Hydrodynamics

Hull Improvements

In an effort to optimize the hull shape one must focus not only on achieving the smallest possible resistance on the design draft. Rather, the optimization process is based upon a matrix which includes complex requirements. The addressed target parameters are:

• minimum calm water resistance across the entire area of relevant load conditions
• minimum drag in shallow water
• minimum added resistance waves
• safe conduct in the sea state with regard to the ship’s structural integrity, avoidance of cargo damage, avoidance of personal injury
• comfortable motion characteristics in waves
• optimal propulsion through ideal placement of the propeller and optimization of the wake field

The far-reaching and carefully executed optimization of the hull shape with regard to these performance parameters is a basic prerequisite for achieving superior product performance, particularly in view of minimal fuel consumption at a maximum loading capacity.

For this reason, optimization of the hull shape is of the highest priority in FSG product development, and is included in all design discussions at a very early stage. In order to facilitate quick and precise conclusions with regard to the performance parameters, FSG employs up-to-date, direct computation- and simulation methods (CFD) which reflect the current state-of-the-art and scientific technologies. Many of the methods applied are FSG’s own in-house developments and are available exclusively to FSG. A team of specialists from the fields of hydrodynamics and software development is continually engaged in the improvement and further development of these methods, in order to be able to implement the latest findings into product development

Improved Propulsion Concepts

During the early design phase varying options for the propulsion concept are initially considered, evaluated and optimized; this is closely coordinated with the aftbody design. For the design process of RoRo- and RoPax ships, diesel mechanical drive systems with controllable pitch  propellers or diesel electric drives are typically in use, with the option of housing the propulsors in an engine pod (“pod drive”). The reason for preference of such drive concepts lies in the increased demand for autonomous maneuverability of these ships, since the routes are relatively short in comparison with other ship types. 

Depending upon the operating profile of the vessel, the required thrust load on the propeller, and the demands on the maneuverability and drive system, this suggests a choice of a single- or twin screw vessel. On the whole, construction- and operating cost must be carefully evaluated in close cooperation with the customer. The propeller geometry, like the hull shape, is thus optimized according to each individual design, since the hydrodynamic requirements for each ship vary considerably. In the optimization of the propeller, serious consideration must be given to optimal performance on the one hand, while the tendency to cavitate must be reduced to an acceptable measure, on the other hand. The propeller’s cavitation, aside from the propeller geometry, depends heavily on the inflow conditions to the propeller, which in turn are influenced by the shape of the hull. In addition to structural damage to the propeller itself, a strongly cavitating propeller may lead to vibrations which will influence the comfort of those on board. This is why FSG has potential-flow as well as viscous computation systems available which facilitate real time optimization through integration into the design software.

Improved Appendages

Any additions to the ship’s hull will represent an encumbrance to the flow conditions and therefore cause additional resistance, which in turn will lead to increased fuel consumption. Consequently we will attempt to avoid any possible additions to the ship’s hull as much as possible, however, there are a few components which are present in almost any ship and are clearly unavoidable. This includes among other things:

• bilge keels
• rudder(s)
• shaft brackets
• propeller shaft
• bossings

At the stern it is particularly important to avoid any unnecessary appendages for yet another reason: to achieve optimal propulsion, the inflow conditions to the propeller must be as homogenous and undisturbed as possible. This places high demands on any hull appendages in this area.

Any unavoidable appendages must be carefully designed and optimized in order to keep interference with the flow to a minimum.

To support the development of optimized appendages, FSG works with the latest direct computation- and simulation methods (CFD) which reflect the current state of the art in science and technology. Many of the methods applied can be credited to FSG’s own in-house developments and are exclusively available to FSG. A team of specialists from the fields of hydrodynamics and software engineering work tirelessly on the improvement and continued development of these methods, in order to be able to apply the latest findings to the benefit of product development.

Improvement of Seakeeping Characteristics

Besides achieving a sufficiently high level of safety with regard to damage and loss of ship, the optimization of a ship’s seakeeping characteristics also serves to secure the cargo and persons aboard and to reduce weather related losses.

With the aid of long-term simulation in typical sea states of the respective maritime territory, conclusions can be drawn as to the roll angle and the accelerations in specified on-board locations.  For example, the question as to how frequently the cargo has to be secured against slipping, can be answered with these means. With a corresponding design of the hull shape and other measures of improvement of the seakeeping characteristics (for example, roll-damping systems) the serviceability of the ship can be optimized in compliance with the customer’s requirements.

FSG has made it its goal to build ships with a level of safety higher than what is required by the currently valid IMO (International Maritime Organization) stability regulations. For this purpose, FSG’s own evaluation index was introduced on the basis of long-term probability occurrence (Insufficient Stability Event Index, ISEI), the use of which permits quantitative evaluations for ships in view of dangerous conditions in waves, such as parametric rolling, for example.

This means that for every design several thousand hours of operating time in the sea state must be simulated, and this for varying cargo situations and speeds of the vessel; this is how a representative portion of the simulated service life will enter into the evaluation. Additionally, critical situations can be examined in detail through short-term simulations (a few seconds) with more complex viscous flow solvers. This is of particularly great interest in the evaluation of risk from wave impact loads.

For the safety of the crew and the ship good maneuverability is another important factor. The evaluation of maneuverability properties, such as course keepping ability and turning rate through direct computations at an early stage of the design process, facilitates a customer specific adaptation of the maneuvering devices.

Improved Structure

The loads which affect the dimensions of a steel structure are not limited to global loads only. Extremely high loads, as well as local and temporary loads acting upon the steel structure must equally be considered for a resistant steel structure. The classification societies have created empirical formulae to design steel structures under consideration of wave impact loads, however, in many cases those cannot demonstrate a sufficiently satisfactory reality.

Extensive damage to the foreship area, such as must be addressed on some ships after just a few years of service life, is a clear indication that an empirical approach is not always appropriate. Ships with varying operating profiles are certainly affected by the wave impact loads; while carrying out their transport assignments they either operate in very heavy seas, where waves above 8m significant wave height are not uncommon, or if the ships are not exposed to such extreme sea states, they nevertheless operate during moderate sea states nearing operating speed. This commendable property is a true advantage for keeping with time tables, but may occasionally result in the permanent deformation of the plate fields on the foreship.

Even the partial global effects of wave impact loads must not be overlooked. The momentum of the foreship will only be moderately stopped by the wave impact loads, as far as the structure’s impact on the water surface is concerned. The amplitudes and distribution of pressure on imerging structure are of paramount importance; these may not be calculated in a reliable manner with the assumption of static pressure distribution. As a result, viscous solvers are employed.

Following is an approach to obtain the required calculations of the above mentioned phenomenon in connection with wave impact loads:

• Identification of critical environmental and operating conditions on the basis of sea state statistics in relevant areas
• Identification of critical relative immersion speeds between the ship’s hull and the water surface
• Identifikation kritischer relativer Eintauchgeschwindigkeiten zwischen Schiffsrumpf und Wasseroberfläche
• Calculations of 2D wave impact loads on representative sections
• Application of loads on a 3 D model
• FEM analysis of loads on the bow door
• Risk analysis based upon sea state loads
• Investigation of options, design modifications, limitations of shipping routes, territories and/or instructions for the crew.

The above described procedural approaches must be repeated iteratively, since critical load conditions may ultimately be determined only with the use of results from the FEM analysis.