During a routine subsea platform inspection in September 2025, a UK North Sea operator identified severe corrosion on bolts in numerous clamped guides attaching a 3-inch methanol riser to a 20-inch gas riser. Bolting in four of the clamps had already failed completely and there was varying corrosion across the remaining clamps. With platform production shut down when the damage was identified, Jee was asked to determine whether the riser could continue operating safely and to define the urgency and focus of remediation.
The immediate technical challenge was to quantify riser integrity with reduced restraint caused by missing or ineffective bolted clamps. With the platform shut down and the survey vessel on station, the operator needed clear and urgent guidance on whether the riser was safe to operate, for the short term at least, in its as-found condition and what repairs were necessary and how quickly they needed to be carried out.
Jee carried out finite element analysis using Abaqus to model the riser from hang off to pipeline tie in, including welded guides and bolted clamps as lateral restraints where present. Wave and current loads were applied to determine both response to maximum storm conditions and long-term fatigue damage. The work was undertaken in 3 phases to support the ongoing developing intervention.
● Initial response analyses were performed immediately following identification of the corrosion issues to establish whether it was safe to restart operations. Given the urgency of the situation, Jee utilised evening and weekend working to provide answers to the operator as quickly as possible. The initial analysis determined that the riser was safe to operate in its as-found condition for a limited period and the operator was able to restart production on the platform less than 48 hours after discovery of the damage.
● Worst-case configuration analyses were undertaken to assess the survivability of the riser assuming subsequent failure of all clamps identified as having severely corroded bolts, with the aim of determining the urgency of remediation. This analysis confirmed that the riser would survive impending winter storms but would have a limited fatigue live and would require clamp reinstatement within the first year.
● As-left configuration analyses were undertaken to assess the resilience of the riser following initial intervention and partial remedial works completed in September 2025. The initial intervention replaced bolting or clamps at 4 locations that had been prioritised by Jee as giving most effective benefit. In the ‘as-left’ configuration, the riser could withstand lifetime worst-case load conditions and would have a fatigue life of 12 years. Whilst the fatigue life was not sufficient for planned remaining operations, it was adequate to allow the operator to schedule non-urgent remedial intervention to reinstate the riser clamps back to the origin design configuration.
Benefits and outcomes
Jee’s analyses and recommendations gave the operator clear guidance and justification for timely and prioritised interventions. As a result of Jee’s rapid response and working flexibility, the operator was able to restart production in less than 48 hours, thereby minimising costs from lost production. With insight from Jee’s recommendations, the operator was able to prioritise activities to optimise the benefit of the immediate remedial response and schedule a phased longer-term intervention to return the riser to its original design configuration.
For more information, visit www.jee.co.uk/integrity-management
To contact Jee’s Technical Authority, Graham Wilson, email Graham.Wilson@jee.co.uk.