In 2011, 63 people were evacuated from the Visund platform in the Norwegian Continental Shelf (NCS) after gas escaped from a leak in a riser that had been closed for inspection. Field operator Statoil immediately set up a specialist team to investigate, while Norway’s Petroleum Safety Authority (PSA) conducted its own report. No-one was injured, environmental pollution was minor. The incident was caused by a new failure mode unknown to the industry, giving another example of the challenges of understanding the complex behaviour of flexible pipes.
The Statoil taskforce brought in to investigate the issue involved 20 leading riser experts over 18 months. The group consisting of Statoil, 4Subsea and DNV GL was assembled to share information to solve issues around failure of multilayer polyvinyl difluoride (PVDF) risers.
Flexible risers are an enabling technology for floating production in harsh environments. These pipes encompass a layered structure in which the various materials have functions such as: withstanding external and internal pressure, coping with tensile forces, preventing hydrocarbon leaks and protecting against seawater.
Norwegian statistics for 2010-13 show at least 1.5% probability of failure per riser per operational year, according to the PSA. There are multiple causes, but the underlying factor is several years of inadequate appreciation of the complexity of flexible pipes and possible failure mechanisms.
Earlier this year, the PSA commissioned a study to update knowledge of un-bonded flexible risers in Norway. DNV GL is also calling for more comprehensive studies in this area through industry collaboration, across commercial boundaries and barriers. We believe that this could increase confidence in the sector and open the market to innovation and standardisation in riser technology.
In 2013, there were about 300 flexible risers offshore Norway. Many were subject to high pressures and temperatures, large fluctuations in operating parameters, and high dynamic loadings. This is a significant reliability challenge. According to an independent engineering company, more than 25% of NCS risers have been replaced historically and few have met their originally predicted service life.
In its 2012 report ‘Risk Level in Norwegian Petroleum Activities’, the PSA acknowledged that the frequency and significance of flexible riser incidents were rising and higher than for steel risers.
Risers that experienced carcass collapse were constructed with three layers of PVDF polymer material. Issues related to carcass collapse in flexible risers with multiple layers of PVDF have been known since the early 2000s. However, being able to predict, detect and understand carcass collapse and tearing has been further investigated in ongoing work continuing from the taskforce established by Statoil in 2011.
One outcome of the 2011 Statoil investigation was a paper presented last year by co-authors Statoil, 4Subsea and DNV GL. It explained the nature of the carcass-tearing problem and suggested load model and operational policy for mitigating risk of new failures. The paper called for further inspection and carcass monitoring to avoid pollution or a safety critical situation, and for new risers to have greater robustness towards carcass collapse and tearing.
The paper and discussions resulting from both the Statoil and PSA investigations have gone some way to address the technical complexities of the issue and to consider the need for enhanced flexible riser and flowline technology at the subsea project design stage, but more is needed.
Standard paves way for subsea certification
In a major drive to improve quality and reduce costs, the oil and gas industry now has its first standard to certify subsea equipment and components.
The goal of the DNV GL standard is to streamline quality control and manufacturing processes and to reduce pressure on global supply chains. This will ultimately shorten lead times and help to deliver projects on schedule.
The certification scheme also aims to help the interpretation of existing API and ISO standards. It will provide operators with confidence that fabrication quality is being controlled and assured throughout the industry.
Bjørn Søgård, segment director, subsea and floating production at DNV GL – Oil & Gas, said: “Standardisation is widely agreed to be the solution to address many inefficiencies in most industries. For operators, this standard will reduce costs without sacrificing quality, innovation or safety. For suppliers, it will increase predictability and enable the strategic stocking of long-lead items.”
 Statoil press release, ‘Routines improved for operation and maintenance’, September 2011
 ‘PSA begins flexible risers study (Norway)’, Subsea World News, January 2014
 4Subsea report. ‘Un-bonded Flexible Risers – Recent Field Experience and Actions for Increased Robustness’, 2013
 ‘Hydrocarbon leak from flexible riser – investigation report’, Officer of the Watch, July 2013
 ASME 2013: OMAE 2013-10210, ‘Carcass failures in multilayer PVDF risers’, Knut-Aril Farnes, Claus Kristensen, Steinar Kristoffersen, Jan Muren and Nils Sødahl