Progression of Ultradeep Subsea Deployment Systems
Authors: Cermelli Christian , Morrison Denby , San Martin Hugo Corvalan , Guinn Mitch 
Published: October 2008

This paper describes various options to deploy subsea equipment in ultra-deepwater. The methodology used to study the feasibility and cost of the various options, and results of measurements conducted offshore to validate the numerical model are presented. Vessels including Mobile Offshore Drilling Units, Offshore Construction Vessels, and Anchor Handling Vessels are considered, and the influence of vessel, environment and deployment system characteristics are presented.

Introduction

The offshore industry is confronted with more complex ultradeepwater developments, and water depths extending to 10,000 feet. Deployment of trees and wellheads, as well as other subsea equipment has evolved from deployments using the Mobile Offshore Drilling Unit (MODU)'s rig, to winch deployments off the MODU to stand-alone deployments from service vessels. A number of subsea equipment deployment techniques have developed in order to reduce costs associated with expensive 5th generation drilling vessels.

This paper presents various vessel and rope system configuration options for deployment. It is shown that the demands of package weights and lowering depths, as well as the small heave tolerable at the mudline have led to the heavecompensated landing systems that are the topic of a companion paper.

This paper presents the methodology used and the results obtained for subsea equipment deployment and installation in the Gulf of Mexico at water depths up to 8,000ft.

The main packages considered in the analysis fordeployment are subsea trees and mudmats. These packages have very different characteristics, and it will be shown that this can have a large impact on their ability to be deployed. This study focuses primarily on deploying the equipment with rope overboard the vessel. The lifting and deployment was considered with either steel wire or synthetic fiber rope. Some cases were run with deployment through the moonpool of a MODU with a drillpipe. This method ensures good uptime as heave motions are kept to a minimum on the very stable rig platform, while package motions are not amplified dynamically due to the high stiffness of the drill pipe. This is a well-established method for deployment, but the MODU must interrupt other drilling activities during deployment. This results in very large cost in ultra-deepwater as the time to deploy increases, and the rig capability and cost is also higher than in shallower waters.

Three of the most critical phases during the installation of subsea equipment in deep water are:Overboarding procedure;Lowering equipment through splash zone (sea surface);Connection / landing of equipment on the sea floor. This work focuses on the third one. The landing of equipment on the sea floor, and the deployment operation immediately preceding the landing, in which the equipment is hanging close to the seabed and positioned above its target location are usually subject to very tight heave tolerances. During the other critical stages, the overboarding procedure and lowering through the splash zone, the package is exposed to large transient loads as it reaches the water line. It is extremely difficult to predict the magnitude of these transient loads.