Offshore Systems Research Articles

Low-carbon energy scheduling for integrated energy systems considering offshore wind power hydrogen production and dynamic hydrogen doping strategy

This paper proposes a low-carbon energy scheduling model for integrated energy systems (IES) considering offshore wind power hydrogen production and dynamic hydrogen doping strategy. The offshore wind electrolysis system, hydrogen mixed gas turbine (HMGT), carbon capture, utilization and storage, and laddered carbon trading are introduced to reflect the values of hydrogen blending in carbon emission reduction for IES. A carbon emission and combustion model for HMGT based on the system thermodynamics and chemical reaction kinetics is presented. The dynamic hydrogen doping strategy is leveraged to consider the impact of gas composition variations on HMGT operating and gas system. Then, an energy balance-based quasi-steady-state model is proposed to deal with the different calorific value of the gas mixture. In order to effectively solve the resulting highly nonlinear and nonconvex problem arising from the varying hydrogen blending ratio, the nonlinear hydrogen production efficiency, and the convex–concave energy balance-based quasi-steady-state model, a tractable solution formulation is proposed. Numerical results illustrate the effectiveness of the proposed model.

Color-restoring and energy-saving imaging monitoring for intelligent offshore engineering

Images play a crucial role in artificial intelligence (AI) for offshore engineering, which provide a large amount of data for machine intelligence to ensure the safe operation and maintenance of offshore structures. However, due to the attenuation of sunlight in water or monitoring at nighttime, underwater imaging monitoring systems need a light source, which consumes a lot of energy and impacts the load capacity of an offshore monitoring system. In addition, due to the nonlinearity of attenuation at different wavelengths, underwater images generally suffer from color deviation, tending to be blue and green, which decreases the accuracy of machine learning and intelligent recognition in unmanned offshore engineering. In order to reduce energy consumption and enhance color restoration, we proposed and designed a three-detector underwater imaging system (TD&TP-UIS) based on a prism, in which three detectors were used to enhance the utilization efficiency of light, and a gated band coating was designed on the prism to enhance the color reproduction. A series of experiments were carried out in water tanks, pools, Qiandao Lake, and other places to verify the advantage of color restoration and energy saving using the TD&TP-UIS.

On Fatigue Damage-Oriented Through-Life Optimization and Control of High-Power IGCT Converters in Wind Energy Systems

Integrated gate commutated thyristors (IGCTs) are critical components in high-voltage, high-current, and high-power conversion systems, particularly in offshore wind energy systems. However, the working environment of offshore wind energy conversion systems is extremely harsh. In this article, we propose an active damage control approach aiming at enhancing the reliability of the conversion system. By employing electro-thermal modeling for the equipment of the offshore wind energy conversion system, the junction temperature and fatigue damage of IGCT are simulated during the operation process. Using the improved model predictive current control (MPCC) method, active damage control effectively regulates the switching frequency of IGCT. IGCTs are symmetrically distributed on each leg of the converter, so the lifespan of the two IGCTs on each leg is also considered to be similar. This method balances the life of the IGCTs on the three legs of the converter and optimizes their utilization to the maximum extent. These measures effectively enhance the reliability of the conversion system and lower the operation and maintenance cost of high-power IGCT converters. The effectiveness of the proposed method is validated by co-simulation results by ANSYS and MATLAB/Simulink.

Offshore and onshore renewable energy system modelling to meet the energy demand of megacity Istanbul

In this study, a renewable energy system was designed using offshore and onshore energy sources to meet the electricity demands of the megacity Istanbul. Istanbul spends about 11 % of Turkey's consumption according to 2022 data, this electricity fee is generally supplied from fossil fuels, which cause large amounts of greenhouse gas emissions. In this study, to increase the usage of clean energy sources, a detailed analysis of the appropriate land was performed to install the necessary wind farms and solar farms, considering the electricity consumption of both the European and Anatolian sides of Istanbul. A new integrated approach using Geographic Information Systems (GIS), Multi-Criteria Decision Making (MCDM) and Analytical Hierarchy Process (AHP) was presented to determine the locations of offshore and onshore power plants to be installed. The analysis with a new approach took into account important criteria such as land cover, power transmission lines, substations, and settlements of the location. Moreover, HOMER software optimized the power plant’s energy production potential and components to be installed on the European and Anatolian sides. The megacity was considered as two sides, the overall power output on the European side was obtained as 2874.6 GWh/yr in HOMER; however, the Anatolian side was 158 GWh/yr. It was obtained that wind energy had a significant potential to produce electricity throughout the entire megacity. According to the results of the cost analysis, the European side with a cost of electricity (COE) of 0.0105 $/kWh is conspicuous, and the COE of the Anatolian side is 0.015 $/kWh.

Analysis of the resonant converter for multi-terminal collection of DC offshore wind farms

Abstract With the rapid development of power electronics technology, the offshore wind power system using DC collection and DC transmission is the best solution to increasing offshore distance. Resonant converters achieving soft switching have become a common choice in the DC/DC power conversion system. This paper proposed a design scheme for the multi-terminal collection of DC offshore wind farms based on the resonant converter. The operating principles are analyzed, and variable-frequency control of the converter is utilized to control the output voltage. Simulation verifies the effectiveness of the multi-terminal DC collection system and its control strategy.

Walrus optimizer-based optimal fractional order PID control for performance enhancement of offshore wind farms

Offshore wind farms (OWFs) play a crucial role in producing renewable energy in modern electrical power systems. However, to ensure that these facilities operate smoothly, they require robust control systems. As a result, this paper employed the newly developed Walrus Optimization algorithm (WaOA) to optimize the design parameters of fractional-order proportional-integral-derivative (FOPID) controllers in the power electronic interface circuits of the studied wind energy conversion system (WECS). In contrast to conventional optimization techniques like GA and PSO, the suggested approach proves more effective. The paper validates the WaOA application in optimizing FOPID controllers within a WECS comprising two, onshore and offshore, VSC stations at the two ends of an HVDC transmission system connecting OWFs to the mainland. The study shows that the WaOA outperforms GA and PSO, improving system stability and enabling quick recovery after disturbances. The study carried out using MATLAB/Simulink highlights the significance of newly recently introduced optimization techniques to ensure efficient and reliable operation of offshore wind energy systems, thereby expediting the transition to sustainable energy sources.

On the local scour around a jacket foundation under bidirectional flow loading

Jacket structure is a new type of offshore wind turbine foundations with advantages for its lightweight construction, high stability, and suitability for various water depths. However, jacket foundations are often subjected to scouring caused by tidal flows, which can significantly affect the stability of offshore wind energy systems. Limited research has been conducted on the scour characteristics around the jacket foundation under bidirectional flow actions due to the structural complexity. To address this gap, this study performs physical model tests to investigate the effects of flow properties and foundation position on the scour around the jacket foundation. This study assesses the impact of a complex upper truss structure and finite pile height on scouring, in contrast to monopile and regularly arranged pile group foundations. It also analyzes the differences in scour characteristics between a monopile foundation and a jacket foundation subjected to identical bidirectional flow conditions. The impact of flow direction on the scour evolution is evaluated by comparing the scour characteristics under unidirectional and bidirectional flow actions with the same hydrodynamic conditions. The results indicate that the equilibrium scour depth increases with the flow strength and water depth, and reaching its maximum at a foundation angle of 30°. Findings are of great significance for predicting the scour extent around the jacket foundation.

Experimental study on the influence of the riser on the gas-liquid flow in the horizontal pipeline of the offshore pipeline-riser system

The study of gas-liquid interface evolution and pressure fluctuation mechanism in the pipeline-riser system is the key to ensuring the safety of oil and gas transportation. This paper investigates the influence of the riser on the gas-liquid interface in the system with a 1657 m pipeline and risers of 16 m, 22 m, and 29 m. In severe slugging, the differential pressure in the horizontal pipeline exhibits a low-frequency large-amplitude fluctuation consistent with those in the riser. During the gas blowdown of the riser, the pressure fluctuation breaks the force equilibrium of the gas-liquid interface and induces numerous liquid slugs in the horizontal pipeline. A strong correlation is found between the differential pressure in the riser and the slug velocity in the horizontal pipeline by the Spearman Rank coefficient. The correlations between the slug velocity and the gas volume fraction and the void fraction under different riser heights are established, and the errors are 15% and 10%, respectively. The linear relationship between the Strouhal number of liquid slugs and the Lockhart-Martinelli parameter is constructed during the gas blowdown.

Performance analysis of massive MIMO offshore system with distributed antenna subarrays

In this paper, we propose a transmission scheme of analog precoding for an offshore communication system with distributed antenna subarrays to serve multiple vessels. The proposed analog precoding scheme relies only on line-of-sight (LOS) components of the fading channels, since the accurate estimation of small-scale fading information is challenging due to the varying sea state of maritime wireless channels, such as reflection, scattering, and ducting effect. We derive an approximate expression for the ergodic achievable rate, and emphasize the multiplexing gain analysis of the whole system with a large antenna configuration. In addition, we derive the probability density function (PDF) of signal-to-interference-and-noise ratio (SINR) and further obtain closed-form expressions of symbol error rate (SER). Monte Carlo simulations verify the theoretical analysis, and numerical results demonstrate that the performance of the proposed LOS-based analog precoding scheme for the distributed multiple-input multiple-output (MIMO) offshore system is extremely near to the limitation of system performance, i.e., the ideal situation without any interference.

Limit hypersurface state of the art Gaidai multivariate risk evaluation approach for offshore Jacket

Current study presents state of the art approach to risk and reliability assessment for multivariate nonlinear dynamic systems. Specifically, novel hypersurface Gaidai risks evaluation methodology has been presented for evaluating offshore structural risks. Advocated approach being particularly suitable for offshore engineering multidimensional dynamic systems, possessing large number of critical components, that have either been physically observed/measured, or numerically modeled over a representative timelapse. Advocated Gaidai hypersurface structural risks evaluation methodology is applicable for a wide range of engineering and industrial systems. This study demonstrates that given in situ environmental conditions, it is well possible to assess accurately risks of system’s failures, damages or hazards, caused by excessive structural dynamics. Multivariate dynamic systems, possessing nonlinear cross-correlations between critical system’s components often present design challenges, when utilizing classic structural risks evaluation approaches, as those are mostly only univariate or bivariate. Dynamic offshore Jacket hot spot stresses have been utilized as an example in this reliability investigation. Modeling system’s excessive dynamics is challenging because of the non-stationarity of the system and the intricacy of fluid-structural dynamic interactions, arising from in situ wave-induced loads, influencing Jacket’s structural dynamics. Nonlinearities greatly affect structural dynamics, e.g., 2nd, 3rd, higher order effects within fluid-structural interactions. Methodology presented in this work offers a straightforward, effective, yet precise means of assessing the risks of failure or hazard for multivariate, non-stationary, non-linear dynamic offshore systems. For bivariate case, verification note has been added.

Simultaneous design optimisation methodology for floating offshore wind turbine substructure and feedback-based control strategy

This research article explores the application of control co-design methodologies for optimising floating offshore wind turbine systems concurrently. The primary objective is to offer insights into concurrent design approaches employing an advanced genetic optimisation algorithm. To achieve this, a reduced-order dynamic model is employed to minimise computational time requirements, complemented by a modified version of the levelized cost of energy equation serving as the cost function. Furthermore, various optimisation scenarios are investigated under diverse wind and wave conditions to assess the advantages and drawbacks of increasing the complexity of dynamic cases used in evaluating the cost function. The optimised system designs are then compared against baseline floating system designs to underscore the advantages of employing this approach to floating wind turbine design.

FPSO/LNG hawser system lifetime assessment by Gaidai multivariate risk assessment method

Floating Production Storage and Offloading (FPSO) unit being an offshore vessel, storing and producing crude oil, prior to crude oil being transported by accompanying shuttle tanker. Critical mooring/hawser strains during offloading operation have to be accurately predicted, in order to maintain operational safety and reliability. During certain types of offloading, excessive hawser tensions may occur, causing operational risks. Current study examines FPSO vessel’s dynamic reactions to hydrodynamic wave-induced loads, given realistic in situ environmental conditions, utilizing the AQWA software package. Current study advocates novel multi-dimensional spatiotemporal risks assessment approach, that is particularly well suited for large dataset analysis, based on numerical simulations (or measurements). Advocated multivariate reliability methodology may be useful for a variety of marine and offshore systems that must endure severe environmental stressors during their intended operational lifespan. Methodology, presented in this study provides advanced capability to efficiently, yet accurately evaluate dynamic system failure, hazard and damage risks, given representative dynamic record of multidimensional system’s inter-correlated critical components. Gaidai risk assessment method being novel dynamic multidimensional system’s lifetime assessment methodology. In order to validate and benchmark Gaidai risk assessment method, in this study it was applied to FPSO and potentially LNG (i.e., Liquid Natural Gas) vessels dynamics. Major advantage of the advocated approach is that there are no existing alternative risk assessment methods, able to tackle unlimited number of system’s dimensions. Accurate multi-dimensional risk assessment had been carried out, based on numerically simulated data, partially verified by available laboratory experiments. Confidence intervals had been given for predicted dynamic high-dimensional system risk levels.

LTFM-net framework: Advanced intelligent diagnostics and interpretability of insulated bearing faults in offshore wind turbines under complex operational conditions

The rapid advancement of intelligent theories and models, exemplified by deep learning, has achieved remarkable success across numerous fields. However, given that the complexity and variability of offshore wind turbine systems, particularly in the context of high-power variable-frequency control of insulated bearings in offshore wind turbines, the problem of fault identification has become a recognized technical challenge. Additionally, fully exploiting the temporal characteristics of insulated bearing fault data poses a key problem that demands urgent resolution. To address these gaps, this paper pioneers a novel Lightweight Temporal Feature-focused framework, named LTFM-Net, aimed at solving the difficult problem of identifying insulated bearing faults in offshore wind turbines in practical engineering applications. Specifically, this framework enables the intelligent identification of insulated bearing faults under harsh operating conditions such as alternating voltage and variable loads, marking a first in this field. Furthermore, an innovative strategy named Weighted Diminish Recurrent Unit (WDRU) was developed, along with the derivation of its backpropagation formula, which is innovatively applied to the feature extraction module of the LTFM-Net framework for the first time. Thus, an efficient method for acquiring fault data from insulated bearings in offshore wind turbines was proposed. Based on a unified dataset, the diagnostic performance of the LTFM-Net framework was evaluated and compared with seven advanced methods. The results demonstrate that the LTFM-Net achieves precise identification of insulated bearing faults, confirming its excellent generalization, robustness, and superiority. The introduction of t-SNE for visualizing the fault characteristics of insulated bearings uncovered by the LTFM-Net framework further enhances its reliability, accuracy, and credibility.

Analysis of Resonance Mechanism of AC Submarine Cable Delivery System for Offshore Wind Power

Abstract Several resonant overvoltage accidents caused by machine-grid interaction in offshore wind power generation systems have led to significant economic losses over the years. To solve this problem, based on the equivalent distributed parameter circuit model of AC submarine cable delivery system, a method combining impedance simulation-time domain simulation and spectrum analysis is proposed to reveal the mechanism of resonant overvoltage and its influencing factors. First of all, considering the phase locked loop (PLL) model and the cable distribution parameter model, an impedance model of AC submarine cable delivery system for offshore wind power is established, which takes into account the dynamic characteristics of PLL and current loop, and provides a model basis for the theoretical analysis. Then, equations for current and frequency at the point of common coupling (PCC) are derived to analyze the mechanism of resonant overvoltage generation and the influence of submarine cable length and power grid strength on resonance. Finally, PSCAD is used to verify the resonance mechanism of AC submarine cable delivery system and its influencing factors.

Optimal Sensor Placement for Improved Prediction Accuracy of Structural Responses in Model Test of Multi-Linked Floating Offshore Systems Using Genetic Algorithms

Optimal Sensor Placement for Improved Prediction Accuracy of Structural Responses in Model Test of Multi-Linked Floating Offshore Systems Using Genetic Algorithms

Risk-informed integrated design optimization for offshore wind farm electrical systems

The present paper develops and presents a risk-informed framework for the design of electrical systems of offshore wind farms. The framework provides a consistent theoretical and methodical basis for the treatment of uncertainties affecting service life performances and accounts for both the direct and indirect consequences related to system failure events. The risk of electrical systems is represented through a probabilistic systems modeling approach in which the states (direct failure, available, and unavailable) of the components and the resulting consequences are duly accounted for. The paper further proposes a risk-informed design optimization framework for offshore wind farm electrical systems (collection and transmission systems). Two risk indicators, the scenario-based net present value, and the robustness index, are formulated and quantified to support decision-making. The proposed framework and associated methods are illustrated through an application example considering an OWF comprised of 57 10MW wind turbines with HVAC transmission.

Investigation of soil damping for offshore wind turbine monopile-soil system in stiff clay on elastoplastic-damage model

Damping plays a crucial role in the design of offshore wind turbine (OWT) monopile foundations. The soil damping of the monopile-soil system (MSS) represents the energy dissipation mechanism arising from the interaction between the pile and the soil. It is typically derived by back-calculating from the overall damping measured in the entire OWT structure. However, few studies have independently examined the soil damping in MSS, and the impact of key parameters such as pile diameter, pile embedded depth, cyclic load amplitude, and load eccentricity on the variation of soil damping in MSS remains unclear. This paper introduces an elastoplastic-damage constitutive model for the numerical simulation of the damping ratio variation in seabed soil and MSS. The model is implemented in ABAQUS software and validated against cyclic triaxial tests on stiff clay soil. On this basis, a three-dimensional finite element sensitivity study was conducted to elucidate the effect of these key parameters on the MSS damping ratio. The results of the study reveal that the MSS damping ratio exhibits a nonlinear and asymmetric trend as the loading cycles increase. The MSS damping ratio decreases with increasing pile diameter and embedded depth but increases with increasing lateral cyclic load amplitude and load eccentricity from the mudline.

Conceptual design and model test of a pontoon-truss type offshore floating photovoltaic system with soft connection

With the growing demand for clean energy in human society, the development of offshore floating photovoltaic (FPV) systems emerged as a prominent research topic. Positive-airgap FPV systems are more practical for engineering applications, however, current designs still face challenges under open sea such as negative airgap risk and significant connecting load. To address these issues, this study presents a novel foundation and connection scheme for an offshore FPV system. First, a pontoon-truss platform was designed and its superior performance was demonstrated by comparing it to a traditional semi-submersible. Further, a four-module offshore FPV system concept was developed, in which soft ropes were first introduced as a solution for FPV module connection. After that, a basin model experiment is employed to evaluate the feasibility of proposed FPV system under wind-wave-current joint effect, where the current-wind load is simplified as a constant force, and the established numerical model is verified by experiments. To improve understanding and design, the impact of several key design parameters on dynamic responses is also discussed. The findings indicate that the airgap performance deteriorates rapidly when single module draught surpasses half of pontoon diameter, and improving soft rope stiffness is more effective and produces less disturbance to other behaviors than enhancing mooring stiffness.

Towards unsupervised fault detection for offshore wind turbine cable protection systems using contrastive learning

As offshore wind power expands globally, it is essential to ensure the reliable operation of components of such critical infrastructures. A less explored instance of such components, which are though essential in terms of operation, is found in subsea turbine cables and their protection systems, whose failure can incur prolonged shutdown periods and costly repairs. We propose a novel unsupervised machine learning approach exploiting use of Distributed Acoustic Sensing (DAS) data and contrastive learning for monitoring offshore wind turbine Cable Protection Systems (CPSs). A Transformer neural network adapted for time-series ingests the high-frequency, noisy DAS CPS time-series measurements, and is trained to learn a coherent representation of the data using a contrastive learning scheme that enforces temporal and positional consistency in the latent space. This latent representation can then be used to perform anomaly detection in an unsupervised manner, alleviating the need for costly labeled offshore anomaly data. We demonstrate that a coherent representation of the data is learnt by the model, which we then use to detect synthetic anomalies and an actual CPS stabilization event.

Development of a robust welding process for electron beam welding of thick plates for construction of offshore wind turbines

AbstractDue to its high energy intensity and the associated high welding depth, electron beam welding is particularly suitable for welding steel plates with high wall thicknesses. These are used, for example, in offshore wind power systems. However, due to the cooling conditions, the required cold toughness for offshore applications is often not achieved. The aim is to develop S355 ML steels – within the EN10025 4 standard – for welding monopiles for wind turbines. For this purpose, three parameters with different energy input are developed on three steels. The tests are carried out on plates with a wall thickness of 80 mm, whereby a full penetration weld must be achieved. The resulting top and bottom bead of the welds meet the standard DIN EN ISO 13919‐1. The parameters have an energy per unit length of 9.5 kJ mm−1 to 15.5 kJ mm−1. The resulting weld seams have an average width of 5.5 mm to 7.5 mm, and burn‐off of the manganese alloy element is observed, particularly on the top side of the seam. In addition, T8/5 times close to the weld seam of 11 s to 27 s are estimated using a simulation.