Design-Engineering

Load Determination

Loads which are relevant to the dimensions of steel structures may be obtained from the global movements of a ship in the sea state, as well as local loads on the foreship and stern which are derived from short-term wave impact. FSG is able to provide a static and transient analysis of both types of loads by direct application of the physical laws, followed by simulation and finally by applying these values through its own, in-house developed interfaces as external fringe conditions on finite element models.

These methods have evolved in such a way that they can be applied iteratively to ship-building design. With the aid of comprehensive measuring data, as well as testing in an experimental aquatic facility, all data is confirmed for the purpose of verifying the limits of load fringe conditions. The results of such measuring-data and calculations will later on provide the ship’s navigation with clear indications as to the use of potential operational profiles, under particular environmental conditions. Furthermore, such data will serve the identification of potential targets and thus enable the customer to implement strategic investments into improving the ship’s structure.

Loads from wave impact
Extremely high local as well as temporary limited loads must be given very careful consideration in dimensioning a robust steel structure. Classification agencies have established empirical formulae for wave impact loads to determine the steel structure; however, in many cases those formulae are an unsatisfactory reflection of reality. Oftentimes enormous damage to the foreship after just a few years of the ship’s service life have shown that the empirical data has regularly reached its limits.

This is why FSG and its ship design software consistently utilize the physical laws of nature for direct computation of a ship’s interaction with its surroundings. Serving as an input variable for fully viscous solution algorithms, sea state spectra as previously observed, or probability distribution of the sea state spectra is used, along with the significant wave heights of the respective shipping routes. Those conditions are examined to determine critical parameters, their frequency is assessed, and extremely critical events are simulated in transient format. Conclusions as to the potential probability of damage risk and the pertinent locations may thus be pinpointed with certainty, so that targeted investments in localized reinforcements may then be offered by FSG.

Loads based upon fluctuations in sea state
When operating a ship, constantly occurring movements of the ship in the sea state create temporary and localized fluctuations in loads due to water pressures acting upon the ship’s exterior surface structure. Moreover, the entire steel structure is subject to movement-induced translational and rotational accelerations. These accelerations and their effects are given insufficient consideration within the body of regulatory work issued by the classification agencies, since the complex heterodyne detection of degrees of freedom cannot be depicted therein.

FSG’s ability to utilize long sea state movement simulations in the time domain, including all degrees of freedom as fringe conditions for the finite element model, facilitates the identification of critical load situations and their frequency of occurrence. An ‘enhanced’ interpretation of critical structural areas, in the form of additionally identified targets for investment, can thus alert the customer to realize an appreciation in product value.

Serviceability Analysis

Every commercial ship should be able to perform dependable service for a minimum of 20 years. Over the course of its service life it covers innumerable sea miles on the world's oceans, encounters great storms and waves and is under constant stress imposed by the sea state. To make sure that the ship can withstand all of these stresses it is necessary to be able to recognize weak structural areas in a timely manner and to eliminate those areas.

Several at-risk design details are known already and may be assessed on the basis of past experience. Other areas, however, require more precise testing since they vary from case to case, in order to enable the experts to make a generally reliable assessment.

Typical weak points in steel design in the ship-building industry are junctions in the frame, sections in profiles and plates, the transition from the hull to the deckhouse, and, from a microscopic point of view, all of the welding seams and joints. For precisely such areas FSG has designed computer models with which the stresses of these types of structures may be identified. The results can then be projected via various statistics onto the ship's entire service life, so that reliable conclusions about the life expectancy of the structure may be made. In so doing, distinctions are made as to whether these are loads are a result of the sea state, loading and unloading cycles, or even factors related to specific parts, such as the engines or the propeller, for example.

These comprehensive calculations will ensure that the ship's operation may be guaranteed to be without any extraordinary service interruptions on account of repairs.

Vibration Calculations

An essential aspect of ship design is testing of the vibrational behavior of the steel structure across the lower frequency realm. Humans have a highly sensitive reaction to this frequency impact onboard a ship, particularly the vertical vibrations in the resonance area of the spinal column and the lower jaw, or mandible.

The cause of such potentially disagreeable vibrations can be found in propeller-induced pressure fluctuations and main engine excitations. To calculate forced vibrations and their causes with the aid of FE-model generation and modal analysis is essentially well under control when it comes to steel structures related to ship building, however, a considerable uncertainty factor is the influence of the body of water, the mass surrounding the ship during its operations. FSG presently employs two comprehensive methods to choose from, in order to depict such influences in mathematical terms.

Finite Boundary Element Method (BEM)
Thanks to its collaboration with the University of Rostock, FSG is able to illustrate the surrounding water in the form of fluid with its mass/inertia effect upon thin walled structures. For this purpose FSG has integrated a special FE-Boundary-Element equation solver which shows the results of all of the complex mass/inertia-interactions of the wetted FE nodes in a velocity-dependent wave image situation.

Modeling a sufficiently large, deformed water surface surrounding the ship with its pertinent ‘impression’ in the water is quite adequate. The distance of the ship to the bottom of the body of water may be part of the demonstration. This interaction is used as a parameter field for modal analysis, and is therefore a component of the mandatory global vibration calculations. Through the use of comprehensive measuring data from natural vibrations during dockyard tests, many new designs were able to benefit from pre-calculations for steel design as far as vibration levels are concerned. In this way, adherence to the specified limits or classification-relevant threshholds can be secured as early as in the pre-design phase.

Direct Fluid Modeling (Accoustic Fluid Elements)
In present times modern FEM software packages are quite capable of illustrating a fluid volume with the aid of special finite elements. To these ends FSG, in collaboration with a distributor, makes use of the know-how from fields outside the ship building disciplines, particularly the field of accoustics. With the use of complete integration of the ANSYS and CFX software packets FSG is able to model this situation in its own ship designs and transfer the results into a solvable mathematical grid. In various load situations, the bi-directional fringe conditions of all relevant degrees of freedom and pressure connections at the juncture of sea water and ship structure are given careful consideration and are then generated in automated form. The boundary layer of water in relation to the surrounding air is also depicted.

All components of a global modal analysis are combined into one singular mathematical model and are thus easy to control.

Engine Vibrations

Man's comfort aboard the ship is of primary importance to FSG; therefore, vibrations created by the main engines are kept to a minimum, as much as possible. The main engines have already been tested by the manufacturer in a test facility, however, they essentially display varying vibrational behaviors in the ship’s foundational structure later on. This is a model which cannot be predicted by the engine manufacturer alone, or ahead of time.

In order to be able to draw reliable conclusions with regard to the free forces and moments occurring in the main engine area, FSG elects to simulate the dynamic behavior of the entire system, of the main engine and foundation of the ship. For this purpose, using a Fourier analysis in the rated speed range, all cylinder pressures, under consideration of bearing rigidity factors, engine geometry and firing sequence are separated out, so that in a Finite Elements Computerized Model they may act as harmonic force fringe conditions at the locations of stress. From the in-phase superimposition of all of the calculated structural reactions it can then be recognized whether or not specific limits or thresholds are being exceeded.

In the process, potentially critical areas of vibration in the ship are being detected, as well as unfavorable structural reactions from the main engines, which can now be recognized. If this is the case, then the structural impedance coordination may be adjusted in the main engine-foundations prior to the request for proposal or tender, or may be counteracted by taking measures with regard to the engine. In this manner, and in collaboration with the engine manufacturer, the total behavior of the main engine will be coordinated with the installation conditions. Time and cost -intensive modifications during the construction phase or when launching start-up operations can thus be avoided.

Ship Vibrations

The comfort of passengers and crew are of the greatest importance to us. A great deal of comfort and work safety is affected by the influence of vibrations on humans. This is why we place the highest value on keeping such influences on human wellbeing to levels as controlled as possible.

Vibrations are primarily created by the main aggregates and the propeller. With the aid of the most up-to-date software tools which are to a great degree developed in our own facilities, the ship’s vibratory behavior is tested as early as during the offer stage. It can be seen from a global FE Model if permissible thresholds or limits are being exceeded in certain areas. Should this be the case, structural modifications can be made even prior to making an offer or, for example, the specifications can be given to the propeller manufacturer with reference to admissible pressure impulses. This prevents time- and cost intensive modifications during the design- and/or construction phase.

One example for excellent low levels of vibratory behavior is the new 748 design, the "Northern Expedition" for BC Ferries, which was “Comfort” certified on the basis of the most stringent vibration requirements ever issued by ABS, the American Classification Agency.