January 2018, Vol. 245, No. 1

Features

Greater Understanding of Flexible Pipes Needed

Since the first flexible pipes came into operation in the North Sea, they have acquired a mixed reputation. While recognized as an essential piece of technology that makes key operations possible in a way that fixed pipe cannot support, they are still seen, rather more widely than is perhaps deserved, as a more risky option that is prone to greater failure rates.

To understand more about the reality of flexible pipe deployments in the global oil and gas industry, and to separate persistent beliefs from empirical fact, the Sureflex Joint Industry Project (JIP) started a two-year project in 2015 to gather, analyze and eventually share information and experience of flexible pipe systems. This work was a follow-on project from a historic program of work that dates back to the late-1990s.

Through the creation of the most comprehensive global database to cover pipeline population, failure and damage statistics, the JIP’s aims were to understand whether the acknowledged complexity and challenges associated with flexible piping had coloured perceptions of its risk potential and damage caused.

More specifically, the JIP’s goals were to:

• Create an up-to-date picture of the quantities and types of installed flexible pipe.
• Improve industry knowledge and understanding of flexible pipe integrity management.
• Develop and update industry guidance for managing the integrity of unbonded flexible pipes.
• Aid personnel responsible for flexible pipe integrity.

During the project, the joint input from the JIP’s 13 industry members, including operators of flexible pipe, flexible pipe manufacturers, certification bodies, and regulatory authorities, was supplemented by contributions from a number of non-member organisations – including accounts of damage and failure experience.

The resulting Sureflex JIP report, Flexible Pipe Integrity Management Guidance & Good Practice, provides integrity engineers and managers with a comprehensive review and technical guidance relating to flexible pipe damage and failure mechanisms. It includes population statistics relating to every flexible pipe manufactured by each of the established manufacturers before the end of 2015.

Research Framework

Headline figures from the report show that 9,787 miles of unbonded flexible pipe had been supplied by the end of 2015 – often in more onerous conditions than rigid steel piping. This installed capacity comprised 17,500 individual pipe sections.  The report also shows that the use of flexible pipe for static flowlines accounts for more than 70% of total supplied length and 50% of pipe sections, while dynamic risers account for just over 20% of supplied length and 26% of pipe sections.

In total, flexible pipe jumpers account for only 2.5% of installed length around the world. However, as it is typically supplied in shorter lengths, it also accounts for 18% of the total number of installed individual sections. Combining these numbers with global damage and failure statistics enabled the JIP to establish comprehensive failure rate statistics.

As a significant advance over other studies in this area, the JIP-focused on accurately classifying different types and the severity of the failure events that had been observed over the years. Broadly speaking, the database identifies categories of failure as follows:

• Damage: An issue or anomaly that has degraded or continues to degrade the flexible pipe’s construction or performance over time – but crucially may still allow the pipe to continue in operation through careful monitoring and management.
• Failure: A failure is considered to be the loss of the primary bore containment (the polymer internal pressure sheath or IPS). Failure is then determined or described as a leak or rupture.
• Leak: A relatively low-level leakage caused by an IPS defect.
• Rupture: The failure of bore containment through a major defect in the IPS such as a large crack, an extrusion of the IPS or a physical separation of the flexible pipe and are considered to be a catastrophic event.

A significant focus of the JIP has therefore been to understand the differences between the types of defect that result in damage or failure. This includes pipes that have been shut down or replaced after raising integrity concerns but with no loss of containment.

This is an important consideration, as some industry studies from the past 20 years have classed all defects or the change-out of a pipe as a failure. By correctly categorizing historical damage and failure reports, the JIP study aimed to provide a more accurate assessment of threats to their specific flexible pipe systems that represent a clear and present threat to overall operations.

The JIP has also intentionally focused on causes and incidents of damage and failure that occur in the installation and operating phases of the flexible pipe life cycle. There are a limited number of damage or failure events that have occurred at the factory acceptance testing (FAT) stage. The JIP found these were more likely to be significant events that were rectified prior to delivery to the operator – either by pipe replacement or re-termination. There were also a small number of damage or failure events at the load-out stage, but the JIP concluded that these were more likely to require just minor repairs to fix outer sheath damage.

Key Findings

The JIP’s research confirmed what many in the industry have suspected for some time, but have not necessarily had the evidence to support. Despite its challenges, the number of incidents leading to the most catastrophic rupture events is relatively low, with only five reported cases worldwide in the past 10 years.

The majority of reported incidents are still dominated by annulus flooding or damage to the outer sheath, which account for just under 40% of all flexible pipe damage and failure. The vast majority of these were classified as either damage or minor defect with no loss of containment.

Looking at some of the top-line results in more depth, the research showed that of the 584 incidents reported: 451 were cases of degradation which did not result in a leak or rupture; 123 were cases which resulted in a leak; and 10 cases resulted in a rupture. Of the 584 incidents, 465 were related to risers and the remaining 119 are attributed to flowlines and jumpers.

Looking at these results in a little more detail, the JIP database shows that:

• 261 of the total number of cases (45%) were classed as damage: an anomaly that had degraded the piping but not caused a loss of containment. Of these, 231 were incidents of damage to risers, and 30 were incidents of damage to flowlines and jumpers.
• 123 cases (21%) resulted in a loss of containment, which was classified as a leak. Of these 123 leaks, 63 were incidents in the risers, and 60 were to flowlines and jumpers.
• In 94 cases (16%), the pipe was operating with a minor defect that was unlikely to materially affect the service-life capability of the original design.
• The two most commonly reported causes of degradation led to annulus flooding, and in overall terms 212 cases of annulus flooding were reported.
• In a further 86 cases (15%) pipes had been shut down because of an integrity concern, but the damage was not necessarily identified during or following the shutdown.

Finally, 10 cases (less than 2%) resulted in a loss of containment that was classified as a rupture. The majority of these occurred in risers; while only one flowline was affected. These were primarily caused by corrosion of armors and tensile armor wire breakage (fatigue, and cases of overload in smooth-bore risers with an impaired IPS)

Results Analysis

The JIP research showed that the safety record of flexible piping to date is more benign than many had previously perceived. When its typical deployment is taken into account – notably harsher conditions than those experienced by fixed pipe counterparts and the necessary role it often plays as an enabling technology – the JIP concludes that flexible pipe integrity experience compares favorably with that of rigid steel pipes.

The database also shows that, although there have been a number of failures, leaks and ruptures, the industry’s experience is that flexible pipes exhibit a good degree of robustness and structural capacity under a number of abnormal conditions. What’s more, the root cause of reported failures tends not to be associated with extreme design or weather events. Specific examples of occasions where abnormal loading has resulted in damage of varying degrees of severity include:

A mooring system failure that led to a number of riser bases being displaced by several hundred metres. Nonetheless, no ruptures were reported and the only loss of containment was caused by the failure of a topside rigid spool, which was overloaded by the riser. Large dropped objects that impacted on flexible pipes near host facilities. However, investigation and analysis showed that no damage was caused, and the pipes re-entered service following re-validation assurance activities.

A number of mid-water arch (MWA) system failures have occurred that resulted in either multiple risers being dropped on to the seabed or significant abnormal loading. Although the risers were damaged in a number of cases, there were no riser leaks or ruptures. Several of the dropped risers were subsequently revalidated and reinstated.

Despite the relative youth of flexible pipe, the overall damage and failure statistics show that even though new failure modes have been observed as the technology matures, the actual rates of failure have been declining since the mid-1990s. The rates of reported damage, principally on risers, have shown some historical increases.  However, this is largely a result of increased monitoring and testing which identified more pre-existing, and in most cases relatively minor, damage.

In addition to the core work of the JIP, a complementary review of some independent studies also showed that when the incident rate per installed pipe is taken into account, flexible pipe has lower rates of containment loss than those of rigid pipe.

Potential Risks

The results therefore are broadly positive. However, the database does also contain the first recorded incidence of a rupture caused by operational fatigue failure in the main pipe section. Previously undocumented failure modes identified in this latest report include:

• Fatigue failure in the main pipe section.
• Carcass-tearing failures.
• Smooth bore riser ruptures as a result of reverse permeation in pressurised J-tubes.

It was also noted that flow-induced pulsations could occur in unbonded flexible pipes. Although flexible pipe itself is only the initiator and is not directly affected, it is important not to underestimate the potential threats to safety and production relating to failure in connected topside pipework.

It is not unreasonable to assume that the rates of both damage or failure arising from corrosion, fatigue and the ageing of ancillary equipment – collectively regarded as late-life failure mechanisms – will increase from this point onwards simply as a function of long-term deployments.

The database shows that the oldest flexible pipes were manufactured 42 years ago and more than half (56%) of all flexible pipes that have ever been manufactured were supplied in the last 15 years. Nine in 10 were manufactured in the last 30 years. It is predicted that 69% of the installed pipe inventory has had less than 15 years in service.

Operators therefore need to remain vigilant as an increasing proportion of their flexible pipe deployments get closer to or enter late-life state. As experience of working with flexible piping remains comparatively limited within the industry, continuous effort is still required to ensure that integrity management and good practice guidance are kept up-to-date.

Conclusions

In light of demands from ongoing projects for greater environmental, structural, and service life performance, the good practice guidance from the JIP report is particularly apt.

Flexible pipe is a specialist product, with distinct differences to rigid steel pipe. Historically, however, flexible pipes have often been inspected, repaired, maintained and assessed by personnel with limited knowledge of flexible pipe technology.

There is now an established acknowledgement within the industry that flexible pipe integrity should be managed by individuals and teams that have demonstrable competence and experience in the life cycle of flexible pipe systems. To that end, the JIP report includes a comprehensive Inspection and Monitoring Technology Review to help operators identify where and how the threats identified in the report can be mitigated.

The report also acknowledges that many operators have now established long track records of applying integrity management techniques detailed in prior JIP reports. Combined with improvements in materials and design of flexible pipe this has led to greater confidence among operators in the effectiveness of their integrity management methods for regulatory compliance and beyond.

However, as the report indicates, new threats are now coming online and so the JIP has also created a standardised reporting template for flexible pipe damage and failure – along with guidance for its use.

Finally, the report itself makes a number of key recommendations on flexible pipe safety as follows:

• Operators should consider the guidance presented in the report relating to the global experience of flexible pipe usage, damage, and failure when assessing threats to their own flexible pipe systems.
• Annulus flooding events remain the most prevalent cause of damage to dynamic risers. However, dynamic risers are increasingly designed for flooded (albeit de-oxygenated) conditions, and flooding of flowline annuli may have limited impact on service life. Operators should make every effort to identify, mitigate and remediate any anomalies leading to annulus flooding, in case it was not fully considered at the design stage.
• As with previous integrity guidelines, operators should perform risk-based integrity management programs to identify the specific threats to a flexible pipe system. The damage and failure mode descriptions detailed in the JIP report provide comprehensive input into such a program.
• The statistics gathered as part of the JIP database should be kept up-to-date to provide an ongoing understanding of technology advances and trends in damage statistics. Finally, the JIP recommends using its standardised reporting template for flexible pipe damage and failure, and in cases of unexpected failure or with significant consequences that a root cause analysis is performed and the findings shared with any relevant manufacturers for the betterment of the wider industry.

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