Staying Safe Whilst Subsea

Over 30 years ago, offshore wind farms started to emerge in Europe, with slow adoption through the turn of the millennium, to almost meteoric growth over the past decade, and ever more coming online as 2030 comes closer. Development focus has led to great gains in the most visible part (the turbine and its blades), but as a result, is the whole system being mistakenly labelled a mature technology not in need of further investment and improvement?

As with any system, it is only as strong as its weakest link, and some are beginning to argue that its very connection to people’s homes might just be that. Given the environment in which these systems are immersed, and the unforgiving nature of coastal waters, small risks are being exacerbated, but through the lessons of failure, new approaches are uncovered to mitigate, analyse, and eliminate them.

To facilitate the world’s burgeoning thirst for renewable electricity, submarine cable production has been increasing like never before to meet with the demand for offshore wind, with the cumulative deployed length of cables now exceeding the diameter of the planet, and an eight-fold increase coming online within the decade. Thus, any risk to their integrity could impact the entire industry and, without cable redundancy on most fields, any single point failure could lead to considerable loss of power to homes and businesses.

Cable protection products

CRP Subsea has spent nearly half a century refining its cable protection products, to either offset cable weights, or to protect them from abrasion, overbending, and fatigue. With over 100 000 buoyancy modules deployed and over 300 km of cable protection supplied, the product’s tenure is not in doubt. But these numbers, whilst impressive, pale into insignificance when compared with the lengths of cables now being deployed globally. However, CRP Subsea’s focus is not the overall cable itself, but arduous hot spots along it where, without additional support, it may suffer catastrophic failure.

One such location is the transition of the cable from the security of burial in the seabed toward its connection to the turbine’s monopile. Cable management during the installation of such inter-array cables sees great care taken in the cable’s submergence and movement toward its connection point, offsetting historical installation failure risks. The cables are laid in such a way as to eliminate overbending or stress – as they are pulled toward the connecting turbine, they are provided with protective cladding (which accurately controls bending radii) as winches on the turbine pull them ever closer, then upward into the turbine’s monopile without a kink. This protective cladding generally meets the point of entry of the monopile, which could be a cut orifice or j-tube, where the cladding becomes permanently affixed to the pile, permitting the subsequent cable to be hauled through it without overbending. This cladding, evolving to support safe cable installations, can then also be used to strengthen the elevated section from the heave and sway of waves and tidal currents as they accelerate around the pile and over the sandbank. Thus, the cladding, or cable protection system (CPS), has a dual purpose: 1) to aid installation, and 2) to mitigate this hotspot’s operational response to the hydrodynamic environment. The primary goal of both purposes is to reduce overbend and excessive movement.

Prior to the cable’s insertion into the pile, a blanket of rock is laid to protect the pile’s foundation from localised erosion, created by the water currents accelerating around it. This scour protection, which the CPS sits atop, introduces the first operational risk to the cable’s integrity: abrasion. The second most prominent operational risk involves a slower, but no less severe, deterioration. As the cable moves back and forth in the water current, it undergoes cyclic tensile strain, which will inevitably lead to cable fatigue failure.

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Read the article online at: https://www.energyglobal.com/special-reports/27122022/staying-safe-whilst-subsea/