Concrete Gbs Platforms And Site Considerations For Southeast Asia: Structural Concepts

Abstract

ABSTRACT Concrete GBS (Gravity Base Structure) concepts are explored with respect to their structural and other merits for small to mid-size offshore field development in S.E. Asia, including Australia. The fact that a concrete GBS can incorporate storage for oil or other petroleum liquids was one of the attractive features suggesting its consideration as a platform alternative for the often isolated potential producing locations in the subject region. Met/Ocean conditions across the region vary from quite mild to about as severe as the Gulf of Mexico. Water depths considered for the study were in the range of 200-400 ft., which covers a large percentage of anticipated producing areas, excluding ultra-deep waters. Earthquakes pose a significant risk in a number of the areas. Soil conditions tend to be somewhat weak, and include areas of soft clays, volcanic silts, and calcareous sediments which can pose difficulties for pile foundations. Promising construction aspects include the possibility of substantial local construction content at a variety of sites around the region, the low potential maintenance with concrete, and the alternatives to launch barges, and other heavy offshore installation equipment. INTRODUCTION The use of concrete offshore structures for petroleum related resource development in the Far East, has to date had little-to-no precedent. There, however, has been in use, for quite some years already, off Indonesia, a concrete barge for LPG (Liquid Petroleum Gas) storage (1). Also currently, there apparently are plans for a concrete GBS to be installed in the Bass Straits off Australia (2). The subject considerations, and associated investigations, were motivated by an interest in exploring the technical viability of concrete as alternative platforms for the Far Eastern region, as well as wanting to obtain some preliminary technical performance data for such structures. The latter data could be potentially useful in comparative technical evaluations of various platform types, or other structural systems, that may warrant consideration. This paper presents in the following some of the significant considerations on the subject, and a sampling of results relating to what may be Labeled as small to mid-size platforms. The information and cases discussed, are believed to provide an informative cross-section of the various concrete platform types and general site conditions that apply to a large portion of the region. DESIGN OBJECTIVES The core significance for any type of platform structure is in Its ability to enable and shelter production activities by a "topside" facility. Hence it was thought important to investigate the suitability of the concrete GBS type with a set of conservative, and indeed demanding, "producing assumptions" for the chosen applications that were to be in the small to mid-size range. Furthermore, the types of environmental conditions, including water depth and soil, wanted to be as widely representative as possible for the appropriate region. Also, construction and installation aspects, and their consequences, as for the region wanted to be included in the considerations.