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Variable Amplitude Resonance Testing of Girth Welds

TWI has developed a new method of fatigue testing, to provide oil and gas operators with greater confidence in the integrity of their assets and infrastructure.

Fatigue design is of major concern for pipelines and steel catenary risers that are subjected to cyclic stresses of variable amplitude (VA), which they are during deployment at sea. TWI has responded to this by developing a method to test full-scale girth-welded pipes under VA loading.

Replicating service conditions

TWI regularly conducts full-scale fatigue testing of girth-welded pipe from steel catenary risers (SCRs) and pipelines for its Members, to ensure the welds have sufficient fatigue strength. These tests are typically achieved by resonance fatigue testing under constant-amplitude (CA) loading (Figure 1). However, these components are subject to VA loading in service, with a loading spectrum typically characterised by very high numbers of low stress cycles due to vortex-induced vibration (VIV). The choice of method for accounting for the fatigue damage due to stresses below the constant-amplitude fatigue limit (CAFL) can have a significant impact on the estimated fatigue life.

Objective

To overcome the shortcomings with existing test methods, TWI set out to develop a method for fatigue testing of full-scale girth-welded pipes under a loading spectrum representative of that experienced by SCRs.

Figure 1. Resonance fatigue testing of full-scale girth-welded pipes
Figure 1. Resonance fatigue testing of full-scale girth-welded pipes

Work programme

TWI used its large resonance testing rigs to develop a new methodology for SCR and pipeline fatigue testing. The development of the testing procedures involved the following steps:

Establishing RPM and strain range relation.

In resonance fatigue tests, the target strain (stress) range is achieved by controlling the speed of the motor, expressed in terms of revolutions per minute (RPM).

Dividing each block into several sub-blocks.

The fatigue data, representing annual fatigue damage, are divided into several sub-blocks to represent variations in sea states.

Establishing the relation between RPM and time for each sub-block.

TWI then determined the number of cycles at each stress range for each sub-block

Finally, data recording and processing.

Controlling the resolutions of the motor using programmed software allowed precise control over the variation of the stress amplitudes. An example of stress–time signal achieved in full-scale VA test trials is shown in Figure 2.

Project outcomes

TWI developed and used this new method in a joint industrial project (JIP)to investigate fatigue behaviour of girth-welded pipes under VA loading and the damaging effect of small stresses in a loading spectrum for long endurance (up to 2x108 cycles). Based on the results of this JIP (Zhang and Maddox, 2013), a change to the fatigue design guidance for girth welded pipes was incorporated in BS 7608 (2014). In particular, the standard requires that Miner’s rule be used in conjunction with the appropriate CA S-N curve, with the slope change from three to five introduced at N= 5x107 cycles.

References and credits

  • BS 7608 (2014) ‘Guide to fatigue design and assessment of steel products’, British Standards Institution, London.
  • Zhang Y H and Maddox S J, ‘Fatigue testing of full-scale girth welded pipes under variable amplitude loading’, J Offshore Mech. Arct. Eng., 136 (2) 2014, pp.021401/1-10 May 2014.

The following companies are acknowledged for their financial support for this project: BP, Chevron Energy Technology Company, ExxonMobil Upstream Research Company, Heerema Marine Contractors, Petrobras, Saipem, Tenaris Tamsa and the UK Health and Safety Executive.

For further information about Integrity Management, please email contactus@twi.co.uk

Figure 2. Example of stress–time signal achieved in full-scale VA test trials
Figure 2. Example of stress–time signal achieved in full-scale VA test trials
Avatar Yanhui Zhang Consultant – Fatigue Integrity Management

Coming from a background in metallurgy, he graduated from the University of Science and Technology Beijing in 1982 with a bachelor’s degree, before obtaining a PhD from the Open University, UK in 1992.

Yanhui worked on Ni-based superalloys as a postdoctoral researcher in Cambridge University before joining TWI in 2001. His expertise includes fatigue design, fatigue and creep life evaluation, engineering critical assessment, fatigue and creep testing, and failure investigation. He is also experienced in establishing relationships between mechanical properties and microstructures of materials. He has published over 50 academic papers in journals and at international conferences.

For more information please email:


contactus@twi.co.uk