An improved, efficient and accurate design methodology

“Coupling between hydrodynamics and structural analysis is what makes this method so unique”, says Ana Maria Tocu, Lead Naval Architect at Nevesbu. “We have developed a method for mapping the response of ships and offshore structures with direct link to the FE (Finite Element) analysis, in order to accurately account for specific environmental conditions. It goes without saying, that once a marine structure is at sea, it should remain intact and be able to properly and safely operate over its entire life time. Therefore, our goal is to ensure that global structural strength, as well as local structural details, are capable of withstanding operational and environmental loads, whilst balancing these requirements against economic and environmental emission-driven demands calling for lighter and more efficient structures and structural arrangements.”

Current practice, risks and limitations

Currently, it is more of a common practice to separately perform hydrodynamic calculations FE analyses. Wave-induced loads, like accelerations and wave bending moments, are commonly determined by linear seakeeping analyses. Thereafter, the load combination approach is typically used to load the structural FE model. This is an example of a rule-based approach in which the FE model is loaded by a combination of extreme wave-induced loads, which will not necessarily give a representative design load condition for the structure. Practice learns that in case of non-compliant results, this can lead to time-consuming discussions with classification societies and sometimes to extreme structural design modifications. Additionally, modern engineering practice implies that in those cases where rules are not deemed sufficient, direct calculation procedures are to be applied.

Hydro-structural link

“When the opportunity arose and we had adequate in-house knowledge and experience, we set up a core team of specialists to develop the methodology for performing the hydro-structural analyses more efficiently", says Ana Maria. “These calculations are by no means simple. They are extremely complex and require specific knowledge of both hydrodynamics and structural aspects. Our main goal was to make use of an integrated software package that covers all relevant hydro-structural aspects, and we succeeded.”

With the new software package, a direct connection is made between the hydrodynamic mesh and the finite element mesh. The pressure exerted by the waves on the wetted surface of the ship can, therefore, be directly linked to the FE model. Consequently, this means that working with (sometimes) unrealistic wave loads is cancelled out. More complex issues such as fatigue behaviour, non-linear excitation of waves and hydro-elasticity can also be simulated. Usually, various software packages are used by different disciplines, but ultimately, everything is integrated in one model, with direct link to the structural FE model. This is of great advantage for Nevesbu as a wealth of knowledge, experience and tools can be reused.

Main focus

The main focus for Nevesbu is to increase the consistency and representativeness of the FE results of marine structures, for both extreme load cases as for fatigue loading. “We use for example a non-linear seakeeping model to calculate extreme wave-induced loads”, says Lennart Buitendijk, Naval Architect at Nevesbu and member of the core team of specialists. “This is necessary to determine realistic wave loads in extreme environments. It is unique that the software package is not only capable of performing this calculation, but can also link this information to the structural FE model. Particularly in extreme wave conditions, it is observed that a non-linear model provides a significantly higher response than a linear model.”

Continuous optimisation

After years of knowledge development and research, the moment finally arrives that the methodology is mature enough. Everything has been thought out and calculated in detail and the required software has been purchased. But does it work in practice as intended? Lennart: “We have conducted various benchmark studies to test the hydrodynamic calculations, and to refine these where necessary. For one of the benchmarks, we have used the available data from a FPSO project. We compared our results with those of the classification society and the outcome was good. The results matched.” Ana Maria: “Our method works. Nevertheless, both research and further development remain an ongoing process. Developments are still in full swing and we strive to deliver the best quality possible.”

The extensive article can be found in our magazine.

Would you like to know more about this subject? Kees will be pleased to share ideas about your engineering issues. Contact him by email or call 088 943 3400.

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