Straight up Savings
Bow forms for modern cruise ships are traditionally raked so as to impart a strong aesthetic look to the vessel but whether that is the best form is debateable. In this article we look not at any particular white paper but take the viewpoint of design and CFD specialists Foreship and look at some recent vessels that have opted for an alternative. “Cruise ship hull designs have become so optimised in recent years that true innovation has demanded fresh thinking because the hard targets to improve performance keep coming,” says Mattias Jörgensen, Foreship Vice President Business Development. “Generation to generation, cruise ships have made massive strides in terms of energy efficiency, but forward-looking owners continue to want more.” Foreship disclosed that the ‘fresh thinking’ Mr Jörgensen is referring to had found commercial form, as it began talking openly about a new approach to CFD which factored real sea states into hull form optimisation. “Discussions with cruise ship owners concerning how to optimise hull forms in real conditions have been ongoing for around five years, but it has taken time to develop the practical tools,” explained Janne Niittymäki, Foreship Head of Hydrodynamics. “A lot has happened in the field of CFD over those years, and it has become practical to look at frictional resistance below the water in a way that considers the complexities of wave motions.” Most designs are optimised for a single sea state or vessel condition but such conditions most definitely cannot be maintained in a dynamic environment affected by weather, sea state and the loading of the ship itself. Acting outside of any client commission, Foreship looked at the methodology of CFD simulation based on real conditions and created simulations including average wave heights that remain constant throughout the cycle of the ship and along the entire length of the hull form. Foreship brought its expertise to bear on advancing the capability of OpenFOAM, the open source CFD software package, developing the coding required for hull optimisation and graphical interfaces that simulate real conditions. Inputs include the baseline 3D hull form and any possible restrictions, with normal operational conditions based on expected wave heights, wave periods and speed range. After a specific request from a client, a more intensive phase was entered into as part of a cruise ship newbuilding project that looked to compare the performance of a bulbous bow versus a vertical stem bow on board a cruise vessel of around 300m in length optimised for 18kn. The beam, draft, block coefficient and submerged hull length were constant. It is received wisdom that the bulbous bow is the optimal solution in calm water, but above a certain threshold condition (wave height and period) the vertical stem becomes the optimal solution. Using the new method, the simulations showed that in calm water the bulbous bow could indeed be considered a marginally ‘better’ solution in speeds ranging between 14-18kn, but that at 22kn the vertical stem was demonstrably better, achieving 2.7% less resistance. However, in conditions simulated to include head waves (Hs = 1.75, Tp = 8s), modelled in intervals of 20s in calm seas and 25s in waves, the vertical stem achieved an improvement in hull resistance of above 2% even in small regular head waves across 14, 18 and 22 knot test speeds. The differences in ship motions were considered insignificant in the context of resistance. “When the first estimates came through, I have to say that I did not believe them,” said Niittymäki. “I believed that the vertical stem would be seen to be the better option at some point, but not at such low wave heights. The resistance in waves was shown to be clearly better across the whole speed range in the case of the vertical stem version. We knew that even small waves might affect how the optimal hull form was decided on, but it is only now that we have the new methodology that we can measure precisely what the effect will be.” Using Foreship’s RANS-CFD method at an early stage will confer more freedom for modification than would be the case after model testing. However, Niittymäki emphasises that fuel consumption gains will be very significant in the practical case worked on and other hull optimisation improvements will achieve the same outcome. For the cruise ship industry, a 2% margin of improvement in terms of frictional resistance is a ‘big deal’. However, Niittymäki adds: “Naturally, this would not be so significant for other vessel types, but the point is that the methodology is in place to simulate other factors, such as seakeeping or slamming, which may be the leading consideration for other owners. The same CFD methodology would be applied to evaluate the optimum hull form at the earliest possible stage.” In the normal course of events, hull form optimisation is verified by model testing at a later stage of design. While this will remain the case, one of the benefits of the new methodology is that no further modifications would be expected. In addition, CFD simulation is repeatable in a way that model testing is not, meaning that small adjustments can be easily modelled. The client in the project is not named but it is interesting to note that at a later date, Foreship was involved in projects involving Polar Class cruise ships for two operators; Crystal Cruises and Scenic. The results will be the first luxury passenger vessels that are purpose-built for waters currently served by old ice breakers and survey ships. Crystal cruises is of course making a pioneering voyage through the North West Passage this year with the 2003-built Crystal Serenity. The ships from the first project are now known under the Exclusive Crystal Class name and are under construction at Lloyd Werft in Germany which, like Crystal Cruises is now also owned by Genting Hong Kong. The ship from the Scenic project – Scenic Eclipse – is also under construction and is being built by Uljanik in Croatia. The Crystal vessels are large ships while the Scenic vessel is described as an expedition yacht. It is interesting to note that both designs have a conventional and recognisable bow form for cruise vessels. Although that should be expected for the large Crystal vessels which may well make only occasional forays into Arctic waters, the smaller ship for Scenic is intended to be used for more frequent luxury tours and scientific expeditions along the Arctic and in Antarctica. The bow form of two other recently announced vessel designs for polar cruising are much more in line with the alternative vertical stem in Foreship’s CFD exercise. They are both smaller vessel types similar to the Scenic Eclipse. The first of these is to be another Crystal owned vessel for launch in 2018 and which will also be built in Germany by Lloyd Werft. Crystal Endeavor (named after Captain James Cook’s HMS Endeavor) will be the first purpose-built Polar Code compliant yacht in the world with a PC6 Polar Class designation. It will also feature DP so as to be able to hold station over coral reefs without anchoring. The other design is an all Norwegian affair with the design coming from Rolls-Royce, construction at Kleven Verft and built for the famous ferry operator Hurtigruten. These vessels will feature the Rolls-Royce wave piercing bow form which has been adopted on offshore ships and the Environship form used by Nor Lines for its freight ferries and other owners. Time will tell which bow form is best suited to the tasks the ships are intended for. It is difficult to know what speeds the ships are planned to be operated at especially as ice conditions rather than schedules are likely to dictate ship speeds for polar cruising. If the vertical stem does not add much to fuel savings it will at least be marginally cheaper to build and maintain.