North Sea Bypass

North Sea pipeline operator, Gassco, had pipe-in-pipe bundle with 17 consecutive, interacting free spans with lengths of 65m-83m. A planned rock dump campaign was canceled based on findings from the Global Free Span service, which resulted in cost savings of 600-900k USD.

Challenge

Based on findings from previous free span assessments performed by engineering companies, it had been concluded that rock dump interventions were necessary for this bypass pipeline section in order to manage the free spans that were under development. Though this solved the problem in the short term, it created interacting free spans and potentially increased scouring around the pipeline.  

Approximately 10 years after the rock installation campaign was performed, the bypass was nearing it's end of design life, and therefore needed to a life time extension assessment as the intention was to keep operation going for several years more. 

DNV solution

The new Global Free Span service was applied to the pipe-in-pipe bundle with an outer diameter of 40’ in the North Sea. The automated calculations from the model saved time consuming interaction calculations and demonstrated that present configuration was acceptable, accounting for interacting multi-spans. 

Outcome and benefits

A planned rock dumping installation campaign was canceled based on the assessment, which resulted in cost savings of 600-900k USD, and the lifetime extension assessment concluded that operation of the pipeline could continue beyond the lifetime design.