Preventing premature chemical dosing valve failure in subsea tiebacks

Paul Boughton

Simon Tattersall examines how specialist valve trim technology prolongs valve life and helps to avoid costly unplanned outages in severe service environments.

In the designing, operating and maintaining of a subsea tieback, an operator has multiple challenges to consider. The way they choose to overcome these challenges will determine the system's reliability and operability, as well as its ongoing profitability. One of the principal challenges in subsea systems is enabling flow assurance through effective hydrate management. To miminise the risks posed by hydrate formation, a continuous MEG injection will be required.

Earlier in 2012, Koso Kent Introl (KKI) were approached by a major oil operator, who required specialist assistance to overcome a hydrate management problem in an existing chemical dosing system within a subsea tieback.

Several manifold MEG dosing valves had failed prematurely within the chemical dosing manifold, and the operator was seeking an ongoing solution that would eliminate the risk of this occurring elsewhere in the system in the future.[Page Break]

KKI engineers were briefed about the problem: prior to the tieback being implemented, pressures in the pipeline had been relatively low (60 barg inlet pressure, with only a modest drop to 30 barg outlet pressure). The dosing valves had therefore been able to cope with the flow. With the introduction of a new, higher pressure well tieback to the field later in its life, inlet pressure had subsequently increased to 350barg. The resultant high velocities and energy levels arising from this greatly increased pressure drop had caused failures in the existing single stage valves well before the end of the commissioning period. Sand erosion was also a major problem, known to be contributing to the valves' poor performance. The failure of the valves was causing costly, unacceptable interruptions to production in the field.

The valve the KKI subsea engineering team proposed incorporated a multi-spline trim, with the flow sleeve and plug constructed as an insertable cartridge, allowing multiple stages of pressure drop to ensure that cavitation would not occur throughout the known flow range. The multi-spline trim option would be able to achieve the required rangeability of 200 to 1, due to the very precise fit of the plug within the seat.[Page Break]

The other major consideration would be material selection. A range of scenarios were modelled, considering the effects of pressure drop for different materials at every stage of let-down, and determining long-term erosion rates for each

Guided by the KKI team, the operator selected the Solid tungsten carbide option, and valve production is currently underway at Kent Introl's facility in Brighouse, Yorkshire.

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Simon Tattersall is with Koso Kent Introl Ltd, Brighouse, West Yorkshire, UK.

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