Marine Structures最新文献

Bending strain data from 3 well-instrumented VIV model test programs on simply supported bare pipes under uniform flow are processed using rainflow counting to get fatigue damage rates directly. These are presented in terms a dimensionless fatigue-equivalent bending strain amplitude, ϵ_e, which for the structurally undamped simply supported span under constant axial force N undergoing small rotations depends on 5 dimensionless parameters, and only weakly on the fatigue resistance curve used. Existing and new approximations are discussed as assessed whereby the number of dimensionless parameters that affect ϵ_e may be reduced from 5 to 1. Thus a scaling approach is developed, whereby any VIV test can be scaled to represent any prototype at a current velocity determined by the scaling procedure. The procedure uses scaling only for the dynamic part of the response, and structural analysis for the static part. It works for single as well as multi-mode response. It is illustrated for a 150m-long prototype span subject to high currents. The test and/or scaling results are also compared to predictions by existing methods, including DNVGL-RP-F105 (“F105”) and Vivana. The example also illustrates how the proposed method can be used for partially strake-covered (“straked”) pipes, provided the strake coverage is similar for model and prototype. The scatter in the data gives an indication of the uncertainties involved in estimating VIV fatigue damage rates from limited experimental data where high-mode and multi-mode response is possible.

In this paper we perform an analytical and numerical study of the performance of a wave cycloidal rotor in irregular waves, with passively morphing foils and variable rotational velocity control. The performance is measured in two ways: Mechanical power, and fatigue damage in a sample stress hot spot located at the fixed end of the hydrofoils. We consider different strategies seeking to both maximise power extraction and reduce fatigue damage. To maximise power, we consider both constant and variable rotational speed. To mitigate fatigue damage, we consider, for the first time, morphing foils in the context of a wave cycloidal rotor. By testing these control strategies in isolation and in combination, and with the aid of high performance computations, we find that variable rotational speed, in combination with morphing foils, offers the best compromise to enhance power production with a reduced structural penalty on the sample stress hot spot. Hence, in this work, we demonstrate that novel control strategies, such as those proposed in this work, can hold the key in reducing the levelised cost of energy and accelerate the commercialisation of the next generation of lift-based wave energy converters.

This study employed an accelerated corrosion method to investigate the impact of corrosion on the mechanical properties and ultimate strength of S420 steel hull plates. Initially, corrosion was accelerated using H₂O₂ as a depolarizing agent, and corrosion parameters of the specimens were measured. Subsequently, ten tensile specimens representing five different corrosion states of S420 steel were prepared to test the mechanical properties after corrosion. Simultaneously, 60 ship hull plate specimens, each measuring 146 mm in length, were prepared for ultimate strength experiments under four accelerated corrosion cycles and five slenderness conditions, resulting in a total of 20 operating conditions. The results revealed a linear decrease in various mechanical properties with increasing corrosion degradation. A bilinear stress-strain model considering the corrosion degradation was fitted. The ultimate strength of the hull plates linearly decreased with the increasing corrosion degradation, and the rate of decrease slowed with greater slenderness. Even at the same corrosion level, there was still a certain variability in ultimate strength, which gradually converged with increasing corrosion levels. A predictive formula for ultimate strength considering corrosion level, slenderness, and variability was fitted. The accuracy of the formula was verified through detailed error analysis, providing a practical reference for future structural design and corrosion management.

Shared mooring brings potential cost benefits to a floating wind farm. In such a wind farm, adjacent floaters are connected by shared lines and the total number of mooring lines is reduced. This paper presents an experimental study on a dual-spar floating offshore wind farm. In the first configuration, two spars are connected by a shared line. In the second configuration, a clump weight is added to the shared line. Irregular wave tests are performed for both configurations under operational and extreme wave conditions. The platform motions of one wind turbine and the mooring tension are measured during the tests. The influence of the added clump weight is found to be insignificant in the operational wave conditions. Under extreme waves, the added clump weight results in smaller platform motions in the wave direction and reduced tension oscillations in all mooring lines. With the additional clump weight, the extreme mooring tension in the shared line can decrease by 30% and fewer snap loading events occur, but the extreme tension in single lines can increase by 6%. This study contributes to an improved understanding of shared mooring systems and facilitates the development of model test methods for floating wind farms.

This study introduces a Simulation-Based and Data-Augmented method for shear force inversion to address the challenge of directly measuring shear force on connector pins in multi-module floating platforms. Stress sensors are strategically placed in adjacent areas. Extensive Finite Element simulation scenarios lead to the identification of optimal features sensitive to both force magnitude and direction. Subsequently, an Artificial Neural Network (ANN) is developed to distill the simulation data into characteristic sensor responses. Fine-tuning with physical measurements further enhances shear force inversion accuracy. Using simulated and experimental data, the method demonstrates a shear force inversion error below 3.2 % and an angular inversion error under 1.4 % across test conditions. This methodology provides essential load data for connector safety assessments and crucial guidelines for the assembly of multi-module floating platforms.

In this study, a two-step stress analysis method to incorporate the global hydroelasticity and local bending effect is developed in the frequency domain. In the first step, the continuous structure is discretized into several rigid modules connected by elastic beams to evaluate global hydroelastic responses, known as the beam-connected-discrete-modules (BCDM) hydroelasticity method. In the second step, the hydrodynamic and hydrostatic pressure as well as inertia forces in step one are mapped on an entire finite element model to estimate the stresses by a quasi-static method. In this method, the boundary value problem solved in the generalized mode is replaced by multi-body hydrodynamics which has been extensively studied. The application of the proposed method is first verified against the results from modal-based method and published experimental data. Then, the effect of local bending and global flexible deformation on the stress is investigated through an intentionally flexible barge with an open-cross section. The results show that the local bending leads to an increase in the stress for some non-resonant frequencies. The global flexible deformation mode contributes significantly to the stress when the resonance vibration is excited, which is caused by the associated inertia forces.

A novel theoretical model is proposed to study the continuum damage mechanical (CDM) behavior of reinforced thermoplastic pipes (RTPs) under bending moments, in which stress analysis of composites, failure evaluation and stiffness degradation are combined in loop calculation. Based on the existing homogenization assumption, the stress distribution of every ply could be calculated according to the equilibrium equations between the RTP and a hypothetical homogenous pipe. Once stresses of composite plies satisfy Hashin-Yeh failure criterion, dominant failure modes are determined by filtering failure coefficients. Subsequently, the stiffness degradation model would be performed, in which a sine weight function is employed to consider the damage distribution along the hoop direction. Meanwhile, the von Mises criterion and Ramberg-Osgood curve are used to simulate the material nonlinearity of liner and coating. Four-point bending tests and numerical simulations were conducted to verify the proposed theoretical model. A user-defined VUMAT subroutine was employed to simulate the progressive failure of 3D composites. Compared with experimental tests and numerical simulations, the proposed model could give accurate predictions on the linear and nonlinear responses, such as the bending stiffness, the stress field and the damage propagation. Furthermore, different methods for the four-point bending test were also compared and good correlation found.

A good understanding of cross-sectional loads of the support structures of floating wind turbines (FWTs) serves as a basis for achieving a reasonable floater design to ensure its safety and reliability over the lifetime at a low cost. While many studies have investigated global motion responses of FWTs, minimal attention has been directed toward the structural load effect analysis of the support structures. This paper deals with an experimental study of a 5-MW semi-submersible FWT in shallow water, in which a multi-segment hull is manufactured to measure the internal cross-sectional loads of the floater based on strain measurements. Model tests of the FWT are carried out at a 1:50 scale in a wave basin under multiple environmental conditions to investigate the characteristics of global dynamic responses of the FWT and internal cross-sectional loads of the hull. The results unveil that the vertical bending moment of the hull's cross-section is notably influenced by resonant responses in heave and pitch motions, as well as the platform motions within the wave-frequency region. Conversely, the horizontal bending moment remains impervious to resonant responses of the hull in the low-frequency range and is mainly governed by platform motions in the wave-frequency region. In addition, the wind loads also exert a certain influence on the vertical bending moment of the hull, but have limited impact on the horizontal bending moment. This paper contributes to establishing enhanced model test methods and provides a basis for improving the design and analysis of semi-submersible hulls of FWTs.

The paper presents a fatigue assessment method for offshore steel catenary risers (SCR) under vortex-induced vibrations (VIV) with multi-directional incoming flow. Firstly, the structural responses are obtained using the finite element method (FEM) in conjunction with a coupled cross-flow (CF) and in-line (IL) wake model. Subsequently, the cyclic stress associated with fatigue life is transformed into the frequency domain, and a corresponding Kriging model is constructed. Further, the structural fatigue life is correspondingly computed using the Miner-Palmgren damage rule and the structural fatigue reliability model is established. To validate the effectiveness of the FEM model and the frequency-domain-based Kriging model, comparisons between experimental and numerical results are conducted. Through the analysis of the structural response, the paper elucidates the mechanism underlying the impact of the incident angle on the fatigue distribution, while emphasizing the contribution of higher harmonics to structural fatigue. Moreover, a case study demonstrates the superiority of the proposed method in predicting the probability of failure and factor of safety compared to models that disregard the incident angle and IL responses. Overall, the proposed fatigue analysis methodology contributes to the reliability and availability of SCRs during the design stage, and to reduce potential fatigue-related failures.

In offshore oil and gas production, the increasingly complex line system and relatively fixed incidence direction of external loadings have led to asymmetrically distributed mooring lines in floating production systems (FPSs). This study presents an approach for the optimal design of asymmetrically arranged mooring systems in FPSs, considering the mooring radius, azimuth, separate angle, number of lines and three segment lengths as design variables. A series of sample mooring configurations were generated via the constrained Latin hypercube sampling method and executed by a time-domain numerical model. An improved particle swarm optimization (PSO) method involving both continuous and discrete design variables in a constrained design space was applied to mooring optimization. The radial basis function (RBF) model was trained by the sample data and applied as a surrogate model to replace the expensive finite element simulations. A case study of a deep-water semisubmersible platform was presented to demonstrate the implementation of the optimization procedure. The results showed that the improved PSO exhibited fast convergence and strong robustness. The offset of the floating platform was reduced by 8.29 % via the asymmetrical mooring pattern.