This paper demonstrates cost savings that can be gained by selecting either slimbore or drill through Xmas Tree types to develop deepwater subsea fields. Field data and a computer model were used to quantify the CAPEX, OPEX and NPV life cycle cost savings these tree types can provide when compared to conventional vertical dual bore and horizontal Xmas Tree designs. The paper considers applications in depths from 2,000 to 10,000 feet.


As operators embark upon development of deepwater subsea fields, long duration drilling, completion and workover cycles are encountered. The CAPEX and OPEX expended on these fields is largely governed by the cost to charter drilling rigs as opposed to the cost of the subsea hardware itself. However, the Xmas Tree type selected does have a major influence upon drilling, completion and workover durations and drilling rig selection. It is therefore imperative that the most suitable Xmas Tree is selected for any given application to minimize installation and workover durations whilst maximizing use of lower cost drilling and completion rigs.

Slimbore and drill through Xmas Trees were compared against standard fullbore vertical and horizontal Xmas Trees currently being installed in deep water. This paper is limited to comparing CAPEX and OPEX variances between the systems. Other technical merits or the relative risks associated with each system are not addressed or discussed. The ability of slimbore systems to utilize lower cost MSV vessels for installation and workover activities is examined and discussed. Time estimates and associated costs are presented for installation, tubing access workover, tubing retrieval workover, tree retrieval, well sidetrack and decompletion for each Xmas Tree. These costs were used to generate a financial model to compare the NPV life cycle costs associated with each system.

This paper is written assuming that readers have a fundamental understanding of subsea well completion operations and associated Xmas Tree hardware.

Xmas Tree Definitions

A brief description of each Xmas Tree type compared in this study is provided below. All Xmas Tree types are guidelineless (GLL) configurations, which are either currently in use or are being proposed for use in deep water field developments. The associated installation and intervention tools, which are used with each system, are also described. To the knowledge of the author, no subsea wells have been completed at water depths exceeding 7,500 feet and some technology development is ongoing to extend well completions to ultra deepwater. This fact will also impact Xmas Tree selection criteria, at least in the near term.

Conventional Dual Bore Vertical Tree (CXT).

The CXT is a dual bore configuration with an ROV installed, pressure containing tree cap. The tubing hanger is an 18-3/4" full bore design that is installed directly to the wellhead. The tubing hanger contains a hydraulically operated valve to provide annulus isolation.

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