OTC 10891
Use of the Dual-Activity Drillship as a Field Development Tool
Johnce E. Hall, BP Amoco; Robert P. Herrmann, Consultant; Charles A. Holt and William J. Straub, BP Amoco; Rickey G.Weber, R & D Technical
Copyright 1999, Offshore Technology Conference
This paper was prepared for presentation at the 1999 Offshore Technology Conference held in Houston, Texas, 3-6 May 1999.
This paper was selected for presentation by the OTC Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Offshore Technology Conference or its officers. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Offshore Technology Conference is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was Presented.
Abstract
The startup of the dual-activity drilling vessel Discoverer Enterprise in early 1999 heralds a new age for deepwater development. For the first time a drilling vessel exists that can perform many of the aspects of deepwater field development cost effectively, including a variety of subsea production equipment installations and flowline/pipeline laying. This capability minimizes the barges and equipment required for field development and yields schedule flexibility for the installation tasks.
Aspects of the dual-activity drillship-related to non-drilling activities are described in the paper. The second rig on the drillship is only required part of the time to support drilling operations. The remainder of the time can be used for other field development activities.
The subsea tree is run with the auxiliary rig while drilling with the BOP and riser with the primary rig. In addition, subsea manifolds, umbilical pods. flowline jumpers and supporting piles can be placed and secured with the auxiliary rig while drilling progresses with the primary rig. Concepts for subsea equipment that can be installed with the dual-activity drill rig are presented.
Some flowline installation work can also be performed while drilling is in progress. The flowline installation technique designated “discrete lay” and/or “reverse lay”, results in sectional flowline placements using an anchor handling vessel and an ROV boat (for inspection and monitoring). This technique can also be used for the placement of “long jumpers”.
Flowlines, as well as export pipelines in deeper waters, can also be installed by the dual-activity drillship in pipelay mode before or after drilling is completed. Using both rigs. Two lines can simultaneously be laid which is an advantage for flowline sets requiring pigging loops or parallel oil and gas export lines. Burial of the lines is also achievable with the dual-activity drillship.
A conceptual timeline of how drilling and non-drilling operations can be achieved by the dual-activity drillship is provided.
Introduction
The concept of a dual-activity drillship has been in existence since the mid-1990's. A year-long study performed by Transocean Offshore Inc. concluded that deepwater drilling efficiencies could be enhanced by design and construction of a single drillship with dual drilling capabilities. Construction of the first dual-activity drillship, the “Discoverer Enterprise”, began in 1997 and is scheduled for delivery in the spring of 1999. Two additional drillships, the “Discoverer Spirit” and the “Discoverer Deep Sea”, are currently under construction and are expected to be delivered in the year 2000.
One advantage identified in association with the dual- activity drillship is that it has the potential to enhance offshore operations in unconventional modes. Because the auxiliary rig is out of the critical path, it can be used for operations which would not be economic if the rig spread cost were applied to the operation. For example, the second rig is capable of being employed to drill a disposal well for flared gas or for produced water, or the second rig can be used to pull in pipelines, run production jumpers, run control lines or other subsea construction work. The extra rig is available for heavy subsea lifts and work as a 5,000 hp ROV. In addition the second rig can help eliminate shallow water flow problems or reduce external hydrates. After the BOP stack is set there is available time at the auxiliary rotary table for making up pipeline segments for installation on the seabed.
