Large offshore platforms with vertical circular cylindrical hull shapes have been designed and employed in recent times. Classical spar platforms and mono-column concepts are just two examples of designs with such simple configuration, supposed to perform limited motion responses in waves. Reports on investigations on the occurrence of parametric resonance of spar platforms have been published recently in which the relevance of Mathieu amplifications have been assessed making use of different mathematical models. However, some uncertainties still remain on the influence of crucial design parameters as, for instance, metacentric height, draft/diameter ratio, associated damping and mooring system. In an attempt to clarify some of these aspects, in this paper the dynamic stability of a vertical cylinder in regular waves is investigated theoretically and experimentally. A coupled non-linear mathematical model is employed to model and simulate the coupled heave, roll and pitch motions. Theoretical aspects related to the development of resonant motions are discussed. In addition to the numerical and theoretical investigations, an extensive series of experiments with a model of a typical mono-column have been recently conducted at LabOceano. The findings of these investigations are compared and summarized.
This study considers the use of a genetic algorithm for the structural design optimization of support structures for offshore wind turbines. Member diameters, thicknesses and locations of nodes are jointly optimized. Analysis of each design is performed with a complete wind turbine simulation, for a load case in the time domain. Structural assessment is in terms of fatigue damage, evaluated for each joint using the hot-spot stress approach. This defines performance constraints. Designs are optimized with respect to their weight. The approach has been tested with the modified 4-legged UpWind jacket from the OC4 project. The weight is quickly reduced, convergence slows after about 100 iterations, and few changes occur after 250 iterations. Interestingly, the fatigue constraint is not active for any member, and it is the validity of stress concentration factors that determines the best design, which utilizes less than 90 percent of the available fatigue lifetime. These results of the preliminary study using the genetic algorithm demonstrate that automatic optimization of wind turbine support structures is feasible under consideration of the simplified load approach. Even for complex, multi-member structures such as the considered jacket a weight reduction was achieved.
Although in most vessels the usual function of a moonpool is a passage for production lines, equipments and divers, the moonpool may be used as a device to reduce the vertical motion of floating structures as it can be observed in published studies. From these published works one may say that changes of the moonpool’s internal geometry modify the behavior of vertical oscillation of the water inside the moonpool. Looking for a “hydrodynamic design of the monocolumn-moonpool”, Petrobras has carried out a set of numerical and experimental tests with monocolumn hulls provided with moonpool. Some of these results have been published by the authors in the literature. To increase the comprehension of the dynamics of the joint motion, fluid and structure, numerical results were obtained using a commercial code based on the potential theory and a commercial code based on the solution of Navier-Stokes equations. The paper presents results of these experiments and analyzes the behavior of the coupled motions of mono-columns and the vertical motion of the water inside the moonpool, comparing the experimental and the numerical results.
The steel catenary risers (SCR) have been studied to be used in ultra-deep oil fields. In Brazil, the oil field is going to deeper water. The geographical characteristics impose this type of production risers, and the necessity of these risers is real. However, in order to avoid risers fatigue problems, some premises related to low vertical motions have to be respected. To reach the necessary good seakeeping responses, new type of hull shape has been proposed, the monocolumn hull, to which some motion minimization devices have been developed. There are several conceptions of monocolumn type hull, but the concept presented in this paper is different from the existing monocolumn hull. To have good seakeeping capability, three main minimization devices were proposed: the structural skirt, the inclined wall, and the moonpool. The inclined wall, a variation of a section area near the water-plane, works together with the skirt, an enlarged bilge keel at the bottom of the platform, modifying the effect of damping, added mass and inertia. The moonpool is the old tank usually found in drilling units, properly modified to work as a motion minimization system. It works as a passive heave motion reduction system, shifting the natural period far from the wave spectrum. Some numerical analyses were made to present the effects obtained from these devices. These numerical results from WAMIT® were compared to experimental showing to be according to them.
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