COMPUTATIONAL FLUID DYNAMICS OF SUBSEA CHOKE VALVE FAILURE
The downstream side of a subsea choke valve was found to have failed, reportedly due to sand entrainment directed against the downstream wall of the valve. This induced a severe wear mechanism reducing the wall thickness to a point failure (wall rupture). EXGROUP Engineering (EXE) was requested by the client to conduct a Computational Fluid Dynamics analysis to run various multi-phase flow cases and scenarios in order to establish the failure mechanism.
EXE approached the problem by constructing a 3D computer model representative of the choke valve and conducted flow simulations to obtain velocity profiles and pressure profiles at the given process conditions while varying scenarios. A sand erosion study was conducted through engineering calculation as well as through particle-study feature inbuilt the Computational Fluid Dynamics package to investigate the extent of sand erosion damage on various valve components at various locations.
Upon extensive modeling and investigation, EXE managed to produce a precise model to determine the failure scenario. The sand erosion damage was initially inflicted at the upstream side of the cage and exposed more port holes. The damage was then further inflicted on the upstream side of the plug, which introduced a flow path directing towards the downstream wall at velocities as high as 25 m/s at impact (refer to Figure 4.4). The continuing exposure of this high-velocity erosive flow with sand entrainment on the valve downstream wall resulted in wall perforation and rupture failure.
The lack of consideration in sand management practice during the sizing and selection of the choke valve proved to be detrimental. The venturi reduction at the inlet of the valve had caused a jet impingement, which accelerates sand entrainment to collide on the valve trim at high velocity (Refer to Figure 4.2). There was no sacrificial liner or flow centraliser, which are commonly used to reduce erosion damage in choke valve design to protect from wall perforation. Based on the investigation findings, it was evident that the valve was fully exposed to sand entrainment damage.
On-site Machining services
EXE proposed a replacement sub-sea choke valve designed to suit to the sandy service condition. The proposed valve features a straight bore, which eliminates the issue of jet impingement on to the cage, experienced on the original valve. The proposed valve is designed to minimize the sand erosion damage by utilising Venturi Liner (Wear Sleeve) as well as the centralizing sacrificial plug nose design.
The Venturi liner is installed on the downstream of the trim, which the primary function is to protect the choke outlet body wall from erosion while also helping to centralise the flow through the outlet. The liner is constructed of 17/4PH stainless steel with full stellite overlay in the outlet trim / body flow area, providing an excellent resistance to erosion and wear. The sacrificial plug nose is Tungsten Carbide coated, and also helps redirect the fluid through the outlet of the choke helping to avoid abnormal flow impingement onto the side wall of the outlet.
Further computational fluid dynamics analysis was conducted to show the comparison between the original valve design to the proposed design. The results were conclusive that the fluid velocity has been reduced significantly up to 83.4%. The analysis concludes that the proposed design is an improvement to the original design, and deemed suitable for the sandy service conditions.
Benefit / Added Value
EXE successfully managed to identify the failure scenario and determine the root cause of the sand erosion problem experienced by the subsea choke valve. More importantly, EXE also provided a working solution that was fully supported by comprehensive engineering examination. This case study strongly demonstrates EXE high expertise in the field of valve engineering. In addition, ‘the downstream wall erosion vs. time’ chart, as shown in figure 4.5 was utilized by the client to predict the remaining life-time of existing identical subsea choke valves that are installed on the field. EXE was notified 6 months later that the simulation result was highly accurate, which enables the client to stop production and retrieve the valve at the best possible time.