The world’s need for energy has resulted in a growing thirst for submarine power lines, some connecting windfarms, some connecting countries.
However the rising number of operations that can see thousands of metres of heavy cable laid out on the sea bed has also thrown up issues for both vessel and onboard equipment.
The biggest issue is that ‘cable’ can mean so many different things, and while existing telecoms vessels seem an obvious choice of ship to reconfigure, there are quite a number of elements to evaluate says Jon Foreman of ETA. Underpinning the challenge is that telecoms cable and power lines have very different characteristics. “Fibre optic cables are lighter, smaller and more flexible,” he explains.
While most submarine cables are coilable designs, which means the cables can accept the twist that is formed with each turn as the cable is stowed, some types of submarine power cables have contrahelically wound armour wires that can’t be subjected to a radial twist; these have to be loaded onto a carousel or turntable, not straight into a tank. This means an operator is faced with nothing less than fitting a carousel inside the tank space, or converting a flat-backed DP2 vessel or barge to take a reel.
Even if the tank storage onboard a telecoms vessel is feasible, “you still have size, weight, stiffness and bend radius to think about” says Mr Foreman: “Whereas you can subject a telecoms cable to a 1.5m bend radius, a power cable can demand a minimum bend radius of between 2m to 5m,” he explains. Further, there is still the coiling radius to consider: as this can be as much as 6m, it can be even more demanding – so you can find the tank needs not the usual 3m telecoms cable cone but a massive 12m centre, “which leaves very much less room for the cable”.
Further, whereas you can load a telecoms cable into a tank using a handful of crew, when it comes to power cable it’s a different story.
Light weight telecoms cable is only 0.5kg to 0.8kg per metre and even the heavily armoured variety is only around 7kg to 9kg/m, very different to its power carrying subsea cousin which commonly weighs anywhere between 20kg and 90kg/m. “This puts it beyond the ability of the crew to handle without extra mechanical assistance; so if using a tank power cables need a coiling arm to help lay the cable down,” says Mr Foreman.
These factors tend to combine into a fairly knotty challenge. For example, converting the Maersk Responder for its contract on the Jindo-Juju HVDC connection project in Korea meant extra capacity, so alongside the coiling arm equipment for two new 20m diameter deck tanks there was also the upgrade to its existing pair of static cable tanks. “All of it fitted in a very tight space,” adds Mr Foreman’s colleague, Keith Morris: “Both designing the below deck coiling arm equipment and fitting it onboard proved tricky. We made each part to get through the available gap but we were still down to less than 10cm clearance each side.”
Another important difference between the two kinds of loading operations is that unlike telecoms, power cable requires loading from some height to allow it to coil: the necessity for such a big drop means a vessel may need reduced stack height in an existing tank or a very sturdy gantry tower. When configuring this last option, ETA favours putting another tensioner on top of the tower. “This helps draw the cable up rather than relying on loading it just by pushing it from the bottom with the deck tensioner,” says Mr Foreman. “It can also help during the lay in pulling the cable out of the tank,” he adds.
However, all this adds another layer of complexity that can reach beyond the equipment itself to impact the entire ship: even a couple of extra tonnes at the top of a 20m gantry has the potential to create further issues. “We have to look at the overall vessel at this point, which means checking the deck strength with regard to dynamic loading and sometimes taking on stability calculations for the whole vessel,” he says.
So, Mr Foreman concludes retrofitting telecoms ships to power cable deployment necessitates “a balancing act”: identifying the challenging details while at the same time keeping an eye on the bigger picture.
By Stevie Knight
