Pipeline companies operating on moveable seabed have long struggled with existing pipeline standards which are not tailored to these conditions. As a consequence, frequent and costly free-span corrections have been carried out, sometimes causing more harm than good. Pipeline operator BBL Company and DNV are now joining forces to explore the pipeline reality in these conditions and reinforce DNV’s recommended practice with tailored standards. This could mean significant savings for pipeline operations on moveable seabed.

Hans Boersma, Manager Offshore Assets and General Affairs, BBL Company V.O.F. and initiator of the BBL DNV Joint Industry Project. photo: DNV/Marianne Wennesland
A typical free span in a trench along the BBL pipeline. ROV based Multi Beam image from the 2011 survey.

A hypothesis based on an extensive CFD (computational fluid dynamics) analysis – conducted by Dutch pipeline operator BBL Company V.O.F. and Danish CFD expert Lloyds ODS – raises questions about existing standards for free-span corrections, indicating them to be too conservative.

Hans Boersma, Manager of Offshore Assets and General Affairs, BBL Company, describes their findings which indicate that a majority of today’s free-span interventions on moveable seabed are in fact superfluous. “On moveable seabed, free spans occur temporarily as part of the pipeline’s self-lowering process. Our analysis shows that in most cases these will correct themselves naturally in due time,” he says. “Scour trenches may also provide qualities similar to those of man-made trenches when it comes to protection against vortex-induced vibrations.”

According to existing standards, pipeline operators are frequently recommended to either reduce vortex-induced vibrations by installing spoilers on free-spanning pipelines, or correct the free spans by building rock berms, trenches or gravel stitches. “These measures often end up creating more problems than they solve,” argues Boersma. “For example, installing a rock berm often results in scouring on each of its ends, creating two new free spans.” In addition, all interventions may pose unwanted interference with local eco-system and fishing activities. Boersma calls for an industry practice that works with the natural processes of moveable seabed instead of trying/failing to overcome them.

In 2012, he approached DNV’s Pipeline Director Asle Venås and proposed a joint industry project to explore free-spanning pipelines on moveable seabed and related vortex-induced vibrations. About to be launched, the JIP aims to expand knowledge, establish accurate calculation methods and reinforce DNV’s RP by including standards tailored to pipeline operations on moveable seabed.

BBL Company is located in the Gasunie Building (Groningen), considered to be one of the most beautiful office buildings in the Netherlands. photo2: Photo: DNV/Marianne Wennesland

“Knowledge is our weapon,” says Boersma, pointing out that any knowledge-based standard builds in safety factors to account for the unknown. “The only way to get accurate parameters in place of such safety factors is to expand our knowledge and prove it sound.” The JIP’s further development of applicable CFD analysis with predictive value will be validated by thorough laboratory testing at Deltares institute for applied research in the Netherlands.

Boersma hopes the JIP will create cooperative networks and enhance performance across the industry. He believes the end results will involve better solutions, significant cost reductions and reduced risks. “We need the best possible industry practice and a recommended practice to document it,” he says, pointing out that pipeline operators answer not only to regulators and authorities, but also to society at large. “Standards tailored to the actual reality of moveable seabed could mean a significant reduction in the number of free-span interventions. This would be beneficial to all users of the ocean.” BBL chose DNV as a collaborative partner because of its objective third-party role, existing RP on free-spanning pipelines and experience of running joint industry projects.

This spring, additional partners will be invited to participate in the JIP with their experiences, questions, survey results and pipeline cases for CFD modelling. The project is expected to start this summer.

Text: Marianne Wennesland, DNV

JIP & Partners

• BBL Company operates the Balgzand-Bacton Line, transporting natural gas from the Netherlands to the UK. Operational since 2006, the 235-km-long and 36-inch diameter BBL pipeline is laid on sandy seabed in fairly shallow water; it can supply 20% of the UK gas demand.

• In 2012, BBL Company approached DNV and proposed a joint industry project to explore the hypothesis that DNV’s recommended practice for free-spanning pipelines (DNV-RP-F105) is too conservative for pipeline operations on moveable seabed. This is with regard to pipelines in trenches, particularly due to the suppression of in-line vortex-induced vibrations.

• The general lack of standards tailored to pipeline operations on moveable seabed has led the industry into frequent and costly free-span interventions, often creating more harm than good. BBL and DNV are now initiating a JIP on Vortex-induced vibrations of free-spanning pipelines in scour trenches. This aims to expand knowledge, develop calculations/analyses with predictive value and reinforce DNV’s RP for free-spanning pipelines with standards tailored to these conditions.

• DNV key deliverables: DNV brings to the table its world-leading recommended practice for free-spanning pipelines (DNV-RP-F105), the software Fat Free, and vast experience on free spans. As an experienced joint industry partner, with other free span JIPs on its résumé, DNV will facilitate and manage the JIP. The company will provide the project manager and project sponsor, along with experts and advisors.

• BBL key deliverables: BBL brings to the table a significant body of already conducted research and state-of-the-art CFD analyses. The company will provide a manager for the JIP’s continuing cooperation with Lloyd’s ODS on further CFD analysis development, and a manager for the cooperation with Deltaris on modelling in their test tanks. Hans Boersma will take on the responsibility of chairman of the JIP steering committee.

• New partners will be invited to participate in the joint industry project in the spring 2013. The project is expected to start in the summer of 2013.