The Garden Banks 388 Horizontal Tree Design and Development.
Authors: Granhaug Otto , Soul John 
Published: October 2008

This paper describes the Horizontal Subsea Production Tree System, later referred to as a "SpooITreeTM," developed for the Enserch Garden Banks 388 field in the Gulf of Mexico. The paper starts with a project overview followed by a comparison between the SpoolTree and the Conventional Tree design. A brief discussion explains why Enserch elected to use the SpoolTree for this field development, including available technology, workover frequency, cost etc. The rigorous safety analysis carried out for the subsea production equipment is then explained in depth. The paper continues with a technical discussion of the main features specific to the SpoolTree design and the Garden Banks 388 field development. Issues discussed include the SpoolTree itself, BOP Adapter Plate (for control during installation, workover and production), Tubing Hanger and pressure barrier design, debris cap design, downhole communication (SCSSV, chemical injection, pressure and temperature) ROV intervention, template wellbay insert design and other relevant issues. The use of computerbased 3-D modelling tool is also briefly described.

The experience and results described in this paper have direct application to numerous subsea development prospects worldwide, particularly in deep water. In addition, the "system development" aspect of the project is relevant to most marine equipment development projects. This includes the use of safety analysis techniques, 3-D computer modelling tools and clearly defined engineering procedures.

A full account of the final design configuration of the SpoolTree system is given in the paper. A summary of the experience gained during the extensive testing at the factory and during the template integration tests is also provided.

PROJECT OVERVIEW

Enserch Exploration, Inc. (ENSERCH), is developing oillgas reserves discovered in Garden Banks Block 388 in the U.S. Gulf of Mexico. The field uses subsea drilled and completed wells tied back to a floating production facility located directly above the subsea template/wells, (refer to Fig 1). Water depth at the location is 2096 ft, and the design capacity of the process facility is 40,000 bbls of oil per day and 120 million set of gas per day. The lightly processed oil and gas is piped to a shallow water facility for further processing before being exported to shore via separate oil and gas pipeline systems

The subsea wells are tied back to a floating production facility through a free-standing production riser with integral production and control lines. The subsea wells are drilled through a 24 slot subsea template by use of the drilling facilities on the floating production vessel.

Four appraisal wells have already been drilled at the template location. Three of these produce through tie-in to the main template structure ladjacent to the wells). The existing wells were drilled through a simple drilling guide frame. The template has provision for drilling and completion of up to 24 subsea wells including tie-back of remote satellite wells using diverless tie-in to connection slots on the template structure.

COMPARISON OF SPOOLTREE TO CONVENTIONAL TREE

Before any comparison of horizontal and conventional subsea trees is made, it should first be noted how alike the two configurations.