Advances in Landing String Technology for Completion and Intervention Operations in Deep Water
Authors: More D.J. 
Published: October 2008

Landing string technology has evolved to prove a more cost effective solution to the demands of subsea completion and intervention operations. However, since the communication distance increases with water depth, simple direct hydraulic control of mudline equipment becomes less feasible because of both the size and cost of the multi-core umbilicals required, and the resultant increased response times for safety, or environmentally, critical operations. This paper will therefore compare and contrast a number of field-proven control system concepts and present the latest developments to address the challenges of deep (&gt;2000') and ultra-deep (&gt;5000') water environments.

Introduction

Performing well completion and intervention operations in deep and ultra-deep water depths provides a number of technical challenges. Not least among these is the costeffective design and provision of control systems for subsea equipment. The need for built-in reliability must be a key feature of any such design, especially in deeper water depths, because of the high costs associated with equipment recovery and replacement. However, the need to limit cost, in comparison to conventional equipment, is also critical since hydrocarbons from deep water prospects carry no intrinsic price premium. Subsea technology has evolved, but equipment design can still be classed as immature, in the sense that considerable scope remains for step-changes in cost effectiveness.

There are of course, many ways in which to approach the problems posed by the need for seabed control in deep water. Speed of response, functionality, equipment costs, weight and size, installation costs, and reliability can all be expected to impact the choice made for each individual project. Therefore no one concept can be expected to provide the solution for every project. Nevertheless, the ability to integrate systems controlling equipment from several production tree and completion vendors and the possibility to mix and match modular components can provide cost reduction in almost all instances.

In many instances, the completion installation programme involves the flowing of the well via the landing string. Thus, in deep water, the string must incorporate a primary well control barrier and an emergency disconnection function. These functions must operate in a swift manner, particularly when operations are being conducted from a Dynamically Positioned (DP) vessel, since the speed of response will determine the safe watch circle that the vessel must operate within. A smaller watch circle would reduce the weather window for operations and potentially increase project costs due to vessel downtime. Reliable operation of such functions is paramount to the potential financial and human costs should the system fail to operate in an emergency.

In addition, the failure of the control system would result in deviation from the installation programme necessitating a time-consuming and costly recovery to surface, repair or replacement, and subsequent re-deployment of the landing string. The provision of a secondary in-built control system must therefore be considered to minimize total project costs.