Design of Well Completion and Intervention Systems For Deployment From Dynamically Positioned Vessels
Authors: Harrold David 
Published: October 2008

Abstract
Exploration wells are currently being drilled in water depths up to 10,000 feet. Completion of these ultra deepwater wells creates demand for innovative well system and intervention technology. Equipment design should not limit rig selection during initial installation or subsequent intervention activities. Selection of optimized intervention packages that function on both moored and dynamically positioned (DP) vessels will provide access to a wider rig market and create the opportunity to make substantial life cycle cost savings. This paper discusses unique design and operational requirements for the well system and intervention equipment to allow safe deployment and operation from DP vessels.

Introduction
Although mooring technology has made tremendous gains in recent years, DP operations still provide advantages for installation and intervention activities on fragmented, deepwater field developments. However, the decision to use dynamically positioned vessels presents far-reaching implications for both equipment specification and operating philosophy. Key operational considerations such as mitigation of DP station keeping incidents, watch circle management, ESD philosophy and vessel interfaces are presented to illustrate the challenges faced to successfully integrate the intervention system to the DP vessel. The design requirements imposed upon the well system and intervention equipment by DP operations are also discussed at some length. Functional requirements for Wellhead equipment, Xmas Trees, open water Completion / Workover (C/WO) risers, Horizontal Tree (HXT) intervention strings and drill pipe installation systems are presented. The paper concludes that the limitations of the intervention system must be fully understood with respect to the DP vessel response and that considerable planning and engineering assement is required to ensure safe installation and intervention operations.

Operational Considerations
The discussion that follows presents some of the key operational constraints and vessel interfaces, which dictate the requirements for well systems and intervention equipment that are deployed from DP vessels.

DP Station Keeping Incidents.
One of the key risks that must be quantified to allow proper specification of equipment and development of operating procedures is the likelihood and associated consequence of DP station keeping incidents. The frequency of a station keeping incident is dependent upon the reliability of the DP system and the exposure time during which the vessel is connected to the well. Various tooling packages will be used throughout the completion or intervention process, each with a different exposure time and associated level of risk depending upon the type of operation being completed. Therefore, it is inappropriate to assume that generalized recommendations will cover all tooling packages and operating procedures.
The performance and reliability of DP systems typically used on semi-submersible and monohull drilling rigs has improved to an extent that a station keeping incident requiring disconnect of the marine riser should be no more frequent than once every ten years1. This data can be used to predict the frequency of DP station keeping incidents that could be encountered during well completion or intervention activities. Figure 1, shows estimated incident frequencies based on exposure time (number of days connected to the well) and level of activity (well completions or interventions per annum).