Sleipner Vest : Cost Effective Marine Operations
Authors: Allen A.P. , Rush P. , Giske S.R. 
Published: October 2008

This paper describes the complex marine operations required for the installation of the Sleipner Vest Treatment Platform (SLT). The largest bucket foundation jacket yet built required installation to within a tolerance of 1.0 metre, just 100 metres from an existing platform. Four additional major lifts had then to be performed. all within a period when statistically the environmental conditions never reduced to .installation' seastates.

Acceleration of the first gas date by six months required examination of every aspect of the marine operations to ensure complete synergy between all activities from fabrication to tie-in. This paper describes the steps taken, from jacket design to optimise installation schedule, module design to minimise hook-up, through installation operability analyses. to final installation and preparation for hook-up and tie-in,

Introduction

The SLT platform lies in 82.5 metres of water, immediately to the West of the existing Sleipner Alpha Platform (SLA) gravity based structure, and is located in block 15/9 of the Norwegian Sector of the North Sea. It is linked to the Sleipner 'B' Wellhead Platform located a further 12 km to the West, from which it receives gas for treatment. The existing in-field layout was complex. Within 500 metres from SLA, separate riser and flare platforms had been installed, and extensive pipeline corridors lay to the North and South East, (Figure 1).

The SLT development consists of a 7500 tonne jacket, two large modules, (MIl and M12, weighing 8300 and 9200 tonnes respectively), a 120 metre high vertical flare, and a 1000 tonne bridge linking the new facility to the existing SLAplatform.

The original project schedule assumed a late May 1996 installation. However, it soon became apparent that significant savings could be achieved with an acceleration of both fabrication and offshore commissioning.

To achieve an early first gas date, the installation period was accelerated to mid April. Based on the installation activities of the existing platforms at Sleipner, and the anticipated available weather windows, this was considered to be the earliest date practical. It was essential therefore that offshore activities could be refined to be performed in short durations, and in view of the existing platform and subsea infrastructure optimised as far as possible.

In the early phases of conceptual engineering therefore both Operator and Engineering Contractor identified marine perations as a key function, and worked to develop an installation friendly design.

To achieve this, key areas were targeted:Application of bucket foundations for jacket installation,Module design to optimise commissioning,Model testing combined with statistical operability analyses to maximise operability.

Design For Installation

Jacket Design

- The potential of the pioneering bucket (or plated) foundation concept had been successfully used on the Snorre TLP foundations and Tordis protection frames. Itsapplication to steel jacket sub-structures was first demonstrated during the Summer of 1994 when the 5000 tonne Europipe 161l1-E (now renamed Draupne E) riser platform was installed.