Subsea Manifold Tee Overpressure Assessment

The Challenge

A brownfield North Sea subsea manifold faced a classic HP/LP interface question: gas-lift overpressure scenarios could expose the low-pressure side to pressures beyond its original design basis. The manifold was already in service, already had measured wall loss, and was being considered for life extension.

Super duplex stainless steel adds a specific concern. Under cathodic protection it is susceptible to hydrogen-induced stress cracking, and susceptibility is governed by sustained stress levels — which is precisely what an overpressure scenario changes.

Our Approach

Measured wall loss in the model, not a nominal allowance — on a life-extension case the as-inspected condition is the design condition.

VORYX team members carried out a combined assessment: analytic pressure component checks to PD 5500 and BS EN 13445 to establish the code position, followed by non-linear finite element analysis where the code checks were either non-conservative or not directly applicable to the barred tee geometry.

Measured wall loss from inspection was built into the models rather than assumed as a nominal corrosion allowance, and geometry sensitivities were run to bound the uncertainty in the actual as-inspected condition.

HISC evaluation to DNV-RP-F112 assessed the super duplex material against sustained stress and strain limits under cathodic protection, closing the specific degradation mechanism that a pressure-code check alone would not address.

Standards Applied

• PD 5500
• BS EN 13445
• DNV-RP-F112
• DNV-ST-F101
• BS 7910

Operator and project names are anonymised for confidentiality and disclosed on request. Experience described is that of VORYX team members.