The primary function of a subsea Christmas tree, is to control the flow of oil and gas or injection fluids to and from a subsea well. Subsea trees incorporate a number of valves in their construction for various functions.
A typical conventional or horizontal Christmas tree will include a production/injection master valve (PMV/IMV) and wing valves on both the production/injection wing of the tree (PWV/IWV) as well as the annulus wing of the tree (AWV).
The AWV is more common on wells which require the supply of gas to well ‘A’ annulus. On a subsea production tree, they are used to control the flow of oil/gas from the well. Equally, they are used for controlling the flow of injection fluid, if the subsea tree is an injection tree. All these valves are typically gate valves.
The subsea tree valves are actuated from closed to open position and back, using hydraulic linear actuators which are attached directly to the valves. The control of these actuators and supply of hydraulic fluid to them is done via a subsea control module (SCM), located on the tree. The SCM in turn is controlled by commands from the host installation.
Extensive subsea operation can result in leakage of hydraulic fluid from the valve actuators, causing their failure and inability to stroke the subsea tree valves. In such cases, an extensive completion work over or subsea intervention is required, to be able to recover the tree to the surface for actuator replacement. This type of operation requires a rig or vessel with the correct hardware and safety case. The consequential period of rectification will result in the well shut in and a loss of production revenue.
An established solution to a leaking/non-functioning actuator is to provide a surface override tool which would typically be installed by a diver, if water depths permit. The thrust power would be provided via a subsea hand pump. The major disadvantage is that the diver has to remain on location to provide hydraulic fluid power to the override tool.
The alternative is to leave the tool locked on to the actuator leaving, the valve permanently open. A valve locked open without the ability to close for Well Control purposes will require a deviation from the Operator's Standard Operating Policies and require dispensation from periodic Well Control Integrity testing. In most cases the regional Government acting body will be notified. For the UK sector this is the Department of Energy & Climate Change (DECC). In vast majority of cases it is impracticable to maintain this level of support.