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Flexible Riser Wire Stress Assessment Using Bflex

Flexible Riser Wire Stress Assessment Using BFlex

Benefits
Atteris assessed the impact of a suspected flexible riser wire break using BFLEX flexible pipe software. The increase in armour wire stresses was calculated under extreme and fatigue loading, allowing continued operation and enabling development of new operating limits for the system.
Background
An outer sheath breach was detected in the bend stiffener region of a flexible riser. Integrity concerns were raised due to potential broken outer tensile wires and high annulus corrosion rate at the breach location. The Operator of the system engaged Atteris to assess the impact of increased corrosion rates and up to 4 broken outer tensile wires on the wire stresses under both extreme and fatigue loading.
Work Performed by Atteris

Extreme Analysis:

  • Stress Analysis, using tensions, bend radii and pressures expected in continued operation
  • Local stress analysis of the flexible riser outer tensile armour layer for all wires intact, including increased corrosion rates post detection of breach.
  • Local stress analysis of the flexible riser outer tensile armour layers for 1 wire broken to establish the uneven distribution of load, in particular the load amplification on wires adjacent to the broken wire.
  • Additional stress analysis for 2, 3, and 4 adjacent wires broken to assess the impact of additional broken wires and the potential for domino effect.

Fatigue Analysis:

  • Local stress analysis of both flexible riser tensile armour layers for the dominant fatigue conditions.
  • Fatigue damage calculation using a H2S and Aerated seawater SN curve, for both no corrosion and considering uniform corrosion.

Recommendations:

  • Provide conclusions on the riser integrity and recommendations on operating envelopes.
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