The challenges of building the biggest cruise ship in the ... the bulbous bow have changed with the

The challenges of building the biggest cruise ship in the ... the bulbous bow have changed with the
The challenges of building the biggest cruise ship in the ... the bulbous bow have changed with the
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Transcript of The challenges of building the biggest cruise ship in the ... the bulbous bow have changed with the

  • The challenges of building the biggest cruise ship in the world Report interviews Loic Morand, Head of STX Europe’s Hydrodynamic Department,

    about whether the industry can go on building even larger vessels and about how

    this pioneering shipbuilder is approaching the quest for a zero-emissions vessel.

    STX France, one of the world’s leading cruise ship and naval yards, has been

    a regular visitor to MARIN for decades. More recently, the company carried out a

    wide-reaching test campaign for the world’s largest cruise ship – the magnificent

    ‘Symphony of the Seas’.

    S TX and MARIN worked very closely together on the Symphony of the Seas, a goliath that pushes all the shipbuilding boundaries, and this meant a rigorous test campaign was crucial.

    The figures are impressive, she is a staggering 228,021 GT, more than 362 m long, boasts 18 decks and has a capacity for 8,000 people. Given this shipbuilding feat, Loic outlines the major challenges that STX and MARIN faced together.

    Highest speed possible Loic explains: “One particular challenge of Symphony and her predecessor Harmony of the Seas was the contractual obligations to achieve the highest speeds possible, whereby the yard is penalised if the ships sail below their expected speed. However, there has to be a very delicate balance between the efficiency demanded and the comfort levels on board. Vibrations and noise from the propeller could be a potential issue and lead to discomfort for the passengers.”

    Several model and cavitation tests were carried out at MARIN and these led to a substantial improvement in ship speed, and at the same time, there was no detrimental impact on comfort levels. Following the tests and calculations, both Symphony and Harmony of the Seas have a different hull form and propeller compared to the former Oasis Class vessels - Oasis of the Seas and Allure of the Seas.

    One key factor was a change in the shape of the bulbous bow. The tools to optimise the bulbous bow have changed with the times. Optimising with potential computations led to the ‘Goose Neck’ bulbous bow type (a very pronounced bulbous bow), which provides a gain in

    wave resistance but not for the viscous component (due to the swirl created by the bulbous bow). Therefore, CFD computations were done to redesign the bulbous bow and these resulted in a more slender and longer bulbous bow.

    Bulbous bow shape STX France also worked on the propeller design, conducting pressure measurements, which were compared to the first Oasis Class ships. The lateral thrusters and ducktails were also optimised.

    The cavitation tests brought some major challenges, Loic explains. “We have three propellers of 20MW each and the central propeller is behind the central skeg, which means the wake impact is more pronounced, meaning that it has even more influence.” Eventually five different designs of the central propeller were tested and optimised. “The main dimensions of the propeller were changed, the blade areas, ratio etc.”

    As to whether the industry can carry on supersizing vessels, Loic says the challenge is not so much about the scale of the cruise ships but the balance between efficiency versus comfort. “We already have quite some experience with enormous ships. The Queen Mary 2, which at 350 m, is longer than even the Oasis Class for example. We can build bigger ships. But the only thing is a larger ship means an enormous amount of power is placed on each propeller, so we are then back to the efficiency versus comfort level compromise again.”

    Comfort vs. efficiency A key challenge and something addressed together with MARIN is the tip vortex. “Given the higher level of efficiency, there is a bigger tip vortex, but is this acceptable for comfort

    levels?” At the moment, he admits, the industry doesn’t have the answer. “Our current knowledge is relying on experience. As cruise ships get larger, the power will of course increase. I think this will be the parameter in the end, not the size of the vessels.”

    STX is also designing for ‘non-optimal’ conditions now, he adds. “Previously we would design a ship for ideal conditions, without waves and wind. But the current trend is to take into account conditions such as wind and waves. Our target is to optimise the hull form and propeller for perfect conditions and take into account waves, current etc. as well.”

    On the day of the interview Loic was at MARIN’s testing site in Ede where cavitation tests were being performed. “Here we are looking at the resistance of the ship, plus the additional resistance for wave and wind, and the impact cavitation has on the propeller.” If a hull form can have reduced added resistance in waves this is very interesting for cruise ship owners, he adds. And already there has been quite some success, with the joint STX/MARIN team reducing the additional resistance by 25%. These findings will be taken into account for the next vessel, which will be delivered in October. The model tests also led to the decision to design a straight bow, which significantly reduces resistance.

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  • Reducing resistance In its efforts to make hull forms and propellers more efficient, the company is also looking at air lubrication to reduce resistance, thereby increasing efficiency and leading to fuel savings, and again, it has carried out tests at MARIN. “We are now working on the system itself, aiming to further limit the losses and improve the efficiency of each system.” Currently, air lubrication systems are implemented on four vessels.

    Zero emissions STX is also designing new concepts such as the recently revealed Silenseas, a carbon-free sailing cruise ship design. The Silenseas initiative highlights another major challenge – the quest for zero emission shipping. Loic thinks that full LNG cruise liners are getting very close. “I think they will be there in the next three to four years.” But zero emissions is still the ultimate target, he stresses. STX has a dedicated team working on minimising the losses of every system. This team has already helped to reduce emissions by 20%, he points out.

    Automation Looking to future hydrodynamic research, a new technology of STX is the automated development of hull forms. However, Loic is not convinced that automation will completely take over the traditional development method based on skills and experience just yet. “It is supplanting traditional methods to some extent; we do a parameter study of a bulbous bow curvature and take the best one for example. We use strategic points and move them around and deploy an optimiser program. Here OPENFOAM is deployed for the solvers, RHINO for the CAD model and DAKOTA for the optimisers.

    These three tools are used in a loop and the best hull form is identified a few weeks later.

    “But automation doesn’t necessarily simplify our life. We already start with quite an optimised hull form. And any improvement is dependent on which parameters you choose on the hull form. There are infinite possibilities! The question is how do you parameterise the hull form? I don’t think we will ever press a button and get the perfect hull!”

    Automation is already evident when it comes to propellers and here STX is for the first time designing and manufacturing its first propeller for the next twin-screw cruise ship. “In the past other companies designed our propellers. But we have a good database and experience. MARIN is helping us optimise the propeller by using the PROCAL program.”

    Interactions between hull and propeller With STX/MARIN having had a relationship for generations, the organisations have seen quite some research challenges. What are the next frontiers for research in Loic’s opinion? “One of the key research areas has got to be to optimise the performance interactions between the propeller and hull form. Ultimately, it has to be the sum of the optimum of the propeller and the hull form, rather than optimising separately. But I think it will be far in the future before we have this answer.

    “We need to continue to focus on reducing the power of the ship, whilst maintaining the speed and achieving zero emissions. And of course, we must improve sailing technology by perhaps using the wind as the main propulsive power. There is an enormous amount of wind power available and it is free! Why not use this?”

    Model in bow

    quartering waves

    Cavitation

    observations

    Hull with three propellers

    and Azipods®, and

    pressure pulse sensors

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