Special Session: Offshore Surface BOP - Newbuild Compact Deepwater Drillship Designed for Surface BOP System
Authors: Claassen Laurens , Hendriks Sjoerd , Zijderveld Govert 
Published: July 2010

In the deepwater areas, typically 5th and 6th generation drilling vessels are mobilized for operations. Driven by economic mo-tives and based on Surface BOP systems, improved functional specifications have been incorporated into a newbuild drillship of a reduced size, with improved capabilities and making operations more efficient and cost effective compared to traditional MODUs. The new vessel has the characteristics in size of an earlier generation, but the functionality of the next generation vessel. The advantages of the design including reduction of capital expenditure, vessel dimensions, dynamic positioning power requirements, environmental footprint and rig time, and reduction in usage of drilling fluids and other consumables are illu-strated. Presented is the design effort encompassing a compact deepwater drillship for Surface BOP operations, initiated in 2003 and leading to two ships currently under construction and scheduled for delivery in 2010.

Introduction

Currently, a number of 6th generation drillships are being added to the worldwide drillship fleet. These vessels have followed the trend of the 5th generation, by growing into larger and larger units. In the deepwater areas, typically 5th and 6th generation vessels are mobilized for operations: drilling and testing exploration, appraisal and development wells, running completions, performing workover and intervention operations. The size of these vessels affects cost effectiveness. Larger vessels come at higher initial investment (capex) and operational costs (opex), but also at a higher cost in terms of sustainability and environmental impact. The alternative compact drillship designs aim to curb the growth of vessel dimensions and associated cost, with a design philosophy focusing on functionality and fit for purpose design, while maintaining flexibility in operational scenarios.

An example of a design philosophy that has led to growth of design dimensions of 5th and 6th generation rigs, is incorporation of future early production capability. This requires crude oil storage capacity, additional topsides, and offloading systems. The crude storage option has numerous consequences for the design, all boosting the dimensions, capex, opex and as a result the break-even day rate. When the hull volume below the main deck is sacrificed for crude storage, it forces the drilling equipment to be located in topside modules above the main deck. These topsides move the center of gravity of the vessel up, compromising the stability, which in turn must be counterbalanced by an increase of the vessel width. Furthermore, the process and drilling topsides impede efficient deck logistics, especially for handling of large tubulars. However, in the existing fleet the storage facilities are hardly utilized, which means that most of these vessels are being operated unduly at a lower level of cost effectiveness. Moreover, although these vessels have general capabilities built in, they still need to be adapted before actually being suitable for crude storage. Cargo handling and inert gas systems will have to be installed, keeping the drillship out of service for a significant period. When the design focus would have been more on the actual functionality of the rig, substantial size and cost reductions would have been possible. The alternative design philosophy is to focus on the intended function of the vessel, aiming at a vessel as compact as possible without compromising the operational integrity of the rig. A typical example of this philosophy is the PRD12,000 design, which combines emerging drilling technologies such as the application of Surface BOPs, saving weight and equipment to reduce vessel size as an effective tool against tight economic circumstances in the current market. In this paper, the compact design approach will be presented and typical examples of the results of this approach:PRD12,000: as a specific example of integrating new drilling technologiesPRD12,000 Arctic: extending operations into isolated and environmentally sensitive areas, including the ArcticPRD12,000 MPT: adopting new hoisting equipment