Ship Shape Shifters

Modular power systems may not be far off, but what about the ships themselves?

Roy van Oosterom of Damen says that in many ways, modular builds “are already here”, pointing to the flexible nature of vessels such as the OSV 9020 which can be adapted to each mission by adding accommodation, cranes, saturation dive or even submarine rescue gear.

But modular vessel design reaches right through the hull. For example the Seasnake ‘sea train’ proposed a series of detachable, semi-cylindrical units, serially connected by a universal coupling which permits limited, individual yaw, pitch and roll movement. Intriguingly, it could tailor its length for the cargo and journey, unhitching it’s modules for either unloading or loading in port.

Sadly, the Seasnake remains more concept than reality – unlike Damen’s modular barges or Alego’s reconfigurable cargo ship.

While the latter’s initial feeder vessel build is starting with a 72m length and a 19m beam, it is hoped it won’t stay that way. As it aims to answer some of Africa’s more pressing logistical problems “the vessel should literally grow with the market,” explains Alego CEO Sondre Sandbye. In fact, this feedership is potentially able to reach 146m in length.

Importantly, both companies utilise watertight hull modules which retain their integrity: “We are not welding or cutting into them,” underlines Sandbye. A shape change is accomplished by separating the mechanically fastened bow or aft and floating midsections into the gap – no drydocking required.

However, there is a related issue: how is the propulsion to meet the needs of a significant increase in vessel size?

Alego’s solution comprises propulsion modules, containing gas gensets in the 500kW to 1.2MW range, each of which drives a fixed-pitch propeller: there’s a further, similarly sized unit incorporated into the bow. The LNG tanks are also separated on modular lines: importantly, there are no pipelines running between them, and each section contains its own, individual ballast system.

Scaling up the ship’s power is therefore a matter of adding another propulsion unit and possibly a further gas tank on the deck. “We’ll be using the same generators and the same one-size propellers across all of the modules,” Sandbye explains.

This is an important point: “You don’t want to change the diameter of the propeller as that is balanced to sit comfortably with the generated power output. Further, it fits with the draught, keeping within the hull’s outline,” he remarks, critical for restricted depths.

Interestingly, it’s a gas-electric hybrid. The gensets are linked with a battery pack: “LNG is only environmentally friendly if you have clean combustion. Unfortunately, change the loads on a gas engine and the result is methane slip… so the batteries main use is peak shaving.”

The company is also considering high pressure hydrogen as a fuel, retained in 700bar tanks. “While there will need to be further safety measures and maybe more tanks, it’s still a matter of installing a prefabricated module,” he concludes.

Damen has also developed a wide range of modular accommodation units, deck equipment, propulsion systems, wheelhouses and so onto suit the various potential configurations and likewise, a proprietary coupling system that doesn’t require drydocking or welded joints “so if the owners want to extend the vessel, they just come back for a couple more sections”, says Van Oosterom.

He adds: “We try to keep the ship’s systems as simple as possible.” For example, the diesel powered propulsion units are rather like oversized outboard engines with a gas tank inside the sound proof canopy, simply bolted onto the rear of the deck – which also allows them to be switched for maintenance.

Within certain limitations – these aren’t ocean-going ships – this type of design makes for a very flexible platform: for example similar barge-type elements have been used to create Damen’s modular workboats, ferries and pontoons. There has even been an inland waterway pipe lay vessel: this adapts a sloping unit at the stern for use as a stinger.

Most importantly, both companies’ modules are transportable, being based on standard ISO containers “as that’s the number one driver, especially for remote operations,” says Van Oosterom.

Finally, full type approval of the base units as individual hulls means that the vessel is easily certified. “Everything is uniform, which maintains a simple design, tight control of the cost, and if you use the same components over and over, you benefit from scale ordering from the suppliers,” says Sandbye.

As a result, “it’s far less expensive than a conventional vessel”, adding his calculations make it around 40% cheaper than an equivalent build.

While full, deep-sea design would require another step change, these vessels stand to play an increasing role in coastal and inland waters.

By Stevie Knight

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