The problem

Our client asked us to complete a lifetime extension assessment for a 12” pipeline system in the North Sea, which was approaching the end of its original design life.

The assessment included the typical time-dependent and independent studies carried out in accordance with ISO/TS 12747. Key aspects of this assessment were the internal and external corrosion assessments, according to DNV-RP-F101, and fatigue assessments of the riser (for direct wave action and VIV).

The first pass riser fatigue screening assessment identified that the level of fatigue damage expected due to direct wave loading was unacceptable for the internal and external environments, and that VIV required more detailed investigation.

The solution

A detailed review of the original design’s riser fatigue assessment identified that the original S-N curves and safety factors used for the design were not consistent with current best practice.

We performed a detailed Paris-Erdogan Law calculation to identify crack growth rates for the stress ranges predicted from the direct wave loading. From this, a risk based inspection (RBI) strategy was determined to ensure that any cracks undetected by previous inspections, or that may have formed between inspections could not grow to an extent that could cause failure within a single inspection interval.

We identified a robust inspection method and regime to safely manage the threat from ongoing direct-wave fatigue during the extended period of operation. A more detailed analysis of the risers using Abaqus finite element analysis (FEA) to identify the mode shapes and frequencies of the risers showed that VIV was not expected to limit the life of the risers.
As part of the wider integrity assessments, the riser fatigue studies allowed the lifetime of the pipeline to be extended beyond the end of the original design life.

The benefits

As an experienced provider of lifetime extension and integrity assessments for mature pipelines systems, we were able to identify the optimal strategy for further assessment following the initial issues identified by screening assessments. The appropriate follow up studies were completed and the client was able to put in place the appropriate steps during extended life of the pipeline system.

Our experience in conducting lifetime extension for dozens of pipelines across the North Sea over a number of years, as well as leading the development of the ISO/TS 12747 standard for lifetime extension enabled this effective solution to be developed.

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