Offshore Wind Turbine Substructures Research Articles

Preliminary Techno-Economic Study of Optimized Floating Offshore Wind Turbine Substructure

Floating offshore wind turbines (FOWTs) are still in the pre-commercial stage and, although different concepts of FOWTs are being developed, cost is a main barrier to commercializing the FOWT system. This article aims to use a shape parameterization technique within a multidisciplinary design analysis and optimization framework to alter the shape of the FOWT platform with the objective of reducing cost. This cost reduction is then implemented in 30 MW and 60 MW floating offshore wind farms (FOWFs) designed based on the static pitch angle constraints (5 degrees, 7 degrees and 10 degrees) used within the optimization framework to estimate the reduction in the levelized cost of energy (LCOE) in comparison to a FOWT platform without any shape alteration–OC3 spar platform design. Key findings in this work show that an optimal shape alteration of the platform design that satisfies the design requirements, objectives and constraints set within the optimization framework contributes to significantly reducing the CAPEX cost and the LCOE in the floating wind farms considered. This is due to the reduction in the required platform mass for hydrostatic stability when the static pitch angle is increased. The FOWF designed with a 10 degree static pitch angle constraint provided the lowest LCOE value, while the FOWF designed with a 5 degree static pitch angle constraint provided the largest LCOE value, barring the FOWT designed with the OC3 dimension, which is considered to have no inclination.

Local joint flexibility of two-planar tubular DY-joints in jacket-type substructures of offshore wind turbines: parametric study of geometrical effects and design formulation

ABSTRACT The present paper discusses the results of a parametric study conducted on the local joint flexibility (LJF) of two-planar tubular DY-joints in jacket-type substructures of offshore wind turbines. A set of finite element (FE) analyses were carried out on 81 FE models subjected to two types of axial loading to study the effects of geometrical characteristics of two-planar DY-joint on the LJF factor (f LJF). The f LJF values in two-planar DY- and uniplanar Y-joints were compared. Results showed that the multi-planarity effect on the LJF is significant and consequently the application of equations already available for uniplanar Y-joints to calculate the f LJF in two-planar DY-joints may result in highly over- or under-predicting results. To tackle this problem, FE results were used to develop a new parametric equation for the prediction of the f LJF in axially loaded two-planar DY-joints and the derived equation was checked against the UK DoE acceptance criteria.

Time-domain fatigue reliability analysis for floating offshore wind turbine substructures using coupled nonlinear aero-hydro-servo-elastic simulations

The present study aims to develop a novel methodology for the fatigue reliability analysis and design for a floating offshore wind turbine substructure using its fully coupled nonlinear dynamic responses in the time domain. The developed methodology offers a computationally efficient yet robust assessment for designing fatigue-critical welded joints on floating substructures. The methodology starts with the sea state selection based on the fatigue damage contribution of all the sea states in the scatter diagram. To this end, the dynamic response is analysed by fully coupled aero-hydro-servo-elastic simulations using OpenFAST. The resulting short-term fatigue loading is then used to estimate the hotspot stress history using the analytical solution for global structural analysis and the empirical solution for the stress concentration factor. The long-term fatigue damage is calculated as the joint probability-weighted sum of the short-term fatigue damage using Palmgren-Miner's linear damage rule. Afterwards, the selected sea states are used to perform dynamic response simulations with multiple random seeds, ensuring no significant accuracy loss. To obtain the probabilistic fatigue life prediction, a novel time-domain fatigue reliability analysis algorithm is developed based on the bootstrapping method, which creates a pool of short-term fatigue responses with different random seeds and combines them within a Monte Carlo Simulation. Finally, the fatigue reliability analysis considering the S-N and damage-tolerant approaches for design is carried out.

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.

Bidirectional tuned liquid dampers for stabilizing floating offshore wind turbine substructures

Tuned sloshing dampers have long been used to stabilize tall and thin buildings and are now being applied to marine structures. In this study, the effectiveness of two new passive control devices at stabilizing floating offshore wind turbine substructures was investigated: the bidirectional tuned liquid damper and bidirectional tuned liquid column damper. Numerical simulations were performed to consider the sloshing effect on the passive control devices and the pitch-induced external moment brought about by wave action. The effects of adding baffles and orifices to the passive control devices were explored to optimize the geometric design. The results showed that the optimal combination of substructure geometry and passive control device type could reduce the pitch motion by 20%–40%.

New hybrid foundation solutions for offshore wind turbines

This paper reviews latest developments of substructures for offshore wind turbines focusing on investigations and applications of hybrid foundations. Model tests and numerical analyses were used to simulate the loading of hybrid piles in sand. The results of pile-soil interaction were investigated to confirm the changes in soil stiffness around the hybrid monopile head. The mechanism and factors affecting the change in lateral stiffness of the hybrid foundation were explained by analysing p–y curves for M+H loading conditions in sand. Based on this research, a new shape of p–y curves for hybrid monopiles was established and a method for determining key parameters was proposed. The effectiveness of new p–y curves was verified by comparing back-calculated results with those from numerical simulations. The conducted tests confirmed that the hybrid monopile displacement is 30–50% smaller when compared to a standard monopile with similar dimensions. The gained experiences can be useful for designers and researchers to enhance the design of foundations for offshore wind turbines.

Influence of wind and seismic ground motion directionality on the dynamic response of four-legged jacket-supported Offshore Wind Turbines

This paper aims at investigating the influence of the direction of wind and seismic ground motion on the structural response of four-legged jacket support substructures for bottom-fixed Offshore Wind Turbines located in areas with non-negligible seismic risk. To do that, a parametric analysis that considers thirteen seismic shaking directions, seven wind directions and four earthquake records is carried out. The results are presented in terms of accelerations, axial forces, bending moments and shear forces in the jacket substructure. The NREL 5 MW wind turbine supported on the jacket designed for the phase I of the OC4 project is considered. The response of the system is simulated using an OpenFAST model that takes into account multi-support seismic input, soil–structure interaction and kinematic interaction. The results show that load combinations with aligned wind and ground motion directions are never the worst-case scenario. On the contrary, the largest accelerations are found when the shaking direction acts along the side-to-side direction, and the most significant internal forces are usually found when the ground motion is aligned with one of the diagonals of the base of the jacket and not aligned with the wind direction. The paper also discusses the specific ranges of the angle of misalignment between wind and seismic shaking directions within which the maximum internal forces are expected to be found.

Design Optimization of Floating Offshore Wind Turbine Substructure Using Frequency Domain Modelling and Genetic Algorithm

Floating offshore wind turbines are becoming increasingly popular as a promising technology for producing cleaner energy. However, in order to be competitive with fixed offshore wind projects or even onshore wind projects, the costs associated with floating offshore wind turbines must be significantly reduced. To tackle this challenge, this paper presents a comprehensive framework for optimizing the design of floating offshore wind turbine substructures and their components. The innovative open-source frequency-domain dynamic model RAFT is used to take into account the aerodynamic, hydrodynamic and mooring forces that impact the stability and dynamics of the floating system. The numerical model is coupled with an efficient genetic algorithm to minimize the structural mass of the floating platform while maintaining its stability and dynamic performance. The geometrical parametrization of the substructure, the implementation of the numerical model, and the overall optimization process are all thoroughly detailed in this paper, along with preliminary results that demonstrate the potential cost reductions that can be achieved through this framework.

Geometrical effects on the local joint flexibility of three-planar tubular Y-joints in substructures of offshore wind turbines

ABSTRACT Although three-planar tubular Y-joints are amongst the most common joint types in jacket- and tripod-type substructures of offshore wind turbines (OWTs), local joint flexibility (LJF) of this type of connection has not been studied so far, mainly due to the complexity of the problem and high cost involved. Results of a parametric study conducted on the LJF of three-planar tubular Y-joints are presented and discussed in this paper. A set of finite element (FE) analyses were carried out on 81 FE models subjected to six types of axial, in-plane bending (IPB) moment, and out-of-plane bending (OPB) moment loadings in order to study the effects of geometrical characteristics of the three-planar Y-joint on the LJF factor (f LJF). FE results were then used to develop a new parametric equation for the prediction of the f LJF in axially loaded three-planar Y-joints, and the derived equation was checked against the UK DoE acceptance criteria.

Impact pressure distribution and characteristics of breaking wave impact on a monopile

A laboratory model study is performed to investigate the characteristics of wave impact pressures on a monopile substructure for offshore wind turbines subjected to breaking waves in intermediate water depth. Laboratory experiments are conducted in a 30m long, 2m wide and 1.8m deep wave flume. Breaking wave was generated by focusing the wave energy from a wave group at a pre-defined time and space in the laboratory flume. High-resolution impact measurements are carried out in a well-controlled wave flume for different wave loading conditions of varying incident wave steepness and wave impact conditions. The evolution of local wave characteristics and geometric properties of breaking waves is investigated along the wave flume. The sensitivity of the vertical distribution and the variation of peak pressures to small changes in time and spatial scale is assessed during the impact. During the breaking wave interaction with a monopile, the wave impact characteristics such as pressure rise time, duration of impact, maximum pressure, and pressure impulse are analyzed. The measured geometric properties and impact characteristics are in good agreement with previous studies. The impact region and its correlation with wave characteristics and geometric properties are evaluated and discussed.

Study on hot spot stress distribution of three-planar tubular Y-joints subjected to in-plane bending moment

The three-planar tubular Y-joint (3Y-joint) is the main part of the fatigue assessment of tripod substructures of offshore wind turbines (OWTs). As typical multiplanar tubular joints, 3Y-joints are affected a lot by multiplanar interaction between braces. Moreover, the locations of hot spot stress (HSS) can vary considerably under different load types. Thus, the distributions of stress concentration factor (SCF) and multiplanar interaction factor (MIF) along weld toe curves are necessary to calculate HSS. Considering these requirements, this study focuses on the 3Y-joint considering the wide application of the tripod substructure of OWT. A finite element (FE) analysis method is introduced and validated. Then, a numerical database is established covering common ranges of parameters used in practice. The SCF and MIF of 3Y-joint under in-plane bending moment are analyzed. Distribution formulas are proposed and proved suitable for calculating HSS in engineering design.

Evaluating operational strategies for the installation of offshore wind turbine substructures

The construction of offshore wind farms requires solving more and more complex logistical problems due to the increasing sizes of turbines and changing environments. In particular, the installation of substructures requires attention due to their significant impact on capital expenditures and the limited literature and guidelines available. In this paper, we develop a decision support tool consisting of a discrete-event simulation that allows for comparing strategies for the installation of offshore wind turbine substructures in terms of time and costs. We identify several combinations of transportation and installation strategies for monopile and for jacket substructures. The differentiation is based on the deployed vessels and the installation sequence of the components. The strategies are applied to the case of a wind farm in the North Sea. For both substructure types, we find that strategies involving a second installation vessel result in the shortest installation times, and those in which the installation vessel(s) take(s) care of both transportation and installation result in the lowest costs. Additionally, we quantify the performance increases as a result of a reduction of the most prominent bottlenecks and the sensitivity of the project performance to the start date. Finally, the results are discussed in relation to future market and technological developments in the field.

Probabilistic temporal extrapolation of fatigue damage of offshore wind turbine substructures based on strain measurements

Abstract. Substructures of offshore wind turbines are becoming older and beginning to reach their design lifetimes. Hence, lifetime extensions for offshore wind turbines are becoming not only an interesting research topic but also a relevant option for industry. To make well-founded decisions on possible lifetime extensions, precise fatigue damage predictions are required. In contrast to the design phase, fatigue damage predictions can be based not only on aeroelastic simulations but also on strain measurements. Nonetheless, strain-measurement-based fatigue damage assessments for lifetime extensions have been rarely conducted so far. Simulation-based approaches are much more common, although current standards explicitly recommend the use of measurement-based approaches as well. For measurement-based approaches, the main challenge is that strain data are limited. This means that measurements are only available for a limited period and only at some specific hotspot locations. Hence, spatial and temporal extrapolations are required. Available procedures are not yet standardised and in most cases not validated. This work focusses on extrapolations in time. Several methods for the extrapolation of fatigue damage are assessed. The methods are intended to extrapolate fatigue damage calculated for a limited time period using strain measurement data to a longer time period or another time period, where no such data are available. This could be, for example, a future period, a period prior to the installation of strain gauges or a period after some sensors have failed. The methods are validated using several years of strain measurement data from the German offshore wind farm Alpha Ventus. The performance and user-friendliness of the various methods are compared. It is shown that fatigue damage can be predicted accurately and reliably for periods where no strain data are available. Best results are achieved if wind speed correlations are taken into account by applying a binning approach and if a least some winter months of strain data are available.

Influence of Corrosion Damage on Fatigue Limit Capacities of Offshore Wind Turbine Substructure

The decrease in structural capacities under the limit states caused by structural corrosion is a potential hazard for the safety of offshore structures. Considering the influence of corrosion factors, a fatigue analysis of the typical tubular joints of an offshore wind turbine (OWT) substructure under operation and wave loads during the service period was performed. The structural corrosion was equivalent to a two-parameter Weibull distribution in the analysis. According to the Miner linear fatigue accumulation criterion, the accumulated fatigue damage of three different typical tubular joints in the initial stage and at operation periods of 10, 20, and 30 years was calculated. The most critical tubular joint of the studied OWT substructure is located at the connection between the pile foundations and braces. Owing to the increase in structural corrosion during the operation period, a remarkable decrease in the tubular joint fatigue capacities under the operation and wave loads was observed.

Pitch motion reduction of semisubmersible floating offshore wind turbine substructure using a tuned liquid multicolumn damper

A tuned liquid multicolumn damper (TLMCD) composed of three liquid columns and bottom connecting pipes is proposed to reduce the pitch motion of a semisubmersible substructure for wind turbine. The pitch damping of the semisubmersible floating offshore wind turbine (FOWT) substructure scale model with TLMCD in regular waves is studied experimentally. The TLMCD with the optimized mass ratio and tuning ratio provides effective pitch motion control especially near resonance period. The OpenFOAM is further developed to simulate dynamic response of the FOWT substructure under internal sloshing and external wave excitations, where the wave elevation and the FOWT substructure response have been validated against the experimental data. The design natural frequency of the TLMCD is equal to that of the FOWT substructure, and the analytical solutions and numerical results of liquid column decay are in good agreements. The motion response of the FOWT substructure, velocity distribution and the wall pressure are used to analyze the damping mechanism of the TLMCD. Based on the hydrodynamic moment, the energy dissipation characteristics of the TLMCD are analyzed quantitatively, where the damping and exciting moment are distinguished from each other. The analyses show that the TLMCD has the best damping effect near resonance frequency, where the 2.0% mass ratio of the liquid can reduce the maximum pitch motion by 10.84% to 18.53%.

Development of a Genetic Algorithm Code for the Design of Cylindrical Buoyancy Bodies for Floating Offshore Wind Turbine Substructures

The Levelized Cost of Energy for floating offshore wind must decrease significantly to be competitive with fixed offshore wind projects or even with onshore wind projects. This study focuses on the design optimization of cylindrical buoyancy bodies for Floating Offshore Wind Turbine substructures. The presented work is based on a previously studied buoyancy body design that allows an efficient manufacturing process and integration into different substructures. In this study, an optimization framework using a particular Genetic Algorithm is developed to parameterize the cylindrical geometry of the buoyancy body and optimize its design in terms of cost, considering loads acting on the structure as well as manufacturing and floater specific dimension restrictions. In this paper, the specific characteristics of this particular buoyancy body are highlighted and the implementation of the optimization process is detailed. Preliminary results for a given study case are presented and discussed before pointing out potential cost reductions and future improvements to be made with regards to the presented optimization work.

Stress influence matrix on hot spot stress analysis for welded tubular joint in offshore jacket structure

The detailed finite element (DFE) simulation of welded offshore tubular joint can give realistic and sound hot-spot stress (HSS) estimates regarding fatigue design of jacket substructures for offshore wind turbines. However, the computational efforts and time costs due to complicated joint configurations and a large quantity of design load cases have prompted the design practitioners likely resort to a simple alternative, the SCF method, which is also rooted in the DFE simulation. The present study attempted to optimize the conventional DFE simulation procedure by applying a stress influence matrix (SIM) method and its modified version gSIM, along with discrete joint formulation techniques, to simplify and make efficient of the DFE process, and then incorporating point positioning techniques to eventually ease the HSS evaluation. Through an implementation case involving a KK tubular joint, the gSIM method proved its robustness, reliability and computationally efficiency in joint HSS evaluation practice.

Development of a Genetic Algorithm Code for the Design of Cylindrical Buoyancy Bodies for Floating Offshore Wind Turbine Substructures

The Levelized Cost of Energy for floating offshore wind must decrease significantly to be competitive with fixed offshore wind projects or even with onshore wind projects. This study focuses on the design optimization of cylindrical buoyancy bodies for floating substructures of offshore wind turbines. The presented work is based on a previously studied buoyancy body design that allows an efficient manufacturing process and integration into different substructures. In this study, an optimization framework based on genetic algorithm is developed to parameterize the buoyancy body’s geometry and optimize its design in terms of cost, considering loads acting on the structure as well as manufacturing and floater specific dimension restrictions. The implementation of the optimization process is detailed, and tested for a given study case. Two structurally different genetic algorithms are considered in order to compare the results obtained and asses the performance of the presented optimization framework.

Development of a 3-legged jacket substructure for installation in the southwest offshore wind farm in South Korea

A suitable substructure for Offshore Wind Turbines plays an important role since it can efficiently reduce the financial costs of wind power projects. According to the track record of the Korean offshore wind farms, the 4-Legged Jacket (4LJ) substructure is the most utilized system; however, this design is still limited, specifically in the economic aspects. Thus, this study aims to design a more cost-effective substructure to enable large-scale deployment for Korean offshore projects. To achieve this objective, the new 3-Legged Jacket (3LJ) substructures together with various bracing topological forms (i.e., Pratt, Warren, and X-bracing) are developed. Results show that under the environment loads, dynamic responses obtained from the developed 3LJs are almost independent to the loading directionality; while there is a strong polarization in the case of 4LJ. Among the three cases of the 3LJs, the X-type topological form has the highest flexural stiffness together with the largest manufacturing and fabrication costs, while the Warren bracing system reaches ultimate strength earlier than the Pratt system. Therefore, the 3LJ substructure with a Pratt bracing system is suggested as a good alternative to the existing 4LJ system for Korean offshore wind farms, with reductions of up to 21% and 25% in total weight and number of weld joints, respectively.

Construction and installation engineering for floating wind turbines

The construction and installation engineering of floating offshore wind turbines is important to minimize schedules and costs. Floating offshore wind turbine substructures are an expanding sector within renewable power generation, offering an opportunity to deliver green energy, in new areas offshore. The floating nature of the substructures permits wind turbine placement in deep water locations. This paper investigates the construction and installation challenges for the various floating offshore wind types. It is concluded that priority areas for project management and design engineers minimising steel used in semi submersible construction, reducing the floating draft of Spars and for Tension Leg Platforms developing equipment for a safe installation. Specifically tailored design for construction and installation includes expanding the weather window in which these floating substructures can be fabricated, transported to and from offshore site and making mooring and electrical connection operations simpler. The simplification of construction methodology will reduce time spent offshore and minimise risks to installation equipment and personnel. The paper will include the best practice for ease of towing for offshore installation and the possible return to port for maintenance. The construction and installation process for a floating offshore wind turbine varies with substructure type and this will be developed in more detail in the paper. Floating offshore wind structures require an international collaboration of shipyards, ports and construction vessels, though to good project management. It is concluded that return to port for maintenance is possible for semi submersibles and barges whereas for Spars and TLP updated equipment is required to carry out maintenance offshore. In order to facilitate the construction and to minimize costs, the main aspects have to be considered i.e., the required construction vessel types, the distance from fit-out port to site and the weather restrictions.