Quality Assurance for Offshore Concrete Gravity structures
Authors: Hove Knut , Foss Ivar 
Published: October 2008

The paper lays out the principles followed by the Society in the quality assurance work of the design and construction of offshore concrete gravity structures, as it has been stated in the Society's proposed "Tentative Rules for the Design, Construction and Inspection of fixed Offshore Structures". The principles of design and the standard of safety applied to the design of the concrete structure and its foundation are reviewed.

The influence on the design criteria of special requirements for serviceability, e.g. tightness of oil containment structures, is discussed on the bases of the Society's experience through certification of the Ekofisk I oil storage tank and the Condeep and McAlpine-Sea Tank structures presently under construction.

INTRODUCTION

During the last ten years the scope of Detnorske Veritas' offshore activities has expanded rapidly. Today, the Society's assignments for control and certification include mobile drilling rigs of several types, pipelines and fixed platforms of the conventional jacket design as well as concrete gravity structures. Among the latter, two Condeep and two McAlpine - Sea Tank drilling and production platforms are presently under construction, and the Ekofisk I oil storage tank was placed at its final location in June of 1973.

The offshore activities is a natural offspring of the Society's traditional ship classification activities. Through this work, extensive know-how in all branches of engineering related to ships and the sea has been developed. In recent years fields such as automation, instrumentation, wave load calculations and advanced structural analysis have developed rapidly. With the addition of new engineering subjects such as reinforced concrete and foundation engineering the Society is well prepared to accept the challenge created by the discovery of oil and natural gas in the North Sea.

For ships, the need for an independent quality assurance has most strongly been advocated by the underwriters, who request a classification certificate in order to issue their charters. The benefits from the services of a modern classification society are also realized by the ship owners and the yards.

For offshore structures the requirements of the public authorities are most prominent. In the British sector of the North Sea, The Department of Trade and Industry (DTI) are about to finalize their regulations for offshore installations which will assume that the quality assurance is to be carried out by independent Certifying Authorities commissioned by DTI. In the Norwegian sector, the Norwegian Petroleum Directorate is authorized to carry out the control and Detnorske Veritas is serving as a principal consultant to the Directorate.

The Society's quality assurance is traditionally based on fixed rules. For ships the Rules contain highly specified requirements which are based on extensive experience and research. The rules for mobile drilling rigs (1), first issued in 1973, are based on the principles laid down in the ship rules, but with several important differences as directed by the special nature of these structures.