|Experience can play a major role in the development of an effective flexible pipe integrity management system (FIMS) and plan.|
Unbonded flexible pipes have been deployed in the global oil and gas industry for more than 30 years, and historically they have proved their extreme reliability. However, like all long serving products, they can suffer integrity problems over time, and a lack if inspection or maintenance can be one of the biggest issues affecting their performance.
One of the most common integrity issues relating to flexibles can be mechanical damage to the outer sheath, or a failing of the venting system leading to a pressure breach of the outer sheath.
Globally, about 35% of all flexibles experience an issue of this kind. This problem particularly affects flexible risers as opposed to flowlines. Given the number of pipes that suffer outer sheath damage, and the history of failures in pipes leading to loss of containment around the world related to this, integrity is clearly an issue that needs to be addressed.
During operation, a buildup of gases can occur within the annulus of the flexible that need to be vented.
Subsea pipelines are less susceptible to this issue because of the hydrostatic support of the seawater around them. A breach in a subsea flowline experiences fewer corrosion problems, due to reduced oxygenation of the seawater and support from the flowline’s cathodic protection (CP) system.
By contrast, risers do not enjoy the same near-sea surface hydrostatic support. When over-pressurised, the outer sheath of the riser will tend to burst at the sea surface, or splash zone, where it is least supported and where there is a higher incidence of oxygenated seawater and sporadic – or no support – from the CP system. This can create significant general corrosion problems and lead to a product which is unfit for service.
The service life of the riser can also be reduced due to corrosion fatigue issues associated with a seawaterflooded annulus.
Corrosion can significantly reduce the operating life of a pipe, which can then lead to failure and potentially a risk to life or the environment.
As an example, for a typical riser of about 350m in the UK North Sea, the financial impact has been estimated by Flexlife at approximately US$5 million, once vessel time and replacement costs have been included. Add to that a current lead time of at least a year, with all the implications associated with deferred production, and it becomes clear that effective repair and ongoing maintenance of risers is a much more cost-effective solution.
|Regular inspection, monitoring and repair can extend the operational lifespan of the flexible, optimizing production and mitigating risk.|
The North Sea has traditionally led the way in developing robust and proactive riser inspection and integrity plans on the back of stringent regulation. However, regulatory requirements around the world differ. An outer inspection by an ROV may miss outer sheath damage leading to internal issues in such a complex structure as a flexible pipe. It only takes a hole the size of a thumbnail in an outer sheath, letting seawater in, to potentially create localized corrosion and corrosion fatigue.
Understanding the risks associated with riser integrity is critical to the optimal effectiveness of the system. By deploying inspection and repair techniques as early as possible the operational life of the pipe can be extended.
Taking a step further back, the development of appropriate integrity systems and planning can actually help prevent problems occurring in the first place.
Approaching the problem
There are a number of approaches a company can take in ensuring its riser integrity is maintained and the pipe performs at the optimum. One approach is to involve a partner that understands flexible pipe, including its design and manufacture, at the earliest possible juncture, to provide a detailed understanding and analysis of the risks that may be encountered by the pipe in operations, and how those risks can be mitigated in its design.
A key element of this approach is the implementation of an appropriate and ongoing risk-based – rather than prescriptive, time based – integrity program. This should start from the very beginning of structure selection through the testing and installation stage and beyond, and will consider such factors as damage during installation, operational risk factors, and the design of operational systems, such as the annulus venting system. It will incorporate a regular inspection program to identify any immediate issues, or warning signs which will require action at a later stage, as early as possible.
Another approach is where a company installs a pipe and decides retrospectively that an integrity system and plan is required, or they are faced with an immediate issue and require a solution. Of course an integrity plan can be created at that stage but activity is very much reactive. Historically within the industry, the retrospective program has been the more commonly adopted approach.
As an example, the majority of Flexlife’s inspection activity relates to older pipes in excess of five years old. Of these, approximately 40% have shown either outer sheath damage or venting system issues.
A third approach is the knock on effect from field life extension, where a client requires to understand the predicted ability of a pipe to cope with potentially new demands. In this situation, a review of the pipe as soon as possible is recommended to assess any damage to it and to calculate the degree of impact that will have on the remaining service life.
All three approaches require the deployment of modern and novel inspection technology.
A relatively straightforward, quick and cost effective method to determine whether or not a pipe has an integrity issue is an annulus test where nitrogen is introduced under positive pressure into the annulus, or a vacuum is created.
In both methods, which involve no reduction in production, the aim is to establish the size of the free volume in the annulus, which will provide information on the outer sheath integrity and therefore any seawater ingress.
It is a test which can be undertaken at any time in the pipe operating life, but the sooner it is carried out the better, preferably at the installation stage. Repeating this test based on a risk-based timeframe can help identify any annulus issues at the earliest possible stage.
|Flexlife is expert in providing life-of-field subsea engineering and technology globally.|
As testament to its experience and expertise in the sector, Flexlife, has carried out approximately 200 riser annulus tests for most of the major operators globally, and the company has also established a strong share of the UKCS market in this field.
In one particular case study, the company carried out an annulus testing project for a client on a North Sea field where seawater flooding was found in some of the annuli of the risers. Flexlife encountered that some of the older risers had a completely blocked venting system, preventing an assessment of their condition.
The project involved the Health and Safety Executive (HSE) because a riser inspection was not possible, contrary to industry practice. Because the condition of the pipes was not known, there was also a risk having to shut down production, a consequential loss of revenue and a real concern that the installation may never resume production.
Flexlife deployed the ultrasonic scanning tool, Neptune, a ROV-mounted subsea inspection tool designed by Oceaneering that utilizes Flexlife’s patented UT scanning technology in order to determine the state of a flexible riser’s annulus with 100% accuracy.
Data gained from Neptune scanning provides invaluable information relating to the condition of the flexible riser annulus. The inspection tool also enables operators to make informed decisions regarding the remnant life of these production-critical assets.
Since Neptune does not require access through annulus vent ports, this method is currently the only means of accurately determining the integrity of flexible pipe annuli on risers with restricted or inaccessible vent ports.
The inspection tool was able to categorize precisely where seawater was prevalent within the riser’s annuli, enabling an inspection of the armor wires during operation to measure for corrosion.
Fortunately in this project minimal corrosion issues were detected. The information was reported back to the HSE, which allowed the pipes to continue to operate. The risers were changed out at the operator’s schedule, thereby minimizing down time, the number of replacement risers and delays in production, saving several million dollars.
While Neptune provides detailed pipe inspection, ongoing oxygenated corrosion can be mitigated through the use of Flexlife’s modular Armadillo system, which can be constructed in bespoke lengths and diameters to encapsulate and seal known outer sheath damage.
The Armadillo will prevent the ingress of further oxygenated seawater into the annulus, and where the pipe structure has been proved to be suitable for continued operation through engineering, allow that pipe to continuing operating.
The Armadillo is clamped around the flexible at the location of the outer sheath damage and is filled with a sealing fluid, which seals off the damaged section of the riser. The repair gel cures to form a solid while the elastic nature of the gel and design of the Armadillo permits the riser to flex as normal whilst continuing to prevent seawater ingress.
For more restricted areas, the company has developed FlexGel, which is intended to be deployed around an annulus breach within an I-Tube or J-Tube, displacing the seawater. In doing so, the gel reduces further corrosion in the structural tensile armour wires of the flexible pipe, increasing its life.
There are a range of solutions available to solve riser integrity issues, from lowcost identification at the early stages to detailed inspection of the condition of the pipe through to the repair of outer sheath damage and an engineering assessment of the pipe’s life expectancy.
However, establishing a program at the outset to appropriately design and regularly inspect a pipe, which would avoid these issues, represents a significantly lower cost and answers the risk question from the very beginning.
Stuart Mitchell serves as global business development director for Flexlife, the life of field subsea engineering and technology firm, of which he is a founder. A degree-qualified chartered engineer, Mitchell has worked in the subsea R&D, technology and engineering space for 18 years and has specialized in unbonded flexible risers for the last 13 years. The inspiration for Flexlife came when Mitchell realized, through his own experiences that the available solutions to some of the most common subsea problems were not fully meeting the requirements of the market. Seeing a gap in the marketplace for an innovative consultancy offering its own patented solutions to these problems, Mitchell, with the help of his partners, established Flexlife in 2007.