To protect an object lying on the seabed – such as a flowline – any impact energy 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’s engineering team recently 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:

Protective structures Use of a protective structure which is not in contact with the flowline will stop a dropped object reaching the flowline. 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 flowline. 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 flowline and the dropped object.

Elastomeric coverings Covering the flowline in soft, elastomeric material which compresses under load offers a number of benefits including:

• The deceleration of the object by slowing it down during the compression of the polyurethane. This does not rely on destructive crushing of the covering itself
• The spread of load over a longer length and greater width of the flowline
• A high resistance to damage resulting in continuous protection of the flowline, 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 flowline for support therefore part of the energy will always be transferred into the flowline. The covering will, however, reduce the force.
Which system should be specified?

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

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 softness.

Duraguard provides protection by acting like a spring on top of the flowline thus reducing the transferred force and potential damage. It is recognised from the ‘Energy into Force’ equation that the softness of the elastomer affects the force transmitted therefore the softest material possible is 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 Duraguard elastomeric covering will give an immediate 30-50% benefit by changing from an impact load to a more slowly applied one. The thickness of Duraguard may be varied to meet protection requirements and budget parameters while larger diameter flowlines will benefit from thicker coating. Duraguard can also be specified to protect anti-corrosion coatings and external sheaths.

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 - standard lengths are 1, 1.5, and 2m. Duraguard is installed using interlocking ends, longitudinal coupling and snag-free fastening to provide continuous lengths.

The system is 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.

Duraguard HD™: Protection and ballast
If your 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 ranging from 2300-11,000kg/m3 can be achieved with Duraguard HD.