Marine Structures最新文献

This paper presents the fatigue reliability prediction for welded planar tubular joints of offshore fixed jacket support structures. A coupled model is employed to calculate the dynamic response of the jacket support structure under wind and wave loads. Stress concentration factors (SCFs) are evaluated using Efthymiou parametric equations, and the hot-spot stresses at eight locations on the chord and brace of the tubular joints are derived by summing the stress components from axial, in-plane, and out-of-plane actions. The stress distribution is determined using the rainflow counting method, fatigue damage from an S–N curve, and Miner’s rule. Uncertainties associated with the fatigue lifetime evaluation procedure are identified, quantified, and modeled. The focus is specifically given to modeling the uncertainty related to the SCFs, and a framework is proposed to evaluate the fatigue reliability of the joint by incorporating individual uncertainties for each degree of freedom of the SCFs (i.e., axial load crown, axial load saddle, in-plane bending moment, and out-of-plane bending moment). The results reveal that assuming the same uncertainty level (Coefficient of Variation of 0.2) as recommended by DNV-RP-C210 for each degree of freedom of the SCFs and including the uncertainty after obtaining the stress distribution from rainflow counting does not consistently yield conservative reliability index values, as it depends on the critical hot spot. It is essential to note that the simplified assumptions derived from the oil and gas sector may not directly translate to the offshore wind industry due to differences in loading conditions. A reduction of up to approximately 24% in the reliability index is observed for one critical hot spot, while an increase of up to 20% was observed for another. Overall, this paper provides valuable insights into the complex interplay of factors affecting the reliability of joints in offshore fixed jacket support structures, shedding light on the need for tailored approaches to address uncertainties in the specific context of the offshore wind industry.

This study introduces a multi-objective optimization method utilizing the NSGA-III algorithm to tailor the fiber shape of variable-stiffness CFRP (Carbon Fiber Reinforced Plastics) shell structures. Combined with offshore oil and gas engineering, this method is employed to perform fiber shape optimization design for CFRP-winding buckle arrestors. Based on cubic polynomial path functions' contour lines, curved fibers are represented and fitted using the Curvature Minimum Bi-Arc (CMBiArc) interpolation NURBS curve method. Employing the NSGA-III algorithm, the fiber shapes of a CFRP laminate are optimized under manufacturing constraints, targeting mean compliance, fundamental frequency, and fiber curvature. With the study of numerical modeling, buckling crossover mode and arresting efficiency, multi-objective optimization is carried out for fiber shape of CFRP-winding buckle arrestors designed for subsea pipelines. With the goal of maximizing arresting efficiency, structural fundamental frequency, and minimizing fiber curvature, the method yields an enhanced CFRP-winding arrestor design, which demonstrates a high practicability through a layered design approach. By parametric study of optimization result and comparison with slip-on arrestor, the feasibility of the optimization method and the practical value of the novel improved arrestor are substantiated.

This study presents normalized three-dimensional mixed-mode stress intensity factor (SIF) distributions for inclined surface cracks in finite-thickness plates under uniaxial tension. Inclination angle, aspect ratio and ratio of crack depth to plate thickness were chosen as variable parameters affecting the problem. Mixed-mode SIFs distributions are given comparatively with independent graphs for different combinations of parameters and their effects are also examined separately. The results indicate that at a constant ratio of a/c, magnitude of the normalized mode-I SIF increases with increasing ratio of a/t and decreases with increasing inclination angle, while mode-II and mode-III SIFs increase up to a specific value of inclination angle and decrease afterwards. Having compiled an extensive solution library of mixed-mode SIFs, as functions of the aforementioned parameters, regression-based equations are also generated for the crack front's free surface and deepest penetration points. It is also presented, using the principle of superposition, that the given solutions can be used for inclined surface cracks in thin-walled plate structures, such as thin-walled cylindrical pressure vessels, ship and other marine structures subjected to biaxial uniform tensile loading.

Glass fiber reinforced polymer (GFRP) has been applied in hull structures for small and medium-size ships due its lightweight, high strength and easy fabrication. When the action load coming from the ship self-weight and the ocean environment becomes extremely large, the GFRP hull structure may present special failure modes and their evolution processes. How to define the ultimate strength study for GFRP hulls is a critical issue for vessel safety design. In this paper, one GFRP girder to represent the structural properties of the actual hull structure is selected as the research object. The GFRP girder is made of plates of varying thickness strengthened by hat stiffeners and foam. The hat stiffener, its attaching plate and core foam are made using an integral vacuum forming technique to avoid local construction defects. The sectional corners of the GFRP girder are provided with sufficient support by a combination of adhesive bonding and bolt secure connections. The failure experiment for the GFRP girder is performed on a four-point bending facility with a single hydraulic actuator to apply the bending moment. The strain variation is recorded by the digital image correlation (DIC) system on deck upper surface and 42 strain gauges on other critical regions. The deformation data is measured by 11 displacement sensors during a step-like increase of the applied load. The deformed shape are also recorded by the DIC system and the camera. Based on the experimental results, curves of the applied load and structural displacement are plotted to obtain the ultimate strength of the experiment girder. The failure modes and their evaluation processes are also discussed based on the video recordings. Finally, the failure process of the experiment GFRP girder is simulated using the nonlinear finite element method to study the effect of local delamination at various locations where the ultimate strength is determined. Both the experimental results and direct calculations demonstrate that the GFRP girder may exhibit a kind of brittle failure after the limit load point. The conclusions obtained in this paper will provide a guide for the design of GFRP ships and the modification of ship rules.

The suction bucket jacket foundation has become a widely choice for supporting offshore wind turbines. Assessing the installation of the suction bucket foundation (SBF) is important as it significantly affects the stability of the offshore wind system and the overall project cost. Currently, experience with suction bucket jacket foundation installations primarily revolves around stiff clay and dense sand, drawing insights from North Sea projects (e.g. the design method reported by DNV-RP-C212). However, there is a shortage of publicly available knowledge concerning SBF installations in the South China Sea, characterised by deep-covered mud (i.e. very soft clay) and extensive layered soils. Existing design parameters may not adequately address these challenges, highlighting the need for a site-specific installation design method. This study introduces a tailored approach for evaluating design parameters of the SBF installation in the seabed conditions of the South China Sea. Utilising field data from 19 suction bucket jacket installations across the region, the proposed method undergoes rigorous validation and testing. The findings of this study aim to contribute to the advancement of offshore SBF installations in the intricate geological setting of the South China Sea. By presenting the SBF installation design method, the research offers practical guidance for both the evaluation of suction bucket jacket foundation installations and the optimisation of turbine locations in the region.

Umbilical, as the lifeline of offshore oil and gas resources exploitation, is composed of a variety of functional components. Due to the differences in physical characteristics of components, the compactness, balance and heat source dispersion of different cross-sectional layouts are also different. So, a multi-objective optimization problem is often encountered. In present study, through a quasi-physical method, different functional components are abstracted as the virtual spheres with different properties lied on a funnel surface. A mathematical description is thereby established according to the interaction force of each two neighbored virtual spheres. Based on that, a new cross-sectional layout optimization formulation is proposed, and solved by using the quasi-physical algorithm. Considering the manufacturing and installation, a layered method is utilized to make the results more in line with the actual needs of the project. Finally, the optimization results of the method are verified by numerical simulation. The verification results show that the cross-sectional layout optimization method based on a quasi-physical algorithm can provide a useful reference for practical engineering design.

The vibration reduction for the low frequency band has always been a challenge for floating raft vibration isolation systems. This paper presents, for the first time, the application of the limited nonlinear energy sinks (L-NESs) and cells to achieve vibration suppression of floating raft isolation systems. In comparison to a traditional NES, L-NES increases the constraint on the NES vibrator by introducing piecewise stiffness. In addition, L-NES cells can not only effectively suppress the vibration of the floating raft system, but also display high practicability and flexible versatility by changing the number of cells. In this paper, a mechanical model of the floating raft isolation system with four degrees of freedom is established. The installation mode of the floating raft system is analyzed. Analysis of the parameters of the damping system with the NESs and L-NESs is carried out respectively. Hyperbolic tangent function is used to fit non-smooth models. Then the harmonic balance method (HBM) is applied as an analytical method to obtain the approximate solution of the system, and the accuracy is verified. The damping effects of the vibration reduction systems coupled with the traditional NESs and L-NESs are compared. Meanwhile, a vibration reduction model of the floating raft system with the L-NES cells is established. The influence of the number of the L-NES cells on the vibration suppression efficiency of the system for the first three modes is analyzed. The results show that the vibration suppression effective of the floating raft can be extremely improved and the problem of achieving damping for the low frequency band can be solved by the proposed L-NESs and cells. In a word, the research in this paper provides a novel and effective idea for the vibration control of floating raft systems.

Under thermal loading, upheaval buckling of subsea pipelines occurs when the axial compressive force exceeds the critical buckling load. In order to accurately predict the upheaval buckling behaviour of subsea pipelines, the axial pipe-soil resistance should be considered more precisely, rather than traditionally treated as rigid-plastic in prior analytical researches on pipeline upheaval buckling. Consequently, this study integrates a bi-linear axial pipe-soil resistance model into the mathematical framework of upheaval buckling. This mathematical model incorporates the von-Kármán type of geometrical nonlinearity and the Euler-Bernoulli beam theory. The research examines typical upheaval buckling behaviour and investigates the influence of axial mobilization distance and ultimate resistance on pipeline upheaval buckling behaviour. The results reveal that incorporating bi-linear axial pipe-soil resistance, in contrast to rigid-plastic resistance, leads the pipeline more susceptible to buckling. Displacement amplitudes increase with the axial mobilization distance during the post-buckling stage. Notably, a larger axial mobilization distance exerts a stronger influence on pipeline buckling. Moreover, the critical buckling temperature exhibits an almost linear negative correlation with axial mobilization distance and a positive correlation with axial ultimate resistance. Additionally, greater axial ultimate resistance magnifies the impact of axial mobilization distance. Therefore, in pipeline buckling design, it is advisable to consider a more sophisticated axial pipe-soil model to accurately account for these complexities.

In the present study, an advanced computational platform has been developed for offshore renewable energy converter systems, considering nonlinear wave input, body dynamics, mooring line dynamics, and kinematic and mechanical constraints. In particular, a hybrid linear and nonlinear hydrodynamic model is formulated through coupling with a numerical wave tank generated by OceanWave3D. A novel nodal position finite element model considering geometrical and material nonlinearities is developed to simulate the nonlinear behaviour of the mooring dynamics. Then, an implicit solver with an iteration process solves the dynamics of the rigid body, the dynamics of the mooring lines, and kinematic and mechanical constraints simultaneously. Therefore, the developed platform is capable of studying the wave body interaction with mooring lines in extreme wave conditions. Wave basin model tests of a single float on a nonlinear mooring line are carried out under a series of irregular waves for comparison. The developed model was also validated against commercial software under identical conditions for idealised cases. In steep wave conditions, the hybrid nonlinear/linear hydrodynamics give improved predictions over the linear hydrodynamic method.