Ferries and composites: commercial and practical challenges


Composites have a place on ferries, writes Stevie Knight, but just how effective these changes will be depends on commitment

Certainly, tightening EEDI regulations will be challenging for a broad swath of larger ferries: “Ten years ago, if the drawings for a ro-ro showed 9.5mm plate but the yard had a 10mm stock, you could substitute it,” says Finn Wollesen of Knud E Hansen (KEH). “Now it’s just not possible to do that and keep within EEDI design limitations.”

And whether ropax, ro-ro or conro, the EEDI stands to demand increased effort from both designers and yards. However, weight has always been “a sensitive subject” he adds.

Therefore, it might be assumed that any load shedding innovation would be grabbed with both hands: but that’s not quite been the case, says Wollesen. KEH was involved in the Stena Britannica conversion back in 2007, the 30m lengthening also added a substantial area to the superstructure.

This might have been the moment that composites entered the picture, as there was the potential for reducing the extra weight by half, saving around 45t.

However, the litany of reasons to drop the idea was pretty comprehensive: too expensive for a conversion which can’t adequately spread the extra price over the vessel’s lifetime; yards aren’t familiar with the technology, potentially increasing build price, and the “party killer”; SOLAS demanded lengthy testing to prove fire and structural equivalence to steel, plus a risk that even then, conservative flag states would reject the vessel. Altogether, it was enough to send everyone back to the comfort of steel.

Despite all this, could composites now be reconsidered?

Possibly. A plethora of projects are aimed at solving the issues, which range from “performance, joining and repairs to general upkeep” says Ville Wejberg of Deltamarin. But Wejberg and Wollesen both note that the single most important challenge is “fire safety regulation”, which makes recent advances in areas such as biocomposites and fire-retardant materials – for example, products from Melodea – very interesting, especially as they are about to enter an assessment phase focused on deck, bulkhead and wall applications. Importantly, a run of presentations at IMO could change the view on SOLAS rules when they come up for revision next year.

DESIGN LOOP

Despite FRP remaining metre-for-metre, “more expensive than steel” says Wejberg, there is another important dynamic at play. As drag on the wet area increases exponentially with speed, reducing weight becomes more of an issue: Wejberg’s colleague Mia Elg points out, “basically… the faster the ship is, the more it makes financial sense to consider composite materials”.

However, according to Marcel Elenbaas of Damen, development needs to follow the “overall design loop” all the way round for FRP to fulfil its potential.

For example, a 40m, 450 pax DFF4010 aluminium fast ferry with an average transit speed around 34kn was translated into a carbon fibre design, reducing the structural weight by 12 tonnes. However, just sticking at that would yield “marginal benefits” says Elenbaas.

Instead, what you are really looking for is a “positive design spiral”, he explains.

“For example, on a fast cat, the engine size is what tends to determine the beam of the demihulls”, therefore reducing the engine width means you can narrow the shape of the wet areas. “That’s the biggest saving, because you have less hydrodynamic resistance,” he explains. Further, in total the weight came down by 27 tonnes.

All together the fuel consumption reduction is predicted to reach around 15% to 20%. “Even given the expense of carbon fibre we see customers realising break-even in three to five years,” he adds.

BIT BY BIT

But – despite the advantages – many owners may want to take it slowly.

In Wejberg’s view, composites will work their way into builds from “easy to hard… and low weight-loss to high”. Cabins and interiors “have been on a weight reduction diet for some time”, he adds. Although FRP could make a difference to balconies, next will be ‘non-loadbearing’ elements such as “funnels, masts, hatches, stores, deck canopies, attractions and so on”, he predicts. This slower revolution has already been initiated by other substitutions, remarks Wollesen: “As aluminium has already shown weight savings it should be easier to introduce new materials.”

The issue, as demonstrated by Stena Brittanica, is the elusive ‘tipping point’ where the positive design spiral begins to mitigate the higher costs. Flowship Design’s Vito Radolovic has worked on modular FRP tweendecks for a car carrier with notable success, but he remarks that given ferries’ typical car deck structures “composites probably won’t make enough of a weight change” to make it worthwhile.

It may take the wider introduction of larger, heavier elements “such as superstructure blocks”, says Wejberg. Logically, he adds this will be followed by entire superstructures created from composites. Despite a few inroads by commercial vessels – including the concept design for a riverboat sundeck which had a 1:1 section pass IMO fire tests – they’re slow to be realised, leaving a ‘chicken and egg’ circularity to composites’ market viability.

MORE THAN SPEED

However, tipping the balance in the other direction are zero-emission ambitions. Even recent Li-ion batteries have energy-densities far below diesel, the increased weight adding hydrodynamic resistance which means in turn more batteries to propel the hull through the water.

Therefore, cDynamics have used composites to break that negative spiral on Norway’s 29.9m Barmøy car ferry: “The weight of a composite ferry is estimated to be 40 to 50% lower than that of a similar ferry in steel,” says Edvardsen, reducing the wet area and lowering water resistance.

Further, on Damen’s drawing board is an electric version of its Waterbus – but this is a carbon fibre design. It’s not the kind of material usually associated with low-speed craft “but here the object is not to save fuel but battery capacity”, says Elenbaas.

YARDS

It all needs buy-in from the shipyards: “Most yards want to do things the way they’ve always done them,” points out Wollesen, Wejberg adding: there’s a lack of prior experience and practices – while steel is ‘tried and true’.” Despite this he notes that “steel also took its time to get into shipbuilding”.

However, a few are ready to jump into a gap in the market.

Despite its focus on slightly smaller craft, the experience of Damen’s specialist epoxy yard in Antalya, Turkey, remains relevant as ship builds will likely be accomplished through the kind of modular approach it has specialised in, as this yields “something between a custom and series construction”, says Elenbaas: “In practice, it means keeping tight control over the vacuum infusion process, because you need these different parts to fit together with only a 10mm tolerance.” That is the maximum that can be used for an adhesively bonded joint according to Bureau Veritas rules, requiring high-quality, 5-axis CNC milling machines for the plugs “otherwise you have to rely on over-lamination, which needs a fatigue risk analysis”, he explains.

So, what about larger builds? Damen’s Vlissingen yard is also home to the RAMSSES project demonstrator build, a shipblock for an 80m seagoing patrol boat that meets SOLAS regulations.

While an enclosed vacuum infusion is less sensitive to the external environment than an open one, there are still challenges, he says: “We want to show that we can do it in a typical steel yard, without a clean room enclosing a large area.”

It required the development of a new, vinylester-based resin by Evonik, suitable for large scale infusions; further, the 6m-high hull section needs “outlets and inlets configured to allow a progressive infusion, from bottom to top, in stages”, says Joe Summers of Airborne UK, plus vacuum bag technology that stops the lower layers distorting under the weight of those above.

Despite the learning curve, there is a demand: “Yards are looking into lighter materials, those with greater investment in R&D more so”, says Wejberg.

DESIGN

Composite structures might give new ferry designs a little more freedom. As Elg points out, stability benefits “if weight from the upper decks can be reduced”, adding that in some cases “this may even be critical”.

However, scaling up vessels may not be straightforward points out Knut Inge Edvardsen of cDynamics. Somewhere around 60m “and you may find the ferry has a greater bending moment”, he explains, calling for internal stiffening.

Moreover, while aluminium and steel welds exhibit close to continuous strength, FRP requires “taking care over the load carrying capacity at every T or cross-joint”, he explains: fibres tend to span internal joins in one direction but not in the other, so strength relies on glue connections instead.

These create unfamiliar, yet critical issues for the yards to consider.

Finally, will the idea fly?

The rules do need to be updated: for SOLAS ships a full risk-assessment on the design is necessary even before signing the contract, “so that is where the regulations have to change, taking out the upfront uncertainty”, says Elenbaas.

There are other factors, such as risk (both real and perceived) to add to the mix. Personality may also be decisive: “You need visionary business leaders”, he adds. With characteristic realism, Wollesen points out that “it also requires a company large enough to absorb some of the risks”.

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