FRACTURE MECHANICS FINDS SAFE CYCLES FOR SUBSEA FLANGE

SUMMARY

A manufacturer of subsea components contacted us to assess three subsea flange designs, which were designed to operate under High-Pressure, High-Temperature (HPHT) conditions. The flanges were to be installed in two oil fields with extreme requirements, including pressures and temperatures of up to 20ksi and 400°F.

PDL performed Finite Element Analysis (FEA) to produce a capacity chart finding upper bounds on safe loading conditions, design verification in accordance with API 17TR8 and ASME BPVC Sec. VIII Div. 3, and fracture mechanics (FM) assessments to API 579-1 / ASME FFS. A new screening method was internally developed to quickly identify the critical paths for crack growth based on the stresses within each component. FM calculations were completed based on the provided load histogram and we showed that the flanges were suitable for HPHT and met the desired operational cycles.

The analyses were required were complex and involved, and the efficiencies introduced by PDL’s in-house tools allowed both a more thorough assessment whilst reducing timescales.

SITUATION

The client had been awarded a contract to provide HPHT flanges for use across two oil fields. Three different sizes were assessed. There is a growing requirement for components to operate at higher pressures and temperature in the subsea industry, with API 17TR8 providing guidance on the procedure for assessing such components. PDL are world leaders in HPHT assessments and have developed several in-house postprocessing and calculation tools to improve the efficiency of obtaining results.

CHALLENGE

Our client needed multiple complex assessments to be performed on three different flanges under two different extreme operational conditions. Design validation and fracture mechanics assessments needed to be performed to give the end client confidence in the compliance of the designs.

We have significant experience in the analysis of highly non-linear and dynamic systems and were approached to provide technical support to our client’s team using FEA.

SOLUTION

Starting with an analysis plan, we summarised the bounding load cases and also identified all other critical parameters, such as the data source for the gasholder geometry, material properties, the assessment code, software of choice and post-processing requirements. The model assemblies all contained a flange, mating block, gasket, internal cladding, with modelled studs and nuts. Different material properties were assigned to each part, which had been obtained through the clients own
physical testing.

Half-symmetry FEA models were created, using the ANSYS Parametric Design Language (APDL) graphical user interface for ANSYS Mechanical, to represent the multiple loading scenarios required for the assessment.

A Pressure-Bending-Tension (PBT) capacity chart was produced, considering both structural and serviceability limit criteria.

The flange design was then validated in accordance with API 17TR8 and ASME VIII Div. 3 for design conditions. This considered protection against global plastic collapse, local failure, hydrostatic pressure test and ratcheting.

Linear Elastic Fracture Mechanics (LEFM) assessments were then performed on all components. LEFM examines highly-stressed regions in the model and predicts how a crack would grow if there was an existing flaw in the material. The initial flaw size is based on the examinable limits of non-destructive testing (NDT) or design code recommendations. The results of FEA can be used to evaluate the crack propagation characteristics such as initiation site, direction and growth rate.

For efficiency, we developed a macro using APDL to identify the peak stress locations in each component and perform fracture mechanics assessments on multiple paths from these points.

Our in-house calculation sheets were able to predict the behaviour of theoretical “cracks” originating from these locations, which accounted for the geometry of the model and the direction of crack growth.

The fracture mechanics assessments required the individual threads to be represented, therefore a sub-model was generated with a refined mesh, to accurately capture the stress distribution around the thread root. This stress profile dictated the crack growth rate and direction of propagation.

Through demonstrating that the model replicated current failure modes, sufficient confidence was achieved to use the model as a virtual test bed for a number of corrective actions and ultimately practical interventions that improved the gas holder performance.

BENEFITS

• The capacity assessment identified the bounding load-limits of the flange, considering pressure, bending moment and tension.
• The fracture mechanics analysis we performed gave confidence for through-life operation.
• Our experience in fracture mechanics allowed a thorough screening of the model, using our in-house calculation sheets to assess multiple potential crack locations without adding to the project timescale.
• A similar methodology could be adopted across the three flange designs, significantly reducing timescales for the client and reducing costs.
• We had a close partnership with the client to address 3 rd party comments and support the review process.