7 minute read
New power generation
NEW POWER GENERATION: NEXT-GEN WTIV SOLUTIONS
Closed bus solutions and ring connections are among the power solutions being introduced aboard the latest generation of wind turbine installation vessels, Stevie Knight hears
Turbine sizes have already reached the 14MW mark, their growth triggering a blossoming of super-sized installation vessel designs with kit to match, capable of taking all the components to site and installing them from foundations to blades.
But taking OIM’s BT-220IU and Knud E Hansen’s Atlas C jackup with their respective 2,600t and 3,000t capacity cranes as an example, what are the implications for onboard power?
“The cranes are getting taller, the sizes larger,” says John Lindtjorn of ABB, pointing to Shimizu’s latest 142m, 28,000gt jack-up. This can carry and complete the installation of three 12 MW wind turbines in five days using a 2,500t capacity crane with a lifting height of 158m. The WITV’s legs will also extend far enough to enable operations in 65m deep waters.
To do all this, it relies on six gensets, four 4.63MW engines, with a pair giving 1.425MW: these power three, 3.8MW main propulsion drives, a couple of 3.2MW tunnel thrusters and a single retractable unit of the same output.
Despite the scaling up, “it’s all well within what’s possible”, Lindtjorn adds, but when you add costs and environmental concerns to the mix, things become a little more complicated.
“Typically, ships divide up the power plant so that a failure will be limited to one bus section,” he comments: “That makes fault handling fairly simple - every block takes care of itself.”
But, he points out, “this has an impact on efficiency, you tend toward low partial loading on each engine, more black smoke and soot, higher running hours and shorter maintenance intervals”. Plus there’s no load sharing, so each section has to be sized for the potential peaks. All this impacts cost and environmental performance.
Alternatively, “you could have a closed bus, or further, a ring connection”, a strategy chosen by a number of these new generation WITVs, including the Shimizu jackup. This results in higher efficiency, a lower carbon footprint - and potentially smaller engines. With ships of this scale, that can make a big difference. Further, it yields flexibility “as you can configure your power in whatever way you want, to suit a given operation”, says Lindtjorn.
A similar argument has also driven the power configuration onboard Jan De Nul’s new 236.8m Les Alizés. The ship has a deadweight of 61,000t and can load foundations weighing up to 5,000t.
But it is very different from more traditional jackup designs because installation takes place while floating. Therefore, apart from the four, 3MW Schottel azimuthing thrusters at the stern - giving a maximum speed of 13kn - there are a sizeable pair of 3.25MW retractable units plus two 2.6MW bow thrusters offering DP support.
“A floating installation vessel utilising DP mode means that you don’t have to consider payload limitations due to the jacking operation... so you can carry more cargo, further out to site,” says Gunter Servaes of Jan De Nul - so it saves jacking and sailing time. However, he adds “on a DP vessel like Les Alizés; everything needs to work together continuously, so the harsher the conditions, the more power you need to minimise movement”.
8 Shimizu’s
supersized jackup can carry and install three, 12MW turbines - or seven 8MW units - in a single voyage
Feeding them are six main generators: these are MAN ES 12V32/44CR four-strokes, (supplied with particulate filters and SCRs), together yielding an impressive 43.2MW. Once again, a closed bus is central to “smart operation” says Servaes, allowing overall load balancing and helping the engines to operate within the optimal efficiency range.
However, as Lindtjorn explains, the flip side of closed bus systems is the entire configuration needs to be thought through. Firstly, to make sure the generators balance each other effectively and secondly, to stop problems cascading from one area to another.
A fault here is not just about reduced available power: “It can lead to dramatic changes capable of overloading the sources that are left, which can lead to a complete blackout,” he says.
Further, he adds, “as these big engines can be slow on larger load steps, you have to be lightning-fast to limit the impact and avoid a subsequent event”.
Problematically, it’s just not that easy to determine where the issue lies. “In a closed ring system, a fault might pull the power in all kinds of directions; it’s not sufficient to use current magnitude as a trigger to tripping the section,” says Lindtjorn.
Therefore ABB’s block based protection also notes the direction of the current, and from that, deduces which area has the issue. “The governing logic has to be extremely robust and the whole thing has to happen in milliseconds,” he adds.
However, it is an investment that pays for itself: “Robust protection means at worst you only lose one small part of the system, and for the same reason you can lower the dimension of the energy sources and propulsion.”
BATTERIES
More can be gained from hybridisation - and this is the way most of these WTIVs are heading: “Rarely do we see people asking for a newbuild without energy storage now,” says Lindtjorn.
Again it comes down to costs. “If it’s a critical operation like DP, you either need to make more engine power available to cover the potential demand arising from a power failure... or you can put in a battery,” he says. Further, he adds: “Batteries don’t mind ramping up in an instant.”
“It also helps green the entire operational range, OIM’s BT220IU installation design, now under build for OIM Wind, will be using a 980kWh battery pack for peak shaving as well as spinning reserve. It allows “better use of the power plant and lower fuel consumption during lifting and lowering, sailing and in-field transit, as well as manoeuvring”, says Oddgeir Indrestrand, President of OIM Wind. He adds that elements such as permanent magnet thrusters – which have fewer moving parts – also assist efficiency.
For Les Alizés, a substantial energy storage element is central to the power configuration. The electric, 5,000t heave-compensated crane especially “makes for very steep power swings in the order of some megawatts per second”, says Servaes: batteries provide the necessary buffer.
But having the onboard ESS allows energy recovery during lowering. Instead of being burned off as heat, a large proportion of the potential energy is fed back into the bank.
Given both these things, battery scaling was “a challenge”, he admits: too fast, or too deep a discharge or charge stands to shorten cell lifetime “so we had to play with size and optimal levels”, explains Servaes.
FUTURE
“The WITVs being designed right now are significantly more ambitious than even just a couple of years ago,” says Lindtjorn. And that applies to the fuel.
Partly driven by the green profile of the wind industry, they’ve also started to embrace fuel cells and hydrogen: admittedly “it’s quite early days for the technology, but our customers say they want to be ready for it”.
It is undoubtedly in OIM’s sights. It’s using Rolls-Royce Bergen B36:45 lean-burn gas engines to reduce emissions on the BT-220IU, says Indrestrand, but most importantly, “these are easily converted to ammonia or hydrogen... or new fuel mixtures as they become available”.
While the configuration will remain similar, integrating fuel cells - and probably larger batteries - on a DC power grid makes sense, says Lindtjorn. Not only is it possible to drop some of the filters and transformers, the footprint is significantly lower, plus, he adds, the engines can operate at variable speeds, cutting fuel and emissions.
Moreover, Lindtjorn notes: “If you have a DC backbone, you can more easily integrate jacking systems... the solution can be quite elegant with a bidirectional power flow and so on.”
So, what about truly huge lifts? With its 30,000t capability, the offshore decom vessel Moonraker stands to dwarf tower requirements “It’s suitable for float-over substation installation in even the largest wind fields, and with minor alterations it can be adapted for turbines,” says designer Morgan Howes-Roberts.
Onboard are eight, dual-fuel gensets, providing a hefty 71MW in total. While the vessel is a semisubmersible requiring ballast pumps, lifting rams and so on, the greater portion of the power can be directed to eight, 4.5MW azimuthing thrusters: “the priority remains the positioning”, Howes-Roberts underlines.
However, there are pragmatic challenges that come with scale. Moonraker - like others incorporating flexibility and reduced fuel consumption - has a closed-bus configuration and would normally be operated in this mode. But closedbus test criteria involve triggering a real, high-level shortcircuit explains Howes-Roberts: therefore, the system has to be designed not just for safety but for full functionality after the ordeal, adding costs commensurate with size.
Further, he adds, it also “introduces a certain nervousness... and very big bangs”.
8 BT-220IU: It’s “the
