This presentation, delivered at the Technology in the Subsea Energy Industry Seminar by Wilde’s Dr. Simon Leefe , Technical Director, and Chris Williamson , Director of Product Development at BEL Valves, demonstrated the closure dynamics of large bore high pressure deepwater gate valves using Computational Fluid Dynamics (CFD).
Subsea Safety Valves
High Integrity Pipeline Protection System (HIPPS) and Subsea Isolation Valves (SSIV) are quick closing valves used for emergency shutdown. The valves trip once the pipeline pressure exceeds threshold value and therefore protects the pipeline from a possible rupture. The shut down time of the valves are <2 seconds topside and between 10 and 15 Seconds Subsea.
Valve Close Criteria
The criteria for the valves are:
- Valve must close
- Valve must close in time required
- Valve leakage shall be minimised
Consequence of Failure
The consequences of the valve failing could cause:
- Injury to personnel
- A rupture in the pipeline
- Damage to the downstream equipment
Practicalities of Full Scale Testing
Full scale dynamic testing is not practical to test in a flow loop because of the flow rate involved. Factory Acceptance Testing (FAT) can only take place on static conditions, so CFD was used to verify closure under dynamic flowing conditions.
Hydraulic System
Pre-Pinch: Coupled System
Coupled Fluid System, simplified as series head losses:
Post-Pinch: Different Model
The flow into the cavity is driven by the rate of loss of stem volume from the cavity. The pressure drop from P1 to Pcav depends on the flow rate and seat gap geometry. Therefore the cavity pressure, Pcav (hence stem ejection form) depends on the gate velocity.
Other Assumptions
Single phase flow calculations (gas and liquid)
- Gas flow assumptions;
- Weakly compressible (low Mach number)
- Isothermal
- Ideal gas
- Instantaneous pressures and forces based on steady flow at the current gate position
- Current calculations assume negligible gate inertia
- For the series of gate locations, calculate the equilibrium velocity which makes hydraulic dump resistance force balance stem ejection, spring and friction forces.
Gate Friction
The contact force on downstream seat (for gate friction) is the differential valve pressure over ‘exposed’ gate area (depending on travel) and the unbalanced pressure under upstream seat, over area of seat gap (depending on travel).
Pre-Pinch Model: Use of CFD
Approach
- Dimensionless pressure and stem force coefficients using CFD
Rationale:
- Avoids the need for complicated moving boundary transient CFD
- Avoids problem of topology change at pinch
- Can infer pressures and forces for arbitrary flow rate and density
Method:
- Impose flow rate by calculating pressures and forces; non-dimensionalise
- Obtain the curve fit across the range and extrapolate up to the pinch point
- Use coefficients and modelled forces in the calculation schemes
- Calculate the pressure over the gate assembly faces (CFD)
- Calculate area integral in vertical and horizontal directions, stem ejection force and gate lateral force (CFD)
- Extract force coefficients as a function of gate travel
- Curve fit across the range and extrapolate to the pinch point
Selected Results
- Based on data for all-gas flow conditions,
- Well flow pressure, temperature and viscosity, etc
- No attempt to model nature or location of downstream trigger event
- Pressure downstream of valve assumed constant at trigger value
- Seawater pressure assumed 60 bar
- Upstream resistance assumed due to choke
- Choke loss coefficient estimated from flow data
- Results quoted for hydraulic pressure dump without control system
- Pressure upstream of choke rises from well flow to shut-in pressure
- Rise assumed linear with gate displacement
- Can specify upstream and downstream as any function of time and/or gate displacement
Detailed Results – No Dump Control
Extraction of closure time
- Time taken to reach given gate displacement, y:
- We have results in the form of y’(gate velocity) vs y (gate displacement)
- We can evaluate numerically the integral above to obtain time taken:
- To pinch
- To full closure
- To any intermediate point
Conclusion
System analysis and design exploration tool uses CFD results but not CFD simulation as the results are quicker to use and more versatile. CFD provides an insight into the detail of pressures and forces over the closure process providing confidence in the performance when full scale test are not feasible. The simulation-based design work is on-going providing details of alternative hydraulic systems and different operational scenarios.
Presentation delivered at the “Technology in the Subsea Energy Industry Seminar” on Monday 29th March 2010 at Newcastle University Business School