Floating Drilling Production Storage and Offloading vessels (FDPSO) are used for the production of oil from remote fields, where the installation of pipelines is uneconomical. Moonpool is the main characteristic of the FDPSO. Barges offer cheap way for transportation and lifting of offshore structures. Installation barges are often fitted with moonpool for functional purposes. Motions of trapped water inside the moonpool are a subject of study for researchers. In the present study numerical analysis of barge with and without moonpool were carried out using commercial software namely WAMIT and STAR CCM+. Two different configurations (circular and rectangular shape) of the moonpool were selected to study its effect on the motions of a barge. The oscillations of water inside the moonpool and its effects were also investigated. The numerical results show that the moonpool oscillation changes the response of a barge and also the vessel responses influence the moonpool oscillation. The sloshing mode was found to be dominating in the rectangular moonpool.
The hydro elastic responses of flexible structures under fluid loading is an important concern during the design of large ocean structures. The two-way coupling between the structural responses and the hydrodynamic loads is a complex problem in large flexible floating structures since the structures can vibrate in longitudinal, vertical, horizontal, or torsional modes. The antisymmetric distortion modes may be coupled depending on the location of the centroid and the shear centre. In the case of thin walled open structures, horizontal and torsional vibrations are usually coupled due to the asymmetry of cross section as well as eccentricity between centroid of the section and shear deformation centres. The acurate estimation of dry natural frequency and modes shapes of structure is indispensable since it helps to validate the accuracy of the structural modelling. A numerical method available from one of the existing literatures is used for the estimation of dry and wet natural frequencies, and mode shapes of horizontal and torsional vibrations of an ULCS. The natural frequency and modes are essential parameters for the analysis of interaction between structural responses and hydrodynamic loads. The numerical method is based on a 1D FEM beam model. Distortion due to warping is included in the numerical model since it is well known that containerships with large hatch opening are susceptible to warping. The numerical model is subdivided into 50 stations and the mass distribution and the sectional properties are calculated in order to match the bending, shear, torsion and warping moduli of the experimental model. The dry and wet natural frequency and mode shapes for the horizontal and torsional vibrations of the ULCS is numerically calculated and compared with the experimental results.
Predicting hull responses and structural loads is essential for the robust design of ships. The Ultra Large Container Ships (ULCS) are subjected to non-linear wave loads due to the low block coefficient and pronounced bow flare. They are highly susceptible to torsional loads because of the large open cross-section which is subjected to unsymmetrical hydrodynamic loading. A time domain method based on strip theory is developed for estimating the coupled rigid body motions and wave-induced loads acting on an Ultra large containership. A 2D-Panel method is followed to calculate the added mass and damping coefficients. A body-nonlinear approach is followed to capture the major source of non-linearity in the structural load estimation due to Froude-Krylov and restoring forces. Case studies are conducted for an Ultra-large container ship in small amplitude regular waves. Motions and structural load (vertical, horizontal, and torsional load) RAOs are being investigated for various wave headings, and the results are compared with the published experimental results. The proposed method is computationally efficient to capture coupled rigid body responses and sectional loads, including torsion.
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