Subsea Well Intervention Operations from a Dedicated Vessel - Cost and Performance Analysis
Authors: Johnston C.A. 
Published: October 2008

Abstract
The paper details a recent subsea well intervention operation that saw the accomplishment of several firsts for contractor, client and the Gulf of Mexico area itself.
The application of a multi purpose semi-submersible dynamically positioned (DP) vessel to live well intervention was a first for the operator in the Gulf of Mexico and also the first application for the vessel since its launch in April 2002. Previous vessel operations had covered construction support and well abandonment work in the Gulf and internationally.
The operational work scope included standard through tubing intervention techniques to overcome a problem associated with sand production and sub-surface safety valve failure.
The pre-planning of the operation saw significant work relating to proving the concept of live well intervention work from a non-drilling based DP vessel particularly with the requirement to flow test the well back to the vessel.
Dual activity operations were pursued with the removal and installation of the tree cap via crane, eliminating the need to run the tree cap on drill pipe. Similarly, unplanned operational requirements during well flowing saw the requirement to flow the well to the platform with the vessel and the intervention riser latched to the well.
The anticipated benefits of multi-purpose DP vessel based intervention operations were not fully maximized, however the success of the operation has provided a sound basis from which future well intervention operations can benefit fully in terms of overall cost reduction of subsea well intervention.

Multi-Purpose Vessel
The vessel used for conducting the well intervention operations is the Q4000 (Figure 1). Launched in April 2002 the vessel has conducted mainly subsea and platform support construction operations, however a single well abandonment of a temporarily abandoned well was conducted using an intervention riser system (IRS) and coiled tubing (CT) in 2002. The subsea intervention discussed in this paper is the first through tubing live well intervention involving hydrocarbon flow back conducted from the vessel since its launch.

General Equipment
The Q4000 supports operations with two kingpost cranes one of 160 tonnes and one of 360 tonnes capacity. The latter having capability to reach 3000m water depth on a single reeve (1500 m on double reeve) for deployment of equipment and loads to the seabed. Typically this is a feature of the subsea construction requirements however applications for supporting well intervention operations are also described.

Derrick Structure
In addition to the cranes the vessel possesses a 600 tonne capacity derrick. The center mounted cantilever box girder construction serves as the derrick on board and is a unique design consisting of a box steel structure housing and supporting the following items;Draw worksRigging and sheavesTravelling block and hookActive Heave Compensation systemsPassive Heave Compensation systemsControl and safety systems For clarification it should be noted that the particular design incorporates features that are not common with conventional drilling derricks on board mobile offshore drilling units (MODU).