Ocean-going merchant ships-which can carry up to 180,000 tons of bulk cargo such as iron ore or coal-are continually slammed, not just by shifting natural forces but also by loads, such as forces of 10 to 14 tons generated by the propellers, and up to 200 ton meters exerted by the engine on the vessel structure. These forces have the potential to generate vibrations that can make life miserable for the crew and, over time, damage the ship.
Delta Marine Engineering Co. (Uskudar-Istanbul, Turkey), a merchant marine designer, uses simulation driven product development from Ansys Inc. (South Pointe, PA) to optimize the design of its cargo ships. The company analyzes its designs with software from Ansys to identify and correct troublesome vibration, in order to comply with international standards as well as ensure the safety of the crew and a long life for the ship.
This design challenge requires a solid understanding of the structural behavior of the ship and its interaction with the surrounding water. Complex fluid and structural interactions govern the performance of a ship, so Delta Marine uses Ansys Mechanical structural mechanics software and Ansys Fluent fluid flow software.
One of the most basic tasks that marine engineers face is eliminating undesirable vibration. Each ship has natural frequencies dependent on the design of its structure, which includes parameters such as size and shape as well as the materials used. In addition, there are forcing frequencies that act upon a ship, most often generated by components that operate at different frequencies. As the propeller moves the water, for example, it exerts forces on the back of the ship, and these forces are large enough that they can cause vibration, particularly if they excite one of the structure’s natural modes of vibration.
Vibration in marine applications is especially complex because it also involves the behavior of the structure as it passes through the water. This fluid structure interaction (FSI) can fatigue the hull’s components and cause vital equipment to malfunction.
“Simulation analysis gives our engineers insight into the complexities of these interactions,” says Dirim Sener, planning director, Delta Marine Engineering Co. “Using Ansys technology, our engineers try to correct problems by modifying the ship during the design stage-instead of discovering vibration problems after the ship is launched.” Dirim adds that changes made after launch could cost millions of dollars, while changing the underwater form of a ship in the design stage can be done at almost zero cost. “Engineers also have much more freedom when making design changes in the early stages.”
Delta Marine engineers use software from Ansys to calculate pressures induced by the propeller on the aft (back) of the ship as well as the loads generated on the propeller shaft. If vibration values do not meet international standards, expensive changes may have to be made. By identifying vibration problems in the early stages of the design process, Delta Marine can make alterations such as adding pillars or strengthening structural components, or changing the propeller, revolutions per minute of the crankshaft or number of blades in the propeller. Then, engineers can update the model to determine the effect of the changes on vibration displacement and velocity.
“The marine industry is challenged to develop and produce new designs at an accelerating pace,” says Dipankar Choudhury, vice president, corporate product strategy and planning, Ansys Inc. “Designing ships is not a straightforward process: The changes are frequent and prototyping can be quite expensive. Delta Marine has shown how simulation-driven product development can be extremely effective in designing vessels that mitigate risk-helping engineers make critical decisions throughout the design process.”
Delta Marine also uses Ansys technology to evaluate the structure and cargo tanks of ships against worst-case load scenarios involving forces exerted by the tank’s contents against the tank itself. For example, the design and construction of sulfur- and bitumen-carrying tankers is complicated by complexity of the cargo tanks, which could either be built independently or as part of an integrated structure.