M Haase

Wave-piercing catamaran transom stern ventilation process

The new class of highly fuel-efficient medium-speed catamarans operate at speeds where the transom is partially or fully ventilated, hence it is important to understand the characteristics of the wake for resistance prediction. Unsteady Reynolds-Averaged Navier Stokes simulations were used to simulate the flow around a 98 m catamaran, at both model and full scale, and compared to model test results for a 1:22 scale model. A non-shedding squashed horseshoe vortex was found to build up in the stagnant zone past the vessel, with the transom running dry at transom draft Froude numbers of 2.5 in model test experiments and at transom draft Froude numbers of 2.4 in numerical simulations. For full-scale Reynolds numbers, ventilation occurred at transom draft Froude numbers of 2.2. Finally, unsteady Reynolds-Averaged Navier Stokes simulations are capable of accurately predicting the recirculating flow in the wake of the vessel and the state of transom ventilation.

A Practical Design Approach including Resistance Predictions for Medium-speed Catamarans

Abstract Medium-speed catamarans are under development as a new class of vessels to meet requirements for highly efficient sea transportation with low environmental impact. Reduced service speed and increased deadweight will increase transport efficiency. Compared to current high-speed catamarans, these new vessels will operate in a transitional speed range between high-speed craft and conventional displacement ships. In this paper, design guidelines to choose appropriate main dimensions for medium speed catamarans with minimum resistance were derived and a preliminary design was made. These vessels will operate at Froude numbers of about 0.35 and have a relatively low prismatic coefficient of about 0.5 in conjunction with a small transom immersion. Different methods are discussed to correctly predict the calm water resistance, with RANSE (Reynolds-averaged Navier-Stokes equations)-based flow simulations being the most promising. It is shown that they are capable of predicting the hydrodynamic characteristics of medium-speed catamarans, such as drag force, trim and sinkage with acceptable accuracy.

Full-scale resistance prediction in finite waters: A study using computational fluid dynamics simulations, model test experiments and sea trial measurements

The development of large medium-speed catamarans aims increasing economic viability and reducing the possible negative influence on the environment of fast sea transportation. These vessels are likely to operate at hump speed where wave-making can be the dominating component of the total resistance. Shallow water may considerably amplify the wave-making and hence the overall drag force. Computational fluid dynamics is used to predict the drag force of medium-speed catamarans at model and full scale in infinite and restricted water to study the impact on the resistance. Steady and unsteady shallow-water effects that occur in model testing or full-scale operation are taken into account using computational fluid dynamics as they are inherently included in the mathematical formulations. Unsteady effects in the ship-model response were recorded in model test experiments, computational fluid dynamics simulations and full-scale measurements and found to agree with each other. For a medium-speed catamaran in water that is restricted in width and depth, it was found that computational fluid dynamics is capable of accurately predicting the drag with a maximum deviation of no more than 6% when compared to experimental results in model scale. The influences of restricted depth and width were studied using computational fluid dynamics where steady finite width effects in shallow water and finite depth effects at finite width were quantified. Full-scale drag from computational fluid dynamics predictions in shallow water (h/L = 0.12 – 0.17) was found to be between full-scale measurements and extrapolated model test results. Finally, it is shown that current extrapolation procedures for shallow-water model tests over-estimate residuary resistance by up to 12% and underestimate frictional forces by up to 35% when compared to validated computational fluid dynamics results. This study concludes that computational fluid dynamics is a versatile tool to predict the full-scale ship resistance to a more accurate extent than extrapolation model test data and can also be utilised to estimate model sizes that keep finite-water effects to an agreed minimum.

Energy-efficient large medium-speed catamarans : hull form design by full-scale CFD simulations

Large medium-speed catamarans are currently under development as a new class of vessel for economically efficient and more environmentally friendly fast sea transportation. Their design is based on current highspeed catamarans to adopt their advantages such as large deck areas and low wave-making resistance, but they will operate at lower speeds and carry a higher deadweight to obtain higher transport efficiency. Transport efficiency is defined as deadweight carried per required engine power at a certain speed and a high value represents low fuel consumption per deadweight tonnes per distance travelled. Large medium-speed catamarans will operate at speeds around the main drag hump, where the highest wave-making drag occurs and hence this speed range is usually avoided by boat designers. Therefore, these vessels require guidelines for their hull form design to efficiently operate in this generally unfavourable speed spectrum. Computational fluid dynamics (CFD) tools were investigated to assess their capability for predicting the vessel drag at model and full scale in deep and in shallow water. A potential benefit is avoiding empirical extrapolation methods, which are required when using towing tank tests to obtain the drag for the full-scale vessel. For an initial design space investigation, hull form parameters providing low drag for catamaran and monohull vessels were sourced from literature. Based on these findings, a hull form family was developed with vessel lengths ranging from 110 m to 190 m for carrying a similar amount of payload and the hydrodynamic performance was evaluated at Froude numbers from 0.25 up to 0.49. To provide more accuracy to the final drag prediction, whilst minimising analysis resource requirements, a novel full-scale CFD-based approach was developed. The key advantage of this method is that the same computational mesh can be used for model-scale verification and fullscale prediction, where the desired Reynolds number is achieved by altering the fluid viscosity. It was verified using results of model scale experiments of a 98 m and a 130 m catamaran and validated with results obtained from sea trial measurements of the 98 m catamaran, in deep as well as in shallow water. The computational full-scale simulation approach was found to be capable of predicting the drag force within 5% of results derived from full-scale measurements and extrapolated model test data. In addition it has been shown to correctly predicting steady and unsteady shallow water effects, which made this tool suitable for resistance extrapolation if either the model or full-scale ship sails in deep, shallow or even transversally restricted water. Furthermore, the local flow phenomenon of transom stern ventilation was studied numerically and experimentally. The feature of the flow in the stagnant zone past the partially ventilated transom was identified as a non-shedding squashed horseshoe vortex and the numerical simulation was capable of accurately predicting the state of ventilation. Finally, it was found that the lowest drag can be achieved for catamarans with demihull slenderness ratios (L/∇1/3) of 11-13. Hulls of 150 m in length provide highest transport efficiency for speeds of 20-35 knots at a draft of 3.1 m, and 170 m and 190 m for 3.6 m and 4.1 m vessel draft respectively. At the lower speeds of this range, a shorter hull may provide comparable transport efficiency. Finally, when comparing the results to contemporary large and fast catamarans carrying equivalent deadweight and travelling at the same speed, fuel saving up to 40% can be achieved if a hull of 150 m instead of 110 m length is used. This demonstrates that large medium-speed catamaran have the potential to be a fuel-efficient alternative for a successful future of fast sea transportation.

Simulating Loads and Motions of Full-Scale Offshore Platforms in Cyclonic Wave Conditions

The commercial Computational Fluid Dynamics (CFD) code STAR-CCM+ was used to simulate the dynamic behaviour of a tension leg platform in extreme weather conditions. The numerical results of surge motions and tendon tensions were compared against the measurements acquired in model tests. The full-scale CFD simulations were then conducted on the basis of the settings performed in modelscale simulations. Both model- and full-scale surge motions and tendon tensions predicted by CFD were in good agreement with the measurements. Using CFD results, it was revealed that the component of the vertical wave-in-deck force caused a slam force on the platform followed by tendon slack situations in the down-wave tendons.