There are many definitions of the choke line which can be found in the literature. It can be defined as '' A high-pressure line connected below a BOP to transmit well fluid flow to the choke manifold during well control operations'' (API SPEC 16D). Also it can be defined as ''A high-pressure pipe leading from an outlet on the BOP stack to the back pressure choke and associated manifold'' (from Slb.com).
Many well control problems can occur at the choke line or downstream the choke line. These problems can be caused mainly by the nature of the fluid which is considered as solid-laden fluid and it is considered as very abrasive fluid.
A typical choke line configuration is presented in Fig-1. The configuration is composed of two valves flanged to the drilling spool. One of these two valves is hydraulically operated when the other one is manual and it is considered as backup. The position of the hydraulic valve should be outboard the manual valve which is used only in the case when the main valve fails to operate. Experience has shown that installing the hydraulic valve next to the drilling spool can cause valve plugging by the drilling solids or barite of the drilling fluids due to the short interval between the outlet of the spool and valve. As a good practice, it is recommended to check and flush regularly the choke line to avoid any plugging during operations and get the system ready when a well control situation will be faced.
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| Fig 1- Choke line configuration |
It is not uncommon to fit the well control system with second choke line when drilling underbalance. Drilling with a mixture of drilling fluids and gas can lead to equipment wear and serious problems can be faced. The second choke line should be configured and reliable as the main choke line and meet the same standards in dimension and pressure.
The choke line from the spool to the manifold should be flanged, straight and with minimum diameter of 4 inches. The bends and curves can erode and wear easily and this can complicate the situation when controlling the well. If turns are inevitable when installing the choke lines, the use of T's and targets will be necessary and they should be deep enough to resist to any force of erosion. Another condition targets is installing them in the direction of the flow.
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| Fig 2- Targets one choke line |
The use of hoses nowadays becomes more popular. They are designed to handle the high pressures and flow rates. They are composed of many layers of rubber and stainless steel. They should be compatible with the whole well control equipment. Many considerations have to be taken when designing the flexible choke line as pressure rating, collapse rating, temperature rating, and resistance to fluid abrasion.
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| Fig 3 - Coflex Choke line (Photo from Huiyahose website) |
References
Robert D.Grace, 2003. Blow Out and Well Control Handbook. Gulf Professional Publishing.
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