HISC ASSESSMENT TO PREVENT BRITTLE FRACTURE OF VALVE

SUMMARY

PDL were contracted by a valve manufacturer to assess their Duplex steel, 15 ksi subsea gate valve using Finite Element Analysis (FEA). The material and operational environment of the valve raised concerns of Hydrogen Induced Stress Cracking (HISC), as this would be subjected to cathodic protection when installed subsea. The assessment was carried out in accordance with the Category 2 criteria of DNVGL-RP-F112.

All components except for the body were found to pass the HISC assessment. Failures were identified in the body gasket groove, exhibiting high levels of strain. We discussed with the client potential design modifications to address these areas, and methods to reduce conservatism in the analysis. Quick identification of failures meant that design issues were discovered before manufacturing had begun, reducing costs and wastage for the client. We also recommended that an additional assessment to ASME BPVC Sec. VIII Div. 2 be performed to fully validate the valve, due to very high stress levels in one region.

SITUATION

Our client was a valve manufacturer, who had produced a gate valve for use subsea in the oil and gas industry. Duplex stainless steel was used for the valve components as this has advantages with regard to corrosion resistance, ease of manufacture, and strength. However, Duplex stainless steel is susceptible to Hydrogen Induced Stress Cracking (HISC), with design guideline DNV-RP-F112 established for the offshore industry. Our client needed to show compliance to this standard for the end customer to have confidence in their product.

CHALLENGE

Subsea metallic components that are exposed or have damaged polymetric coating will experience electrochemical reactions due to cathodic protection. Hydrogen is one of the products, which can penetrate hardened steel and diffuse into the bulk of the material. This can potentially lead to brittle fracture in materials that are normally ductile. High tensile stress is a main contributor to HISC and valves operating in extreme environments are particularly susceptible due to the high pressures and thermal gradients present. Since it is impractical to examine components without bringing them out of service HISC assessments are an important screening procedure prior to installation.

SOLUTION

We received a 3D CAD model of the valve from the client and prepared this for the analysis. On inspection of the geometry and loading, it was identified that a half-symmetry model could be used. This reduced solving time, whilst maintaining accuracy. Components of interest were the Valve Body, Bonnet, Pusher and Housing. The flange connection, bolts, and seal ring were included in the model to create an overall representative setup, although these parts were not directly assessed. Non-linear material properties were assigned for the assessed components to comply with the Category 2 criteria of DNVGL-RP-F112.

The analysis considered design loads in the open and closed conditions to identify the worst-case combination. Hydrostatic test conditions were not assessed, as this is assumed to take place topside, prior to the application of cathodic protection.

Convection effects were applied to the external surfaces of the valve which represented heat transfer to the surrounding seawater. The calculated (temperature dependent) coefficients were generated using PDL’s in-house tool and considered the geometry and orientation of the valves faces. The results of the thermal analyses were imported into the structural load cases to determine the stresses and deflections due to thermal gradients.

At locations of high maximum principal strains, paths were considered through the wall thickness. Linearised stresses were taken for each path.

Significant failures against DNVGL-RP-F112 were identified in one component, and as such PDL provided multiple potential design recommendations to address the issue. Within the same component there were locations with a more marginal failure, and for these failures we also suggested modifications that could be made to the analysis to reduce some conservatism.

A pass against the HISC assessment criteria was demonstrated for the Housing component, however, the stresses were found to be beyond the material yield stress through-section in some regions. Further structural assessment, in line with an applicable design code such as ASME BPVC Sec. VIII Div. 2 was recommended.

BENEFITS

• PDL’s investigations showed which components and locations were most at risk of HISC, and whether these regions passed the criteria of DNVGL-RP-F112.
• Quick identification of failures meant that design issues were discovered before manufacturing had begun, reducing costs and wastage for the client.
• Potential design changes were recommended to the client.
• We identified that an additional ASME BPVC Sec. VIII Div. 2 assessment was required for the valve to be acceptable for use.