Operational Aspects of Mooring a Drilling Vessel in Water Depths of 4,200 to 4,700 ft
Authors: Barker J.W. , Day R.M. 
Published: December 2007

Mooring operations in deep water require detailed planning and special techniques for rigs and anchor-handling vessels (AHV's). Techniques developed by a multidisciplinary task group were used successfully and safely during two mooring operations in water depths ranging from 4,200 to 4,700 ft.

Introduction

During recent years, the petroleum industry has continued to expand deepwater-drilling efforts in many areas of the world. As drilling water depths increase, the mooring limits of mobile drilling units are being tested. The complexity of mooring operations increases significantly with water depths greater than 3,500 ft. In late 1989, Exxon Co. U.S.A. began planning two exploration wells on the outer planning two exploration wells on the outer continental slope of the Gulf of Mexico. One was in the eastern Gulf of Mexico in 4,356 ft of water; the other was in the western Gulf of Mexico in 4,645 ft of water. Unusual well equipment requirements combined with the operational time flame eliminated the option of using a dynamically positioned rig. Thus, rig selection focused on available semisubmersibles and their capability to moor safely and reliably in about 4,500 ft of water. A key factor in these mooring operations was the AHV's capability to handle the expected heavy loads. After the best rig and AHV candidates were identified, a task group was organized to study the feasibility of mooring a semisubmersible in 4,500 ft of water. Once feasibility of the operation was established, the task group generated all mooring implementation procedures and contingency plans. The task group also supervised field plans. The task group also supervised field operations for the two critical deepwater-mooring operations. The mooring operations were safely implemented in early 1990. The operations extended the proven world water-depth record for rig mooring by about 26%, almost 1,000 ft.

Well Locations

The eastern well was located in 4,356 ft of water, roughly 110 nautical miles southeast of Grand Isle, LA. Water depth at the anchor ranged from 4,250 to 4,510 ft. The western well was located in 4,645 ft of water, about 135 nautical miles due south of Galveston, Tx. Anchor water depth ranged from 4,385 to 4,720 ft.

Operational Constraints

One of the most significant operational constraints was the varied environmental conditions of the Gulf of Mexico during the different seasons. Hostile conditions encountered during a summer hurricane can easily exceed those expected in the rough winter months. Thus, all planning was based on completing both wells before hurricane season to reduce overall risk for the two-well program. Because the eastern location was program. Because the eastern location was in an area affected by the Gulf of Mexico Loop Current, we decided to drill this well first so that the Loop Current could be easily and closely monitored by means of satellite infrared photography. The location of the other well was much farther west, so the risk of being affected by the Loop Current or eddy was lower. Another major operational constraint was the unusually large casing strings planned for the eastern well. Its casing program required use of an 18 3/4-in. blowout-preventer (BOP) stack. A dynamically positioned rig equipped with this size BOP was unavailable; therefore, we decided to moor the rig. Another constraint to mooring operations was the potential for chemosynthetic communities to exist along escarpments. Mooring operations were planned to avoid these areas completely. The final constraint on the mooring design was the desire to avoid piggybacking rig anchors. When the rig's primary anchor does not develop adequate holding power, the usual response is to add a piggyback anchor (see fig. 1). In deeper water, this procedure is very difficult to implement. procedure is very difficult to implement. The tremendous amount of wire rope, the connective hardware, and the required large buoy make this procedure very costly and time-consuming. The holding capacity of the rig's anchor and mooring system was an important rig-selection consideration. Also, on the eastern well, the proximity of the primary anchor pattern to the potential primary anchor pattern to the potential chemosynthetic community would have made piggyback anchors much more difficult to deploy safely and effectively.