Flexible Riser Repair and Dissection Following Exceedance of API Fatigue Limit

Flexible Riser Repair and Dissection Following Exceedance of API Fatigue Limit
October 2019
Pedro Viana, Renan Tapias, Mehrdad Mansour -Tullow Oil

This paper presents detailed learnings into the dissection of a flexible riser section, which had been operating for seven (7) years and subjected to high rates of fatigue damage, after operational field changes. Detailed global and local fatigue analyses were performed to assess the response of the riser system taking into account significant changes between the design and the operational history. These environmental and operational changes were shown to have a negative impact on the fatigue performance of the risers, in particular the top section located inside the bend stiffner.

Fatigue analyses are performed using field measured data such as vessel headings, internal pressures and environmental conditions, aiming to replicate the riser response history and calculate fatigue usage to date. Actual field data are shown to be more onerous than design data and therefore a detrimental fatigue response is expected. Results from detailed fatigue analysis shows that flexible risers fatigue usage were above the maximum allowable of 0.1, as outlined in API [1,2]. The most critical location was the pressure armour wires of the top section, located inside the bend stiffener. A mitigation plan was proposed, including an immediate reduction of internal pressure to reduce the risk of riser failure, and the removal of the fatigued riser top section. After removal of the fatigue critical section, the riser was re-terminated and its fatigue performance reinstated.

The pipe section removed from operation was carefully dissected. Following the dissection, each riser layer was investigated to determine possible signs of degradation. Additionally, fatigue testing of both pressure and tensile armour wires were performed to determine potential fatigue degradation and confirm analyses findings. Dissection and fatigue tests have demonstrated that the pipe condition was better than expected, highlighting conservatisms in design and analyses methodologies.

In-service inspection of flexible riser internal layers is highly complex, with no detailed insight of all the respective layers. Dissection of a flexible riser, in service for seven (7) years and exposed to high rates of fatigue damage, provides valuable information about the state of the different layers comprising the cross section. This is particularly useful since analytical work have shown the riser to have exceeded the fatigue limit of 0.1 as defined per API. The good state of the flexible riser, on the contrary to the prediction, highlights a good level of conservatism in flexible riser design and methodology.

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