.

To provide basic fatigue design data on the stresses actually experienced in the underwater structure during normal and extreme storm conditions, strain gauges were installed at many points on the structure and these have been continuously monitored throughout the life of the barge. The paper describes the measurement programme and presents in tabular and graphical form analysed stress data for over 1100 days of operation. This data is compared with calculated stresses and conclusions are drawn.

Some tentative proposals are made for assessing the fatigue life of these structures and a specimen fatigue calculation based on the Palmgren-Minor hypothesis is included. Attention is drawn to the paucity of basic design data for the fatigue life of welded joints in sea water.

Introduction

Over the past five years, major improvements have been made in the structural integrity of semi-submersible rigs and the design techniques have advanced to a level which gives the industry much greater confidence is the units used for offshore drilling and exploration. It is not the purpose of this paper to review the historical development of present day design methods but a few comments of this kind would be germaine to the technical content of this paper.

An awareness of the need for a rigorous review of the design approach became apparent in 1965 when semi-submersible rigs first began to operate in rough sea areas and structural weaknesses were cruelly exposed. The industry mounted vigorous and searching enquiries into all facets of the design of these structures with a thoroughness and speed which was most impressive.

In 1966, when British Petroleum took delivery of the "Sea Quest", a semi-submersible drilling unit, the Company was acquainted with the research being done in the U.S.A. It was felt however that the total lack of factual information on the actual stresses experienced by these structures under service conditions was an 9mission which should be rectified without delay. With a view to providing this information, BP arranged for strain gauges to be fitted at a number of critical points on. the "Sea Quest" structure whilst the unit was under construction in Northern Ireland. The strain gauging techniques proved to be' completely satisfactory and, as the value of the measurements exceeded our expectations, additional gauges were fitted at various times. It is with the full scale stress measurements obtained there from that this paper is principally concerned.

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