Uncertainties in the design of support structures and foundations for offshore wind turbines

Offshore wind industry has exponentially grown in the last years. Despite this growth, there are still many uncertainties in this field. This paper analyzes some current uncertainties in the offshore wind market, with the aim of going one step further in the development of this sector. To do this, some already identified uncertainties compromising offshore wind farm structural design have been identified and described in the paper. Examples of these identified uncertainties are the design of the transition piece and the difficulties for the soil properties characterization. Furthermore, this paper deals with other uncertainties not identified yet due to the limited experience in the sector. To do that, current and most used offshore wind standards and recommendations related to the design of foundation and support structures (IEC 61400-1, 2005; IEC 61400-3, 2009; DNV-OS-J101, Design of Offshore Wind Turbine, 2013 and Rules and Guidelines Germanischer Lloyd, WindEnergie, 2005) have been analyzed. These new identified uncertainties are related to the lifetime and return period, loads combination, scour phenomenon and its protection, Morison e Froude Krilov and diffraction regimes, wave theory, different scale and liquefaction. In fact, there are a lot of improvements to make in this field. Some of them are mentioned in this paper, but the future experience in the matter will make it possible to detect more issues to be solved and improved

Establishing storm thresholds for the spanish gulf of Cádiz coast

In this study critical thresholds are defined for storm impacts along the Spanish coast of the Gulf of Cádiz. The thresholds correspond to the minimum wave and tide conditions necessary to produce significant morphological changes on beaches and dunes and/or damage on coastal infrastructure or human occupation. Threshold definition was performed by computing theoretical sea-level variations during storms and comparing them with the topography of the study area and the location of infrastructure at a local level. Specifically, the elevations of the berm, the dune foot and the entrance of existing washovers were selected as threshold parameters. The total sea-level variation generated by a storm event was estimated as the sum of the tidal level, the wind-induced setup, the barometric setup and the wave-associated sea-level variation (wave setup and runup), assuming a minimum interaction between the different processes. These components were calculated on the basis of parameterisations for significant wave height (Hs) obtained for the oceanographic and environmental conditions of the Gulf of Cadiz. For this purpose real data and reanalysis time-series (HIPOCAS project) were used. Validation of the obtained results was performed for a range of coastal settings over the study area. The obtained thresholds for beach morphological changes in spring tide conditions range between a significant wave height of 1.5 m and 3.7 m depending on beach characteristics, while for dune foot erosion are around 3.3 to 3.7 m and for damage to infrastructure around 7.2 m. In case of neap tide conditions these values are increased on average by 50% over the areas with large tidal range. Furthermore, records of real damage in coastal infrastructure caused by storms were collected at a regional level from newspapers and other bibliographic sources and compared with the hydrodynamic conditions that caused the damage. These were extracted from the hindcast database of the HIPOCAS project, including parameters such as storm duration, mean and maximum wave height and wave direction. Results show that the duration of the storm is not critical in determining the occurrence of coastal damage in the regional study area. This way, the threshold would be defined as a duration ≥30 hours, with moderate average wave height (≥3.3 m) and high maximum wave height (≥4.1 m) approaching from the 3rd and 4th quadrants, during mean or spring tide situation. The calculated thresholds constitute snapshots of risk conditions within a certain time framework. Beach and nearshore zones are extremely dynamic, and also the characteristics of occupation on the coast change over time, so critical storm thresholds will change accordingly and therefore will need to be updated

Automatic extraction of shorelines from Landsat TM and ETM+ multi-temporal images with subpixel precision

A high precision geometric method for automated shoreline detection from Landsat TM and ETM+ imagery is presented. The methodology is based on the application of an algorithm that ensures accurate image geometric registration and the use of a new algorithm for sub-pixel shoreline extraction, both at the sub-pixel level. The analysis of the initial errors shows the influence that differences in reflectance of land cover types have over shoreline detection, allowing us to create a model to substantially reduce these errors. Three correction models were defined according to the type of gain used in the acquisition of the original Landsat images. Error assessment tests were applied on three artificially stabilised coastal segments that have a constant and well-defined land-water boundary. A testing set of 45 images (28 TM, 10 ETM high-gain and 7 ETM low-gain) was used. The mean error obtained in shoreline location ranges from 1.22 to 1.63. m, and the RMSE from 4.69 to 5.47. m. Since the errors follow a normal distribution, then the maximum error at a given probability can be estimated. The results confirm that the use of Landsat imagery for detection of instantaneous coastlines yields accuracy comparable to high-resolution techniques, showing the potential of Landsat TM and ETM images in those applications where the instantaneous lines are a good geomorphological descriptor. © 2012 Elsevier Inc.The authors appreciate the financial support provided by the Spanish Ministerio de Ciencia e Innovacion and the Spanish Plan E in the framework of the Projects CGL2009-14220-C02-01 and CGL2010-19591.Pardo Pascual, JE.; Almonacid Caballer, J.; Ruiz Fernández, LÁ.; Palomar-Vázquez, J. (2012). Automatic extraction of shorelines from Landsat TM and ETM+ multi-temporal images with subpixel precision. Remote Sensing of Environment. 123:1-11. doi:10.1016/j.rse.2012.02.024S11112

Significant wave height estimation from nautical radar data sets

Nautical radars scan the water surface at grazing incidence with HH polarization. Under these conditions the backscatter is mainly due to multipath scattering effects on small scale breaking waves. The pattern of returned electromagnetic power is modulated by the larger oceanic structures, such as swell and wind sea waves. The final pattern shown on the radar screen is commonly known for navigation purposes as sea clutter. Unlike other remote sensing systems, as Synthetic Aperture Radars (SAR) on board satellites or airplanes, navigation radar images cover smaller areas, but this sensor is able to obtain short-term temporal information about sea states using consecutive rotations of the antenna. Hence, the spatial and temporal structure of sea states can be known. Each measurement consists of a temporal sequence of consecutive radar images, where the sampling rates in space and time depend on each marine radar features. The temporal resolution sampling rate is given by the antenna revolution period and the spatial resolution is related to the effective antenna aperture and the radar pulse length. The nautical radar data are directly sampled from the radar video signal by using a Wave Monitoring System (WaMoS II) developed by GKSS research centre. This work deals with an improved method to estimate the significant wave height from sea clutter image time series. This method is based on similar techniques developed for SAR systems. The basic idea is the significant wave height is linearly dependent on the root square of the signal-noise ratio, where the signal is assumed as the radar analysis estimation of the wave spectral energy and the noise is computed as the energy due to the sea surface roughness. (orig.)Available from TIB Hannover: RA 3251(98/E/28) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

The European coupled atmosphere wave ocean model ECAWOM

The European coupled atmosphere wave ocean model ECAWOM and its application to a North Sea storm are described. The model was integrated for the 1 month period of February 1993. Preliminary analysis of the results suggest that the model is capable of reproducing the basic features of the atmospheric, the wave, and the surge characteristics for this 1 month period. However, further analysis and integrations are needed to fully assess the skill of the developed model, its potential for long term integrations, and the impact of sea state dependent air-sea fluxes on the atmospheric and the oceanic circulation. (orig.)Available from TIB Hannover: RR 1347(238) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman