Scope
A 10” HPHT gas export pipeline, installed circa 2013 in Dutch waters, was designed for a maximum operating temperature of 135 °C. Since hot gas production commenced in 2018, the line has been transporting wet gas at operating temperatures between 90-100 °C.
Prior to undertaking the detailed upheaval buckling (UHB) assessment, the as-surveyed pipeline burial depths were reviewed against the minimum depth of cover requirements specified in the pipeline detailed design report.
This initial screening identified several locations where the measured cover was lower than the design requirement. These locations were considered potential UHB hotspots and were therefore selected for further assessment using imperfection screening methods and finite element analysis. A detailed finite element (FE) analysis to DNV-RP-F110 was undertaken to assess the likelihood of UHB under current and design conditions and define appropriate mitigation and monitoring strategies. The scope of work included:
- FE analysis of the 10” export pipeline under current and full design operating conditions.
- Comparative assessment of mitigation options including increased downforce and temperature reduction.
- Identification of critical sections of pipeline at greatest risk.
- Development of recommendations for survey and monitoring requirements.
Our Solution
Jee structured the study around the following objectives:
Finite Element Analysis
- Applied DNV-RP-F110 methodology using the surveyed pipeline profile.
- Modelled thermal and structural behaviour under operating and design conditions.
- Tested sensitivity to soil resistance, seabed profile, and downforce.
- Under current conditions (99 °C inlet / 22 °C outlet), UHB risk cannot be ruled out in the first 2 km of pipeline.
- The most critical locations identified were KP 0.638 and KP 1.133.
- Increasing downforce by 50% (uniform or patch-based) did not prevent predicted vertical movement.
- Reducing inlet temperature to ≤ 79 °C prevented upheaval without additional downforce.
- No vertical movement has been observed in practice, indicating the FE model is conservative - likely due to soil property assumptions and the use of an operational survey profile that exaggerated out-of-straightness.
Key Findings
- Under current conditions (99 °C inlet / 22 °C outlet), UHB risk cannot be ruled out in the first 2 km of pipeline.
- The most critical locations identified were KP 0.638 and KP 1.133.
- Increasing downforce by 50% (uniform or patch-based) did not prevent predicted vertical movement.
- Reducing inlet temperature to ≤ 79 °C prevented upheaval without additional downforce.
- No vertical movement has been observed in practice, indicating the FE model is conservative - likely due to soil property assumptions and the use of an operational survey profile that exaggerated out-of-straightness.
- Ensure works are scheduled to minimise disruption to marine life during sensitive ecological periods.
Conclusion
The assessment concluded that UHB is a possible risk within the first 2 km of the pipeline at current operating conditions, and additional downforce exceeding 50% would be required to pass DNV checks. Reducing inlet temperature provides a practical mitigation strategy.
To support safe and responsible management, the study recommended:
- Continuing vertical profile surveys with focus on the first 2 km.
- Rock dumping this section if vertical movement is observed.
- Evaluating operational feasibility of limiting inlet temperature to ≤ 79 °C.
- Refining soil and seabed profile modelling assumptions to reduce conservatism.
Email our Head of Decommissioning, Richard Stark at richard.stark@jee.co.uk or visit our Decommissioning page here.
Jee Ltd
