Mooring an Ocean Victory Class MODU on a Pre-laid Mooring in 3,200 Feet of Water at Green Canyon 254
Authors: Ettle R.E. , Bamber P.J. , Dove P.G.S. , Wilson H. 
Published: October 2008

An Operator wanted to drill one or two additional delineation wells in 3,200 feet of water at Green Canyon 254. Because there were no MODUs capable of this water depth available to fit the Operator's schedule, the Operator elected to increase the water depth capability of an existing Ocean Victory Class MODU. The modification consisted of increasing the number of riser tensioners, installing longer BOP control hoses, guidelines and television cable, and using pre-laid moorings to enhance the mooring capability. This paper describes the pre-laid mooring procedures. Each prelaid mooring leg consisted of one 10 metric ton anchor, 1,000 feet of ground wire, 2,500 feet of dip-zone chain and 3,000 feet of catenary wire. Prior to the arrival of the MODU, each leg also had 3,500 feet of 2 inch riser wire and a surface buoy. The moorings were laid using a single 6,140 brake-horsepower, anchor-handling, towing, supply vessel with a large A-frame at the stern. This same vessel was used to connect the chain from the MODU to the pre-laid moorings.

Using pre-laid moorings to extend water depth capability gives the Operator greater flexibility in selecting MODU'S for deep water drilling and can solve scheduling problems. The use of an A-frame and a ground wire adjacent to the anchor allowed use of a smaller vessel than would otherwise be required. The MODU can be moved to nearby deepwater locations without fully recovering the pre-laid moorings.

This operation demonstrates that the water depth capability of an all chain MODU, normally capable of 2,000 feet of water, can lx. extended to over 3,000 feet by using wire inserts and pre-laid moorings.

Introduction

From March 2 to July 15, 1995, ENSERCH Exploration, Incorporated (EEI) drilled one discovery well at Green Canyon 254, using a dynamically positioned semi-submersible, the Ocean Alliance. Later, EEI wanted to drill delineation wells in this field by setting the center slot of a three well template over the original well. However, by this time, the Ocean Alliance had mobilized to the North Sea and was on a long term contract. There were no other MODUs capable of the 3,200 foot water depth available to meet the EEI's schedule.

EEI decided to modify an existing Ocean Victory class MODU to carry out the drilling program according to the desired schedule. The Diamond Offshore MODU, Ocean Voyager, was selected for this program. The modifications of the Ocean Voyager consisted of:additional riser tensioners so that the capacity increased from 640 kips to 960 kips,installing new BOP control hoses which were 4,000 feet in length,installing longer guidelines and higher-capacity, guideline, air hoists,installing a longer TV cable,increasing the mooring leg lengths by adding 4,000 feet of 3 1/2" wire rope to each mooring leg.

Using chain-wire combination mooring legs in this water depth required site specific mooring analyses. Because the MODU had only chain winches, special procedures for laying the moorings were necessary. EEI selected Aker Marine, Incorporated (AMI), as the contractor to plan, coordinate, and manage the installation of the pre-laid moorings.