Accuracy of nonlinear finite element collapse predictions for submarine pressure hulls with and without artificial corrosion damage

Nonlinear finite element (FE) collapse pressure predictions are compared to experimental results for submarine pressure hull test specimens with and without artificial corrosion and tested to collapse under external hydrostatic pressure. The accuracy of FE models, and their sensitivity to modeling and solution procedures, are investigated by comparing FE simulations of the experiments using two different model generators and three solvers. The standard FE methodology includes the use of quadrilateral shell elements, nonlinear mapping of measured geometric imperfections, and quasi-static incremental analyses including nonlinear material and geometry. The FE models are found to be accurate to approximately 11%, with 95% confidence, regardless of the model generator and solver that is used. Collapse pressure predictions for identical FE models obtained using each of the three solvers agree within 2.8%, indicating that the choice of FE solver does not significantly affect the predicted collapse pressure. The FE predictions are found to be more accurate for corroded than for undamaged models, and neglecting the shell eccentricity that arises due to one-sided shell thinning is found to significantly decrease the resulting accuracy of the FE model.     

A review of physical flexible ship models used for hydroelastic experiments

Hydroelasticity of ships has been established as a necessary form of investigation for both slender ships and high-speed craft. Experimental investigations have spanned various topics, including symmetric and antisymmetric, harmonic and transient, linear and nonlinear responses. Models have varied in size and the way the structure is modelled, depending of the focus of the investigation. The multitude of interacting physical mechanisms introduce almost-impossible-to-resolve scaling issues, and the eventual compromises depend on the aim of the investigator. This publication provides a comprehensive review of the evolution of these experimental techniques, from the first appearances of the field to the modern state-of-the-art and potential future directions.     

Dynamic response of a SWATH vessel for installing pre-assembled floating wind turbines

New and efficient installation concepts which can reduce the cost of developing an offshore wind farm are of particular interest. This paper explores a promising concept using the small water-plane area twin-hull vessel (SWATH) to install pre-assembled wind turbines (OWT) onto floating spar foundations. A focus is placed on the hydrodynamic performance of the SWATH and the response analysis of the coupled SWATH-spar system. Firstly, the numerically calculated difference-frequency wave force effect and damping forces of the original SWATH were verified with experimental data. Secondly, the original SWATH was modified to satisfy the criteria of weight-carrying capacity and hydrostatic stability. Thirdly, a multibody numerical model for the SWATH-spar system was developed, in which the hydrodynamic and mechanical couplings between the SWATH and a spar were considered. The SWATH is equipped with a dynamic positioning system to counteract the slow-drift wave force effects. The nonlinear time-domain simulations were carried out for the mating stage when a wind turbine is lifted above the spar foundation. Based on the analysis of statistics of the relative displacement and velocity of the tower bottom and the spar top, the installation concept with SWATH is found to be of decent performance. Finally, recommendations are provided for future research on this concept, which contributes to developing next-generation installation concepts for bottom-fixed and floating wind farms.     

Stability of submarine bi-material pipeline-liner system with novel polyhedral composites subjected to thermal and mechanical loading fields

This study is concerned with the thermal and mechanical instability behaviors of composite novel liners encased in deteriorated pipelines. A liner may contain many connected segments, and two adjacent segments may become disconnected after long years of service. In such a case, the single disconnected segment may reduce to a ring. An innovative polyhedral configuration is introduced to improve the bending stiffness of the composite ring that is confined by the pipeline. The radius and bending rigidity of the ring are simplified analytically to facilitate the derivation of the critical buckling load. By employing the classical shell criteria, and defining an admissible displacement function, the expression of the potential energy function is obtained with only two unknown parameters. Taking the first derivative of the energy function to the two unknowns respectively generates two equilibrium equations. By solving the two equations, the analytical buckling load is obtained for a composite polyhedral ring in a thermal variational field. Then, two comparisons are taken between the present analytical predictions and results from elsewhere, and good agreements are obtained. An amplification coefficient is defined as the ratio between the buckling load of the polyhedral and circular rings. Finally, parametric evaluations indicate the amplification coefficient reduces with the increase of thickness-to-radius ratio, the increase of the number of sides, and the increase of the temperature variation, respectively. Therefore, a polyhedral ring with a low thickness-to-radius ratio is recommended in engineering applications.     

Efficient optimization design method of jacket structures for offshore wind turbines

With the gradual implementation of offshore wind energy production, the future tendency is to expand into the deeper water. The jacket foundations will take the place of the present monopile foundations when the water depth increases. The foundations account for the majority of the construction cost for offshore wind farms, and the structural optimization of jackets will bring lucrative economic benefits. Structural optimization is a complex iterative process that requires huge computing costs. Therefore, this paper proposes a structural optimization method based on surrogate models to solve this problem effectively and swiftly obtain optimized design schemes of lightweight jackets for offshore wind turbines. The structural responses of jacket wind turbine systems under the equivalent static extreme loads with a recurrence period of 50 years are mainly considered in structural optimization design, and the key optimization variables of jackets are determined by parameter sensitivity analysis. The finite element models of jackets are transformed into surrogate models, and the genetic algorithm is employed to optimize the surrogate models directly. The optimized jackets are additionally verified through coupled dynamic analysis, besides, buckling strength and fatigue life are also checked. And local refined optimizations are carried out for the failure members. According to the optimized design schemes of lightweight jackets for 30 m, 50 m and 70 m water depths, it is demonstrated that the structural optimization design method is adequate and efficient for jackets of wind turbines. Parameter sensitivity analysis can cut the number of optimization variables in half to improve the optimization efficiency. Furthermore, the application of surrogate models can significantly speed up the optimization process by saving about 98.61% of the original time consumed. The optimization design method of the jackets for offshore wind turbines proposed in this paper is suitable for practical engineering, with high precision and efficiency.