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Stability requirements for floating offshore wind turbine (FOWT) during assembly and temporary phases: Overview and application
Abstract
The renewable energy targets set by the EU governments is pushing the wind industry toward deeper and further sites, making the floating option the preferred technical and economic choice. Certification authorities are developing new guidelines, recommended practices and standard specific for this industry, starting to recognize the importance, beside the operational phases, of the temporary phases such as sea transport and installation on site (DNV, 2013). The aim of the present work is to propose an approach to analyse the assembly and temporary phases, namely when the turbine is assembled on the floating support structure and the following transport phase from the shipyard to the operational site. After a review of the approaches for industries near to the offshore wind industry, a framework is proposed where criteria, standards and environmental and loading conditions are discussed. Taking the FOWT NOVA as a case study, the proposed framework is focused on semi-submersible FOWT, assessing the design of this floating support structure, highlighting the impact of these rules and evidencing an overall good performance. The proposed approach can be used as a starting point by administrations and certification authorities and as guidelines in the conceptual/preliminary design phases of the design of an FOWT structure.
... Several studies have investigated the stability of FOWF components under various conditions using different numerical methods. For instance, Collu et al. (2014) developed a framework to analyse the stability of a WTG during assembly and temporary stages. This framework was applied to a case study involving a semi-submersible FOW structure and numerical stability analysis was performed using the SESAM software package with HydroD (DNV, 2023d,g). ...
... Recent studies investigated the stability of TRN and foundation assembly units and substations during towing operations. The studied considered various foundation types, including semi-submersible (Collu et al., 2014;Liu et al., 2023c), spar (Dymarski et al., 2019), and TLP (Ding et al., 2017;Dymarski et al., 2017). For the substation towing stability analysis, wide-shallow bucket jacket foundation (WSBJF) was considered . ...
... One crucial aspect of designing WTGs is ensuring their stability during assembly and temporary installation stages. Collu et al. (2014) developed a framework specifically for analysing WTG stability during these critical phases. The framework was applied to a case study involving a semi-submersible foundation and numerical stability analysis was performed using the SESAM software package with HydroD (DNV, 2023d,g). ...
The deployment of floating offshore wind farms marks a pivotal step in unlocking the vast potential of offshore wind energy and propelling the world towards sustainable energy solutions. Despite the compelling prospects of floating wind technology, its implementation is challenging. Complex installation procedures, associated high costs, and evolving regulations can hinder widespread adoption. However, these challenges present opportunities for innovation and cost reduction. This paper delves into the technical, operational, and economic aspects of floating offshore wind farm installation, providing a comprehensive overview of the current state-of-the-art. The analysis goes beyond simply describing the current landscape by critically examining the complexities involved in floating offshore wind farm installation. It identifies critical research areas for advancing floating wind technology towards broader adoption and greater efficiency. The findings underscore the critical need for standardised foundation designs, advanced installation methods, and robust collaboration between academia and industry. By fostering such collaboration, for example, by creating research consortiums or knowledge-sharing platforms, the floating wind industry can accelerate advancements and unlock its full potential as a clean and sustainable energy source.
... While dedicated rules and guidelines for assessing the intact stability of FOWT systems are currently lacking, certain principles can be adapted from the extensively developed standards within the oil and gas industry, which have been in use for an extended period. Collu et al. [9] conducted research in this area, proposing stability guidelines for fully assembled FOWTs during the transportation and installation phases. Furthermore, there has been a growing interest in investigating the motion dynamics of platforms during towing operations. ...
... The righting arm (GZ) at various draughts (T) was calculated using an opensource tool called BEMRosetta [21,22]. These values are plotted in Figure 2. As mentioned before, the stability rules applicable to FOWTs are still not fully developed, and in the presented study, stability checks were performed using the general IMO stability criteria [8,23] and the MODU (Mobile Offshore Drilling Units) rules [9,24]. The wind turbine was assumed to be kept in a parked-feathered condition to minimise the wind loads on the structure [25]. ...
... Even in this turbine configuration, the wind loads could be significant, depending on the wind speed, but it takes dedicated research to understand the effect of wind, which is beyond the scope of this paper. The stability criteria were provided by [8,9,23,24] as follows: ...
The offshore wind sector is moving into deep waters and using floating platforms to harness the higher wind speeds in exposed locations. There are various floating platform types currently in development, but semi-submersibles are considered the most prominent early movers. Such floaters need to be towed to and from wind farm locations for installation, special cases of repair and decommissioning. As with any other offshore activity, metocean limits exist for towing operations which can impact the development of a wind farm. It is important to calculate the motion and loads of the platform before commencing the towing operations and to check whether they exceed the defined limits to enable safe execution. In this paper, two approaches using two different numerical tools to predict the motion of a fully assembled floating wind platform under tow are presented and compared. A potential flow-based method derived from a low forward speed approach and a hybrid approach combining potential flow and Morison equation methods are investigated, and the numerical predictions are compared and validated against experimental results. Both methods demonstrate accurate predictions, depending on the wave condition and towing speed, albeit differing in execution time and the simplicity of the simulation setup. The first method was found to provide good predictions of the motion in low-speed (0.514–1.543 m/s) towing conditions. The second method provides better results for all the towing speeds and wave heights. As the wave height and towing speed increase, deviations from experiments were observed, signifying non-linear phenomena that are difficult to analyse using the mentioned potential-flow-based methods.
... Collu et al. [40] proposed non-operational stability requirements for semisubmersible floating wind turbines after synthesizing the existing stability guidelines and guidance on offshore floating platforms, which are applicable in the present study. the metacentric height has to be 1 m at least; 5. ...
... Usually, offshore wet towage is carried out under good weather; however, considering that the process may last for several weeks, extreme weather conditions should also be taken into account. The wind speed for tow stability assessment is chosen as 36 m/s under normal weather and 51.5 m/s under extreme weather, with the wind turbine blades feathered [40,42,43]. The heeling moments under different weather conditions are calculated and shown in Table 7, with the wind load acting on the floating platform neglected. ...
For the exploitation of offshore wind resources in areas with intermediate water depths, a novel semi-spar floating foundation is introduced to combine the superiority of the conventional semisubmersible and spar-type floater. It consists of an upper floater and a hanging weight, which are connected through 12 suspension ropes. Such a floating foundation can be wet-towed as a semisubmersible floater, which features a large waterplane moment of inertia to increase stability and reduce transportation costs. After being anchored on site, it behaves as a spar floater with moderate draft and superior hydrodynamic characteristics. The stability of the proposed semi-spar platform during wet towage is analyzed. Afterward, a fully coupled aero-hydro-servo-elastic simulation is conducted to evaluate its hydrodynamic responses in comparison with the responses of the well-acknowledged OC3-spar and OC4-semisubmersible platforms. Then, the ultimate strength of the mooring lines and suspension ropes under extreme conditions was numerically investigated, as well as the relationship between the ropes’ tension and wave direction. Eventually, a cost-effectiveness analysis is conducted in terms of power generation and steel mass. The results demonstrate that the proposed semi-spar design meets the safety criteria in transportation and exhibits a smaller response in surge and pitch motions. In addition, the ultimate strength of mooring lines and suspension ropes satisfies the safety requirements, and simulation reveals that the lateral suspension ropes parallel to the propagation direction are sensitive to the environmental conditions of winds and waves. This study confirms that the newly proposed floating wind turbine exhibits excellent hydrodynamic and power generation performance, which is of great significance for the sustainability of the energy and electricity industry.
... Offshore floating platforms must satisfy specific stability criteria, as shown in the intact stability check curve in Figure 9, where θ 1 is the first intersection angle between the wind heeling moment and the righting moment; θ 2 is the down-flooding angle; θ 3 is the second intersection angle between the wind heeling moment and the righting moment; and θ 4 is the stability intercept angle [23]. Table 8 ...
... Offshore floating platforms must satisfy specific stability criteria, as shown in the intact stability check curve in Figure 9, where 1 θ is the first intersection angle between the wind heeling moment and the righting moment; 2 θ is the down-flooding angle; 3 θ is the second intersection angle between the wind heeling moment and the righting moment; and 4 θ is the stability intercept angle [23]. Table 8 These requirements ensure that column-stabilized platforms meet the necessary stability criteria under different environmental conditions. ...
Floating offshore wind platform (FOWP) has become the economically favored option for supporting wind turbines in deep waters. It is urgent to propose new concept designs for FOWPs that can be effectively deployed. Additionally, the extensive use of steel in such platforms significantly escalates costs, necessitating the optimization of steel utilization. Motivated by these challenges, a V-shaped floating semi-submersible platform equipped with NREL 5 MW wind turbine is designed and analyzed based on the potential flow theory and the blade element momentum theory. Fully coupled time-domain simulations are conducted using the F2A program, which couples NREL FAST and ANSYS AQWA via a Dynamic Link Library (DLL), to compare the hydrodynamic performance and stability of the V-shaped floating platform with the original triangle-shaped model of “Fuyao”. Various sea conditions have been considered, including combined wind-wave action and wind-wave-current action at different incidence angles. The results show that the V-shaped floating platform has better economic and hydrodynamic performance (e.g., a reduction of 40.4% and 12.9%, respectively, in pitch and yaw motions, and a 17.4% reduction in maximum mooring tension), but lower stability than its triangle-shaped counterpart.
... Other requirements are also advised for stability assessment of semi-submersible wind turbines ( Collu et al., 2014), including: ...
... It must be noted that the total wind heeling moment reaches the maximum when the heel angle is 0°. The wind speed considered for the normal towing condition and extreme weather condition are 36 m/s and 51.5 m/s, respectively (BV, 2010;Collu et al., 2014). It can be observed that the maximum heel angles of the STLP in normal towing condition and in extreme weather condition are about 3.8° and 6.7°, respectively. ...
This paper proposed a submerged tension leg platform (STLP) for an offshore wind turbine in moderate water depth (70–150 m). During the transportation, the platform is semi-submersible and self-stabilized because it has a relatively large water plane area. Therefore, it can be wet-towed out together with the wind turbine from the quayside to the offshore installation site. During the operation phase, the platform is submerged with a relatively small water plane area, which improves its hydrodynamic performance. Hence, this STLP wind turbine requires low transportation and installation cost. It can also achieve a potentially good dynamic behavior during the operation phase. In this paper, the stability of the STLP wind turbine during the transportation phase is first assessed without considering the mooring lines. The results show that the STLP wind turbine has a good stability to ensure a safe wet towing. The dynamic responses of the STLP wind turbine during the operation phase are then studied with emphasis on the effect of second-order wave loads, wind-wave misalignment and water depth. Based on fully coupled time domain simulations, it is found that the effect of second-order wave loads on the dynamics of the STLP wind turbine is slightly larger in an extreme sea state than that in a moderate sea state. The standard deviations of the surge, sway, roll and pitch motions and the tower base bending moment are dependent on the wind-wave misalignment, while those of the heave and yaw motions, the blade root bending moment are not. In addition, a larger water depth leads to larger standard deviations of the platform motions and a smaller standard deviation of the tower base bending moment. The effect of water depth on the blade root and tower top bending moments is negligible.
... time-domain dynamic response of FOWTs through diverse methodologies. After referring to the safety rules of the domestic and international maritime merchant ships and the design standards of oil and gas industry, Collu et al. (2014) summarized the methodologies employed in stability research within related industries, and put forward a novel set of standards for the stability and safety evaluation of the wet towing of the FOWT, offering valuable insights for the conceptualization and preliminary design of the FOWT. Based on the physical towing tests, Hyland et al. (2014) analyzed the towing motion response and towing resistance of GICON®-TLP through various towing schemes and dissimilar working conditions via towing tank tests. ...
This research investigates the towing dynamic of a semi-submersible floating offshore wind turbine (FOWT) towed by a tugboat. Initially, the towing resistance of the FOWT towing system in still water is analyzed through a combination of the Computational Fluid Dynamics (CFD) method and empirical formulas. Subsequently, the preliminary towing scheme is determined. After that, the numerical model of the towing system, encompassing the FOWT, towing cables and the tugboat is established in ANSYS-AQWA. Furthermore, the dynamic responses of the towing system in varied working conditions are evaluated. Conclusively, the feasibility of the towing scheme is assessed based on the numerical simulation results. The designed model considers the effects of the towing speed and environment loads during wet towing which may be potentially helpful in offering a reference to the practical engineering.
... The first step is to check the intact stability of the FOWT at the given draught of interest. Collu et al.[3] have defined the stability requirements of FOWTs during towing operations. A ...
A robust pipeline of floating wind energy has emerged with a general trend of projects becoming larger, further from shore, and placed in increasingly energetic seas. The installation process for these farms involves the pre-assembly of components onshore or in sheltered waters before towing the platform to the operational location using tugs. It can be expected that such marine operations will be repeated in reverse at the time of decommissioning. The cost and safety of these operations will be influenced by the tugs used, towing speed, the local metocean conditions, the platform/turbine characteristics and other factors. This paper investigates the hydrodynamic characteristics of a large semi-submersible floating offshore wind turbine (FOWT) under tow. The motions of the FOWT are analysed using a numerical tool and validated using a towing test. A framework is proposed for the assessment of FOWT towing operations. Various limiting factors have been identified and the hydrodynamic performance of the system has been evaluated using the framework.
... These advantages encompass time and cost savings by allowing the integration and commissioning of modules to take place on land, instead of at sea. Additionally, decoupling the fabrication schedules of the topside from the availability of heavy-lift vessels is another significant benefit [25][26][27][28][29][30]. ...
This paper presents the performance of a new, floating, mono-hull wind turbine installation vessel (Nordic Wind) in the installation process. The vessel can transport pre-assembled wind turbines from the marshalling port to the offshore installation site. Each assembled turbine will be positioned over the pre-installed floating spar structure. The primary difficulty lies in examining the multibody system’s reactions when subjected to combined wind, current, and wave forces. Time-domain simulations are utilized to model the interconnected system, incorporating mechanical coupling between components, the mooring system for the spar, and the installation vessel. The primary objective is to focus on the monitoring and connection stages preceding the mating operations between the turbine and the floating spar. Additionally, it involves examining the impacts of wind, current, and wave conditions on the motion responses of the installation vessel and the spar, as well as the relative motions at the mating point, gripper forces, and mooring forces. The simulations show that the resulting gripper forces are reasonable to compensate. The relative motion at the mating point is not significantly affected by the orientations of the turbine blades, but it is influenced by the prevailing wave conditions. In addition, vessel heading optimization can minimize the relative motions at the mating point and gripper forces. Given the examined environmental conditions, the presented installation concept exhibits a commendable performance.
... 3.1 FOWT phases The main non-operational phases can be identified, ref [6], whose features are briefly detailed in the following: Turbine installation on the floating support structure: this phase is composed of the platform construction, the procedure of turbine positioning on the deck (when the structure is still either in a dry dock or in a fit-out basin), the inclining tests and the sequence of ballasting and of deballasting the support structure. This phase ends when the tugs start to tow the structure toward the operational site. ...
Floating wind turbines are becoming an important part of renewable offshore power generation, offering an opportunity to deliver green energy. The floating nature of the substructures permits wind turbine placement in deep water locations, probably out of sight of land. This paper presents the tow out design method requirements for the installation of floating offshore wind turbines.
Most existing floating offshore wind turbines substructures are barge, semi submersible, TLP and Spar types and their installation methods have been developed from those used on offshore oil and gas structures. Whilst the turbines are derived from those used on fixed bottom offshore wind turbines. The paper summarises the weather window limitations for the various substructure types and installations phases, including the transportation to and from the offshore site and during the connection of mooring lines and electrical cables. The choice of construction materials i.e. steel or concrete, influence the draft of the substructure at the fit-out quay and hence during the tow offshore.
Semi submersible and barge types are of shallow draft and can be fitted out alongside a quay. Spar types typically require deep water for construction in sheltered inshore waters. The Tension Leg Platform (TLP) floating wind turbine has minimum water plane area and hence has low intact stability during ocean tow and thus TLPs may require modified crane vessels for offshore installation.
The paper will present recent advances in the tow out requirements of floating offshore wind turbines. Data will be provided on intact stability, damage stability, tow forces and motions during tow from fit out port to the offshore location.
... They adopted the optimized spectral kurtosis and the ensemble empirical mode decomposition combined algorithm to examine the corresponding vibration of offshore wind turbines. Collu et al. [6] conducted a comprehensive review of FOWTs and established the guidelines for the FOWT preliminary design. Acero et al. [7] established a numerical model to simulate the dynamic responses of a tripod-type offshore wind turbine during its installation progress. ...
Numerical simulations are performed within the time domain to investigate the dynamic behaviors of an SPAR-type FOWT under wave group conditions. Towards this goal, the OC3 Hywind SPAR-type FOWT is adopted, and a JONSWAP (Joint North Sea Wave Project)-based wave group is generated by the envelope amplitude approach. The FOWT motion under wave group conditions, as well as the aerodynamic, hydrodynamic, and mooring performances, is simulated by our established in-house code. The rotating blades are modelled by the blade element momentum theory. The wave-body interaction effect is calculated by the three-dimensional potential theory. The mooring dynamics are also taken into consideration. According to the numerical results, the SPAR buoy motions are slightly increased by the wave group, while the heave motion is significantly amplified. Both the aerodynamic performance and the mooring tension are also influenced by the wave group. Furthermore, the low-frequency resonant response could be more easily excited by the wave group.
... The new system can be rapidly recycled when the connection to the manifold is unleashed and float to the sea surface through its own buoyancy. The subsea suspended cluster manifold weighing 250 tons is shown in Fig. 2, consisting of a major structure and four buoys layered design (Collu et al., 2014). ...
The subsea suspended manifold designed to replace the traditional foundation structure with the buoys is a new generation subsea production system that can be suspended at a certain height from the seafloor and rapidly recycled by its own buoyancy. Due to complex environmental conditions, its hydrodynamic performance in the splash zone is extremely important for the safety of the whole installation process. In this paper, the mathematical model for the dynamic analysis of the seawater ingress process of the single-layer pre-set horizontal cabin is proposed based on the different center of gravity positions of the buoy. Meanwhile, the theoretical analysis of fiber cable is divided into infinite differential units by the discretization method, and the formulae of the horizontal displacement of the subsea suspended manifold are presented. In addition, the simulations are carried out to verify the rules of the dynamic responses on the subsea suspended manifold system with the consideration of the environmental conditions in the South China Sea. Comparing with the calculated value of the mathematical model of the cabin water ingress, the error of the simulation result by use of FLUENT is about 5.47%. Furthermore, the wave height is greater than the current impact on the lowering manifold system and the azimuth angle of the installation vessel is aligned with the direction of the environmental load.
... However, the towing stage of the installation process is not covered individually in sufficient detail (Bureau veritas, 2015;DNV GL, 2018a). The lack of rules for the hydrostatic stability of FOWT structures during pre-installation stages has led to Collu, et al. (2014) to propose guidelines regarding stability during these stages to apply during the conceptual design phase. Furthermore, Roddier, et al. (2010) who tested the WindFloat design in a wave tank have also highlighted the lack of rules for stability calculations during towing conditions of FOWTs. ...
An important condition of any port-assembled floating offshore wind turbine concept is the de-ballasted transport stage. As the hydrostatic and dynamic stability may vary greatly from the operational condition, it needs to be carefully investigated in early stages of the design-phase. In this work, physical modelling of the transport of the de-ballasted OrthoSpar device was carried out to determine roll and pitch RAOs, as well as load characteristics of the towing line. Towing was simulated with a stationary model being subjected to currents. To examine the influence of wave direction, a range of model orientations towards the incident waves were tested in still water and together with the simulated towing state. Roll and pitch motions were found to be highly dependent on the wave frequency and a result of a low damping ratio. The towing load amplitude was found to be influenced by the towing direction regarding the wave direction.
... In recent years, scholars have carried out preliminary research on the towing operation process for FOWTs with different types of foundation. Collu et al. [13] studied the static stability criterion of the towing process of FOWTs under a normal and severe environment and provided the calculation guidelines for the maximum values of the metacentric height and the maximum height of the towing lines, and then applied the results to design the semi-submersible floating foundation for NOVA FOWT to evidence the overall good performance of those rules. Myland et al. [14] conducted an experiment to study the towing stability of the GICON ® -TLP under two different carriage speeds in the calm water and two different regular wave conditions. ...
One of the advantages of floating offshore wind turbines (FOWTs) is that they can be designed to be easily wet towed and installed to reduce the cost of offshore construction. In this paper, a fully coupled towing system numerical model is established for a novel 10 MW FOWT concept, namely, a submerged floating offshore wind turbine (SFOWT) to investigate the towing performance. Firstly, the numerical simulation is validated by comparison with model experiment results. Then, a series of numerical simulations are conducted to predict and compare the towing performance for a three-column SFOWT (TC-SFOWT) and a four-column SFOWT (FC-SFOWT) under different wave conditions. The results show that the two forms of SFOWT have good towing performance when the significant wave height is less than 5 m, which is the maximum wave height for the allowable towing condition. The FC-SFOWT shows relatively better performance in heave motion and roll motion, but the towing force is relatively larger compared with the TC-SFOWT under the same condition. When the significant wave height is 5 m, the maximum values of heave motion, pitch motion, and roll motion of the TC-SFOWT are 2.51 m, 2.14°, and 1.38°, respectively, while they are 2.25 m, 2.70°, and 1.21°, respectively, for the FC-SFOWT. Both the roll motion and the pitch motion are satisfied with the requirement that the roll and pitch are less than 5° during the towing process. The mean towing force of FC-SFOWT is 159.1 t at the significant wave height of 5 m, which is 52.8% larger than that of TC-SFOWT. The peak period mainly influences the frequency where the response peak appears in power spectra. The findings in this paper could provide some guidelines for wet towed operations.
... Some considerations are set to assure the proper operation of the turbine and the optimum platform behavior. The decline angle should not increase more than 6 degrees to avoid any negative effect on turbine operation and performance [36]. The structure must also have sufficient static stability as the design standard of Det Norske Veritas DNV-OS-J103 states that offshore platforms have to be capable of maintaining stability of the wind turbine at the wind speed producing the largest rotor thrust [37]. ...
Floating desalination plants are fairly new technologies and are not as common as the traditional land-based desalination plants. Almost none of the proposed nor installed projects' designers indicates that the design is environmentally driven, and only few designs are environmentally assessed. This paper aims to highlight the significant role of the environmental practices to achieve a sustainable design, where most of the environmental impact assessment procedures are performed prior to the design phase. Throughout the research, comparing alternatives and analyzing the baseline provided reliable technical help in the tasks of selecting the proposed project's location, desalination technology, power source and platform configuration. Thus, detailed technical descriptions of different systems are presented. Finally, environmental impacts associated with the operation of the proposed floating desalination plant in the selected location are assessed to give guidance on the monitoring and mitigation processes necessary to enhance the process performance, minimize the adverse environmental impacts and ensure the project's sustainability.
... In Ras Ghareb, a 24° angle generally leads to an increase of 10% in the area, due to freestanding PV from shading [47]. The total number of PV panels is 3300 panels, and the layout of the PV array is shown in Figure 14, where L is the deck width of 87.5 m, W is the panel length of 1.64 m, d is the distance of the array equal to 1.8 m, β is the tilted angle of 24° [48], and Wcosβ equals 1.5 m. ...
The supply of freshwater has become a worldwide interest, due to serious water shortages in many countries. Due to rapid increases in the population, poor water management, and limitations of freshwater resources, Egypt is currently below the water scarcity limit. Since Egypt has approximately 3000 km of coastlines on both the Red Sea and the Mediterranean Sea, seawater desalination powered by marine renewable energy could be a sustainable alternative solution, especially for remote coastal cities which are located far from the national water grid. The objective of this research work is to evaluate the feasibility of a floating desalination plant (FDP) concept powered by marine renewable energy for Egypt. A novel design of the FDP concept is developed as an innovative solution to overcome the freshwater shortage of remote coastal cities in Egypt. A mobile floating platform supported by reverse osmosis (RO) membrane powered by marine renewable power technology is proposed. Based on the abundant solar irradiation and sufficient wind density, Ras Ghareb was selected to be the base site location for the proposed FDP concept. According to the collected data from the selected location, a hybrid solar–wind system was designed to power the FDP concept under a maximum power load condition. A numerical tool, the DNV-GL Sesam software package, was used for static stability, hydrodynamic performance, and dynamic response evaluation. Moreover, WAVE software was used to design and simulate the operation of the RO desalination system and calculate the power consumption for the proposed FDP concept. The results show that the proposed mobile FDP concept is highly suitable for being implemented in remote coastal areas in Egypt, without the need for infrastructure or connection to the national grid for both water and power.
... This number is the position where two of the lower columns that have a current freeboard of 3.65 meters get submerged and the remaining two start exiting the water as shown in Figure 10; accordingly, it can be adjusted by altering their height. For instance, (Collu et al., 2014;Yanqing et al., 2017) advice reserving a 15-degree angle for positive stability. However, vanishing angle on its own is not an indicator of a platform's dynamic stability. ...
The work explains the rationale and principles behind the hydrodynamic design of a dynamically self-stable hybrid platform to host a 10 MW turbine that is a free-float capable tension leg platform, the CENTEC-TLP. It forgoes the often-studied single column hull form combined with spokes and investigates an alternative closer to the conventional TLPs while adhering to a limit hull weight. After an overview of the static stability characteristics of ocean platforms, an appropriate alternative is selected, resulting in a barge-TLP hybrid structure. In transport, it can float on a shallow draft with minimal response to wind and wave forces, and it safely functions as a TLP when installed. The safety check of the installed state is carried out by ensuring that slack moorings, mooring line breaking, and excessive surge motion does not occur. Each of these conditions is studied in rated and above rated operational conditions. A 50-year extreme case is also examined to verify that the structure survives.
... Collu et al. [19] describes the preliminary development of the NOVA concept and the optimisation of its support structure (figure 1c). The primary design criteria for this turbine were to reduce the overturning moment acting on the support structure while maintaining sufficient power output [28][6] [14][20] [40][41] [42]. A gyroscopic analysis of the turbine rotating in response to rolling and pitching wave motions showed the benefits of the low centre of gravity design even in harsh sea conditions [14]. ...
Wind energy has experienced a consistent expansion in the past decade especially with the move to offshore generation. There is an increasing need to further exploit offshore wind resources which is pushing wind farms into deeper water locations where the current popular horizontal axis wind turbine configuration may not be entirely suitable. Subsequently, there has been a resurgence in the interest in the vertical axis wind turbine configuration for a deep offshore floating application due to its inherent design attributes, higher range of power generating capacity and ultimately lower cost of energy. In this paper, a detailed review of the state of the art regarding floating vertical axis wind turbines has been presented, detailing the various designs and concepts currently under development.
Also a comprehensive overview of vertical axis wind turbine aerodynamic modelling methodologies in terms of accuracy and computational expense are presented with recommendations made regarding a model for preliminary iterative design for an offshore floating vertical axis wind turbine.
... This requires a good understanding of the performance of the installation vessels in waves. Therefore, numerical methods and models have been developed to estimate dynamic responses of the system during installation, including Collu et al. (2014), Li et al. (2015) and Li et al. (2016), in which static, steady-state as well as non-stationary dynamic responses of the installation systems were analysed. ...
In this paper, a summary of the recent work at NTNU on the installation of offshore wind turbines using jack-up and floating vessels will be reported. The wind turbine components considered here are the monopile foundations and the blades. The detailed discussions are given to the crane operations for installing wind turbine blades as well as novel installation methods for pre-assembled rotor-nacelle-tower. It includes numerical modelling and analysis for global dynamic responses of the installation system (installation vessels plus wind turbines) and for local structural responses of the blades in case of contact/impact. In particular, the stochastic nature of the environmental conditions (mainly wind and waves) and their influence on the global dynamic responses of the installation system will be assessed based on time-domain simulations. In addition, tugger line tension control is introduced for the final connection to the hub in order to reduce the motions of the blade and therefore the potential damages to the blades. It is then followed by a discussion about nonlinear structural analysis of the blade in contact with tower or surrounding structures using ABAQUS. Damages in the composite plies and sandwich core materials of the blade due to contact/impact for a given initial velocity are then estimated. The obtained damage distribution formulates the basis for a probabilistic assessment of structural safety during installation. Novel installation methods in which the rotor-nacelle-tower structure is pre-assembled onshore and installed on top of the foundation offshore, and the corresponding installation vessels are discussed at the end of the paper. Finally, the main conclusions and the recommendations for future work are drawn.
... To achieve this, accurate assessment is desired of the performance of the installation vessels and installation methods, and numerical methods and models have been developed to estimate systems' dynamic responses during installation. Most of the studies focused on static [17] or steady-state dynamic responses [18], whereas in a few studies, the nonstationary features of the installation process were also considered [19,20]. Based on numerical simulations and response-based criteria, methodologies for assessment of allowable sea states for installing OWTs can be established. ...
Installation of floating wind turbines is a challenging task. The time and costs are closely related to the installation method chosen. This paper investigates the performance of an efficient installation concept – a catamaran wind turbine installation vessel. The vessel carries pre-assembled wind turbine units including towers and rotor nacelle assemblies. Each unit is placed onto a pre-installed offshore support structure (in this paper a spar floater) during installation. The challenge is to analyse the responses of the multibody system (catamaran-spar-wind turbine) under simultaneous wind and wave loads. Time-domain simulations were conducted for the coupled catamaran-spar system with mechanical coupling, passive mooring system for the spar, and dynamic positioning control for the catamaran. We focus on the steady-state stage prior to the mating process between one turbine unit and the spar, and discuss the effects of wind loads and wave conditions on motion responses of the catamaran and the spar, relative motions at the mating point, gripper forces and mooring forces. The relative motion at the mating point is less sensitive to the blade orientation, but influenced by the wave conditions. Under the investigated sea states, the present installation method shows decent performance.
... Therefore, numerical methods and models have been developed to estimate systems' dynamic responses during installation. Most of the studies focused on static (Collu et al., 2014) or steady-state dynamic responses , while in a few studies, the nonstationary features of the installation process are considered . Li (2016) developed a numerical method for analyzing the dynamic responses of the monopile when it is lowered from the air into the water, considering the submergence-dependent hydrodynamic loads on the monopile and the vessel shielding effect. ...
COMMITTEE MANDATE
Concern for load analysis and structural design of offshore renewable energy devices.
Attention shall be given to the interaction between the load and structural response of fixed
and floating installations taking due consideration of the stochastic nature of the ocean
environment. Aspects related to prototype testing, certification, marine operations and total
cost of energy shall be considered.
... The novelty factor of such structures poses the greatest challenges in the assessment of their stability performance. Collu et al. (2014) mention that on of most difficult tasks lays with selecting the appropriate criteria to assess stability of these structures. In fact, only recently have classification societies published their first set of requirements concerning these specific structures. ...
Stability plays a key role in the design of floating structures. Its impact on safety and operational aspects render it a fundamental consideration in the preliminary design stages. Stability assessment of floating structures intended to work offshore, here generally referred to as offshore floating structures, present several specific challenges. These arise from diverse sources: from the departure of classical ship hydrostatic behavior due to different shapes and sizes, to the multitude of operational modes and specific loads and loading conditions inherent to the many different missions these units are intended for. This paper revisits several of such challenges faced in the stability calculations of offshore floating structures as well as its approval by regulatory authorities. Particular attention is paid to the one concerning the identification of the “most critical axis” for GZ curve calculation and a free-twist method is discussed as a preferred method for stability calculations of freely floating structures.
... An approach to analyse the hydrostatic stability of the floating wind turbine in the assembly and temporary phases was proposed according to the experience, criteria, and standards from the offshore oil and gas industry. 10 Experiments on the towing stability of GICON TLP in the transportation phase were conducted. 9,11 Towing points effects on the stability of the GICON TLP and tow resistance in the wave only condition were studied. ...
In this study, a submerged tension leg platform wind turbine (STLPWT) which can be constructed near the quayside and then wet-towed to the installation site as a unit was proposed. This transportation method will consequently reduce the use of heavy offshore cranes. However, as a high-rise structure, the floating wind turbine may sustain large overturning moments induced by wind, wave, current, and towing force in the transport phase. In order to study the stability of the floating wind turbine and the towline force in wet tows, a numerical model of the towing system including a towboat, towline, and STLPWT was established based on multi-body dynamics. Then, the environmental load effects on the towing stability of the floating wind turbine were investigated. In addition, the comparison of the bollard pull and the height of towing points was performed. The results show that the STLPWT was stable under the rough sea towing condition: a significant wave height of 5 m and a wind speed of 17 m/s. An appropriate bollard pull should be chosen, in practice, as it involves the time and costs of the towing process. In addition, it may be a better choice to set the height of the towing points near the mean sea level to lower the pitch motions of the STLPWT as well as the towline force.
... In case of large water depths, the option of either semisubmersible, TLP (Tension Leg Platform) or SPAR floating supports are being considered (Breton and Moe, 2009;Dvorak et al., 2010;Esteban et al., 2011bEsteban et al., , 2011cEsteban et al., , 2015aEsteban et al., , 2015bHoulsby et al., 2006;Lozano-Minguez et al., 2011;Zaaijer, 2006;Zhao et al., 2012). There is a lot of research studies focused on different types of foundations (Benassai et al., 2014;Chang and Jeng, 2014;Collu et al., 2014;Dunbar et al., 2015;Ha and Cheong, 2016;Perez-Collazo et al., 2015;Rogan et al., 2016;Schafhirt et al., 2016;Zhang et al., 2015Zhang et al., , 2016. ...
Offshore wind industry is having a great development. It requires progress in many aspects to achieve the sustainable progress of this technology. One of those aspects is the design of foundation, sub-structures and support structures. The most used at present, with more than 80%, is the monopile. Typical piles used in quays in maritime engineering have a maximum diameter about 2 or 3 m. In offshore wind, the diameter can be more than double. There is a risk associated with the difference in scale. Some formulas used for the design of typical piles with diameter less than 2 m can be unsuitable for larger diameter piles. This paper is focused on giving a first estimate of length and weight of piles for knowing its diameter. There are formulas for that for piles with diameters up to 2 m, but there are doubts about whether they can be used for piles with larger diameters. To achieve it, a database gathering offshore wind farms in operation with monopiles is prepared in order to obtain simple formulas relating those parameters. Furthermore, the results of that formula are compared with traditional formula used in maritime engineering for piles with diameters less than 2 m.
... Another design aspect of the SFP is that it can be assembled with the tower and wind turbine at quayside and then towed to the installation site as shown in Fig. 7. During the transport to the operational site phase, the stability of the entire structure has to fulfill the following requirements (Collu et al. 2014): ...
This study uses a time-domain simulation to investigate the effect of aerodynamic damping on a freestanding bridge tower under the joint action of wind and wave loads. The physical model of the bridge tower, which has been tested in laboratory, is numerically reconstituted considering structural nonlinearities obtained from free oscillation tests. Random wind fields are generated using the harmonic superposition method, and the wind flow characteristics, such as mean wind velocity and turbulence intensity, are identified based on the data collected in the laboratory. The wave loads measured at the bottom of the foundation during the experimental model tests are used for the simulation. The dynamic responses of the tower under the joints action of wind and wave loads are successfully simulated. A clear reduction in structural vibration in the wave-controlled region is confirmed by simulation results. Further numerical investigation indicates that an increase in the mean wave velocity produces additional aerodynamic damping in the structural motion system, which significantly dampens the responses of the wave-induced vibration. A simple method for determining the dynamic responses of a bridge tower under the joint action of wind and wave loads, using the data obtained from wind- and wave-only cases, is proposed.
Motivated by soial and environmental reasons, water scarcity has become a global top agenda item. Egypt is one of the countries suffering from an acute shortage of freshwater. A promising novel and efficient solution to overcome Egypt’s freshwater shortage, especially in remote coastal areas far from the national grid of freshwater and electricity, is a mobile floating desalination plant (FDP) powered by offshore renewable energy. The proposed new FDP concept powered by an offshore wind turbine needs a special floating platform to provide enough buoyancy to support the weight of the desalination plant and to restrain the six degrees of freedom motions within an acceptable operational limit for a wind turbine. Based on hydrodynamics, the main objective of this study is to select the suitable offshore platform that can meet the novel FDP concept operation’s requirements at a specific deployment location in Egypt. Determining the safe natural frequencies zone necessitates taking into account the new FDP concept operation constraints and the Egyptian environmental loads to select platform far from the dynamic amplifications responses in the structure. Numerical modelling results show that the cylindrical platform with a heave plate configuration demonstrated the best dynamic and static performance for Egypt.
The design of floating support structures for wind turbines located offshore is a relatively new field. In contrast, the offshore oil and gas industry has been developing its technologies since the mid 1950s. However, the significantly and subtly different requirements of the offshore wind industry call for new methodologies. An Energy Technologies Institute (ETI) funded project called NOVA (for Novel Vertical Axis wind turbine) examined the feasibility of a large offshore vertical axis wind turbine in the 10–20 MW power range. The development of a case study for the NOVA project required a methodology to be developed to select the best configuration, based on the system dynamics. The design space has been investigated, ranking the possible options using a multi-criteria decision making (MCDM) method called TOPSIS. The best ‘class’ or design solution (based on water plane area stability) has been selected for a more detailed analysis. Two configurations are considered: a barge and a semi-submersible. The iterations to optimise and compare these two options are presented here, taking their dynamics and costs into account. The barge concept evolved to the ‘triple doughnut-Miyagawa’ concept, consisting of an annular cylindrical shape with an inner (to control the damping) and outer (to control added mass) bottom flat plates. The semi-submersible was optimised to obtain the best trade-off between dynamic behaviour and amount of material needed. The main conclusion is that the driving requirement is an acceptable response to wave action, not the ability to float or the ability to counteract the wind turbine overturning moment. A simple cost comparison is presented.
It is widely recognized that offshore wind farms are a key factor to fulfill renewable energy targets set in Europe : for example, in UK, the target is to have 15% of final energy consumption coming from renewable sources by 2020.
In general, with respect to onshore, offshore wind farms have several advantages, like the availability of larger areas and a higher energy potential, due to greater wind velocities and lower turbulence levels. On the other hand, one of the major drawback is the higher cost of the marine foundations, together with their higher installation cost.
Interestingly, the offshore oil & gas industry already managed to fulfill a similar opportunity: as drilling in deeper waters became both technically feasible and economically advantageous, several kind of offshore support structures for oil rigs have been developed. Starting from this experience, the present work focus on the preliminary design of two support structures suitable for medium to deep waters (depth > 30-40 m): a fixed structure, a jacket, and a waterplane- ballast stabilized floating structure.
The two structures are designed for the same 5 MW offshore wind turbine , and the design methodology is presented step by step. As a result, a technical and economical comparison analysis is presented.
StatoilHydro, the developer of the HyWind project, and Principle Power, which is working on the WindFloat concept, are partnered with existing commercial offshore wind turbine manufacturers and are designing their floating foundations to be compatible with many kinds of turbines. This reduces the technical and financial risks significantly, since the hulls are designed according to offshore oil and gas rules, supporting the knowledge base of an industry with decades of experience in building floating structures. The design of floating structures usually involves hydrodynamics tools such as WAMIT Inc.'s software for studying wave interactions with vessels and platforms, or Principia's DIODORE, to predict the hydrodynamic quantities, such as added mass, damping and wave exciting forces, which are used as a kernel in the time domain simulations. There are currently no commercial numerical design tools on the market capable of calculating the complete response of a floating wind turbine and substructure fully coupled.
This manuscript summarizes the feasibility study conducted for the WindFloat technology. The WindFloat is a three-legged floating foundation for multimegawatt offshore wind turbines. It is designed to accommodate a wind turbine, 5 MW or larger, on one of the columns of the hull with minimal modifications to the nacelle and rotor. Potential redesign of the tower and of the turbine control software can be expected. Technologies for floating foundations for offshore wind turbines are evolving. It is agreed by most experts that the offshore wind industry will see a significant increase in activity in the near future. Fixed offshore turbines are limited in water depth to ∼30–50 m. Market transition to deeper waters is inevitable, provided that suitable technologies can be developed. Despite the increase in complexity, a floating foundation offers the following distinct advantages: Flexibility in site location; access to superior wind resources further offshore; ability to locate in coastal regions with limited shallow continental shelf; ability to locate further offshore to eliminate visual impacts; an integrated hull, without a need to redesign the transition piece between the tower and the submerged structure for every project; simplified offshore installation procedures. Anchors are significantly cheaper to install than fixed foundations and large diameter towers. This paper focuses first on the design basis for wind turbine floating foundations and explores the requirements that must be addressed by design teams in this new field. It shows that the design of the hull for a large wind turbine must draw on the synergies with oil and gas offshore platform technology, while accounting for the different design requirements and functionality of the wind turbine. This paper describes next the hydrodynamic analysis of the hull, as well as ongoing work consisting of coupling hull hydrodynamics with wind turbine aerodynamic forces. Three main approaches are presented: The numerical hydrodynamic model of the platform and its mooring system; wave tank testing of a scale model of the platform with simplified aerodynamic simulation of the wind turbine; FAST, an aeroservoelastic software package for wind turbine analysis with the ability to be coupled to the hydrodynamic model. Finally, this paper focuses on the structural engineering that was performed as part of the feasibility study conducted for qualification of the technology. Specifically, the preliminary scantling is described and the strength and fatigue analysis methodologies are explained, focusing on the following aspects: The coupling between the wind turbine and the hull and the interface between the hydrodynamic loading and the structural response.
A state-of-the-art, unified treatment of the dynamic analysis of ocean structures and the natural forces which cause structural motion. Discusses the physical ideas and mathematical methods needed to analyze the dynamic behavior of offshore structures, including platform towers, cables, pipelines, and moored ships. Integrates nonlinear and statistical models throughout. Includes example problems drawn from research and practice.
Offshore Wind Turbine Installation. BMT, 2012a. Global Wave Statistics Online (www Document) URL 〈http://www. globalwavestatisticsonline
- American Bureau
- Shipping
American Bureau of Shipping, 2010. Offshore Wind Turbine Installation. BMT, 2012a. Global Wave Statistics Online (www Document). URL 〈http://www. globalwavestatisticsonline.com/〉. BMT, 2012b. Global Wave Statistics Online (www document).
Offshore Installation Practice
- J Crawford
Crawford, J., 1987. Offshore Installation Practice. Butterworth-Heinemann Ltd.,
Oxford.
Column-Stabilised Units (DNV-RP-C103)
- Det Norske
- Veritas
Det Norske Veritas, 2012. Column-Stabilised Units (DNV-RP-C103).
Design of Floating Wind Turbine Structures Germanischer Lloyd, 2005. IV 2 1–13 Guideline for the Certification of Offshore Wind Turbines
- Dnv
- Os-J103 Dnv
DNV, 2013. DNV-OS-J103 Design of Floating Wind Turbine Structures. Germanischer Lloyd, 2005. IV 2 1–13 Guideline for the Certification of Offshore Wind Turbines. Germanischer Lloyd, 2012. Note on Engineering Details—Marine Operations in Offshore Wind Project Certification.
A floating foundation for offshore wind turbines. J. Renew. Sustain. Energy, 2. Standards Norway
- D Roddier
- C Cermelli
Roddier, D., Cermelli, C., 2010. A floating foundation for offshore wind turbines. J. Renew. Sustain. Energy, 2. Standards Norway, 2004. NORSOK Standard N-004: Design of Steel Structures. WeatherLab Ltd, 2012a. Weather Lab (www document). URL 〈www.weatherlab.co.uk〉. WeatherLab Ltd, 2012b. Weather Lab (www document).
Dynamics of Offshore Structures Loading condition 7—Average weather—Draught 13.738 m—Angle of static equilibrium þ 5.511. Fig. 13. Loading condition 8—Average weather—Draught 17
- J F Wilson
Wilson, J.F., 2002. Dynamics of Offshore Structures, 2nd ed. John Wiley & Sons, Hoboken, New Jersey. Fig. 12. Loading condition 7—Average weather—Draught 13.738 m—Angle of static equilibrium þ 5.511. Fig. 13. Loading condition 8—Average weather—Draught 17.201 m—Angle of static equilibrium þ 5.941.
Note on Engineering Details-Marine Operations in Offshore Wind Project Certification
- Germanischer Lloyd
Germanischer Lloyd, 2012. Note on Engineering Details-Marine Operations in
Offshore Wind Project Certification.
NORSOK Standard N-004: Design of Steel Structures
- Standards Norway
Standards Norway, 2004. NORSOK Standard N-004: Design of Steel Structures.
Stability and Watertight Integrity (DNV-OS-C301)
- Det Norske
- Veritas
Det Norske Veritas, 2011. Stability and Watertight Integrity (DNV-OS-C301).
Offshore Wind Turbine Installation
American Bureau of Shipping, 2010. Offshore Wind Turbine Installation.
A comparison between the preliminary design studies of a fixed and a floating support structure for a 5 MW offshore wind turbine in the north sea. RINA, Royal Institution of Naval Architects -Marine Renewable and Offshore Wind Energy -Papers
- M Collu
- A J Kolios
- A Chahardehi
- F Brennan
Collu, M., Kolios, A.J., Chahardehi, A., Brennan, F., 2010. A comparison between the
preliminary design studies of a fixed and a floating support structure for a
5 MW offshore wind turbine in the north sea. RINA, Royal Institution of Naval
Architects -Marine Renewable and Offshore Wind Energy -Papers, pp. 63-74.
DNV-OS-J103 Design of Floating Wind Turbine Structures
DNV, 2013. DNV-OS-J103 Design of Floating Wind Turbine Structures.
IV 2 1-13 Guideline for the Certification of Offshore Wind Turbines
- Germanischer Lloyd
Germanischer Lloyd, 2005. IV 2 1-13 Guideline for the Certification of Offshore
Wind Turbines.
Weather Lab (www document)
- Weatherlab Ltd
- a
WeatherLab Ltd, 2012a. Weather Lab (www document). URL 〈www.weatherlab.co.uk〉.
WindFloat Prototype (www document)
- Principle Power