Design, Preparation, and Operational Aspects of the First Horizontal Subsea Trees Installed from a Jack-Up

Abstract

Introduction The Johnston Gas Field lies in blocks 43/26a and 43/27 of the UK sector of the Southern North Sea, 11 km to the Northeast of the Raven spurn North production platform (Figure 1). The water depth at this location is approximately 145 feet. The field was discovered in April 1990 by the 43/27-1 well and successfully appraised in July 1991, by the 43/26a-8 well. The field (Figure 2) is formed by a structural trap bounded to the Southwest by a NW-SE trending fault and by structural dip to the north and east. The reservoir itself comprises Permian Lower Leman sandstone exhibiting excellent porosity and permeability characteristics. Recoverable reserves are estimated to be in the range of 155 to 195 BCF. The gas sales contract requires a DCQ of 53 MMSCF/D for a six year plateau, with a peak rate of 90 MMSCF/D. Two wells located near the crest of the structure satisfy the production contract requirements, however a third well may be required in the third year of production. The overall field life is expected to be 13 years. This paper describes the selection, design and installation, in this field, of the first horizontal subsea trees to be installed from a jack-up. Development Plan Two primary development options were considered for developing the Johnston Field; a "not-normally manned" satellite platform or a subsea installation, both of which would be tied back to the Ravenspurn North processing facilities. Evaluation of both these options concluded that the subsea installation was preferred, due to shorter lead time for fabrication and lower initial capital expenditure. The Johnston subsea structure contains a three slot drilling template, a protection frame incorporating a spare fourth slot, production and methanol injection manifolds, and the subsea control equipment. The template and protection frame were designed to be installed either sequentially or in one piece, the choice of which was to be determined by delivery constraints. The Johnston field development was 'fast-track', first gas was produced ahead of schedule and under budget, some 14 months after project sanction. The procurement of the trees was also 'fast-track', the wellhead equipment was tendered, design finalised, manufactured, tested and installed within 12 months. Prior to placing the order for the trees, the operator in conjunction with the tree manufacturer, carried out a detailed technical review and HAZOP of the horizontal tree concept. Both parties were fully satisfied that the spooltree design was the preferred option for the Johnston development. These advantages of horizontal trees are discussed throughout this paper, however, the primary advantages were considered to be:–Reduced capital and installation costs–Reduced delivery times–Reduced size allowing easier handling Horizontal 'Spooltree' The Johnston mudline spooltree is shown in Figures 3 and 4. The 5000 psi spooltree comprises the main spool body with an integral hydraulic 5-1/8" production master valve (HMV), and an integral manual 2" annulus master valve (MAV). The 5-1/8" hydraulic production wing valve (PWV), the 2" hydraulic service wing valve (SWV), the 2" hydraulic annulus cross-over valve and the 2" hydraulic annulus drilling valve are in separate valve blocks bolted to the main spool body. The tubing hanger is landed within the spool body and is ported horizontally to allow flow into the production valves. The production master and the production wing valves provide the main well control barriers in the production flow path. The well barriers in the vertical bore are provided by two independent, positively sealing plugs. P. 11