Hydrodynamics and hydro-structure
Technical bulletin 2011

The fourteen papers in this section provide an overview of our recent achievements in the understanding and modelling of hydrodynamic loads, and their interactions with the structural response.

HYDRODYNAMICS

In the area of hydrodynamics a lot of effort has been made to introduce dissipation in the classical potential flow. Some theoretical analyses and numerical developments have been carried out. The global effect of fluid dissipation can now modelled in a relatively correct way. This innovation leads to predictions of wave load and induced responses with much improved accuracy in the range of wave frequencies where fluid resonance due to confinement between multiple structures occurs.

HYDRO-STRUCTURE

Building on the work of previous years, the emphasis was put on the global hydro-elastic behaviour of large ships and the so called “springing” and “whipping” phenomena. The papers cover various aspects, dealing with theoretical developments, measurements and model tests and improvement and validation of numerical tools.

The first paper presents work done in cooperation with the University of Zagreb on the development of an improved beam structural modelling of Ultra Large Containerships. The usual Timoshenko beam model is an efficient simple tool to model the global ship deformation. This model has been improved to represent torsion more accurately, taking into account the special properties of the open sections of the containerships.

The hydro-structure tool Homer, developed by Bureau Veritas several years ago, deals with the coupling between hydrodynamic loads and structural ship response. Recent developments have been made in the inclusion of tank pressure, and the interaction between the hydro-elastic deformations of the hull girder and the liquid motion inside the tanks. An example of application is shown with a LNG ship.

Even if they appear to be very simple, hydrostatics are still a research field. The hydrostatic properties of a rigid body are well known, but things get much more complex when elastic deformations are included in the seakeeping problem. This paper compares the various existing approaches to computing hydrostatic restoring stiffness, and proposes a more consistent formulation.

The fourth paper compares the results of the hydro-elastic numerical model with some model test results done within the WILS project. An elastic segmented model of an Ultra Large Containership has been tested in regular and irregular waves. The paper shows that good comparisons can be achieved, provided that the relevant data are compared. It emphasises the difficulty of analysing hydro-elastic model tests when torsion is modelled.

The last hydro-structure paper covers results of model tests performed on a 13,000 teu containership, dealing with the effect of whipping and springing on fatigue damage and extreme loading. It shows that the high frequency content of the ship response, caused by whipping and springing, has a very important impact on the design loads.

SLOSHING

Particular attention is given to sloshing and to the structural response of the cargo containment system.

In the first paper, some of the results of the SlosHel project are used to verify that the structural capacities of NO96 cargo containment system (CCS) calculated following BV guidelines for strength assessment of cargo containment system under sloshing loads (BV NI564) are coherent with the fact that no damage was found to NO96 boxes during the SlosHel project. In the second part, hydro-elastic effects on the structural response of MarkIII CCS are demonstrated through the use of dedicated hydro-elastic tools developed by BV during SlosHel. It is demonstrated that these hydro-elasticity effects should not be ignored in a strength assessment procedure.

An important study has been done on the statistical distributions of sloshing pressure. This study is based on a very long duration model test (480 hours full scale). Based on this test, several statistical distributions are fitted and compared to the real data. Generalized Pareto Distribution is shown to be the best suited statistical fitting distribution for the case considered.

The third paper compares CFD simulations with model scale experiments. The comparison is done on the global forces acting on the tank in very violent sloshing conditions. The agreement between CFD and experiments is very good for real sloshing test duration (i.e. 5 hours at full scale).

A more local comparison between CFD and experiments is done in the case of drop tests with and without invar edges. The numerical results, using OpenFoam, are in good agreement. However, for full scale, compressibility may play a role.

The last paper is a comparison between experiments and different kinds of CFD calculations for sloshing flows and sloshing impact pressures for a 2D tank. Results obtained by BV are in very good agreement with experiments.

The Chinese version of Professor Molin’s book on Offshore Hydrodynamics has recently been published. The book crystalizes Professor Molin’s more than 30 years theoretical work on nonlinear hydrodynamics (drift forces, slow drift motion, high frequency loads and response) and experience in the French offshore industry. The book has been widely used as a text book in French engineering schools and as the guideline reference in French offshore companies. Its Chinese version has been welcomed in the Chinese offshore industry.

China is developing rapidly in the offshore field and Chinese scholars are working all around the world. As an international classification society, Bureau Veritas has supported the translation of Professor Molin’s work as part of that development. Dr. Chen (a previous Ph.D student of Professor Molin) has organized the translation and verification work of the book.

We are confident that all Chinese students and engineers working in offshore industry will find this book useful and helpful in their work.

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