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Full-scale sour fatigue testing machine for riser girth welds

TWI designed and built a unique custom rig capable of testing full-scale pipe welds in an internal sour environment, to support its Industrial Members in the oil and gas sector.

Steel catenary risers are commonly used within deepwater oil and gas developments, and fatigue performance is often a critical factor in their overall design. Resonance fatigue testing of full-scale girth welds has become standard industry practice to demonstrate adequate performance.

However, these tests alone take no account of aggressive service environments such as sour production fluids. In these instances qualification testing is usually a two-stage process involving full-scale resonance fatigue testing to demonstrate the required performance in air, and small-scale (strip) fatigue testing (in air and in the sour environment) to determine a fatigue life reduction factor that is then applied to the base design curve. This approach accounts for geometry effects (ie the difference between strip and full-scale testing) and environmental effects individually, and has been adopted on many projects. However, the validity of the approach had not been demonstrated, so TWI launched a JIP with that aim.

Breaking new ground

The project had several key objectives:

  • Elimination of excessive conservatism, providing significant cost savings and greater design flexibility
  • Greater confidence in likely material behaviour in service, resulting from a more direct means of quantifying material performance
  • Improved understanding of fatigue in sour environments, leading to enhanced safety by reducing the risk of corrosion fatigue-related failure 
Fig 1. The custom-built machine at TWI
Fig 1. The custom-built machine at TWI

Test machine

Phase one of the project involved the design and manufacture of a rotating bending fatigue test machine capable of testing full-scale pipe welds with an internal sour environment. This can be seen in Figure 1.

The machine incorporates several novel features:

  • Orthogonal pairs of hydraulic actuators permit in-plane bending, rotating bending, or anything between the two, up to +/-250kNm bending moment.
  • Large diaphragms support the pipe at its ends, permitting angular rotation while maintaining axial alignment.
  • A unique internal jack/column system allows up to 2MN of axial preload to be applied to the pipe.
  • An internal annular sour cell permits circulation of a controlled environment around the weld root region without compromising the internal column system.

To discuss how this equipment could be used to support your business, please email

Fig 2. Configuring the machine
Fig 2. Configuring the machine
Avatar Graham Slater Consultant – Fatigue Integrity Management