Deep Water Locations Research Articles

Definition of a Baseline Rotor Design for a 25MW Floating Offshore Wind Turbine

Abstract Wind turbine sizes have grown rapidly in recent years with machine ratings of 15-16 MW available from multiple manufacturers for offshore wind turbines. While the industry advances large-scale turbine designs, offshore developers have initially focused on siting in shallow waters on bottom-fixed foundations; however, eyes are also on deep-water locations where floating systems are required. The present study presents the definition for an initial baseline rotor design at 25 MW scale, designed for a floating offshore system. The purpose of this paper is to document this 25 MW blade definition for use as a reference design for future floating wind technology development by industrial and academic researchers and developers. As this is an initial reference design, some opportunities (and plans) for further mass or cost reduction are also noted. In summary, the paper documents the initial baseline rotor design including aerodynamic and structural design of the rotor blades, along with key details about the control system and floating system designs for the floating 25 MW wind turbine system.

Deep vs shallow: GPS tags reveal a dichotomy in movement patterns of loggerhead turtles foraging in a coastal bay

BackgroundIndividual variation in movement strategies of foraging loggerhead turtles have been documented on the scale of tens to hundreds of kilometers within single ocean basins. Use of different strategies among individuals may reflect variations in resources, predation pressure or competition. It is less common for individual turtles to use different foraging strategies on the scale of kilometers within a single coastal bay. We used GPS tags capable of back-filling fine-scale locations to document movement patterns of loggerhead turtles in a coastal bay in Northwest Florida, U.S.A.MethodsIridium-linked GPS tags were deployed on loggerhead turtles at a neritic foraging site in Northwest Florida. After filtering telemetry data, point locations were transformed to movement lines and then merged with the original point file to define travel paths and assess travel speed. Home ranges were determined using kernel density function. Diurnal behavioral shifts were examined by examining turtle movements compared to solar time.ResultsOf the 11 turtles tagged, three tracked turtles remained in deep (~ 6 m) water for almost the entire tracking period, while all other turtles undertook movements from deep water locations, located along edges and channels, to shallow (~ 1–2 m) shoals at regular intervals and primarily at night. Three individuals made short-term movements into the Gulf of Mexico when water temperatures dropped, and movement speeds in the Gulf were greater than those in the bay. Turtles exhibited a novel behavior we termed drifting.ConclusionsThis study highlighted the value provided to fine-scale movement studies for species such as sea turtles that surface infrequently by the ability of these GPS tags to store and re-upload data. Future use of these tags at other loggerhead foraging sites, and concurrent with diving and foraging data, would provide a powerful tool to better understand fine-scale movement patterns of sea turtles.

Control Co-Design of Wind Turbines

Wind energy is recognized worldwide as cost-effective and environmentally friendly, and it is among the fastest-growing sources of electrical energy. To further decrease the cost of wind energy, wind turbines are being designed at ever-larger scales. To expand the deployment of wind energy, wind turbines are also being designed on floating platforms for placement in deep-water locations offshore. Both larger-scale and floating wind turbines pose challenges because of their greater structural loads and deflections. Complex, large-scale systems such as modern wind turbines increasingly require a control co-design approach, whereby the system design and control design are performed in a more integrated fashion. This article reviews recent developments in control co-design of wind turbines. We provide an overview of wind turbine design objectives and constraints, issues in the design of key wind turbine components, modeling of the wind turbine and environment, and controller coupling issues. Wind turbine control functions and the integration of control design in co-design are detailed with a focus on co-design compatible control approaches.

Investigation of the influence of sinusoidal internal waves on a SPAR buoy structure

Offshore wind has a great potential as a competitive source of renewable energy, especially in deep waters where wind speeds are more consistent and fewer restrictions apply for running large wind turbines. Previous analyses of Floating Offshore Wind Turbines (FOWTs) mostly considered obvious sources of loading: surface waves, currents, wind and mooring. However, in some deep-water locations, internal waves can occur and these have been shown to significantly affect floating structures. Since the hydrodynamic response of an FOWT governs the structure’s general stability, the aim of this research is to investigate the impact of sinusoidal internal waves on the platform motion of a free-floating SPAR-type cylinder. A potential flow model and Morison’s equation are applied numerically to calculate the forces acting on a free-floating cylinder in an oscillating flow. The commercial Finite Element Analysis software OrcaFlex is verified by the potential flow model of the oscillating flow and is then used to mimic sinusoidal internal waves acting on a free-floating cylinder in a stratified flow. Three different internal wave amplitudes and peak velocities are analysed, and the nine resulting cases are investigated for the oscillating and stratified flow each. It has been found that the pitch rotations of the SPAR cylinder were small (< 0.1°) in all cases and can likely be disregarded. The surge displacements of the free-floating cylinder were substantial in both oscillating and stratified flows, with maximum surge magnitudes of 423m and 120m, respectively. Therefore, significant additional mooring loads due to internal waves could be sustained by SPAR-type FOWTs.

Hydrodynamic Investigation on Floating Offshore Wind Turbine Platform Integrated with Porous Shell

As the siting of wind turbines increasingly transitions from shallow water to offshore deep-water locations, improving the platform stability of floating offshore wind turbines is becoming a growing concern. By coupling a porous shell commonly used in traditional marine structures, with a FOWT (floating wind turbine platform), a new spar-buoy with a porous shell was designed. A numerical model investigating the coupling effect of the aero-hydro-mooring system is developed, and the results of the motion response are compared with the OC3-Hywind spar. The motion response of the two platforms was simulated in the time-domain with the incident wave period varied in the range of 5~22 s. The exciting wave force with added mass and radiation damping of the spar with the porous shell is compared with the OC3-Hywind spar. The results demonstrate that the motion response amplitude of the spar with the porous shell decreases in all three main motion freedoms (i.e., surge, heave and pitch, etc.), among which the heave motions are most significantly attenuated. The study shows that the coupling of porous shells with a floating platform to achieve the reduced motion responses is feasible and can be an innovative structure for the development of deep-sea offshore floating wind turbines.

A diatom-inferred water-depth transfer function from a single lake in the northern California Coast Range

This study examines the relationship between water depth and diatom assemblages from lake-sediment-surface samples at Kelly Lake, California. A total of 40 surface-sediment samples (integrated upper 5 cm) were taken at various depths within the small (~ 3.74 ha) 5.7 m-deep lake. Secchi depths, water temperature, pH, salinity, conductivity, and total dissolved solids were also measured. Some diatom species showed distinct association with depth (e.g., Fragilaria crotonensis, Nitzschia semirobusta). The relationship between the complete diatom assemblages and water depth was analyzed and assessed by depth-cluster analysis, a one-way analysis of similarity, principal components analysis and canonical correspondence analysis. Statistically significant differences were found between the assemblages associated with shallow depth (0–1.25 m), mid-depth (1.25–3.75 m), and deep-water (3.75–5.2 m) locations. The relationship between diatom assemblages and lake depth allowed two transfer models to be developed using the Modern Analogue Technique and Weighted Averaging Partial Least Squares. These models were compared and assessed by residual scatter plots. The results indicate that diatom-inferred transfer models based on surface-sediment samples from a single, relatively small and shallow lake can be a useful tool for studying past hydroclimatic variability (e.g., lake depth) from similar lakes in California and other regions where the large number of lakes required for traditional transfer-function development may not exist.

A deep-water in-situ power generation system based on chain-driven hydrokinetic turbine

Harnessing hydrokinetic energy from deep water to increase the operation periods of autonomous underwater vehicles and underwater moored platforms has received considerable attention in recent years. Hydrokinetic turbines should have a reliable startup performance for deployment in deep water with a prevailing average current speed of 0.5 m/s. In the present study, a deep-water in-situ power generation system based on a novel chain-driven hydrokinetic turbine is proposed for such low-speed current application. A numerical simulation was performed on the proposed turbine to investigate its hydrodynamic performance at different initial setting positions and tip speed ratios. No static dead band exists in the operation of the turbine, indicating a superior self-starting advantage over traditional drag-type turbines. Multi-objective optimization was performed on the proposed turbine to improve its power coefficient and startup performance. Compared with the initial design (Cp = 0.234, Ct,s = 0.520), the Cp and Ct,s in the Pareto front range from 0.103 to 0.325 and 0.565 to 0.712, respectively. All-process simulation was conducted based on Simulink to estimate the power supply capacity of a deep-water in-situ power generation system using the optimal chain-driven hydrokinetic turbine. The result shows that this system can generate the electricity at 218.45 Wh/day in the deep-water location with an average current speed of 0.334 m/s, providing enough energy to the power units of autonomous underwater vehicles.

Insights from detailed numerical investigation of 15 MW offshore semi-submersible wind turbine using aero-hydro-servo-elastic code

Offshore wind turbines are getting larger in terms of dimensions to maximize power generation. Large wind turbine offshore structures can be installed in deep water locations, where they have the advantage to generate higher power due to high wind speeds. To achieve this, wind turbine manufacturers are introducing larger and scaled-up designs to meet the global energy demands. However, these models suffer from large structural loads and deformations when subjected to varying environmental loadings. Therefore, it is necessary to fully comprehend the multibody coupled dynamics of such large offshore structures before proceeding towards their installation in open oceans. This paper carries out an extensive investigation on the ultimate load analysis and fully coupled dynamic response of the 15 MW reference turbine mounted over a UMaine VolturnUS-S semi-submersible floating platform. We used OpenFAST to carry out the stability and full-system linearization studies followed by a detailed time-domain analysis for the power production, parked and fault conditions. We began with a brief background theory on linearization and eigenanalysis for identifying the natural frequencies followed by discussion of the Campbell diagram showing the full-system natural frequencies. It was observed that the multi-blade coordinate code does not correctly pick the natural frequencies. Additional free decay tests have been carried out to verify the full-system natural frequencies. The variation of mean, minimum and maximum values of various characteristic loads and platform motions with respect to different environmental conditions are also presented. We also discussed the change in dynamic response due to the yaw misalignment, wind–wave misalignment, and second-order wave forces. The parked and fault conditions were found to be the critical design load cases (DLCs) for the design of IEA 15 MW wind turbine mounted on Volturn US-S semi-submersible. Furthermore, this paper gives an insight of the important DLCs to be considered while computing the design loads for similar large wind turbine structures.

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.

A truncated, translated Weibull distribution for shallow water sea states

Reliable probabilistic information of the long-term ocean wave climate is important in many coastal and ocean engineering applications such as the design of marine and coastal structures. There is therefore a need to establish accurate models for the long-term distribution of relevant metocean variables. One of the most important wave parameters in marine design is the significant wave height, which is a measure of the severity of the sea state, and wave loads are typically described in terms of this and other variables. Hence, a lot of effort has been put on the proper probabilistic modelling of significant wave height. However, probability distributions proposed to model significant wave height have mostly focused on deep water conditions and less efforts have been put on shallow water conditions. This paper proposes a truncated, translated Weibull distribution for the long-time distribution of significant wave height in shallow waters and demonstrates that the model fit shallow water data quite well. In particular, the proposed model describes shallow-water wave data better than the commonly used 3-parameter Weibull distribution, which is often the preferred distribution for deep water locations.

Floating Offshore Vertical Axis Wind Turbines: Opportunities, Challenges and Way Forward

The offshore wind sector is expanding to deep water locations through floating platforms. This poses challenges to horizontal axis wind turbines (HAWTs) due to the ever growing size of blades and floating support structures. As such, maintaining the structural integrity and reducing the levelised cost of energy (LCoE) of floating HAWTs seems increasingly difficult. An alternative to these challenges could be found in floating offshore vertical axis wind turbines (VAWTs). It is known that VAWTs have certain advantages over HAWTs, and in fact, some small-scale developers have successfully commercialised their onshore prototypes. In contrast, it remains unknown whether VAWTs can offer an advantage for deep water floating offshore wind farms. Therefore, here we present a multi-criteria review of different aspects of VAWTs to address this question. It is found that wind farm power density and reliability could be decisive factors to make VAWTs a feasible alternative for deep water floating arrays. Finally, we propose a way forward based on the findings of this review.

Randomized greedy algorithm for helicopter optimization in the energy industry: a practical approach to model development and solution deliverance

Randomized greedy algorithm for helicopter optimization in the energy industry: a practical approach to model development and solution deliverance

Impact of Geotechnical Uncertainty on the Preliminary Design of Monopiles Supporting Offshore Wind Turbines

AbstractThe growing demand for clean renewable energy sources and the lack of suitable nearshore sites are moving the offshore wind industry toward developing larger wind turbines in deeper water locations further offshore. This is adding significant uncertainty to the geotechnical design of monopiles used as foundations for these systems. Soil testing becomes more challenging, rigid monopile behavior is less certain, and design methods are being applied outside the bounds of the datasets from which they were originally derived. This paper examines the potential impact of certain elements of geotechnical uncertainty on monotonic load–displacement behavior and design system natural frequency of an example monopile-supported offshore wind turbine (OWT). Geotechnical uncertainty is considered in terms of spatial variability in soil properties derived from cone penetration tests (CPT), parameter transformation uncertainty using the rigidity index, and design choice for subgrade reaction modeling. Results suggest that spatial variability in CPT properties exhibits limited impact on design load–displacement characteristics of monopiles as vertical spatial variability tends to be averaged out in the process to develop discrete soil reaction-lateral displacement (p-y) models. This highlights a potential issue whereby localized variations in soil properties may not be captured in certain models. Spatial variability in CPT data has a noticeable effect on predicted system frequency responses of OWTs employing a subgrade reaction model approach, and the influence of subgrade reaction model choice is significant. The purpose of this paper is to investigate the effect of uncertainty in soil data, model transformation, and design model choice on resulting structural behavior for a subset of available design approaches. It should be noted that significant further uncertainty exists and a wide variety of alternative models can be used by designers, so the results should be interpreted qualitatively.

Incorporating stochastic operation and maintenance models into the techno-economic analysis of floating offshore wind farms

Floating offshore wind is rapidly gaining traction in deep water locations. As with all new technologies, to gain the confidence of developers and investors, the technical and economic feasibility of this technology must be proven and robust cost estimates are necessary. In this paper, the authors present a methodology to calculate the capital and operational indicators of a floating wind farm over its project lifetime. A set of computational models is used to reduce the uncertainties in the estimation of the technical and economical parameters. In particular, the effect of using detailed operation and maintenance models and strategies allows a better estimation of operational cost. The paper highlights the requirements and specific adjustments considered for floating offshore wind technology. The methodology is demonstrated for two case studies inspired by real floating wind installations in the United Kingdom, namely the Hywind and Kincardine projects. The related input data, gathered from publicly available sources, constitute a reference database for future studies in the floating offshore wind sector. Results are presented for the two case studies. These show that availability and energy production are in line with typical values for offshore wind projects, and highlight the substantial contribution of operational expenses to the cost of energy. Results are also compared against previous estimations for floating offshore wind projects, showing satisfactory agreement for the overall project costs but an underestimation of operation and maintenance costs in previous studies. This highlights the importance of using detailed operation and maintenance models to adequately capture operational expenses.

Design and Automated Optimization of an Internal Turret Mooring System in the Frequency and Time Domain

A cost-effective mooring system design has been emphasized for traditional offshore industry applications and in the design of floating offshore wind turbines. The industry consensus regarding mooring system design is mainly inhibited by previous project experience. The design of the mooring system also requires a significant number of design cycles. To take aim at these challenges, this paper studies the application of an optimization algorithm to the Floating Production Storage and Offloading (FPSO) mooring system design with an internal turret system at deep-water locations. The goal is to minimize mooring system costs by satisfying constraints, and an objective function is defined as the minimum weight of the mooring system. Anchor loads, a floating body offset and mooring line tensions are defined as constraints. In the process of optimization, the mooring system is analyzed in terms of the frequency domain and time domain, and global and local optimization algorithms are also deployed towards reaching the optimum solution. Three cases are studied with the same initial conditions. The global and local optimization algorithms successfully find a feasible mooring system by reducing the mooring system cost by up to 52%.

Enhanced Estimation of Significant Wave Height with Dual-Polarization Sentinel-1 SAR Imagery

Sentinel-1 synthetic aperture radar (SAR) is one of the most advanced open-access satellite systems available, benefitting from its capability for earth observation under all-weather conditions. In this study, more than 280 Sentinel-1 SAR images are used to derive significant wave heights (Hs) of the sea surface using a polarization-enhanced methodology. Two study areas are selected: one is located near Hawai’i in a deep water region, and the other is in transitional water off the U.S. west coast, where the U.S. National Oceanic and Atmospheric Administration (NOAA) buoy data are available for validations. The enhanced Hs retrieval methodology utilizes dual-polarization SAR image data with strong non-Bragg radar backscattering, resulting in a better estimate of the cut-off wavelength than from those using single-polarization SAR data. The new method to derive Hs is applied to SAR images from 2017 taken from both deep water (near Hawai’i) and coastal water locations (off the U.S. West coast). The assessments of the retrieved Hs from SAR images suggest that the dual-polarization methodology can reduce the estimated Hs RMSE by 24.6% as compared to a single-polarization approach. Long-term reliability of the SAR image-derived Hs products based on the new methodology is also consolidated by large amount of in-situ buoy observations for both the coastal and deep waters.

Company Integrates Offshore Windfarm Into Subsea Field Development

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 30721, “Floating Offshore Windfarm Integrated Into Subsea Field Development: Case Study of the Saipem Windstream Concept,” by Benjamin Mauries, Giorgio Arcangeletti, SPE, and Christophe Colmard, Saipem, et al., prepared for the 2020 Offshore Technology Conference, originally scheduled to be held in Houston, 4-7 May. The paper has not been peer reviewed. Copyright 2020 Offshore Technology Conference. Reproduced by permission. Operators are moving active production and injection equipment onto the seabed with the aim of reducing capital expenditures (CAPEX) or topside space requirements. Moreover, they want to minimize new production floating facilities. Given this scenario, overall electric power needs may become an issue because of the extra power demand caused by the increasing number of electric consumers placed subsea. The complete paper discusses a floating wind-turbine solution that is particularly cost-competitive for deepwater locations and that can unlock the possibility of deploying large wind-powered generators far from the coastline in deep water. Introduction Saipem launched an initiative aimed at finding a solution for management of subsea field power demand bearing in mind two primary considerations: Minimize CAPEX by reducing the distance between the subsea-field production location and the topside equipment supporting this production Decarbonize the field by adopting a renewable energy source Concept Background and Potential Application The operator has developed a floating substructure technology for offshore wind farms known as HexaFloat. This concept uses a minimal floating hexagonal tubular substructure supporting wind-turbine tower and providing necessary floatability. The substructure is connected by tendons to a basket counterweight filled with solid ballast providing stability with pendulum-restoring forces. The assembly of the basket and the substructure behaves as a rigid body if all tendons are loaded. This assembly provides flotation with excellent stability thanks to the distance between the center of gravity and the center of buoyancy. Because this stability is provided by weight, large hydrostatic stiffness is unnecessary. As a result, only the central cylinder is exposed to the wave energy. The whole floating system can be anchored with three to six low-tension mooring lines, depending on environmental conditions.

Anguillosyllis (Annelida: Syllidae) from multiple deep-water locations in thenorthern and southern hemispheres.

Numerous specimens belonging to the syllid genus Anguillosyllis Day were collected during several deep-sea studies conducted between 1970 and 2011 in depths ranging from 180 to nearly 5000 m. These quantitative studies demonstrated that members of this genus are not as uncommon as once thought but can be among the 10 most common infaunal species at some deep-sea locations. Study areas included Georges Bank (New England), the North, Mid, and South Atlantic Continental Slope and Rise (east coast of North America), the Gulf of Mexico (off Louisiana), off San Francisco, California, the Clarion-Clipperton Fracture Zone (Pacific Ocean), the South China Sea (off Brunei), the abyssal plain in the South Atlantic Ocean, and Antarctica (Scotia Sea, Weddell Sea). Sixteen new species are described, bringing the total number of described Anguillosyllis species to 20; one additional species that cannot be fully characterized is also noted. The new species include eight with palps that are entirely or partially free (Anguillosyllis aciculata n. sp., A. acsara, n. sp., A. carolina n. sp., A. denaria n. sp., A. hadra, n. sp., A. hampsoni n. sp., A. taleola n. sp., and A. truebloodi n. sp.) and eight species with completely fused palps (A. andeepia n. sp., A. blakei n. sp., A. bruneiensis n. sp., A. elegantissima n. sp., A. enneapoda n. sp., A. hessleri n. sp., A. inornata n. sp., and A. sepula n. sp.). Type specimens of A. palpata (Hartman) from off Cape Horn and A. pupa (Hartman) from off New England were examined; A. capensis Day from South Africa and A. lanai Barroso, Paiva, Nogueira Fukuda from Brazil are discussed. All adult Anguillosyllis have a fixed number of setigers, either 8, 9, 10, or 11, with 11 setigers being the most common. Species differ in the number of setigers, degree of fusion of the palps, degree of development of parapodial lobes, structure and number of internal and external glands, number of setae in anterior setigers, shape and size of proventricle, and annulation of the dorsum. All species for which the anal cirri had not been lost were found to have four, including two long, thin, ventromedial ones and two shorter, usually oval, lateral ones. Reproduction is poorly known in the genus; the present study shows eggs are found in the posterior (post-proventricle) setigers of most species and enter the parapodia from the coelom of several specimens; egg diameters generally ranged from 50-85 µm with A. sepula n. sp. having eggs up to 120 µm, the largest reported for the genus.

Some probabilistic properties of surf parameter

• Average statistical properties of surf parameters. • Surf parameter for individual waves. • Spectral surf parameter representing a sea state characteristic. • Results using data typical for field conditions are provided. • Example of calculating wave runup elevation. This article is supplementary to Myrhaug (2018) and presents some probabilistic properties of the surf parameter for individual waves and the spectral surf parameter for sea states by using distributions based on data from the Norwegian continental shelf. The average statistical features given by the mean value and the standard deviation of the two surf parameters are considered. Examples of results for the surf parameter are provided for a Phillips spectrum and a family of JONSWAP spectra for wind sea, and for sea states using a joint frequency table of significant wave height and mean zero-crossing wave period for combined wind sea and swell. The spectral surf parameter results are obtained by using a joint distribution of significant wave height and spectral surf parameter, and the mean statistical properties are given for joint frequency tables of significant wave height and mean zero-crossing wave period from three deep water locations on the Norwegian continental shelf. It is also demonstrated how the results can be applied to calculate the vertical wave runup elevation for breaking waves.

Multi-objective optimization of orthogonal TLCDs for reducing fatigue and extreme loads of a floating offshore wind turbine

Floating offshore wind turbines have substantial potential to harness energy in deep-water locations, however, compared to fixed-bottom offshore wind turbines, they also have more complicated dynamic behavior that may lead to larger structural loads. In this paper, structural control using tuned liquid column dampers (TLCDs) is applied to a tension leg platform type floating offshore wind turbine in order to improve the structural response and reliability of the system. A structural control module in the aero-hydro-servo-elastic wind turbine simulator, FASTv8, is developed that enables the analysis of passive orthogonal tuned liquid column dampers with variable cross-sectional areas. Multi-objective optimization using the nondominated sorting genetic algorithm II is solved to provide an optimal Pareto front that can be used to select designs of the orthogonal TLCDs. With the optimal designs, non-linear fully-coupled time marching simulations are conducted to evaluate the impacts of the orthogonal TLCDs on reducing fatigue and extreme loads. The simulation results demonstrate the effectiveness of the orthogonal TLCDs on reducing the fore-aft and side-to-side fatigue and extreme loadings.