HMPE Mooring Line Trial for Scarabeo III
Authors: Corbetta I. , Sloan F. 
Published: October 2008

This paper describes the deployment of a high-modulus polyethylene (HMPE) mooring insert line on the Scarabeo III MODU. Several new technologies are now available that can increase the maximum rated water depth of 2nd and 3rd generation drilling rigs. The purpose of this trial was to qualify one such technology, the use of lightweight synthetic fiber rope mooring lines. The desire to deploy and recover the lines on existing boats and equipment led to the selection of HMPE as the trial rope material. HMPE ropes are almost neutrally buoyant, have high abrasion resistance and exhibit a strength-to-diameter ratio similar to that of steel wire rope. The HMPE line used in the present study, a field-repairable Plasma® 12×12-strand braided rope, served without incident for 3 years. After damage was discovered at one of the rope terminations, the rope was returned to the manufacturer for retermination and residual strength determination. The rope was also inspected and any damaged strands repaired prior to being returned to service. The study demonstrated the durability and ease of handling of these lines, in particular the 12×12-strand braid, for MODU mooring applications. The scope of the paper includes static mooring analysis, deployment procedures, mooring load data and residual strength data. Recommendations are made regarding deployment methods for synthetic ropes.

Introduction

There is a growing desire on the part of operators to utilize 2nd and 3rd generation MODUs in deeper water, to take advantage of their wider availability and lower day-rates. Generally speaking, increasing the water-depth capability of an existing rig requires extensive upgrades to several operational areas, including the draw works, mud system, and buoyancy. These upgrades typically require 6-12 months in drydock and 10's of millions of dollars in capex, particularly for the addition of high-buoyancy sponsons.

Because of the time and cost involved in a traditional drydock upgrade, many drillers and rig owners are exploring other options. Emerging technologies that can extend depth capability include new mud systems, artificial seabeds, and synthetic mooring lines. Many of these upgrades can be used immediately on existing rigs with minimal or no drydocking and relatively low downtime costs.

One such simple upgrade technology is the use of lightweight high-performance-fiber mooring lines as a replacement for much heavier steel wire rope. The use of synthetic fiber lines can greatly reduce the vertical loads on the rig, enable the use of existing winches in greater water depths, and/or increase the amount of available variable deck load. This is particularly important when drilling with older rigs in locations far from an established offshore logistical infrastructure. Environmentally induced offset can also be reduced, in part because of the higher fairlead angles possible with synthetics.1,2

As with many other proposed new drilling technologies, actual field experience specific to MODUs is limited. The principal goals of the present study were to gain experience with one type of high-performance fiber rope, high-modulus polyethylene (HMPE). Specifically the study set out to identify handling issues with these types of lines, to verify their potential for weight savings, and to gage their ability to withstand typical operational environments.