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Pipeline Walking Mitigation

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Pipeline walking is unique challenge for industry caused by thermally driven cyclic expansion resulting in the pipeline walking progressively away from or toward fixed end terminations.

Walking is typically arrested by either applying anchor piles clamped to the pipeline, conventional concrete mattresses or rock dumping. One disadvantage of rock-dumping and concrete mattresses is that the bulk of the additional mass applied is wasted because this is transferred straight to the seabed instead of the pipeline. Approximately 80% of the weight transferred subsea is wasted meaning logistics and and installation effort is also wasted.

Piles typically have to be applied at the pipeline end point, increasing the total restraint load required by up to 10 X with a proportional increase to installation and material costs.

In response to these disadvantages, Subcon and Shell Global Solutions (Shell) developed a new walking mitigation method, the “Pipe Clamping Mattress” (PCM). PCMs address all the challenges faced by operators implementing conventional methods with one simple solution that can be retrofit using common light construction vessels and without removing coatings.


The key design driver of the PCM is to mobilise 100% of the anchoring weight into the pipeline and using the seabed – pipeline friction coefficient to provide the walking restraint load.

This greatly reduces the material required for restraining the pipeline, providing significant cost savings in materials, transport and installation.  Typically  up to 80% of the weight of concrete mattresses or rock goes onto the seabed, not into pipeline restraint.

In addition to this, the PCMs can be installed precisely at the virtual anchor point.   The designer can optimise the restraint load requried by selecting a virtual anchor point requiring the the least amount of restraint load to mitigate / arrest the pipeline walking.

The PCM to seabed friction coeefficient depends on the drainage characteristics of the soil type.  Typically soft clays drain slowly and seandy seabeds drain quickly.

Centrifuge testing we have performed at UWA’s Centre for Offshore Foundation Systems (COFS) has investigated the following soil types: Kaolin Clay, Philippine Angolan Clay and Calcareous Sandy Soils from Australia’s North West Shelf.

The testing has shown that whilst the PCM element may have a reduced friction coefficient, the important response is that of the PCM-Pipeline system as a whole.  Testing has shown that embedment causes the axial restraint of the system to increase as the soil consolidates under cyclic loading.

For a sumamry of test results please contact one of our engineers here.

The PCM clamping surface is tailor made specifically to suit the OD of your pipeline including coatings.  The clamping surface is lined with rubber to account for any local deformations and coating OD tolerances.  We recommend a full scale FAT be performed to ensure the clamping angles and tolerances are correct prior to commencing a full production run.

The clamping loads are less that 1MPa which is considerably less than typical tensioner and stinger installation loads.

See also “Can the pipeline or its coating pull out from under the PCM”?

To discuss FAT results specific to your application please contact one of our engineers here.

This is a common misconception.  The restraint is generated by the pipeline to the seabed friction, not by the PCM to seabed (as per a traditional mattress) and so very little axial load is applied to the PCM.  The PCM  is in fact sitting “piggy-back” on the pipe, so if the pipeline does move, the PCM simply moves with it.

Even in the event that the pipeline and PCM buries, the soil generates very little axial load into the PCM.

In actual fact, the PCM’s stop all movement and so no burial or axial loads can even occur.

To discuss further PCM’s specific to your application please contact one of our engineers here.

The rubber clamping surface is bonded with an adhensive to the concrete jaw.  this bond is only requried to withstand transport and installation loads.

Once in place, the PCM to pipeline connection experiences minimal shear loads and the clamping force ensures the rubber stays in the jaw.

For more details s on how the PCM piggy backs on the pipeline refer to:  “Can the pipeline or its coating pull out from the PCM?”

To discuss further how PCM’s can be applied specific to your application please contact one of our engineers here.

Seabed slope matters for all restraint methods.  Mattresses, rock, piles and PCMs are all impacted by seabed slope.  PCM’s are unique in that in some cases they can provide benefit from the slope!  However it is typical that virtual anchor points do not occur on signifcant slopes or in steep valleys.

Often its the rogue buckles which occur where there is a change in seabed gradient.  In these cases we’ll go looking for the Virtual Anchor Point which invariably will be a suitable place to locate the PCMs.

The direction and angle of the slope may or may not act in the direction of walking.  Small angles has small impact.  Large angles mean we would look for a more optimal location to place the PCMs.

Predicting buckles and walking is a fickle business.  As not every pipeline will experience walking PCM’s allow operators to take a wait and see approach to Walking Mitigation.  PCMs can be readily applied to live pipelines after the pipeline walking is demonstrated.

Alternatively, they can be deployed during pipeline installation.  THey can even be used to restrain pipeline end expansion, thereby reducing expansion spool size and installation costs even further!

Shell holds the patents for the PCM technology and retains ownership of the intellectual property.

Shell has granted Subcon the exclusive, global license to supply the PCMs to market.

The technology is then shared with contractors and operators on a project by project basis.

The good news is all our design innovation, project experience and test results can be made available to all operators so that PCMs can be deployed not only on Shell projects, but world wide for all field developments.

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AOG2020 Subsea Forum Presentation
Featured Case Studies
Pipe Clamping Mattress Brochure

Featured case studies