New ships must meet the EEDI or EEOI requirements and existing ships are often redesigned to meet the environmental and economic requirements.

Bulbous Bow retrofit

To minimize the cost of fuel, ships are now operated at lower speeds than a few years ago. Whereas with a 5-year-old container ship the Froude number is based on the design speed in a range from 0.21 to 0.25, these ships are now in a Froude number range of about 0.14 to 0.16. The speed reduction alone already results in significant fuel savings. In most cases, however, the bulbous bow is no longer optimal for the new, reduced speed range.

The original bulbous bow, which had been optimised for a Froude number of 0.21, but is running at a reduced Froude number of 0.145, produces an unfavourable wave system. The use of an optimised bulb for the new, reduced speed, in this case has led to a power saving of up to 8 %.

The SVA Potsdam uses the FRIENDSHIP-Framework for bulbous bow optimisations. Structured and unstructured grid topologies are generated for CFD calculations based on the parametrically described geometry. The numerical calculations are performed with the ANSYS CFX program. In a multi-point optimisation, the ship lines are optimised with the required power and uniformity of the wake field as a target function.

Propeller Re-Design

The lower thrust loading of the propeller in slow steaming results in greater freedom for propeller designs. Through increasing the propeller diameter, reducing the frequency of rotation, reducing the number of blades and the area ratio of the propeller, as well as the fine optimisation of the propeller blades, significant efficiency improvements can be achieved compared to the existing propeller, which was designed for higher vessel speeds. For large container ships, 6-bladed propellers are replaced with 5-bladed propellers with the transition to slow steaming. With a simultaneous increase of the propeller diameter by up to 5 % and reduction of area ratio by a third, power savings of 9 – 14 % can be demonstrated in experiments.

For the significantly slower bulk carriers, 4- and 3-bladed propellers are increasingly designed and investigated for both re-design projects as well as for the construction of new vessels. The area ratios of these propellers are in the range of AE/A0 = 0.35 to 0.45. Systematic investigations have shown that the 3-bladed propellers generate marginally higher pressure fluctuation amplitude in the first blade harmonics number. If the 3-bladed propeller is designed with a sufficient skew angle, the pressure fluctuation amplitudes of the higher blade number harmonics are in the range of propellers with a higher number of blades.

The trim of a ship influences the power consumption. Through an optimal trim several percent power savings can often be achieved.

Trim optimisation tests have already been carried out as standard procedure since 1970. As a result of the R & D project “Effective Trim Optimisation for Cargo Ships” [1] it has been possible to improve the predicting methods of power savings (calculated trim optimisation) as well as the experimental trim optimisation. The methods have different approaches from a purely statistical analysis of extensive model test series up to CFD simulations of entire trim matrices:

• Prediction of the resistance of the ship for different draughts with or without combinations of various trim states using the resistance method according to Danckwardt and / or the SVA-LSR method
• Formulas for determining the influence of partial discharging and / or trimming characteristic values on the propulsion
• Hybrid procedure: Predicting of resistance and propulsion for partial discharging and / or trimming by coupling parts of the process of resistance according to Danckwardt / SVA LSR method with experimental results
• Trim optimisation through pilot projects (EFD)
• Trim optimisation through CFD calculations (CFD)

As a result, master and officers may set the optimum trim condition for the ship using the information provided by the methods mentioned above. The 3D contour plot shows an example of the dependence of the power saving on displacement and trim.

 

Context Related References / Research Projects

[1] Heinke, C.: Effektive Trimmoptimierung für Frachtschiffe, Bericht 4394, Schiffbau-Versuchsanstalt Potsdam GmbH, Juni 2015, Abschlussbericht

For slamming tests, a hydraulic slamming simulator was developed in the SVA. With the help of this system, the model is made to heave and pitch and produce coupled motions using two longitudinally arranged, vertically movable, hydraulic pistons. Depending on the model size, amplitudes up to 0.1m are reached at frequencies up to 2Hz. Thus slamming loads can be simulated, in which the critical immersion speeds are significantly exceeded. Both hull slamming of planing boats and bow flare slamming on all types of vessels with significant bulkhead failure are examined primarily in the bow area. The system allows the simulation of regular and irregular movements. The advantages over slamming measurements in a conventional sea state tests of the targeted replication of slamming scenarios is that defined immersion conditions can be assigned, and also in the exact reproducibility of the experimental conditions.

The equiping of models with miniature pressure sensors of different sizes allows the measurement of slamming pressures on vulnerable positions of the hull in the model test under extreme sea conditions, both when driving and when stationary. Moreover, slamming pressures can also be measured with regular seakeeping tests.

Slamming phenomena are also simulated with existing CFD tools. The UTHLANDE program serves to determine slamming prevalence. For a quick estimate of slamming pressures the SVA has developed a simplified procedure. It is based on results of systematic model tests and allows the determination of slamming pressure for any hull shape, for a given speed and location on the vessel. An important parameter is the velocity normal to the hull and the fluid at the point of interest of the hull surface. Areas with air entrapment (hull slamming) can therefore not be detected.

As a result, the processing of various R &  D projects [1], [2], the [3] SVA has a wealth of experience in investigating slamming phenomena.

In recent years particularly, investigations of the roll behaviour, especially the roll damping of ships, has developed into a special field of hydrodynamic research. Due to high deck loading, modern container ships have low stability reserves and, in high seas, are exposed to the risk of extreme rolling motions. The development of specialised vessels and supply ships for the offshore sector is characterised by strict specifications in terms of positioning and motion stabilisation to reduce the risks of coming along side under rough conditions.

The SVA Potsdam has advanced testing methods along with the latest associated measurement methods. The SVA has offered innovative methods for improving and optimising the rolling behaviour of ships, mainly for stabiliser fins, rudders, Voith-Schneider equipment and roll damping tanks.

The SVA has a system that causes forced rolling motions of ship models with variation of rolling amplitude and frequency, with and without forward motion. By doing this, the rolling moment can be measured.

Parameterically excited rolling can be simulated with the ROLF method (nonlinear strip method), which has been integrated into the UTHLANDE program system. For a quick estimate of the risk of a ship with respect to parameterically excited oscillations, an SVA developed method is used which provides, based on fluctuations in metacentric heights, an indicator as to whether a rollover impulse is to be expected for a combination of ship speed, wave height and wavelength.

For the selection of roll damping tanks, calculation methods are available that allow both Frahm and box tanks to be analysed.

For large yachts and cruise ships in particular, stabiliser fin systems are available with corresponding geometries which damp rolling even while the vessel is stationary. The roll damping rates of these and similar systems are determined in the towing tank by applying different wave conditions to the model.

The seakeeping of ships and offshore structures is determined by model tests and numerical simulations. Model tests in a seaway are carried out in the towing tank. For this purpose, a hydraulic wave generator is available with which various wave spectra as well as regular waves and transient wave packets can be generated. Significant wave heights are possible in model scale to 0.23 m and at modal periods up to 2.5 s. For a model scale of about 1:40, wave conditions can be simulated up to gale force with about 10 m significant wave height at 18 s modal period in ship scale. To avoid unwanted wave reflections there is a beach with a natural inclination and an additional lateral wave damper to absorb diagonal waves. Sea state testing done with free-running, self-propelled models is used for detecting movements and accelerations, the shipping of water, relative motion, propeller ventilation and slamming phenomena. The effectiveness of controlled, zero speed, fin stabilisers, regarding their roll damping is tested with models having no forward motion. The determination of force coefficients is done in experiments with a tethered model.

Seakeeping tests can be carried out in countering and following seas as well as quartering seas at an angle of up to 35° off course. The latter takes the form of zigzag driving, which is recognised by the classification societies as an experimental technique. The motion behaviour of tested objects is detected with a high resolution optical tracking system. This system allows contactless measurement of the coupled motions of two-body systems in all 6 degrees of freedom as well as speed and acceleration at defined positions.

For numerical simulations of sea keeping the UTHLANDE program system is available. Based on linear and non-linear strip method, this system allows the calculation of the motions and loads of mono hull ships and catamarans for all 6 degrees of freedom. The program provides the statistical characteristics (short and long term statistics) for long and short crested seas represented in Cartesian or polar diagrams and a video animation based on a “ship motion viewer”. The process provides valuable insights by varying different sea state specific parameters in the optimisation process and serves as a valuable support of model experiments.