Subsea Well Intervention
SubSea Well Intervention is a specialised intervention area although the principles of operation are identical to a platform or onshore based intervention service.
The main differences are as follows:
- Requirement for specialised intervention mono hull vessels OR
- Utilising existing high cost semi submersible rigs or jack ups
- Specific intervention equipment including risers / emergency disconnect packages / surface trees / coiled tubing lift frames , deployment lubricator and circulation lines, HP electro / hydraulic control systems
Other factors particularly environmental make Subsea operations technically challenging, these include - weather & wave conditions, sea currents, heave, pitch and roll characteristics of the intervention vessel. Access to Subsea wells may also be difficult due to sea bed architecture, Subsea flowlines, umbilicals and manifold layouts.
Applications
Subsea well interventions covering all the techniques/services used elsewhere in the oilfield can be applied namely: Coil Tubing, Wireline, and Fracturing & Stimulation.
Equipment (Semi submersible/ jack up - riser to surface)
Equipment levels depend on the type and complexity of the Subsea intervention being planned. However this could include the following:
- Pressure Retaining Riser (either single or dual bore) of sufficient length to run from sea bed to surface
- Emergency Disconnect & Lower Riser Package (EDP/LRP) - this provides well isolation and disconnect facilities in the event of an emergency onboard the vessel or within the wellbore
- Surface test tree / wear joint - this provides surface isolation for tool string change-outs and bleed down facilities
- High pressure hydraulic control unit (HPU) and umbilical hose reel - provides remote hydraulic functions to valves and actuators
- High pressure flexible flowlines - connect to the surface tree for pressure testing and bleed down operations
- Surface skid mounted bleed down package - base unit features separator , choke manifold , stock tank and transfer pump
- Intervention equipment for service ie Coil Tubing equipment, wireline equipment etc.
Equipment (Mono hull Light well Intervention Vessel (LWI) - Subsea lubricator (SIL) system
For mono hull or light well intervention service the pressure retaining riser to surface is not required due to the use of a Subsea lubricator system which is deployed to the Subsea xmas tree and the service is performed with the 'wireline through water'.
- Subsea lubricator (SIL) System c/w EDP/BOP package
- High pressure hydraulic control unit (HPU) and umbilical hose reel - provides remote hydraulic functions to valves and actuators
- Intervention equipment for service ie Wireline winch unit / power pack etc
Whilst still in the early stages of development, coiled tubing intervention would use a slightly different method of application. Several companies are developing such technology to enable 'coil through water' applications.