‘Ripping out the old and coming in with new’ isn’t always the most cost-efficient approach when it comes to lifting gear upgrades but tying Active Heave Compensated (AHC) equipment into pre-existing kit risks losing potential savings amongst the technical difficulties. Till now.
Firstly, Johan Sol of Netherlands based Seatools told MJ that all lifting equipment – recent or not – inevitably comes with its own ‘character’, so the company’s easy-integration HeaveMate compensator has been designed to make up for any imperfections. For example any lagging hydraulic valve behaviour or ‘dead’ zones can be automatically corrected, even on third party apparatus and it can equally be applied to electric winches, cranes, LARS and gangways.
Moreover, it is able do the job even if the new AHC gear is based on an older passive heave device. The attraction is that “putting the two together can be a cost-effective solution”, he explained: “By using the existing passive heave compensator to take, say 80% of the load then the active heave side only has to take the remaining 20% and the dynamic element can therefore be relatively small.”
Plus, he added the actual conversion requires less time, there’s a far shorter delivery lead-in compared to a complete new AHC system, and more, the vessel can be returned to the initial configuration if need be.
But it is complex and resonance can be an issue. This can be thought of as the ‘trampoline’ effect of having two systems both working at the same time and on new-build systems the design is adjusted to mitigate any possibility of a clash. “However, this is often not possible in retrofit projects – but as the HeaveMate package is designed for both new systems and retrofits, we have armed the controller to avoid these resonance problems.”
Secondly, even larger gains can be made because the company uses simulation software to prep the ‘brain’ of the device – the controller - before the hard engineering begins.
“If you don’t use a simulation you’re then starting with a ‘blank’,” explained Mr Sol. This often means a long iterative commissioning process, “changing the settings step-by-step”. Worse, it’s dependent on the environment: “The main deficit with this methodology is that you cannot test out all kinds of sea states as you can’t dictate the weather. So the controller might be performing well in certain conditions but you still won’t be certain how it performs in others.”
The simulation is pretty sophisticated and very thorough: “We start with the basis of the design, collect the primary requirements including load, vessel behaviour, wave amplitude and spectrum and so on... then we go on to select actuators and interfaces so we have the outline of the components – and further we programme in mechanical inertia, valve behaviour, the impact of temperature changes and so on, and if it’s a gangway, then we extend the simulation to reach between its tip and the platform.”
“Next, we go on to run the designed system in ‘virtual’ conditions that mimic the real world. It means we can see at an early stage exactly how the system will behave even in extremes - and we can simulate various failure modes, whether this is sensors, components or other elements and that way we refine the redundancy strategy,” he explained. All this happens “before we start with the real engineering”.
Further, it’s a system that can take on the extremes: “We have delivered a control system for an offshore gangway which was able to compensate more than 7m of heave in both directions - a total of over 14m,” he said.
Cutting to the chase – how much time will be saved on commissioning using these strategies? “We think it is fair to say you’d save at least a couple of days,” he said, but, it could be far more than that if, for example, the alternative was completely altering the controller strategy, “something which can take weeks”. Further, in reality there’s often a limit to set up time: “Of course, without running a simulation, there is also the chance that in the end the vessel has to get to work and further tuning is skipped,” he said, the result is “sub-optimal performance”.
Lastly, it’s not just the equipment that gets prepped up: “The way our controllers are programmed allows using the same mathematical models for building realistic training simulators,” he explained, so operators’ efficiency stands to gain too.
By Stevie Knight