There are many definitions of choke manifold. It can be defined as '' An arrangement of piping and special valves, called chokes. In drilling, mud is circulated through a choke manifold when the blowout preventer is closed; a choke manifold is also used to control the pressures encountered during a kick. In well testing, a choke manifold attached to the wellhead allows flow and pressure control for test components downstream. (Source: IADC website)
Also it can be defined as '' manifold used to lower the pressure from the wellhead. It consist of a set of high pressure valves and at least two chokes. These chokes can be fixed or adjustable or a mix of both. The redundancy is needed so that if one choke has to be taken out of service, the flow can be directed through another one. By lowering pressure the retrieved gases can be flared off on site'' (Source: Wikipedia)
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Fig 1 -Choke manifold at rig site |
Many configuration of the choke manifold can be arranged. The minimum requirement for choke manifold is illustrated in the Fig-2. It is mentioned that it is composed of outboard valves and inboard valves and also it consists of flanges, hydraulic and manual chokes and pressure gauges.
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Gate valves are used in manifolds due to their high resistance to wear and sufficient seal. They are designed to be fully opened or closed, they are not designed to restrict flow. Restricting flow by the gate valves can damage the valve body due abrasive nature of the fluid which is considered as solid-laden fluid. The damage can be occur on the valves downstream the choke due to the high level of velocity when the fluid pass through the choke which can increase the chances for the erosion.
As a general standard, the valves downstream the choke have the half pressure rating of the valves upstream the choke. For instance, if the inboard valves are 10000 psi working pressure, the downstream valves can be 5000 psi working pressure.
The redundancy in the choke manifold is more than necessary when the complex and difficult situation are predicted to be faced during operations. In the figure Fig-2, an example of a choke manifold consists of two hydraulic chokes and two manual chokes. It can be noticed that on this manifold, any component can be isolated and replaced, the operation can be carried out and continued on the other side.
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Fig 3 - 10K psi choke manifold |
The drilling chokes which are used in the drilling manifolds are categorized in two main types according to the system used to achieve choking. In order to achieve choking in the first category, two surfaces with openings are rotated against each other to allow or restrict the flow. Swaco Super Choke falls in this category, it is composed of two tungsten carbide plates with half-moon openings. One plate is fix while the other one can be rotated hydraulically. In the second category, the choking is achieved by inserting a tungsten carbide bean into tungsten carbide sleeve. The Cameron Drilling choke falls in this category.
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Fig 4 - Swaco Super Choke (Source: ALMOGI webiste) |
There are some recommendations for the installation of the choke manifold to perform the operations safely:
- The upstream valves and the chokes should at least have the same working pressure as the working pressure of the ram BOP's in use.
- It has to be installed in accessible zone and it will be better to set it outside the rig substructure.
- For 5000 psi manifold and higher, it is recommended to put two valves between the BOP and the choke manifold.
- One of the drilling chokes should be hydraulically operated.
- The manifold should be protected from freezing by heating or using appropriate fluid when low temperature is expected.
- Choke panel or choke console should deliver all the data needed to control the well efficiently such as the drill pipe pressure, casing pressure, pump stokes etc.
- The air used for operating the choke should be steady in pressure and volume. Rig air system should be checked to deliver the required parameters. The choke console should be equipped with an emergency backup system such as manual pump.
References
Robert D.Grace, 2003. Blow Out and Well Control Handbook. Gulf Professional Publishing.
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