Offshore wind energy projects are increasingly adopting concrete platforms due to their enhanced corrosion resistance, cost efficiency, and suitability for large-scale structural configurations in harsh marine environments with high uncertainty. Despite this, conventional design approaches for offshore wind turbine (OWT) structures typically treat uncertain variables as deterministic, which can lead to suboptimal or overly conservative designs. This study develops a reliability assessment framework tailored for concrete floating offshore wind turbines (C-FOWTs), integrating fully coupled structural analysis, response surface modelling, and probabilistic reliability analysis. A comprehensive coupled structural analysis framework for an offshore reinforced concrete substructure is first developed, incorporating a computational model that accounts for the detailed design of reinforcement configurations. Then, a quadratic response surface is constructed to approximate structural responses as functions of key stochastic parameters. This surrogate model is coupled with the First-order Reliability Method (FORM) to identify dominant variables influencing safety margins under Ultimate Limit State (ULS) conditions. The results revealed potential failure modes of C-FOWTs related to different reinforcement and concrete properties. This study demonstrates the feasibility of using a reliability-informed approach to optimise the structural design of C-FOWTs, providing guidance beyond conventional deterministic codes.

This study presents a numerical investigation, using advanced nonlinear time-domain models, of the hydrodynamic performance and energy generation of a semi-submersible floating offshore wind turbine (FOWT) and a hybrid concept obtained by integrating oscillating water column (OWC) wave energy converters into the same platform. The primary aim is to assess the coupled aero-hydro-servo-thermodynamic behavior of the systems, quantify the impact of OWC integration on platform motions and combined energy conversion output, comparing the performance of wind power only platform and hybrid configurations. The coupled model resolves six degrees of freedom for the platform and heave motion for multiple OWCs, combining blade element momentum theory for aerodynamic loads, linear potential flow theory with viscous corrections for hydrodynamics, and a thermodynamic model for the OWC impulse turbine. Analyses include hydrostatics, frequency-domain characterization, and fully coupled time-domain simulations under regular and irregular wave conditions with wind-induced loads. Results indicate that OWC integration reduces initial stability, modifies natural periods, and increase pitch motion by approximately 28% to 70% near rated wind speeds. At wind speeds above rated, the average wind power output may even experience some decrease due to amplified platform motion. At low wind speeds (<5m/s) the OWCs may supply a small additional power equivalent to 0.65% of the wind turbine output. Despite introducing additional hydrodynamic damping and enabling some extra wave energy generation, the findings suggest that merely adapting a designed platform for supporting wind turbine to hybrid concept without proper redesign is not advantageous for the examined conditions. However, potential improvements through targeted redesign strategies—such as optimized OWC geometry or alternative PTO tuning—could enhance the overall hydrodynamic and energetic performance.

Floating offshore wind turbines (FOWTs) deployed in ultra-deep water require mooring solutions that reduce cost and seabed footprint while maintaining robust limit-state performance. This study proposes an anchor-saving shared taut mooring system for a three-turbine array of 15 MW semi-submersible FOWTs at 1500 m water depth, benchmarked against a shared-anchor configuration and a classic individual taut mooring baseline. Coupled time-domain simulations are conducted for representative operating and extreme parked (survival) conditions to quantify peak line tensions, six-degree-of-freedom motions, and sensitivities to chain-polyester segmentation and shared connection-point position. Relative to the individual baseline, the proposed system reduces the peak chain tension of the most heavily loaded shared line by 34.7

As offshore oil and gas exploration advances into deeper waters, double carcass hoses (DCHs) are subjected to increasingly complex combined loading conditions, necessitating enhanced reliability and durability in extreme environments. This paper presents a theoretical analysis methodology for evaluating the stress and deformation of DCHs under concurrent internal pressure and axial tensile forces. The approach, based on the laminated plate theory and Mooney-Rivlin model, incorporates the nonlinear characteristics of the rubber matrix and geometric nonlinearity within reinforcement layers. Through iterative loading processes, material parameters and reinforcement layer winding angles are systematically updated. The failure criteria are established using the maximum tensile strength of the cord and Von Mises criterion for helical steel wires. The model’s validity was verified through axial tensile tests on a DCH with a 500 mm inner diameter. The analysis reveals distinct variations in load-bearing contributions between helical steel wire and cord layers at different internal pressure levels. The hose demonstrates complex nonlinear behavior under combined loading conditions. Comprehensive sensitivity analyses examined the influence of critical parameters, including cord winding angle, layer count, hose diameter, helical steel wire pitch, and wire diameter, on hose failure characteristics. A failure envelope for DCHs under various parameter conditions was developed, providing a theoretical framework for optimizing DCH structural design.

Deepwater sandwich pipes (SPs) offer high collapse resistance and thermal insulation, making them promising for hydrocarbon transport under high-pressure and low-temperature conditions. However, mechanical damage such as local dents increases cross-sectional ovality and can substantially degrade their external pressure capacity. This study develops a numerical model using ABAQUS to assess the collapse pressure of dented deepwater SPs under hydrostatic loading. The model is validated against existing reference data. A total of 2316 FE models are constructed to investigate the effects of material properties, geometric configurations, and dent characteristics on collapse performance. Results show that the collapse pressure decreases significantly with increasing dent depth, and spherical dents have a more pronounced effect than planar dents. Enhanced collapse resistance is observed as both the thickness ratio and the core thickness of the sandwich structure increase. The use of higher-strength materials in the core layer and the internal and external layers also improves compressive capacity. Drawing on these results, a simplified formula for estimating the collapse pressure of dented sandwich pipes is proposed.

Accurate prediction of tridirectional vortex-induced vibration (VIV) in long tapered flexible cylinders under linear shear flow remains challenging. This study develops a unified semi-analytical model based on EulerBernoulli beam theory to predict nonlinear in-line (IL), cross-flow (CF), and axial (AX) responses. The formulation incorporates spanwise-varying structural properties, linearly varying tension, three-dimensional hydrodynamic force projection, and lift- and drag-related Van der Pol wake oscillators. The coupled system is solved by combining the Sturm-Liouville Eigenvalue Using Theta matrices (SLEUTH) method with the Generalized Integral Transform Technique (GITT), and validated against published experimental data, finite-element results, and OrcaFlex simulations. Results show that IL motion dominates the amplitude level, CF vibration exhibits the strongest modal reorganization, and AX motion remains dynamically relevant despite its smaller magnitude. Geometric taper and shear gradient reorganize response envelopes, spectral structures, synchronization patterns, and nonlinear states rather than simply scaling amplitudes. The proposed model provides an efficient and physically interpretable tool for tridirectional VIV prediction of variable-cross-section flexible cylinders.

The pursuit of renewable energy sources has driven the development of innovative wind and wave energy technologies. Two promising technologies that have received special attention are floating offshore wind turbines (FOWT) and wave energy converters (WEC). These devices are known for their simplicity, and minimal environmental impact, making them promising for sustainable ocean energy generation. This study analyzes the hydrostatic properties of a hybrid floating wind-wave structure and presents the hydrodynamic performance of the system in the frequency and time domains. The floating wind-wave system includes a semi-sub wind turbine and a three oscillating water column (OWC) WEC which principal mode of operation involves capturing the rise and fall of water within a chamber caused by the movement of waves. Moreover, this study examines the impact of OWC on natural periods and hydrostatics of the floating wind system. The analysis focuses on the impact of the OWC's chamber size on the system performance and power production. A comprehensive comparison of the hydrostatics of the floating wind systems platform on the pitch angle is conducted. Understanding these aspects is important for the design and optimization of hybrid platforms that effectively integrate wind and wave energy technologies to maximize energy production.

This study investigates the complementarity between the ocean thermal gradient and offshore wind energy in the Potiguar and Campos basins of Brazil. Ocean thermal energy conversion (OTEC) operates through the Rankine thermodynamic cycle using the temperature gradient between the ocean surface and deep waters, while offshore wind energy harnesses atmospheric wind flows. To assess their potential, ocean's temperature and wind speed data from HYCOM and ERA5 models, spanning 2000-2019, are analyzed. The complementarity between these sources is evaluated using Kendall and Pearson correlation coefficients and the Coefficient of Variation (CV). Results reveal that OTEC demonstrates significantly lower CV values compared to wind energy, indicating greater stability and reliability. For instance, in the Potiguar Basin, OTEC exhibited a CV of 0.001, contrasting sharply with wind energy's CV of 0.45. Moreover, negative correlation coefficients, such as Kendall -0.38 and Pearson -0.51, highlight an inverse relationship between the two sources, emphasizing their complementary nature. OTEC consistently delivers around 15 MW of energy output across seasons, effectively balancing wind energy variability, which peaks in spring and declines in autumn. These findings underscore the potential of integrating OTEC and wind energy technologies into hybrid systems to ensure a reliable and sustainable energy supply while significantly reducing the carbon footprint of offshore oil and gas operations.

Offshore renewable energy is a clean and inexhaustible energy resource with the potential to supply over 2TW of energy worldwide. Floating offshore wind turbines are emerging as a key solution to harness this renewable energy. With the rapid expansion of the offshore wind energy market, there is increasing potential to integrate diverse technologies to optimize marine energy utilization, particularly wave energy. The existing design approach for offshore structures typically treats uncertain variables as deterministic, leading to either over-engineered or under-engineered designs. This study presents a purpose-developed reliability assessment framework for a hybrid wave-wind floating platform that incorporates three Oscillating Water Columns (OWCs) into a spar buoy. This framework is significant as it provides a systematic approach to account for uncertainties in the design parameters, thereby enhancing reliability and safety of the structure. A coupled aero-hydro-structural dynamics model was developed using tools such as AQWA ANSYS©, verified against the results reported in the literature. The structural responses obtained from the model simulation are employed in deriving the Limit State Function (LSF), which is then incorporated into the iterative First-Order Reliability Method (FORM) algorithm to calculate the fatigue reliability indices of the structure. The S–N curve-based fatigue reliability results revealed that, under inherent stochastic conditions, the structural assembly could safely withstand such conditions as the reliability index values remain within acceptable limits, while the Fracture Mechanics method suggested otherwise. Consequently, an optimization exercise of critical components was subsequently performed based on fracture reliability assessment to achieve target reliability.

The stability of variable cross-section cold-water pipes (CWPs) is crucial for ensuring the reliability and safety of Ocean Thermal Energy Conversion (OTEC) systems under complex marine conditions. This paper introduces a novel semi-analytical framework to assess the stability of CWPs subjected to multiple clump weights, which is ignored by previous works. The dynamic response equation is established based on the Euler-Bernoulli beam theory, accounting for both inertia forces and gravitational effects of the clump weights. To efficiently and accurately compute the dynamic stability of variable cross-section CWPs, a hybrid method integrating the SturmLiouville Eigenvalues Using Theta matrices (SLEUTH) with the Generalized Integral Transform Technique (GITT) is proposed. The problem is transformed into an auxiliary eigenvalue problem and a second-order differential equation with time-dependent coefficients. Eigenvalues and eigenfunctions are computed at discrete points using the SLEUTH method, while coefficients of the differential equation are determined through numerical techniques like the Newton-Cotes formula, Gaussian functions, and exponential functions. The GITT method is employed to solve the transverse vibration equation. Validation with numerical examples demonstrates rapid convergence and high accuracy. Further investigation reveals that the weight, position, and number of clump weights significantly influence CWP stability, providing key insights for OTEC design improvements.

A subsea control module (SCM) hydraulic system is an important element of subsea control system. Fault diagnosis of SCM hydraulic system is challenged due to the complex system structure. To identify the faulty components and distinguish the fault types, this study develops a dynamic Bayesian networks (DBN)-based fault diagnosis methodology of SCM hydraulic system. The methodology generates evidence from SCM internal sensors and uses the information to update the process knowledge. The dynamic degradation process of the valve is simulated and the conversion relationship between time slices are determined by Markov models. Based on EM algorithm, the probability parameters of BN nodes are calculated, and transfer probability distribution between time slices is determined. A multi-time slices DBN fault diagnosis model of SCM hydraulic system based on reverse analysis was established. Thirty-four fault diagnosis cases including high-pressure and low-pressure hydraulic system of SCM are investigated to illustrate the methodology. The results show that the posterior probability of all cases has changed from 10 % to more than 50 % when failure occurs, and DBN model can correctly diagnose the faults that occurred, with an accuracy rate of 100 %, and failure rate of DCV valve is related to the hydraulic oil circuit flow and pressure. The fault diagnosis cases validate the accuracy and effectiveness of the proposed methodology.

Long flexible cylinders with variable cross-sections are extensively used in engineering applications, yet their vortex-induced vibration (VIV) behavior under linear shear flow remains insufficiently investigated. This study proposes a novel semi-analytical method that integrates the Sturm-Liouville Eigenvalues using Theta Matrices (SLEUTH method) with the Generalized Integral Transform Technique (GITT), enabling efficient and highly accurate VIV predictions. This approach addresses the computational challenges of traditional numerical methods while accurately capturing key structural responses including the structural displacement, structural frequency, displacement envelope, and displacement evolution. Validation against existing experimental data and finite element simulations confirm the method's superior accuracy and computational efficiency. Results reveal that variations in cross-section significantly influence VIV behavior. Tapered, waist-shaped, and stepped cylinders exhibit distinct displacement distributions and frequency responses under shear flow conditions. Tapered cylinders experience amplified displacements in regions with increasing flow velocity, waist-shaped cylinders exhibit localized vibration attenuation, and stepped cylinders display abrupt transitions in displacement envelopes at geometric discontinuities. These findings lay a robust theoretical framework for analyzing VIV in variable cross-section structures, providing essential design guidance for mitigating flow-induced vibrations in offshore and marine engineering applications.

Despite significant progress in floating offshore wind turbine (FOWT) technology, there are still several challenges, including the design of a cost-effective system. Considerable research has been dedicated to optimizing the floating platform geometry, layout, dimensions, and weight over the past few years, with some focusing on semisubmersible platforms, where steel is often used for both the platform and the tower. However, concrete FOWTs may be more cost-effective and reduce carbon footprint. Other areas requiring further research include the impact of the tower material, the maximum inclination angle, and confining ballast water within dimensionally variable compartments during optimization. The study aims to address these points through a hydrostatic optimization of a novel 15 MW concrete semisubmersible FOWT using a genetic algorithm method. The results show that the platform mass reduction for pitch angles larger than 6^∘ is lower compared to that for angles smaller than 6^∘ regardless of the tower material. Moreover, confining the ballast water inside dimensionally variable compartments leads to a lower semisubmersible platform weight. Finally, an initial comparison of raw material costs shows that a concrete platform with a steel tower offers the most cost-effective solution compared to a FOWT entirely made from steel or concrete.

This study presents a unified semi-analytical framework for predicting the stochastic vibration of long flexible cylinders with variable cross-sections subjected to spatially distributed white noise excitation. Distinct from most existing analyses that focus on geometrically uniform fluid-conveying pipes, the proposed approach explicitly incorporates geometric non-uniformity, internal fluid dynamics, and spatially varying inertia into a physically consistent formulation. By integrating the Sturm-Liouville Eigenvalue Using Theta matrices (SLEUTH) modal solver with the Generalized Integral Transform Technique (GITT), the method yields high-accuracy, closed-formlike solutions for displacement variance, stress variance, reliability index, and failure probability, while maintaining high computational efficiency compared with conventional simulation-based approaches. The framework captures the effects of mass ratio, damping ratio, and geometric variation on dynamic reliability, revealing nonlinear and non-monotonic trends not previously reported. Validation against pseudo-random simulations confirms its accuracy in both deterministic and stochastic regimes. Parametric results show that increasing mass ratio suppresses vibrations and improves reliability; moderate damping maximizes reliability gains, whereas excessive damping can induce adverse modal interactions; and geometric variation plays a dual role, enhancing stiffness yet potentially intensifying local stress under high flow velocities. The proposed framework offers a computationally efficient and physically interpretable tool for reliability-based optimization of non-uniform flexible marine structures.

Vortex-induced vibration (VIV) poses significant challenges to the structural reliability of Cold-water pipes (CWPs) in Ocean Thermal Energy Conversion (OTEC) systems, particularly when geometric variation and non-uniform axial tension are present. This study develops a high-fidelity semi-analytical framework that combines the Generalized Integral Transform Technique (GITT) with Sturm-Liouville Eigenvalues Using Theta matrices (SLEUTH) method. To analyze the VIV of CWPs with variable cross-sections under three realistic flow profiles: uniform, linear shear, and exponential shear. Validation against finite element simulations and existing literatures confirms the accuracy of the proposed model. Results demonstrate that while uniform flow produces stable standing-wave responses, linear and exponential shear flows introduce progressive mode transitions, nonlinear mode coupling, and broadened excitation spectra. Notably, exponential shear induces spatially asynchronous vortex shedding and chaotic lock-in behavior, revealing energy transfer from dominant to higher-order modes. These phenomena are strongly influenced by the interplay between velocity gradients and structural heterogeneity. The findings provide critical insights into the physical mechanisms governing VIV and offer design guidance for mitigating fatigue-critical responses in CWPs. This work addresses gaps in the current literature by accounting for coupled geometric-tension-flow effects and proposes a robust computational tool for future engineering optimization of marine energy systems.

Tanker ships have a length greater than their breadth and depth and should be designed for withstanding high longitudinal bending moments of hogging and sagging. Furthermore, other situations may arise that damage the bottom or side panels. These damages can cause severe loss of structural capacity on the ultimate longitudinal bending moment. Therefore, studying this damaged structure type is useful for measuring the safety reserve through the residual strength under the bending moment. This paper analyzes a box girder on a small scale under two sequential loads. In the first step load, the box girder is indented through a spherical indenter such that the damages are applied at the upper panel, which will support the compressive loads during bending. In the second step load, a bending load by four points contact is applied to the box girder. These applied loads are used to measure the residual strength to different levels of indentations. The objective is to study the residual strength of the box girder with the intact model, a model without a central stiffener, and another without the side plate. The buckling mode of the plate is used to introduce levels of initial imperfection in the box girder plates. Numerical models are developed using the Abaqus program with shell elements, implicit solutions, and nonlinearities.

Offshore composite rubber hoses play a crucial role in marine oil and gas transportation systems. This study aims to determine the bending stiffness of these hoses by establishing stress-strain constitutive equations that account for the cord-rubber composite layer, helix steel wire-rubber composite layer, and buoyancy layer. Based on the constitutive model, equilibrium equation, and geometric equation of anisotropic multilayer composite pipes, the theoretical model for the bending stiffness of floating hoses is derived. A nonlinear mechanical model of the floating hose under bending loads is developed using the Rebar element and Embedded technique. The accuracy of the theoretical analysis and numerical model was validated through bending stiffness tests on the floating hose. By solving geometric, displacement continuity, and stress continuity equations, the axial, radial, and circumferential stress distributions, as well as the cord tension distribution under bending loads, are obtained. The bending stiffness of the offshore composite rubber hose is analyzed under varying parameters using both theoretical methods and numerical simulations. The findings of this research provide theoretical and technical guidance for designing new floating hoses and developing design specifications.

Brazil has abundant natural resources and a largely renewable electricity matrix, with about 90% of its capacity from clean sources. Despite strong offshore wind potential, its economic viability remains uncertain due to the lack of a domestic supply chain and reliance on international cost estimates. This study assesses offshore wind competitiveness in Brazil using the investment decision model (IDM), which minimizes expansion and operational costs through 2031. Capacity factors (CF) from ERA5 data support monthly energy production estimates across load levels. Three scenarios were analyzed: (i) a reference case based on Brazil’s 10-Year Energy Plan (PDE 2031); (ii) mandatory addition of 500 MW/year of offshore wind to assess cost impact; and (iii) a breakeven case with gradual CAPEX and OPEX reductions until offshore wind became cost-competitive. The results indicate that offshore wind energy can become economically viable with a CAPEX range of approximately USD 1500–1550/kW and an OPEX of USD 50–55/kW·year in locations with a CF above 60%. These cost levels have already been observed in global markets and may be achievable in Brazil. However, challenges, such as the lack of a domestic supply chain and volatility in the exchange rate, remain significant barriers.

Floating hoses are critical components in offshore oil and gas transportation systems, engineered to withstand various loads such as internal pressure, tension, torsion, and bending. In this study, an anisotropic constitutive model for cord-rubber and helix wire-rubber composite materials was proposed based on the Mooney-Rivlin hyperelastic formulation, taking into account the nonlinear mechanical behavior of rubber. Linear systems of equations describing the displacement and stress fields within the composite layers were established, incorporating unknown integration constants. The accuracy of the proposed theoretical model was validated through full-scale burst tests. Furthermore, the mechanical behavior of the hose under a design pressure of 10 MPa-five times the rated working pressure-was analyzed. Parametric studies were conducted to examine the effects of key structural parameters, including the cord winding angle, number of cord layers, helix wire diameter, pitch, and hose inner diameter, on the ultimate internal pressure and axial stiffness. The findings provide valuable technical guidance for the structural optimization and performance improvement of offshore floating rubber hoses.

In recent years, as the development of marine resources continues to develop in deep sea areas, offshore wind turbine pile foundations have been driven to create in the direction of larger diameters and bearing capacities. As a result, during pile driving, the soil around the piles is disturbed and weakened, thus affecting the vertical bearing capacity of the piles. To investigate the weakening phenomenon of pile-soil interaction in soft clay under continuous vertical loading, a series of indoor model tests were conducted using a self-made loading device. The distribution of pile-side friction and variation of pile-top reaction force with pile displacement were obtained. The experiments verified that the exponential formula is more suitable for describing the vertical soil weakening effect. Subsequently, the exponential formula was used to correct the pile-soil vertical hyperbolic model, and a vertical pile foundation weakening model was established. The vertical pile foundation weakening model was implemented using the FRIC subroutine in ABAQUS, providing a reference for the design analysis of weakened bearing capacity caused by the piling of offshore wind turbine foundations.