Multi-Service Vessels for Deepwater Subsea Well Interventions
Authors: Scott R.A. Andy 
Published: October 2008

Abstract
As the Deepwater Gulf of Mexico development trend continues to accelerate with the use of subsea wells, the importance of reliable subsea well interventions becomes critical to the success of the field. Traditionally, deepwater drilling rigs have been used for intervention operations. However, the availability and flexibility of these drilling rigs can have a significant impact on the timing, costs and length of time to perform subsea intervention operations. The development of deepwater Multiple Service Vessels (MSV) now offers viable options.

Introduction:
Intervention philosophy for deepwater subsea developments crosses multi-disciplines from reservoir, completions, facilities to operations engineers. In every case, intervention becomes an integral part of the field plan to recover the projected reserves. Risk assessment of the access to the well, equipment design, reliability of downhole hardware, and intervention intervals, are critical to the overall field architecture layout and specific equipment design. Purpose built MSV's designed for deepwater subsea well intervention operations will be well suited for multiple tasks. These semi-submersible, dynamically positioned vessels are designed with large, open deck areas are particularity well suited for multiple activities. MSV's, such as the Q4000, offer an efficient and an effective deployment platform for subsea well interventions and subsea well plug and abandonment operations. With the use of a purpose built intervention riser, the Q4000 will be ideal for through tubing intervention operations using coiling-tubing, electric wireline or slickline. Properly equipped MSV's will able to perform wellhead conductor batch setting, well cleanrapid deployment of MSV's will be key to safe, successful deepwater subsea operations from start-up through field abandonment.

Results and Observations:
Deepwater subsea wells have always presented a multitude of challenges and difficulties. MSV's have proven to be safe, efficient and cost effective tools in deepwater applications. MSV's are able to deploy low cost intervention risers, which do not require the use of conventional BOP assemblies and thereby eliminating the need for a fully capable drilling rigs. Properly equipped MSV's should have heavy lift capacity cranes and derricks with large open area deck work areas, which allow for full and rapid deployment of all the necessary tools and equipment for multiple service contractors. Flexible response times and short mobilization periods will add to the MSV's operational effectiveness for deepwater applications. These efficiencies will be significant factors in the initial subsea well and field design. MSV's flexible capabilities allow for design enhancements that improve safety, lower capex development costs, increases the subsea well's up-time performance, and lowers workover frequency requirements and costs.
The Q4000, as a purpose built deepwater MSV for subsea well operations, construction and j-lay pipeline was designed with flexibility, adaptability, and reliability. Q4000 is designed and equipped for:General10,000' ft water depth rate4,000 deck load capacityAccommodations for 136 menLarge deck area (35,000+ SF)Float Under Docking SystemMaterial barge up to 250' × 72' moored under vesselTop DriveHydralift APS650E/AC600 MT Rating