Use of the Dual-Activity Drillship as a Field Development Tool
Authors: Hall Johnce E. , Herrmann Robert P. , Holt Charles A. , Straub William J. , Weber Rickey G. 
Published: October 2008

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.