In the early 1970s, a model test facility, complete with a towing tank and a manoeuvring basin, were built at the Otaniemi campus in Espoo. VTT’s Ship Laboratory used its model test facilities to perform applied research in hydrodynamics and contract research for shipyards and design offices. Helsinki University of Technology used the model test facility for basic research and education.
The 1980s were an era of strong growth for the Ship Laboratory. Finnish shipyards were building a high number of vessels for the Soviet Union and kept the test facility busy. VTT also focused on numerical methods and developed software for measuring hull resistance, manoeuvring and seakeeping. Using its own vibration calculation software based on the finite element method, VTT created passenger ship vibration predictions for Finnish and international customers.
A significant offshore industry grew up in Finland. VTT was involved in developing the first Finnish vessel simulator, used to expand research on ship manoeuvring into the area of shipping safety.
It was also the golden age of research into marine ice loads and ice-going for Arctic shipping. Studies of ice loads on hulls and propulsion devices could involve long-term measurements lasting years in the Baltic Sea, Arctic and Antarctica. This enabled VTT to develop on-board autonomous measurement and data acquisition systems, which were supplied to Finnish, Russian and Canadian ships. Research funding was used to develop ice load models, write doctoral dissertations and widen publishing activities.
The 1990s were characterised by rapid internationalisation and the beginning of EU research. VTT established itself in international organisations such as the ITTC (International Towing Tank Conference). A strong focus on Computational Fluid Dynamics (CFD) was begun at this time. VTT achieved remarkable resultsresults, especially in propeller flow field modelling.
Use of computational methods and simulations
VTT was one of the first research institutions to develop CFD methods for marine applications. It currently uses the CFD methodology to study more challenging flow phenomena. The most recent applications include the additional resistance caused by waves, and underwater noise due to propellers. Potential flow methods are used to calculate ship motions in waves, while disregarding flow friction. This markedly accelerates the calculation process.
Ship structures are typically modelled and analysed using the finite element method. Structural models can be used to examine strength, fatigue or vibration. Large models are needed for modelling passenger liner structures, and long calculation times are required for vibration analysis. A distinct model is used to take account of the effect of fluid.
VTT uses a ship manoeuvre simulator to study vessel performance. The simulator includes a ship manoeuvring model, a bridge and a visualisation application. The manoeuvring coefficients required for the model are defined using model tests or (nowadays) CFD. The marine environment around the bridge is visualised for the user. For example, the simulator is used to study whether the vessel is manoeuvrable enough to be steered through narrow sea routes in difficult conditions.
VTT still has strong expertise in the study of Arctic shipping and marine structures. The operating capability of ice-going vessels is forecast using a semi-empirical methodology. VTT’s methodology involves the use of results from a number of test runs of vessels in ice. The company has been heavily involved in developing ice rules for propulsion devices, enabling the calculation of propeller design loads for icy conditions. In addition, a simulation method has been used for calculating propeller ice loads, which have been carefully validated using full-scale trials on ships and in laboratory conditions.
VTT has a scientific Ice-structure-interaction-spearhead program under way to model ice loads on off-shore structures. When ice moves due to the wind or ocean currents, for example, high loads are placed on a structure. In addition to the thickness and strength of the ice, such loads are affected by the geometry of the structure and vibrations within it.
Environmental issues are being highlighted in shipbuilding. VTT has developed life-cycle analysis of the environmental impact of shipbuilding and shipping. This method measures the environmental impacts of the construction, use and scrapping of a ship. Shipyards and design offices can use the method when reducing the environmental footprint of their products.
Towing tank used in model tests
Concept design for hull hydrodynamics mainly involves the use of computational tools. However, model tests are performed as early as the concept phase. In almost every case, model tests are used to ensure the performance of the final hull version prior to construction. Shipyards and design offices want to avoid the risk of a failed hydrodynamic design when engaging in expensive ship investments.
VTT has Finland’s only open water tank for testing ship models – its towing tank. The tank is 130 metres long, 11 metres wide and 5.5 metres deep. At one end is a wave maker, which creates the desired sea state in the tank. Ship models tested in the tank are usually 7–9 metres long and weigh 2–3 tons. In the above picture, a ship model is undergoing propulsion tests in the towing tank.
Hull resistance in calm water is measured for almost all ship models tested. Following this, model propellers corresponding to real ones are fitted and used to propel the model ship. This enables the measurement of the ship’s and propulsion system’s energy efficiency and its comparison with computationally determined values.
The ship’s seakeeping properties are studied in wave conditions created in the tank. Motions and accelerations of the studied ship must remain below certain defined limits. Seakeeping tests can also be used to measure wave loads on the hull for hull-strength or vibration analyses. In addition, VTT’s model test tank is used to study off-shore structures and various wave power plant innovations.
Manoeuvring tests measure the forces needed for various manoeuvres and a ship manoeuvring simulator model is created on this basis. A simulator model created from model tests behaves very similarly to a real ship.
Validation of the flow calculation methodology is a key model testing task. Measurements can be made with extreme precision and iteratively through model tests. This is essential when seeking to determine the precision of theoretical and computational models compared to test results.
VTT’s most recent research projects involving the towing tank have used model tests to explore how various ship technology innovations function. Model tests have also been used in Ph.D theses by company researchers in recent years. The model test tank is also being used for both teaching and research by Aalto University and Turku University of Applied Sciences.
A broad package for sea trials
Full-scale tests and long-term measurements on ships remain necessary to understanding various phenomena and validating numerical methodologies. Sea trials are performed on all ship types to determine the performance of new builds. In the most typical cases, the ship’s speed on main engine power and its manoeuvring characteristics are tested in the calmest possible conditions.
The programme also involves measurements of vibrations and noise caused by propeller and engine excitation. VTT offers a broad range of services for sea trials of new builds and the related special measurements.
The study of complex phenomena in natural conditions, such as a ship’s performance in waves, the hull load caused by waves, or ice loads on vessels, require special skills in measurement technology and data acquisition. VTT has specialised in the development of wave and ice load measurement systems for almost four decades. The systems in question are currently being used on icebreakers and passenger ships.
Over the years, the capacity of data acquisition and analysis systems has developed to the point where very large-scale systems can be implemented and time signals can be collected over long periods. Because wave and ice conditions vary, very long time periods must be measured to obtain high statistical representativeness in load measurements. VTT uses wave sensors and buoys to measure the waves encountered by ships. Long-term measurements of ice conditions remain a challenging development objective.
Impact of digitalisation and environmental regulation
The energy efficiency design index adopted by the IMO is guiding ship design towards lower energy consumption and engine power. The reduction of
power margins will require greater precision in hull resistance predictions based on computational models and model tests – including in sea keeping situations.
Underwater noise emissions caused by ships is more frequently highlighted in the design of ships and propulsion devices. The measurement and modelling of underwater noise, a field of research in which VTT has been strongly involved, are extremely challenging.
High passenger comfort is required on modern cruise liners. A new field of research lies in low-frequency vibration caused by wave impacts, in addition to vibrations caused by propeller and engine excitation. To forecast this, VTT has developed special software which uses pressure measurements derived from model tests and numerical structural models based on cruise vessels.
New energy solutions, such as natural gas and fuel cells, are proliferating rapidly in shipping. VTT has strong expertise in integrating these with marine applications. Environmental technical issues in the marine industry remain an important research topic.
Digitalisation is having a strong impact on the shipping industry. R&D efforts have focused on development of the autonomous operation of shipping in recent years. VTT has a unique research offering in ship technology, telecommunications, digitalisation and safety for the development of autonomous shipping.
The study and theoretical modelling of various phenomena and the continuous development of computational methods, field measurements and the research infrastructure will keep VTT among the top players in marine technology research. Extensive applied research in marine technology and close cooperation with industry will ensure that Finland’s marine industry cluster is a player in the global competition.
Tuomas Sipilä, MSc (Technology), Lic.Sc.(Technology) is the Research Team Leader of VTT’s Ship and Arctic Technology Team. His research activities have mainly involved using computational methods to model propeller flows. He became Research Team Leader at the beginning of 2014. He is a member of the Specialist Committee on Hydrodynamic Noise of the ITTC (International Towing Tank Conference).
Senior Principal Scientist Seppo Kivimaa, MSc (Technology) and MBA works at
Digital Engineering research area. Kivimaa’s career at VTT started in the Ship Laboratory in 1981. He began as a specialist in the study of ship vibrations and ice loads. Kivimaa has worked at VTT as a Group Manager, Head of Research Area, Technology Manager and Key Account Manager. He is a member of the ITTC Advisory Committee and is the Chairman of ECMAR, the European Council for Maritime Applied R&D.
Photos: Paula Bergqvist, Sirpa Levonperä, VTT