As FRP projects addressing fire, joining and flexibility issues progress, Stevie Knight asks whether the time is ripe for wider adoption of large-structure composites in shipbuilding.
The 7,000-car carrier Siem Cicero is the first, and at time of writing, the only ship with composite tweendecks. Built in Croatia, these cover a 12,600m2 area, “the same size as two-and–a-half football fields” says Vito Radolovic of Flow Ship Design. While this reduced the deck weight by 230 tonnes, Radolovic underlines that the advantages are far more radical: in the case of Cicero, “the changed stability resulted in a reduction in ballast in the bottom of the double hull”. As a result, Cicero gained capacity for another 800 tonnes of cargo – but it also knocked between 4% and 5% off the fuel bill for the same load.
How much did it cost? In total, it came out slightly more expensive than a traditional deck “but given that it’s saving around two tonnes of fuel a day, payback’s inside a year or 18 months”, he says.
Unsurprisingly, others are also eyeing the advantages: Oshima Shipbuilding and Compocean have developed a moveable tweendeck: this could be paired with composite hatch covers – both innovations promise shorter port stays as composite is much lighter and easier to handle.
There are also numerous benefits from incorporating wetroom, aircon and electrical channels into a moulded internal structure, making it particularly useful for cruiseship outfitting. However, while Carnival demonstrated FRP (fibre reinforced polymer) components result in a steep weight reduction, reportedly a tonne a cabin, these went back to traditional, piecemeal metal fixing approaches. “A much more effective method is to create a big ship block incorporating all the fittings, noise and thermal insulation, including the strip for attaching it to the steel hull,” says Laurent Morel of InfraCore.
GOING ALL THE WAY
But what about taking FRP further? While it makes sense to outsource replicating items such as multiple cabin components and the thousand-odd, 11m FRP sections that make up Siem Cicero’s tweendeck, others have decided to hit the learning curve in its entirety.
It may be a big step outside the comfort zone for those that get involved: “You only have to mention composites in a regular shipyard, and everyone starts looking really worried,” says Radolovic. “You have to convince people that composites are both safe – and reliable,” adds Morel.
In fact, the FibreShip project (funded by the ERC/Horizon2020 programme), has created a demonstrator ship block for an 85m special-purpose vessel at the iXblue shipyard.
It was no small task: iXblue project manager Edouard Waldura explains that the demonstrator bulkheads and decks were constructed from around 1,000m² of flat panels, while the hull sections were made from two, massive 100m² resin infusions with carbon fibre reinforcements.
Putting it together, he says was “a kind of lego”… except the yard had to begin by making the various components from scratch. Waldura adds that comparing it to the traditional manufacturing approach, “it’s as if we had to start by producing the metal sheet in the shipyard”.
Likewise, the RAMSSES hull-section demonstrator (another ERC/Horizon2020 funded project) will also be built at Damen’s steel production facility in the Netherlands. As with the FibreShip programme, the elements of a midship block for an 80m patrol vessel will be created onsite: InfraCore is to construct the decks, bulkheads and helideck.
Morel’s composite infrastructure experience gives him confidence that the processes are transferable: “People say, there’s dust, no climate control… but dealing with these things isn’t rocket science: there are quality management protocols in place and material solutions available,” he explains. Neither is size an issue: while these shipbuilding projects are investigating the build of an 80m or 85m hull, some of InfraCore’s bridge projects dwarf this by thousands of cubic metres, one in particular seeing 25 tonnes of resin pumped in a single infusion.
Still, ship development demands more than scale; it requires finesse. FibreShip lead, Alfonso Jurado of Técnicas y Servicios de Ingeniería (TSI), underscores that the design strategy can’t directly be translated from that of traditional ships, mainly because composites aren’t as stiff as steel.
This has serious implications. Too much flex and the vessel will be stressed by the humps and troughs of long-period waves: the so-called hogging-sagging effect. Rigidity, therefore, has to be designed-in.
To beat this, “efforts were focused on increasing the inertia of the structure by improving the hull girder response” explains Jurado. That resulted in twin-walled carbon fibre elements that lend the design the necessary reinforcement.
Despite the innovative nature of the build, the 11m by 11m by 8.6m ship block shows an impressive 70% drop in weight against the steel version. This would, carried through to the entire vessel, result in a 30% overall lightship reduction.
While the more standard sandwich approach takes structural foam and surfaces it with an FRP skin, that’s not the only, or arguably the best, method.
For example, Fibrecore has developed a technology which takes non-structural foam bricks, wrapping FRP around them and stacking these in a specific way: the resin infusion then “creates a multi-beam, sandwich-like structure”, explains Morel.
It does have impressive resistance to impact fatigue – even on heavy-duty infrastructure installations: “The foam is nothing more than a filler, so denting it has no effect. We’ve proved that even a heavily damaged and fatigue-cycled motorway bridge could still keep going for at least 50 years without repair,” says Morel.
There are other benefits: it’s generally both cheaper and lighter than a conventional sandwich structure and it also answers a few quality worries for class societies. During vacuum infusion cures, fabrics have a nasty habit of pulling toward each other: unfortunately, the resulting wrinkles create issues for the final product’s load distribution. “This is minimised by our use of smaller, discontinuous segments,” says Morel, adding “it also means delamination becomes inconsequential as cracks can’t propagate across the structure”.
Interestingly, the internal webbing creates pockets that can be utilised in creative ways. For areas of high load transfer, it’s possible to co-infuse part of the structure with a resin: this results in monolithic zones which can, for example, be used for bolted connections: a possibility explored on a composite containership rudder.
Other technologies are also stepping across into ship construction. Pultrusion, explains Radolovic, is a continuous manufacturing process which can create a beam of any length in a fraction of the time of other methods. It works by impregnating fibres (straight, matt or multiaxial) with a thermoset binder and pulling them through a heated die to cure – the profiles are simply sawed to length on exit. While pultrusion increases weight in comparison to a sandwich vacuum infusion, it’s still lighter than steel and this process promises to reduce deck construction costs even further, potentially to a half or even a third of typical, composite builds. As a result, Flow Ship Design is aiming to utilise the technique for its subsequent deck developments within the RAMSSES project.
In the past, bonding has been limited to non-structural elements, but taking composites forward means the joints now have to support the global strength of the vessel, points out Stephane Paboeuf of Bureau Veritas.
Interestingly, the FibreShip demonstrator was built in two separate parts to experiment with the block junction, explains Waldura. The flange and resin infusion strategy came through yard tests successfully.
There are many different types of join, and Morel says that crossovers from bridge infrastructure demonstrate that a mixture of joining approaches, bolting, bonding and shape-locked, yield the best results overall “as this gives you a number of levels of redundancy”. Further, he adds it can also allow for dismountable structures, which makes repair or replacement a lot easier.
Obviously, load-bearing joints hold a particular interest for class societies. “The challenge is to correctly assess the design and effectiveness of the bonding zone,” says Paboeuf.
This doesn’t just apply to large composite structures, it’s also necessary for metal and FRP hybrid joints, especially as combining materials promises to open up the market.
Uniting different materials does demand a little care: “For example, if you make a fully infused, rigid hybrid connection, you need to make sure the thermal expansion characteristic is the same as the surrounding steel,” says Morel. Likewise, the composite’s vibration transmission should “stay out of the excitation frequency of the adjoining structure”, but he adds that dampening is relatively easily achieved.
QUALIFY, a co-funded INTERREG 2SeasMers Zeeën project (properly titled Enabling Qualification of Hybrid Joints for Lightweight and Safe Maritime Transport), aims at a realistic, predictable joining strategy. “We want to develop an engineering approach to bonding that can be practically applied by the shipyards. And for class to be confident that it’s robust,” says Paboeuf.
Still, these yards are aggressive environments. One issue is control of the joining surfaces, protecting them against chemical or water ingress. Here, innovation is coming to the rescue: moisture-tolerant resins crossing over from the aerospace, automotive and railway industries are yielding both better mechanical properties alongside a reduced demand for surface preparation.
But it’s not all about using expensive resin or carbon fibre: for example, InfraCore’s strength lies in the structure “so we tend to stick with cheap, effective glass-fibre and polyester wherever possible”, says Morel. Of course, a lot comes down to technique: a one-shot resin infusion at scale has to be extremely thorough.
Despite the learning curve, it’s ‘do-able’ says Morel. He underlines that the desired results are currently achievable and above all, replicable in real-world conditions: for example, InfraCore has successfully installed a Dutch bridge which had to cope with bonding temperatures at minus-10C.
However, QUALIFY is looking beyond approval and design guidelines to joints’ long term behaviour and failure modes. Certainly, saltwater, UV and other elements can degrade the structure over the 25 or 30 year lifetime of the ship, but the effects may not be as apparent as old-fashioned rust. According to Paboeuf, accelerated ageing tests should, therefore, yield a crucial data set: “If you know what the mechanical properties of your structure will be at the end of life, you can optimise the design while still retaining your safety coefficient.”
The SOLAS rules for fire safety today mandate the building of structural elements in steel or apply a long, costly ‘equivalence’ safety analysis, which may even then be rejected by the flag state. This, says Paboeuf, “is a challenge for development”. Although revision of the MSC 1574 interim guidelines on FRP structures is due in 2021, there’s a lot to prove.
Of course, the big concern is that a ship could behave like a plastic bottle on a hotplate – this is a misconception “as fire is typically a local event and global softening of the ship hull and other load-bearing structures is improbable”, says Tuula Hakkarainen of VTT. Further, she adds the matrix polymer of an FRP is usually a thermosetting material that does not melt.
But as Hakkarainen admits, whereas steels can retain their strength up to around 600C, FRP can lose its load-bearing capacity at relatively low temperatures and can itself combust, potentially contributing to the fire and production of noxious gases.
Hakkarainen’s colleague Antti Paajanen points out there’s a very wide range of composite materials with different characteristics which could be developed further: positive results came from tests on specially developed fire retarding components under the EU Fire-Resist programme.
As a result, Paboeuf explains there’s a concerted move to introduce the more adaptable European construction industry classification: the notation including fire resistance (R), integrity (E) and insulating properties (I).
But Paajanen underlines it’s not quite as simple as “this plastic will lose these mechanical properties at this temperature” as FRP could be put together in multilayered sandwiches to ensure that the load-bearing capacity is maintained – even if the initially exposed layers have already softened. While it does require further testing at scale, both the FibreShip and the Fire-Resist projects demonstrated that laminate structures (with rockwool insulation or intumescent coatings) could pass the ‘load-bearing fire-resisting bulkhead 60’ test, holding up for over an hour against a standard time-temperature curve.
Still, it’s a balance: the big motivation for composite construction is to get the weight down, so a fire-proof design has to avoid accumulating the kilos.
Given this, it’s more a case of how, not if, composites can be used: for example, both Paajanen and Hakkarainen stress “the fire performance and thermal insulation of both load-bearing structures and joints deserve special attention”.
As such, ship design demands multiple risk mitigation strategies: it’s not just choosing the correct components and structure, but it will also require evacuation planning and the use of active fire protection measures such as sprinklers. “It needs ship-level as well as material-level safety consideration,” concludes Paboeuf.
FibreShip http://www.FibreShip.eu/ (grant agreement 723360) and RAMSSES https://www.ramsses-project.eu/ (grant agreement 723246) both receive European Research Council (ERC) funding under EU Horizon 2020
QUALIFY https://www.qualify-euproject.com, Enabling Qualification of Hybrid Joints for Lightweight and Safe Maritime Transport”, co-funded by the INTERREG 2SeasMers Zeeën programme
Fire-Resist http://www.Fire-Resist.eu (grant agreement 246037) funded under EU FP7-NMP programme