Design Considerations for Mooring a Drilling Vessel in 5000 Feet of Water
Authors: Towns T.K. , Christman S.A. , Kwan C.T. , Ettle R.E. 
Published: December 2007

This paper reviews the engineering basis for extending the use of a moored drilling vessel to 5,000 feet of water. It examines steps taken to perform the mooring operations in a reliable and efficient manner (e.g., upgrading mooring wire, anchors, and boat winches). It shows the usefulness of mooring design standard for the mooring analysis and equipment inspection.

This paper discusses the design considerations used as well as other options considered. The findings, including field experiences, should assist future efforts to extend this technology to deeper water. The use of personal computer (PC) to perform complex mooring calculations introduced a powerful tool which helps increase the utilization of existing mooring equipment.

A step by step installation procedure was developed to optimize sharing of mooring line loads, up to 315 MT, between the winches on the drilling and anchor handling vessels. Using this procedure and two anchor handling vessels, total installation time on the first well location in 4,864 feet of water was 54 hours or 2-1/4 days.

Planning for deep water mooring requires a careful review of the mooring equipment to evaluate the technical/economic feasibility. As operators move to develop fields in deeper waters from subsea templates, a moored drilling vessel may be a cost effective alternative to dynamically positioned drilling vessels.

Introduction

Due to the limited availability of dynamically positioned drilling vessels. Exxon contracted a moored drilling vessel, the Ocean Valiant owned and operated by Diamond Offshore Drilling, Inc. (DODI), to drill wells in the East Breaks area of the Gulf of Mexico where water depths range up to 4,925 feet. This Contract included upgrading the existing 4,000 feet rated mooring system to 5,000 feet as follows:Extending the existing 3 1/4 inch chain length from 3,700 feet to 4,550 feet.Replacing the existing 30,000 lbs. anchors with 22,000 lbs. high holding power anchors.Replacing existing 3 1/2 inch mooring wire with new 3 1/2 inch, galvanized, higher strength, mooring wire.

In addition, Exxon contracted an anchor handling vessel (AHV), Ensco Kodiak I, from Ensco Marine to run the moorings along with its sister AHV, the Ensco Kodiak II. This Contract included an upgrade of the AHV winch from 250 MT line pull to 450 MT line pull. The winch upgrade will occur when the AHV schedule permits.

Design and implementation of the mooring procedures were a joint effort that combined the knowledge of DODI, Exxon Company, International (ECI) Drilling Organization, Delmar Offshore anchor handling & mooring systems, Ensco Marine, and Exxon Production Research Company (EPR).

ECI engineers and operations supervisors assisted DODI marine personnel during actual mooring operations. EPR provided technical assistance to ECI and DODI during the mooring analysis as well as field supervision during the mooring equipment inspections. The wells were being drilled for Exxon Exploration Company (EEC).

MOORING DESIGN STANDARD

The ECI design standard, Mooring Systems for Mobile Offshore Units, is based on references 2 through 8 listed at the end of the paper and the mooring experience of Exxon personnel.

The objectives of the recommended practices in the ECI standard are to help ensure that the mooring equipment is:

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