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Cable and flowline protection: A dropped object research study

Hundreds if not thousands of kilometres of cable are used by the energy sector carrying power and communication signals to and from installations and vessels. Cables lying on the seabed require protection from many things including dropped objects and the energy generated by such an impact must be deflected, reflected, absorbed or spread over a larger area.

 

In order to provide clients with a thorough understanding of the benefits of polyurethane elastomer cable and flowline protection, Balmoral Offshore Engineering’s technical team undertook a review into the various methods of cable protection available. A brief summary of the research results follows below:

 

Recognised protection systems

There are three generally accepted methods of protection:

Concrete protective structures

Use of a protective structure which is not in contact with the cable will stop a dropped object reaching the cable. This is an expensive option and consequently not generally specified.

Protective covers

Protection is achieved by placing concrete block mats directly on top of the cable. Generally, these mats are not sufficiently tensioned to create a structural assembly meaning that the protection is created by the presence of a concrete block between the cable and the dropped object.

Elastomeric coverings

Covering the cable in soft elastomeric modules or matting which compress under load and offer a number of benefits including:

  • The deceleration of the object by slowing it down during the compression of the polyurethane elastomer. This does not rely on destructive crushing of the covering itself
  • The spread of load over a longer length and greater width of the cable
  • A high resistance to damage resulting in continuous protection of the cable, coating or external sheath.

The elastomer covering absorbs part of the dropped object energy as heat, due to hysteresis in the material, therefore the dropped object will be partly reflected back.

Elastomeric coverings are not structural as they rely on the strength of the cable for support therefore part of the energy will always be transferred into the cable. The covering will, however, reduce this force by a significant factor.

 

Balmoral cable protection
Balmoral Duraguard undergoing drop weight impact testing

 

Balmoral cable protection FEA analysis
Computer analysis of a 100mm diameter bar dropping onto a 250mm protected pipeline. The pattern around the bar clearly illustrates the energy absorption and spread provided by Balmoral Duraguard coating.

 

Which system should be specified?

Theory shows that a sudden impact load will create up to twice the pipe stress than if the same load is applied more slowly. This is a complex subject but it does suggest that installing elastomeric cable protection will give an immediate 30-50% benefit by changing from an impact load to a more slowly applied one.

Balmoral Duraguard™ provides protection by acting like a spring on top of the cable thus reducing the transferred force and potential damage. It can be seen from the ‘Energy into Force’ equation that the suppleness of the elastomer affects the force transmitted therefore the softest material possible is specified.

The thickness of the protection may be varied to meet project requirements and budget parameters; larger diameter cables will benefit from a thicker module. Duraguard can also be specified to protect corrosion coatings and external sheaths.

A combination of methods can be used to optimise the overall protection; for example, concrete mats may be laid over an elastomer protected flowline. The mats will provide load spread and protection in the case of gross impact while the elastomer will provide further impact reduction, shock absorption, load spread and protection. The seabed may also absorb a large proportion of dropped object energy depending on its formation.

An effective alternative to the concrete protection described above is elastomer matting. During the manufacturing process polyurethane matting is filled with barites that provide ballast and prevent tidal movement while the grooved element design allows flexing and separation of the protected lines. Elastomer matting is more easily handled than concrete and is ROV and diver installable.

If a project requirement demands cable protection with added ballast, Duraguard HD (High Density) provides much more than straightforward abrasion and impact resistance. Heavy filler materials are added to the Duraguard mix to increase density and overall weight. The ballast provided gives extra on-bottom stability while the added mass improves dynamic response thus reducing the risk of clashing.

Densities of up to 2000-8000kg/m3 can be achieved with Duraguard HD.

The options described above provide a range of benefits and have differing cost implications therefore the engineer must review the requirement and select the most appropriate system for the project.

 

Manufacture and installation

Duraguard is manufactured from resilient, fatigue-resistant polyurethane elastomer and is available in a range of diameters and thicknesses depending on the level of protection required. Duraguard is supplied as standard lengths of 1.0, 1.5, and 2.0m and is installed using interlocking ends, longitudinal coupling and snag-free fastening to provide continuous lengths.

The system can be installed prior to or during laying procedures and is fitted by placing the two modules around the cable or flowline and banding them into place, usually at 300mm intervals, using integral or circumferential banding systems.

 

 

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