Drilling In Water Depths In Excess Of 3,000 Feet From Moored And Dynamically Positioned Vessels
Authors: Smith Glynn D. , Parlas Solon C. 
Published: December 2007

Smith, Glynn D., Member SPE-AIME, and Parlas, Solon C., The Offshore Company Parlas, Solon C., The Offshore Company Copyright 1979, American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. Petroleum Engineers, Inc. This paper was presented at the 54th Annual Fall Technical Conference and Exhibition of the Society of Petroleum Engineers of AIME, held in Las Vegas, Nevada, September 23–26, 1979. The material is subject to correction by the author. Permission to copy is restricted to an abstract of not more than 300 words. Write 6200 N. Central Expwy., Dallas, Texas 75206.

Deep water exploration drilling has associated with it many unique problems including effects of weather, special equipment, logistics and specially trained personnel. However, deep water wells have personnel. However, deep water wells have been successfully drilled by moored and dynamically positioned drillships. Comparisons are made between two sister self propelled drillships; the Discoverer 534, propelled drillships; the Discoverer 534, which has drilled in 1,055 m (3,461 ft.) of water moored by anchors, and the Discoverer Seven Seas currently completing a well in 1,487 m (4,876 ft.) of water utilizing dynamic positioning and various aspects of the ever increasing technology and required expertise of deep water drilling. These drillships are owned by subsidiaries of The Offshore Company and each holds the current respective world water depth record; for a moored drilling unit in the case of the Discoverer 534 and a dynamically positioned unit for the Discoverer Seven Seas. Although the predominate number of deep water drilling units are dynamically positioned, exploration wells can be drilled in excess of 900m (3,000 ft.) if special equipment is available such as large mooring capacity anchor handling vessels and particular attention to detail is paid for the design of the mooring system. Both drilling units have operated in geographic areas around the world for various operators thus accumulating an interesting and successful history.

Introduction

The frontier for deep water drilling depths has progressed steadily from the 200 m (600 ft.) barrier in 1965 off the Southern California Coast to 1,487 m (4,876 ft.) in 1979. The 600 m (2,000 ft.) mark was exceeded in 1974 by the Sedco 445 dynamically positioned drillship and in late 1976 the 900 m (3,000 ft.) depth was surpassed by the Discoverer 534 in 1,055 m (3,461 ft.) of water offshore Thailand. The 1,200 m (4,000 ft.) barrier was surpassed in early 1978 by the Discoverer Seven Seas drilling in 1,325 m (4,346 ft.) of water off the Congo in West Africa with subsequent records of 1,354 m (4,441 ft.) off Spain and then 1,487 m (4,876 ft.) off Newfoundland, Canada in 1979. Figure 1 demonstrates how deep water exploration has progressed rapidly during the 1970's with floating drilling units now equipped to drill in excess of 1,500 m (5,000 ft.) of water. The capability of extending this to possibly 2,400 m (8,000 ft.) exists and dependent upon the results of deep water drilling, the frontier could be pushed that far during the 1980's. pushed that far during the 1980's.

BACKGROUND

Exploration drilling can be accomplished in deep water from either a moored drilling vessel or one that is dynamically positioned; however, in water depths of 900 to 1,200 m (3,000 to 4,000 ft.) or beyond, the practicality of drilling from a moored unit is doubtful since such a large array of mooring wire, chain and pendent wire is involved deployment is complex, costly, and time consuming. Special large capacity, anchor handling supply boats are required to adequately store thousands of feet of chain, mooring wire rope extensions and pendent wire rope. Table 1 shows the specifications of the workboats utilized for deep water anchor handling with the Discoverer 534. Additional rig time is also involved to anchor up and pick up anchors on each well. With a dynamically positioned drilling unit, it is possible to drill in deeper water depths with smaller workboats but with greater fuel costs to provide the power requirements for maintaining station. The Discoverer Seven Seas is not equipped with a mooring system or a mooring turret, but relies completely on dynamic positioning for station keeping.