Deepwater Dynamically Positioned Drillships - How Shallow Can They Go?
Authors: Ambrose Bill , Keener Chip , Farr Dan 
Published: October 2008

In the last four years, 5th generation dynamically positioned (DP) deepwater drillships have come on line and operated reliably around the world in their design water depths. While targeted to drill in deep water, they also possess the ability to work in shallow water areas provided a contractor-operator "partnership" is formed at the onset of the drilling program to establish safe operating limits.

This paper outlines a philosophy that enables deepwater rigs to safely move into shallower water not typically associated with these units. By thorough well planning and design, technical limits can be established and crew training addressed that sufficient reaction time will exist to permit safe operation of a DP drilling unit in as little as 800-ft of water. For some programs, moving efficient 5th generation drilling packages to shallow water exploration areas, where 3rd and 4th generation moored semisubmersibles tend to operate, can potentially reduce overall well construction cost and time for the operator and ease logistic issues when operating in remote areas.

Included in this paper is a description of the station keeping limit philosophy, risk mitigation measures to consider when in shallow water, and benefits that are achieved with 5th generation drilling equipment. Additionally, comments on operations during a four well program conducted with the Deepwater Discovery, a 10,000-ft rated DP drillship, in 923 ft to 2674 ft of water offshore Mauritania will be provided showing the high level of performance delivered by today's DP systems. Finally, a performance and commercial comparison to a 3rd generation moored semisubmersible conducting the same four-well drilling program will be used to highlight potential gains of a 5th generation unit in shallow water.

Introduction

At first glance, operating a deepwater dynamically positioned drillship in shallow water (less than 2000 ft) may seem risky. In large part this is due to a reduced reaction time available to disconnect from the well in the event of a DP upset or blackout. However, with proper design of the riser/wellhead system and station keeping management, reaction times to safely disconnect from the well can be established in water depths as shallow as 800-ft. While there may be a reduction in the maximum environment to conduct safe operations, the payback for assuming the risk of limited waiting on weather downtime may outweigh this downside for certain operating areas in the world.

Working together to plan a shallow water program, the operator and contractor can establish safe operating limits and drill challenging shallow water wells with 5th generation DP drillships. The following sections will outline a station keeping methodology that allows DP drillships to work in shallow water, discuss risk mitigation measures, and provide insights from operations on the Deepwater Discovery during a four-well program in 923 ft to 2674 ft of water offshore Mauritania. Finally, a cost and time comparison between using the Deepwater Discovery and a 3rd generation rig for the same four-well program will be shown.

Basis of DP Station Keeping Limits

A systematic strategy to define station keeping limits (watch circles) based on vessel drift-off and reaction time has been established and put into practice [ref. 1].