The diverter is installed on top of the wellhead to enable flow from shallow formations to be diverted away from the work area in case of a shallow gas kick. However, current diverter equipment is not yet designed to withstand an erosive shallow gas kick for a prolonged period. The diverter system is still seen as a means of "buying time" to evacuate the drilling site.
Dirverters are not used for land operations, unless there is risk of shallow gas.
For offshore drilling operations diverters are used when drilling 17.1/2in and 16in hole, Any hydrocarbon returns shall be directed away from the rig via a dedicated line, configured to have the minimum elbows, bends and tees that are practically possible.
In principle, a diverter system must be installed on each well when both of the following conditions apply:
- There is a possibility of loosing primary well control which may result in a kick situation.
- The well cannot be closed-in with a BOP stack, because the formation below the stove pipe/marine conductor, conductor string, or surface string is too weak. Fracturing of the formation will occur if the well is closed-in.
1 Diverter equipment specifications
Flow restrictions in diverter systems should be avoided where possible, because they may lead to formation breakdown and cratering of the well in case of a shallow gas blowout.
The minimum required nominal ID of diverter outlets/lines is considered to be 304.8 mm (12").
In principle two outlets are required on the diverter spool. They should face opposite directions to be able to vent flow downwind of the rig. However, one outlet only may be considered, in case there is a prevailing wind direction and the vent line extends a sufficient distance from the rig to permit safe venting. Rigs which can 'weather vane' (i.e. dynamically positioned or turret moored rigs) can have just one diverter line. Diverter lines should be as short as possible, but long enough to conduct flow past the extremity of the offshore drilling structure, or away from any obstacle in land operations. Rig structure and/or cellar design may have to be modified to accommodate straight diverter lines.
The minimum rated working pressure of diverter equipment is based on the anticipated backpressure during a shallow gas blowout and therefore largely depends on the size of the diverter lines. The minimum rated working pressure of the recommended large bore diverter line system should be 3450 kPa (500 psi) WP. One must remember that dynamic forces are much higher in the initial stage of diverting a well, when the expanding gas is forcing the mud out of the diverter system.
The following considerations should be made when selecting diverter equipment:
- The equipment shall be selected to withstand the maximum anticipated surface pressures.
- Welded flange or hub connections are mandatory on diverter systems; quick connections in diverter lines are not allowed. Diverter lines should be straight, properly anchored (especially at the end of the lines) and sloping down to avoid blockage of the lines with cuttings, etc.
- Installation requirements for wellhead and BOP equipment also apply to diverter equipment.
- A diverter system can be a BOP stack system with diverter spool, or a specifically designed and developed diverter system, although the faster closing diverter unit is preferred above a large and slowly closing annular preventer. In any case, the diverter and mud return lines should be separate lines, not partial integrated lines, to avoid gas entering the rig system in case the separating valve between both lines fails to operate properly.
- Diverter valves shall be full opening valves with an actuator (pneumatic or hydraulic). The bore of the diverter valves shall be equal to the bore of the diverter lines.
- Each diverter system should incorporate a kill line facility (including a check valve) to be able to pressure- and function test the system and to be able to pump water through the diverter system.
- The diverter control system should preferably be self-contained or may be an integral part of the BOP accumulator unit and control system. It shall be located in a safe area away from the drilling floor and have the control functions clearly identified.
- When a surface diverter system and a subsea BOP stack are employed, two separate control/accumulator systems are required. This will allow the BOPs to be operated and the riser disconnected in case the diverter control system gets damaged and looses pressure. The diverter control system should be capable of operating the diverter system from two or more locations, one to be located near the driller's position.
- It should contain the minimum of functions. Preferably, a one-button or lever-activated function should operate the entire diverter system.
- A 1 1/2" hydraulic operating line should be used for diverter systems with a 1 1/2" NPT closing chamber port side. The hydraulic line for the opening chamber port may be 1".
- All spare operating lines of the control system and connections which are not used should be properly plugged off.
- Control systems of diverters/annular preventers and BOPs should be capable of closing the diverter and annular preventers smaller than 508 mm (20") within 30 seconds, and annular preventers of 508 mm (20") or larger within 45 seconds. Diverter valves should be opened before the diverter element is completely closed.
- It should be possible to control pumping operations at the pumps as well as on the drilling floor.
- Telescopic joints should incorporate double seals, to improve the sealing capability when gas has to be circulated out of the marine riser.
- All fans should be stopped automatically in case of a gas alarm, including the fans inside the accommodation.
- At least one windsock should be installed at any drilling location prior to spudding a well.