Shallow Seismic As An Aid To Locating Offshore Installations

Abstract

Summary The geotechnical characteristics of the North Sea soils are often highly variable even over small distances. To derive a three-dimensional picture of an area earmarked for platform location is only possible by integrating seismic data and information from soil borings. This information is needed to select the best location for the structure and to optimise its design. INTRODUCTION The final choice of location for the placement of large production platforms in the North Sea is mainly dependent upon the size, depth and lateral extend of the hydrocarbon-bearing reservoirs. However, the soils of the North Sea have been found to be highly variable and often weak in geotechnical terms at shallow depths. This variability in soil types makes it imperative that optimum sites, from an engineering design and installation viewpoint, be found for a particular structure under consideration. It has been shown that a small shift of a platform site can have a considerable impact on both type of platform and the costs involved in construction and installation. Geophysical techniques have been used in an attempt to determine the lateral continuity of soil layers revealed in site investigation borings. Problems of inadequate seismic resolution and penetration have had to be contended with but it has been found that provided the appropriate tools to suite a particular soil stratigraphy are selected much useful data can be obtained to aid platform site selection. This effort has stimulated the development of new shallow seismic tools and recording techniques which have improved the capability of the industry. This paper describes methods used in the North Sea to obtain the soil data needed for platform location selection and stresses the need for good consultation between the reservoir engineering team, the soil specialists, and the construction engineering team before a final decision on the platform site is take. RESERVOIR UNCERTAINTIES Experience in the North Sea. has shown that there is a certain amount of flexibility (sometimes as much as lkm) in locating a platform when only the optimum drainage pattern of the reservoir is considered. The drainage area of a single platform varies in diameter from 6 to 8 km depending on the depth of the reservoir. For fields whose areal extent in one or more directions is larger than the maximum drainage circle and for fields which are much smaller than this circle there is usually no unique optimum platform site. For nearly all fields in the North Sea there is an even greater apparent freedom of choice if one realises that at the time when a decision for the site has to be taken the field is generally still not well defined. Small shifts of the location of the platform can there fore nearly always be absorbed in the flexibility allowed in the development plan of a given field.