The designer must ensure that there is critical flow through the choke in order to eliminate the effects of downstream pressure variations on the formation. This is achieved when the FTHP is approximately 1.7 times the downstream flowline pressure.
Actuated chokes
By its very nature the choke is subjected to very high pressure drops, which can lead to mechanical problems. These are dealt with briefly below and are drawn from experience with chokes in several Opco's world wide.
Start up and shut down
With zero pressure in the flowline and maximum CITHP the choke is subjected to the maximum pressure drop it will experience. It is essential that during this phase of production the choke is capable of moving off seat. During the latter part of the lifespan of the well, this pressure difference will gradually reduce as the field depletes.
When selecting a choke the tendering vendors should be able to prove by calculation or demonstration that the offered choke and actuator combinations will perform at the extremes of the operating envelope and not 'freeze' in one position. Experience has shown that with multiple orifice chokes this problem may be overcome by fitting "concave" front and back discs. These discs are machined to give a smaller area of contact between the fixed and moving discs, thus reducing the effort needed to overcome the effect of the pressure difference. Several designs of chokes do offer a positive shut off, however, for isolation purposes:-
Under no circumstances is the choke to be regarded as a positive shut off device.
In cases of chokes with a rotary action, a high pressure differential will cause wear and at times failure due to seizure of any thrust bearings fitted. The construction of the thrust device should be examined to determine if failure would cause production down time or migration of hydrocarbons to the atmosphere.
In the selection of seals and seal material the normal guidelines should be followed to ensure that all elastomeric seals are compatible with the fluids produced. Consideration should be given to metal to metal seals between the choke 'bonnet' and body.
Where the choke has a linear movement the effect of the differential pressure should be determined: the direction in which it acts; its effect on the valve. High differential pressure may cause control valve type chokes to "bind" or "freeze" in one position, although pressure balance ports should resolve this problem.
There are three common types of control choke trims, the Plug and Cage, Internal Sleeve and the External Sleeve Trims.
The selection of the actuator for the intended service is critical. It should be able to open and close the choke under all anticipated operating conditions. When signalled to close by either an ESD/OSD alarm or normal control the choke should be able to close smoothly and quickly. Normally the choke is first to close on a sequenced closure of all the well valves. This prevents undue erosion of the other valves in the system.
Normal operations
The choke should operate smoothly under control of the actuator. The control system should not "hunt" or generate any random choke movements. The choke should still be able to move smoothly if held in a fixed position for long periods.
Due to the nature of the choke, very high turbulence is generated directly downstream of the device. This problem must be considered at the design stage, otherwise erosion of the downstream flowline may occur in a relatively short time span. A standard arrangement is to fit a hardened pup piece downstream of the choke.
One advantage of the control choke is that the turbulence it generates can be directed into itself, which limits its downstream effect. The major turbulence is in the centre of the fluid flow and not impinging on the internal surface of the choke. In this case the need for hardened pup pieces or wear sleeves may be eliminated.
Positive (fixed bean) chokes
The significant features are:
- the bleed screw assembly does not allow removal of the blanking plug prior to depressurisation;
- if erosion of the blanking plug threads or seat occurs, the screw assembly may be replaced;
- the metal to metal blanking plug seat has an elastomeric back-up seal;
- the blanking plug is tightened using a spanner as opposed to a hammer union.
All new designs of positive chokes should exhibit the above features. In addition there should be competent persons trained to examine and gauge the internals of the choke. The threads and sealing surfaces form primary barriers against loss of pressure containment, internal corrosion and wear.
Maintenance
At some stage in the life span of the well maintenance will be required on the choke. Therefore when the choke is selected initially a check should be made on the space needed to fit special tools for internal maintenance. Removal tools for choke internals can be heavy and quite long (depending upon the size of choke), therefore adequate access space is important. Prolonged maintenance caused by incorrect positioning of chokes or space constraints increases well downtime and operating costs.
For safety considerations there should be adequate valving to ensure that chokes can be isolated and depressurised.