Improvements in the Design of a Swage Pile Connection System for Jackets

Abstract

ABSTRACT In the North Sea, weight considerations have become an important issue in the design of jackets. This has largely arisen due to the ability of the new superbarges to lift install jackets which would otherwise have required design for barge launch. The swaged pile connection system which has been used for several years on subsea structures offers several advantages for such structures. Nevertheless in its existing form, although the system offers considerable advantages for jackets the potential weight penalty could influence its acceptance by the industry. A development programme has been undertaken which has demonstrated how the weight of the pile sleeves used for the swage system can be reduced by 35% - 40%. This has been achieved with the minimum of changes to the already established and certified designs. The paper discusses this work. The paper also describes the design and installation of the first jacket to be installed using the revised system. This jacket, conventionally barge launched, was installed for Elf Aquitaine in approximately 500 ft sea water in the Gulf of Mexico. 1.0 INTRODUCTION Swage pile connections, which have now been available for almost 10 years, have gained considerable acceptance in the securing of subsea templates (l) (2). As experience has been gained, their versatility and advantages have been recognised by jacket designers who have also considered and applied the system. In recent years the system has been used for several jacket type structures in the North Sea and Gulf of Mexico (3) (4). The design of the swage connection requires the pile sleeve to be manufactured from steel substantially thicker than the pile. Although this has not presented problems with the small piles encountered in subsea structures it offers potential problems with the larger and hence thicker piles required for jacket structures. This was further exacerbated by the introduction of vertical piles with their attendant increase in bending capacity and more recently by the popularity of lighter jackets. The swage pile connection offers considerable advantages for these lightweight structures, and in particular provides a means of rapidly improving stability of the jacket during the critical installation phase. Inevitably, during the design phase of these structures, any feature perceived as adding excessive weight to the structure, particularly at the "heavy end" will lead to its rejection. It became clear, therefore, that in order to overcome this problem and to allow the industry to benefit from the many advantages of the swaged system a weight reducing refinement of the overall pile sleeve geometry was required. From previous analysis undertaken, it is well known that a large proportion of the sleeve thickness becomes redundant once the connection has been formed. Also, in the early days of design the finer points of weight reduction had not been critical in gaining industry acceptance for the concept. There was therefore considerable scope, within the limitations of the "approved" designs of the connection, to refine the geometry, minimise weight and reduce sleeve cost.