Abstract

From July 1999 through May of 2000, five horizontal subsea Christmas trees were installed in 4,750 feet of water in the Diana field in the Gulf of Mexico. The previous depth record for horizontal trees was about 3,400 feet. A wet parking scheme for tree installation was used to save money by reducing BOP trips and by using a less costly installation vessel. The final horizontal section of up to 1800 feet of the reservoir was drilled through the tree. Innovative techniques that saved time and money were used to contract, engineer, and test the tree, and bring the well on production. These techniques and lessons learned can be applied by others using horizontal trees in deepwater applications.

Introduction

The Diana and Hoover fields are located approximately 150 miles east of Corpus Christi and 160 miles south of Galveston, Texas, in the Gulf of Mexico. The Diana Field consists of East Breaks Blocks 945, 946, 988, 989 and Alaminos Canyon Block 21. Initial exploration of the area began in 1990 and two further delineation wells followed in 1996. The Diana reservoir features a slanted crescent-shaped formation with a thin oil rim along the down dip region. However, Diana's remote location and the extreme waterdepth of 4,750 feet made the field challenging. The discovery of the Hoover field (Alaminos Canyon Blocks 25 and 26), 16 miles away, provided additional incentive to develop the necessary infrastructure and co-develop both fields simultaneously.

Hoover would be produced from a deep draft caisson vessel (DDCV) (Figure 1) and the Diana field would be developed via subsea completions tied back to the DDCV. The Diana field consists initially of 5 oil wells that will be redrilled and completed higher as gas wells later in field life. Each tree is tied via a nominal 5" hard pipe U-jumper to one of two gathering manifolds (two at the northern drill center, three at the central drill center). The manifolds are in turn connected together in a daisy chain fashion by 10" hard pipe pipelines, in a round trip loop, to accommodate pigging. The pipelines extend back 16 miles to the DDCV, terminating in steel catenary risers (SCRs). Project timing permitted the Diana field to be drilled, completed, and ready for production when the DDCV was commissioned.

Systems Engineering And Trade-Off Studies

When work began in 1996 for Diana it was recognized that it would be deeper than the then-current subsea world record of about 3,300 feet. However, two vendors were building vertical tree (VXT) systems for water depths in excess of 5,000 feet. Both systems were installed in mid 1997. In addition, if a horizontal tree (HXT) were chosen, it would bein record water depths for an HXT. Consequently the two vendors building the deep water systems were chosen and paid for early trade-off studies for Diana. The results included their recommendations of HXT versus VXT, comparative specifications, procedures, and costs.

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