Installation and Workover Time Savings: Key Drivers for Deepwater Tree Selection
Authors: Wester Randy J. , Ringle Eric P. 
Published: October 2008

This paper presents the results of a study that was performed to determine the impact of different subsea tree configurations on deepwater well completion and workover costs. Four stateof-the-art, guidelineless tree designs are compared over water depths ranging from 2,000 to 10,000 feet. Results are presented for single and multi-well completion scenarios and for single and dual activity rigs. Recent applications of timesaving, deepwater tree technologies and installation methods are also presented. Factors that significantly influence the deepwater tree selection process are discussed.

Introduction

Operators are developing subsea oil and gas fields in rapidly increasing water depths. Over the past ten years the water depth of the deepest subsea completions has increased from 2,000 to 6,000 feet. Exploration wells are presently being drilled in water depths approaching 10,000 feet, and development projects are in progress for water depths exceeding 7,000 feet. So the demand for deeper water completions is right around the corner. As water depth increases, the operational costs associated with completing and working over subsea wells increases at a significantly higher rate than the cost of subsea tree hardware. As shown in Figure 1, the ratio of the installation and hardware costs for a subsea tree in 2,000 feet of water is roughly 1:1, but that ratio increases to 3:1 in 10,000 feet of water. Therefore, the focus for achieving significant cost savings on subsea tree systems in deepwater needs to be on operational time savings duringwell completions and workovers.

The configuration of a subsea tree dictates the sequence of well completion and workover operations and therefore has a significant impact on the cost of those operations. When a subsea tree is selected for a given application, a thorough understanding of the installed cost (CAPEX) and the life-offield operational costs (OPEX) for that tree should be developed. Those costs can be compared for different types of subsea trees to ensure that the most cost effective system is selected for the application.

The purpose of this paper is to evaluate subsea tree designs that are being used in the deepwater theaters of the world today on the bases of CAPEX and OPEX. The most suitable tree designs for a range of applications and decision drivers are identified. The applications that are considered include completing wells: immediately after drilling, after a period of temporary abandonment, and in batch operations. The influence of using single and dual activity drilling rigs is also considered. The impact of using moored or dynamically positioned vessels is addressed.

Two configurations of conventional (vertical) trees and two configurations of horizontal trees are used for the CAPEX and OPEX evaluations. The evaluations are made using a computer-based well completion and workover model. The model uses actual running procedures and field data to predict well completion and workover times. CAPEX comparisons are generated by applying drilling rig day rates to the installation times and adding typical costs for tree hardware and rental equipment, such as running tools, completion risers and IWOCS