Project Code: 36686
Start date and planned duration: January 2026, 15 months
Objective
- Use the sub-sets of data that are relevant to welds in risers in the offshore oil and gas industry, and welds in monopiles used in offshore wind structures in BS7608 and BS7910 in order to produce industry-specific SN curves, FCGR laws and industry-specific guidance for ECA.
- Compare fatigue lives calculated using the BS 7910 guidance to fatigue lives from full scale fatigue tests. Recommend adjustments to the BS 7910 guidance that would result in more representative fatigue lives.
- If there are gaps in the existing data, identify what additional information is needed (new test methods, data, simulations, analysis) to generate the required industry-specific guidance.
Project Outline
The project will use two case studies: Girth welds in risers and circumferential welds in monopiles.
Data in TWI’s girth weld database of full scale resonance fatigue test results will be re-analysed, with a focus on the influence of weld geometry on fatigue performance and linking fatigue life calculated using fracture mechanics with that observed in full scale fatigue tests.
There is less data currently available for welds in thick sections, so a review will be carried out to document dimensions relevant to the calculation of fatigue life for welds in offshore wind structures, published SN and FCGR data that are relevant to welds in thick structures for offshore wind applications. The gaps in available data will be identified. This case study will also review and comment on the background to the BS 7608 SN curves and their relevance to welds in thick structures, and the basis of the DNV-RP-C203 wind turbine SN curve and the thickness correction factors in BS 7608 and DNV RP C203.
The project will also review BS 7910 guidance for fracture mechanics fatigue life calculations in the context of the relevant case studies, including:
- SIF solutions
- Residual stress distributions
- R ratio assumptions in ECA calculation
- Scatter in FCGR curves
Industry Sectors
- Oil and Gas
- Wind
Benefits to Industry
British standards BS 7608 and BS 7910, used for fatigue design and fatigue assessment of welded joints, are general guidance documents. They can be safely applied to welds in any industry.
However, the documents have evolved over many years, and much of the background data for BS 7608 is from welds relevant to the construction/ bridge and some of the guidance in BS 7910 traces back to the requirements of the nuclear industry. The nuclear industry has a clear need to ensure the highest levels of safety in its structures due to the nuclear hazard, which means some of the guidance in BS 7910 is conservative when applied to welds in other industries.
Similarly, the SN curves and FCGR laws were established from databases of results determined from welds made in different grades of steel plates and a variety of welding procedures. The materials, welding consumables and joint preparations are different in other industries.
This leads to a lower estimate of fatigue lives using these documents eg both at the design stage using BS 7608 and when calculating the remaining fatigue life of an existing flaw using BS 7910 and has significant financial consequences for industry because the calculated allowable stresses are relatively low, necessitating thicker structures (and more steel equates to more cost). Assessment of existing flaws predicts shorter remaining fatigue lives. As a result more frequent inspections may be carried out than necessary (at significant cost) and structures may even be scheduled for early decommissioning.
The discrepancy between calculated fatigue lives using BS 7910 guidance and the actual fatigue performance of welded joints can make justification for life extension significantly more difficult.
Tailoring the guidance in BS7608 and BS 7910 to target specific industries would produce less conservative design life prediction and higher allowable stresses, and this helps to justify longer operational lives, whilst embedding a level of safety that is suitable for the particular industry.
The project will produce a report which can ultimately be referred to or incorporated in the future revisions of the BS 7608 and BS 7910 standards.