ABSTRACT

In the twelve years since exploratory drilling has begun on the Gulf of Mexico's continental slope, mobile offshore drilling units' mooring systems have become increasingly sophisticated and the technology for their design and use has become well established.

With the advent of sophisticated computer analysis software, improved weather forecasting, seafloor surveys, high holding capacity anchors, permanent chain chaser systems, upgraded anchor boats, Loran-C based rig positioning systems and effective planning and coordination, it is now possible to routinely moor a rig quickly and safely in from 600 to 2000 feet of water in the Gulf of Mexico.

There is increasing interest and competition in drilling on the Gulf of Mexico's continental slope. This paper reviews the new equipment and technology being used in the Gulf to safely and efficiently moor mobile offshore drilling units on the continental slope.

INTRODUCTION

The Gulf of Mexico's continental slope begins at the 600 feet of water depth line where the continental shelf ends and extends out to over 10,000 feet of water. Only depths to just under 3,000 feet have been drilled from moored, mobile offshore drilling units (rigs).

The continental slope from Texas' southern border to southern Florida is under the dominion of the united States government which has divided it into 12 lease areas each of which is subdivided into many approximately 3 mile square lease "blocks"1. These lease blocks are shown in Figure 1. With the exception of the Destin Dane area, Florida's continental shelf area is not a promising area for future drilling and, for this reason, these lease areas are not designated in the figure.

Of the over 280 exploratory wells drilled on the continental slope from moored rigs, the majority have been drilled in the Mississippi and Green Canyon areas as tabulated below 2

Particularly since the 1983 federal lease sale in these areas, exploratory wells are being drilled in increasingly deeper water as shown in Figure 2. Note that most of the wells were drilled from chain moored rigs even in the deeper water areas. This is due to several factors.

One factor is that the majority (65%) of rigs are chained moored vs a combination of wire rope and chain and that most of the rigs available in the Gulf of Mexico are also chain moored3 .Another factor is the introduction in recent years of higher strength chain commonly designated Super Oil Rig Quality or K4 chain which allows chain moored rigs to safely drill in over 2000 feet of water under most environmental conditions.

With the increasing rig competition the emphasis on efficient and safe mooring operations has intensified. In these deeper waters large amounts of mooring equipment must be recovered, stored and redeployed each time the rig is moved. To improve mooring efficiency and safety several trends have developed in the industry.

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