Tower Jackets As Cost-Effective Design Solutions
Authors: Lee Ming Shyong , Brooks J.C. , Tate Geoff , Robinson R.W. , Beer E.O. 
Published: October 2008

The use of a tower jacket configuration in shallow water depths of up to 30 meters is well established as a cost effective design solution, whereas battered jackets have tended to be adopted in deeper water applications. However, in the Amoco Central Graben Development (CGD), Central Area Transmission System (CATS)project, two drilling/production platforms and one riser platform, each with a tower jacket configuration and vertical skirt pile sleeves, have been found to be highly cost effective, while meeting the functional and environmental requirements of the central North Sea. The jackets have been successfully installed during summer 1992 in water depths approaching 90m (295 ft).

This paper identifies the benefits and cost Advantages for these tower jackets in the areas of weight, design, fabrication and installation.

1.0 INTRODUCTION

The two primary features of the CGD/CATS jackets are the tower configuration combined with grouted leg piles.

The tower configuration was introduced during the evolution of the conceptual design after a study was carried out comparing the benefits of a battered structure with a tower configuration. In general, a jacket with battered legs would present the following advantages over the tower concept:The jacket unpiled stability is generally improved through the enlargement dimensions thus enhancing the installation weather window and potentially reducing the cost of installation.The pile penetration can be reduced due to reduction of the axial loads by the increase in moment-arm length at the base.In the case of battered piles, the pile wall thickness may be reduced at the mudline due to a component of the horizontal loads on the platform being resisted by the axial pile capacity.The jacket dynamic response is reduced as the structure is stiffer.

However the advantages of the "battered" jacket Concept were offset by two main considerations for the drilling/production structures.

First there was a topside operating weight requirement of around 16,000 tonnes for the integrated drilling/production decks. Thus there was a need for a large deck plan dimension which in itself resulted in leg spacings of 30m × 25m. These leg spacings alone would provide sufficient base for the jacket footprint.

Secondly the jack-up tender drilling method, required the distance between the platform deck and the jack-up to be minimized to allow ease of transfer of the drilling derrick. However, adequate clearance had to be maintained between the jack-up leg spud cans and the jacket piles to avoid soil disturbance and undermining of the platform piles.

Both these considerations assisted in the Development of the vertical tower arrangements for the two production/drilling jacket structures which was also adopted for the CATS Riser Platform, as shown in Figures 1 and 2.

The other main feature of the jacket configuration was the adoption of leg piles which when grouted worked with the jacket legs as composite sections. This design feature had the following identifiable cost benefits.