Integrity challenges for outer sheath damage on flexible pipes

I've been involved in flexible pipe technology for nearly 20 years now and specifically over the last 12 years I have been heavily involved with companies who manage the integrity of flexible pipelines once in operation. This area of operational management is highly interesting but also incredibly challenging.

Flexible pipes differ in many ways to their rigid cousins, although the ultimate goal for both technologies is the transportation of oil or gas medium under potentially high pressure and temperature from one source to another, however a flexible pipe needs to undertake this operation within dynamic conditions.

Rigid pipelines have been utilised over a long period. They have been used extensively onshore and in shallow water offshore with testing that can be easily replicated or accessed. Therefore almost all rigid pipe failure modes can be accurately documented and shared amongst the oil and gas community. Flexible pipes however are a relatively 'new' technology in comparison, although their existence dates back beyond the second world war they have really only been used in anger since the mid 1990s. In addition, integrity solutions and lifetime predictions are still hindered by an air of secrecy and a reluctance to share lessons learnt between the manufacturer and operators which stems from the limited number of competitors in this market place.

The Impulse Group set out nearly 5 years ago now with the mission to try and close this gap, to provide integrity management which was sensible for the operator and to communicate these lessons to the manufacturer so that more can be achieved within a stretched budget. It's been a hard slog to say the least and we are still a long way from understanding and working together for the good of integrity but one area in particular interested me greatly and that is armour wire corrosion within the splash zone.

For many years, I have seen companies involved in flexible pipe integrity use tactics to scare an operator to utilise their technology with the most obvious being the use of splash zone corrosion on pipelines with damaged outer sheaths. The basic understanding of flexible pipe integrity is to utilise the design data, operational conditions and dynamic loading to understand the true risk of failure to a pipeline, then undertake a sensible and cost effective approach to testing those theories.

Annulus testing is the first stage for most operators to toe dip into flexible pipe integrity management, probably because it is a recommendation of API to do so. However, undertaking these tests and interrogating the results can provide an operator with valuable data. Completed with competence and knowledge the annulus test can provide one of four potential conclusions for the flexible integrity. 1. A closed system with no breach, 2. An open system with a breach in air, 3. An open system with a breach in the splash zone (<20m water depth) and 4. An open system with a breach below the splash zone.

It's in these latter two scenarios where flexible pipe integrity management and intervention play a key role. Splash zone integrity is key to understand, predict and repair if necessary in order to continue to operate the pipeline successfully to its design life and beyond. Our annulus testing has been developed with this in mind and is robust and accurate enough to analyse these scenarios without the need for further scanning or expensive interrogation.

So, what if annulus testing identifies that you have a breach in the shield within the splash zone?

This is a scenario many operators face, the nature of a flexible pipe means that it is tied in after the installation of the production unit. This means pull in is under dynamic motion and the pipe is often supported through the most dynamic and high fatigue regions with the use of a J-tube or conduit. Installation is challenging and often results in damage to the pipeline within the splash zone and it is important for operators to understand that the risk of failure is higher for pipes with damage in this region.

Why is a pipe more at risk of failure with a breach in the splash zone?

The flexible pipe outer sheath is designed to prevent the ingress of seawater, unlike umbilicals the wires which are providing the hoop and axial strength to the flexible pipe are high strength carbon steel and so will corrode under certain conditions. The sheath helps to protect this from occurring. In water depths beyond the splash zone, the wires are protected by anodes, there is continuity to the end fittings and cathodic protection kicks in and corrosion will arrest when the oxygen within the annulus from manufacturing exhausts. In the splash zone, the water is constantly re-oxygenated and the area of damage will not be effectively protected by the continuity to the anodes and so cathodic protection will be redundant. It is in these areas where solutions need to be found to protect the carbon steel wires from excessive corrosion. This corrosion will not necessarily lead to pipe failure but coupled with gas permeation from the bore and dynamic fatigue it can and has lead to catastrophic failure of flexible pipelines.

What options do operators have in these scenarios?

Significantly damaged pipelines may need to be replaced, the annulus test coupled with an integrity review of the pipeline operation over its service life will give us a good idea of what risk there is to failure. In some scenarios there may be an option to perform a partial recovery but with head height being an issue for over-pull and the expense of mobilising personnel to undertake such an operation and the cost of shutting down the pipeline for repair often makes this method too costly. The most cost effective solution is to undertake a repair of the outer sheath so that the area is protected for the remainder of the service life. Obviously a breach in the pipeline will flood the riser annulus and there is no methodology available to exhaust this water ingress but ensuring no further re-oxygenation occurs and allowing annulus gases to purge past the breach to the topside system allows a much more robust and predictable model for operation.

How can I repair the outer sheath?

Well I'm glad you asked that question! What Linked-in article would be complete without a free plug for a product. It just so happens that The Impulse Group in partnership with our polymer product supplier Supergrip, have been able to design and fully test an outer sheath repair system which not only seals the flexible pipe but allows the annulus to re-establish. We do not need to flood the product with gel or epoxy which would block the annulus thus allowing communications to be re-established with topside integrity. This allows the operator and their integrity management consultants to reduce the risk of failure and operate a pipe to a successful decommission. Our products are fully tested in house and witnessed by third party, we have an extensive record of providing seal and polymer products to many of the major operators and we are growing our portfolio rapidly.

We have aptly named our system the 'Flexible Repair System' or FRS for short, it consists of interlocking polymer modules similar to those used for bend restrictors. Each interlocking unit is sealed against the next providing a water tight seal across the whole string of components.

Installation is simple, no requirement to depressurise or stop production, easy to install by rope technician, diver or we can incorporate ROV grab handles. The system simply fits over the breach and is energised by bolting up the end clamps, nothing more complicated than that. The clamping operation energises all of the seals across the unit and that's it, ready to go. If you would like further information then please visit our website www.flexiblepipes.co.uk or drop us an email at [email protected] and we would be glad to help.

On a final note, integrity management of flexible pipes is complex, the nature of the product makes the models of failure prediction difficult. As an industry we have lots to learn and lessons to share. Flexible pipe integrity and operation is something we are passionate about here at TIG and we very much hope we can stay central to these learnings and continue to work together to build intelligence, products and services which are cost effective, efficient and appropriate for the challenges we face in our industry today.