Kent Introl

June 2017

Advancements in Subsea Choke Valve Trim Technology

Subsea choke valves are generally installed on Subsea Christmas trees or in Subsea Flow Manifolds. The subsea choke is the primary pressure and flow regulator on the subsea installation.

On particularly onerous wells, the flowing parameters can be quite complex due to the different mixture of hydrocarbon gases and liquids. In addition to the flowing medium, there is also the added technical challenges of handling the very high pressure drops that in turn lead to very high velocities and then with the added difficulties of entrained solid contaminates in the flow (sand).

Velocity Control Trim Technology

It is quite common for subsea choke manufacturers to select a single stage pressure reduction trim solution to try and handle some of the most severe flow applications. These solutions, in most applications, will incorporate a solid tungsten carbide trim to try and avoid erosion. If the application is quite benign then the single stage solution, in most instances, should perform satisfactorily. However on really high velocity applications, erosion can happen very quickly, if the velocity control methodology is not followed.

The VECTORTM “Velocity Control” trim technology has been used successfully for a number of years on topside applications. The well fluid is guided through a series of passages at each level on the VECTORTM trim. The well fluid then passes through the three dimensional flow-path. Each of the flow path areas expand after every turn, allowing for the expansion of the gas phases and thus controlling the velocity at each 90 degrees turn.

On liquid applications the “Velocity Control” principle can ensure that the pressure profile through the choke does not go below the vapour pressure of the medium. This will prevent the onset and harmful effect of cavitation. In turn the velocity is controlled so it does not go above the threshold for vibration, erosion and noise.

Non-Collapse Subsea Choke Valve Trim Design

Most subsea choke trim designs would have the main control / throttling components manufactured from solid tungsten carbide. Certain grades of tungsten carbide have excellent characteristics to prevent erosion; however tungsten carbide can be quite brittle. So if it is not suitably protected the subsea choke, complete with the unprotected tungsten carbide trim, could experience a catastrophic failure.

The introduction of the “Brick-Stopper”, or composite guide, provides three basic functions. (01) It is primarily there to hold the whole trim component in a cartridge form, to assist subsea insert retrieval. (02) As the tungsten carbide is shrunk fit into the more ductile “Brick-Stopper”, the tungsten carbide is forced into a state of compression. When tungsten carbide is placed in tension, its mechanical performance is reduced quite significantly. However when it is in a state of compression, the mechanical performance is enhanced. (03) The “Brick-Stopper” prevents direct solid impacts onto the tungsten carbide components, preventing catastrophic failure of the trim.

Over the last 20 years Kent Introl has conducted various choke trim impact tests to ensure that the tungsten carbide choke trim components would not suffer catastrophic failure when hit by solid contaminates in the flowing medium. The Kent Introl standard “Brick-Stopper” has been successfully tested up to an impact value of 100 Joules. In addition to the standard Tungsten Carbide trim design. Kent Introl has supplied trims that have been tested to 1320 Joules, these trims generally meet the onerous requirements of NORSOK P100.