Xiao-Bo Chen

Consistent Hydro-Structure Interface for Evaluation of Global Structural Responses in Linear Seakeeping

The difficulties related to the equilibration of the 3D FEM structural model, in the context of hydro-structure interactions in linear seakeeping are discussed. Different philosophies in modeling the structural and hydrodynamic parts of the problem, usually lead to very different meshes (hydro and structure) which results in unbalanced structural model and consequently in doubtful results for structural responses. The procedure usually employed consists in using different kinds of interpolation schemes to transfer the total hydrodynamic pressure from hydrodynamic panels to the centroids of the structural finite elements. This approach is both, very complex for complicated geometries, but also rather inaccurate. The method that we propose here is based on two main ideas: 1. Pressure recalculation instead of interpolation; 2. Separate transfer of different pressure components (incident, diffraction, radiation & hydrostatic variation). The first point removes the difficulties related to the interpolation techniques, and allows for a very robust method of pressure transfer. The second point ensures the perfect equilibrium because the body motions are calculated after integration over the structural mesh, which means that the equilibrium is implicitly imposed. It should be noted that this procedure is not completely straightforward and several numerical “tricks” need to be introduced. However, once these difficulties are solved, the final numerical code is extremely robust and can be easily coupled with any of the general 3D FEM packages.Copyright

Some Aspects of Hydro-Structure Coupling for Combined Action of Seakeeping and Sloshing

The techniques for hydrodynamic load transfer from combined action of waves (seakeeping) and internal liquid flow (sloshing), onto 3DFEM structural model are discussed. The problem is relevant both for a ship transporting liquids in tanks (LNG carriers, tankers...) as well as for any ship sailing in ballast conditions. Correct pressure transfer to the FEM model is essential for spectral fatigue assessment of structural details. The methods that are used in practice do not appear to be very clear and different levels of approximations based on some empirical considerations are usually employed. In this paper, a fully consistent method is proposed in the context of the linear frequency domain model.Copyright

Hydroelastic analysis of global and local ship response using 1D–3D hybrid structural model

ABSTRACT For relatively ‘soft’ floating bodies (very large ships, floating airports, etc.) using the classical rigid body seakeeping analysis and modelling the hydro-structure interface by the transfer of the pressure is not sufficient. The most appropriate method for rational design is the hydroelastic coupling by solving the interaction between the surface waves and the rigid-elastic body. In this paper, a hybrid structural model based on one-dimensional (1D) beam elements and three-dimensional (3D) finite element elements is developed and used for the analysis of wave induced ship vibrations. Using a hybrid model, both global and local structural response can be determined. The coupling between structural and hydrodynamic model is performed using the modal superposition method. The complete hydroelastic response (motions, accelerations, stresses, …) of a floating unit is performed by using 1D–3D hybrid structural model with 3D hydrodynamic model and compared with full 3D structural model coupled with 3D hydrodynamic model.

A numerical study of dynamic response of crane semi-submersible along TLP in tender-assisted drilling operation

ABSTRACT Dynamic response for tender-assisted drilling is crucial due to the safety concern and impact on operability. Notably, the hydrodynamic interaction between two free floating units is believed as the main factor that threatens the operation. However, this urgent issue has not been thoroughly studied yet. We aim at establishing an accurate and efficient numerical model to compute the coupled dynamic response between crane SEMISUB and alongside tension leg platform (TLP). In terms of crane and lifting, an ideal pendulum model is employed; the lifting cargo is assumed as an independent mass-body in the air. The important physical variables e.g. the load motion at boom tip location, swing angle and tension are computed. The impact of cargo weight and sling length are investigated parametrically through sensitivity study. The time-domain analysis is performed in order to study the response under irregular waves, and the standard deviation as well as the most probable maximum value by Weibull distribution fitting are obtained.

Validation of Hydro-Structure Analysis Using Partial Structural Model

Nowadays direct Finite Element Method (FEM) calculation using partial or full length model is necessary for checking the structural integrity of ship and offshore structures under given environmental conditions. The main advantage of using hydrostructure analysis on partial model is to obtain better accuracy than usual computation based on rule loads and also a consistent decrease of the time necessary to build a complete ship model. The comparison of different three cargo hold models with the complete ship model and the improvement of our partial FEM models are the main objectives of the work. Unlike the classical partial FEM models approach, our hydrostructure analysis is based on creating an equivalent full FEM model from the partial model. The equivalent full FEM model is built by adding to the partial model two concentrated masses in the center of gravity of missing aft and fore parts. The mass and inertia properties of the equivalent full FEM model are the same as full ship FEM model. By using an equivalent full FEM model the problem of balancing the partial model transforms into the same problem for the corresponding full model. Instead of using the traditional method for interpolating the pressure from hydrodynamic mesh to structural mesh, the pressure components are recalculated over structural mesh. The inertial loads are then determined by motion equations integrating all pressure loads. In this way, the structural model is fully balanced. The balancing of the 3D FEM structural models represents one important issue to avoid unphysical structural response induced by an unbalanced structural model. This paper is focused on the validation of hydro-structure analysis methodology by comparing the results on a FSO unit using an equivalent full FEM model and a complete ship model. INTRODUCTION For ships and offshore structures operating in rough waves different physical phenomena are involved (waves, large ship oscillations, slamming, sprays, wind...) and in order to consider real seakeeping loads in their design and analysis stage hydrostructure coupling is required. By using a partial structural model engineers can quickly obtain structural response using real environmental conditions instead of performing three cargo hold analysis, based on rule approach, than can reduce accuracy. The advantage of using direct hydro-structure analysis on partial model is the important reduction of time necessary to build the full FE model. The fact that fore and aft part are replaced by two concentrated masses introduces some technical difficulties related to the balancing of the FE model. Adding to the partial model two concentrated masses and linking them with rigid elements to the partial model the mass and inertia properties of so called equivalent full FE model are the same with the complete ship model. To remove the difficulties related to the interpolation techniques for transferring the pressures from hydrodynamic model to the structural model the pressures are recalculated in each node of structural model. After these difficulties are solved the current procedure leads to completely balanced partial model and the results are compared with the results obtained using a complete ship model. Proceedings of the ASME 2016 35th International Conference on Ocean, Offshore and Arctic Engineering OMAE2016 Due to copyright restrictions only the first page is available June 19-24, 2016, Busan, South Korea