Rapid Evolution of Ultra-Deep Water Drilling Rig Designs
Authors: Bums T.S. , Bennett W.T. 
Published: October 2008

Abstract
Large shuttle tanker based drill ships have proven to be the first commercially successful new build design alternative for ultra-deep water drilling application. Historical trends indicate that more cost-effective semisubmersible drilling units will follow the drillship designs in ultra-deep water drilling applications. Several suitable designs are being developed and cover a broad range of capabilities, performance, and cost. In evaluating the most cost effective design alternative for a particular application, it is important to understand the cost sensitivity and operational efficiency of various vessel design variables. Detailed cost, performance, and capability comparisons of new ultra-deep water semisubmersibles designs indicate significant cost sensitivity to variables such as structural complexity, design environment, vessel motion performance criterion and transit speed requirements. Design compromises in these areas may not significantly effect the drilling performance for certain applications but can result in significant cost reductions to the drilling unit. This cost reduction potential must carefully be evaluated against the compromises with consideration given to the duration, flexibility, and total information known about the projected drilling program. As more information and knowledge is obtained about specific drilling programs and consistent long term drilling programs evolve, cost effective "fit for purpose designs" will become justifiable. The evolution of commercially successful rig designs are anticipated to be developed in a conservative step by step process that can be accelerated by increased knowledge and experience. With strong deep water demands, this evolution process may occur with considerable overlap significantly reducing the overall time in which cost effective designs are perfected and proven. Future potential cost improvements in ultra-deep water drilling units will continue to strengthen the economic attractiveness of deep and ultra-deep water exploration and development projects.

Introduction
Except for a few isolated projects, new construction for the offshore drilling industry has been dominant for the past IS years. During the last few years however, the industry has committed to many major hull conversions and has announced orders for the construction new vessels involving over 30 drilling vessels. Several of these vessels are being built or converted for deep water drilling applications but the majority are being built to explore and develop prospects in ultra-deep water. In this discussion, deep water is referred to as water depths ranging from 1,000 to 2,000 meters and water depths greater than 2,000 meters is referred to as ultra-deep water. For the most part, the trend has followed the patterns set previously when the industry has ventured into deeper water. First existing vessels are upgraded to meet the immediate needs of the operators. Then the new building vessel of choice has once again been the drillship or monohull designs. These vessels are easily adapted to carry the heavier equipment and increased variable loads required for supporting activities in deeper water. The characteristics of the larger monohull shows promise of superior motions and increased operability when compared with the smaller vessels of the previous generations The design of the ultra-deep water mobile offshore drilling units (UDWMODU) is rapidly evolving as Operators become more familiar with their long-term requirements and rig designers optimize the variables to increase overall drilling performance in this new market.