Deepwater Rig Design for 10,000 Feet of Water
Authors: Wilburn M.D. , Steddum R.E. , Shaughnessy J.M. 
Published: October 2008

Transocean Offshore has designed their newbuild drillship Discoverer Enterprise to drill in up to 10,000' of water. The current deepwater drilling record is 7,612'. This paper addresses the equipment upgrades and corresponding additional investment required to outfit a newbuild drillship' for 10,000' of water versus one outfitted for 7,000', which approaches the current limit of existing rigs. Much of the equipment selected for the Discoverer Enterprise had not previously been built in the size or capacity rating required to operate in 10,000' of water. Some upgrades, such as the riser, impacted several other items such as the rotary, buoyancy, tensioners, derrick, and hoisting equipment. In most cases it would not be practical to retrofit existing deepwater rigs for 10,000' of water. The technology developed setting water depth drilling records worldwide in the last 20 years is being applied and expanded operate successfully and efficiently in 10,000' of water.

Introduction

The Discoverer 534 is the current deepest water depth rated rig at approximately 7,800'. Equipped with 21" riser and an 18 3/4" BOP stack, it holds the water depth drilling record of 7,612' set in the Gulf of Mexico in 1996. The Discoverer Seven Seas has drilled in 7,520' of water, but this was using a 16 3/4" BOP stack and nominal 19" riser. Using riser smaller than 2 I" allows drilling rigs to get in deeper water depths without upgrading their equipment, but that will not be considered since the merits of using a 21" riser system are well documented. In addition to the Discoverer 534 and Seven Seas, there are other rigs nominally rated to water depths of approximately 7,000' that have not had the opportunity to demonstrate their capability. The typical drilling equipment found on a deepwater drilling rig such as the Discoverer 534 or the Discoverer Seven Seas is summarized as follows:0.625" wall riser with 2000 kip couplings, 3" ID 15 ksi choke and kill lines44" - 47" OD riser buoyancy (riser is fully outfitted with buoyancy)16 X 100 kip wireline riser tensioners1 X 3000 HP drawworks using I 3/4" drill line3 X 1600 HP Mud Pumps650 ton derrick capacity49 1/2" rotary table and diverter housing400 or 600 kip in-line heave compensator

These vessels are about 534 feet long, have an operating variable load of approximately 7500 tons, and are equipped with six 2500 HP thrusters for dynamic positioning. The vessels' size, equipment, capacities and basic structural limitations all combine to limit their water depth capability to approximately 8,000'. The basic reason for this limitation is their inability to effectively store, run, and support long strings of the type of riser needed for water depths in excess of 8,000'. Operators are now looking to deeper water depths due to the success seen in the last few years between 3,000' and 6,000'. Improved seismic technology has increased the deepwater potential for discovering commercial reserves. High permeability reservoirs and improved completion technology has made flowrates of 20,000 BOPD from oil wells and 100 MCFD from gas wells possible. There are currently 485 blocks leased in greater than 7,000' of water in the Gulf of Mexico.