Assessment of fatigue of a subsea pipeline including analysis of dents

A major UK continental shelf operator planned redevelopment of two mature fields to increase production and extend asset life. This required higher throughput and operating pressures in existing gas export trunklines that were not originally designed for these conditions.

 

Jee was engaged to assess the impact of these changes on pipeline fatigue life and confirm ongoing integrity under revised operating scenarios.

 

Jee was tasked with evaluating fatigue life for both pipelines under historical and proposed operating pressures.

 

A further consideration was a historic anchor strike on one trunkline, which damaged the external coating and raised concerns about potential underlying steel deformation. The assessment therefore considered the possible presence of dents and their effect on remaining fatigue life.

 

Jee carried out a structured fatigue assessment using historical operating pressure data from commissioning to present. Fatigue damage was calculated using the rainflow method in accordance with DNV RP C203 with remaining life assessed following DNV OS F101.

 

To understand future performance, pressure data was scaled to reflect increased maximum allowable operating pressure and fatigue calculations were repeated under these conditions.

 

A sensitivity study was performed to assess the potential impact of anchor strike damage. A range of dent sizes was assumed in the absence of confirmed measurements and fatigue damage was evaluated using recognised methods, allowing comparison across credible damage scenarios.

The analysis demonstrated that both pipelines retain sufficient fatigue life under the proposed higher operating pressures, exceeding their design life.

 

While fatigue performance is sensitive to dent size, even conservative damage assumptions showed acceptable remaining life. Jee concluded that the historic anchor strike is unlikely to present an immediate integrity threat.

 

Recommendations were provided to confirm dent sizing through inspection to further refine the assessment and support long term integrity planning.

Jee provided the technical assurance required for redevelopment, enabling the client to proceed with confidence that increased operating conditions can be safely accommodated without compromising pipeline integrity.