Wp 2 Extreme Wave Statistics from Time-series Data
نویسندگان
چکیده
This work is a representation of wave time-series data used in the Maxwave project, with sample time series and statistics of wave height, crest height, trough depth. Data are included from the Ekofisk and Draupner oil fields, and from Belgian coastal waters. In addition, extreme value analysis, and also studies of corrected and evolved time series was performed. INTRODUCTION This report is prepared within the MAXWAVE Work Package 2 (WP2), which is to provide a data set on extreme wave statistics from in situ data (e.g. wave buoys, laser, and vertically-pointing microwave instruments), in order to analyse casualties, review design criteria used in the shipping and offshore industries, and to facilitate decisions regarding the safety of operations and personnel. Time series from fixed moorings can underestimate the heights of extreme waves occurring in the vicinity, and the quasi-Lagrangian motions of wave buoys can underestimate the heights of the highest crests. In this work package, time series from various locations, in particular the Ekofisk field in the North Sea, are being analysed to derive analytical formulae for these underestimates. The parameters to be extracted from the wave data include ratios of maximum to significant wave height and wave height to peak period, the distribution of extreme wave crest height and trough depth. This paper is structured as follows: Section two deals with the description of the data series analysed within the WP2. Section three describes the sensors used in this work. Section four explains the analysis techniques used in WP2. Section five shows some of the achieved results. Finally, the obtained conclusions appear in section six. DATA SERIES ANALYSED Ekofisk This data set has been collected by Phillips Petroleum from the Ekofisk field in the North Sea. Observations have been stored in the environmental database at DNMI since 1980, and comprise wave profiles sampled at 2 Hz and other wave parameters calculated from 20-minutes time series. They were stored at 3-hourly intervals until 1985, and in the period 1985-1995 the sampling was also made continuous in storm conditions (wind speed above 20 m/s or significant wave height above 4 m). Since 1995 the wave profiles from all 3 profiling instruments are stored continuously at the sampling rate of 2 Hz. The instruments comprise one Datawell Waverider buoy and two vertically pointing lasers. The position of the Waverider buoy relative to the platform complex has varied, depending upon ship traffic in the area. The impact of the structure on the waves measured at the buoy has been neglected. Two downward-pointing Optech radars have been deployed on bridges near Flare North and Flare South in the platform complex since 1991. The measurements are considered to be point measurements (the diameter of the area at the water surface from which the signals are reflected is negligible). The Waverider buoy has an eigenfrequency of 30 seconds. Draupner Three time series of water surface displacement at the Draupner oil field, sampled at 2.1333 Hz using a downward-pointing laser instrument, were kindly provided by Statoil. 2048 samples were provided from a storm on 1995 January 1 at each of the following times: 15:20 UTC, 16:20 UTC, and 23:00 UTC. Belgian locations The Belgian coastal zone is characterised by the presence of large sand bars, the Flemish banks. Tide is considerable with an amplitude of 4 to 5 m. Also tidal currents are relatively strong. They are mainly directed parallel to the shore and their magnitude is in the order of 1 m/s. The Coastal Service of the Ministry of the Flemish community operates a hydro-meteo service. Amongst other data, wave data are recorded for immediate use, but also stored for further analysis such as wind and wave statistics. Besides a number of Waverider buoys, also two directional buoys are in operation. One directional buoy is located at Westhinder, where the approximate water depth is 30 m relatively far off-shore. The other one is located close to the harbour of Zeebrugge at location Bol van Heist, where the approximate water depth is 10 m. Figure 1 gives the bathymetry of the area and the location of the buoys. The system has been in operation for about 20 years. At that time data-storage was of major concern and not all sampled data are stored. Besides the oneor two-dimensional spectrum, only a limited number of parameters, such as significant wave height, and peak period are stored. For the Waverider buoys, also the maximum wave height in the 15 minute record is stored. It is remarkable that many records have a maximum wave height which is larger than twice the significant wave height. Why this is the case, is not clear at this moment. Possible mechanisms could be shallow water effects and interaction with currents. Fortunately there are 10 Hz time series from Waverider buoys close to the Zeebrugge harbour. These measurements were and are taken in the framework of the prototype monitoring program of the rubblemound breakwater at Zeebrugge by the University of Gent. Some of the data have been provided for further analysis in the framework of the Maxwave project. A typical record is shown in Figure 2. Figure 3 shows the largest wave in that record. It is intended to screen these data for extreme waves. The data provided by the hydro-meteo system, will then allow to identify other parameters, such as the existence of different wave systems, ebb or flood conditions, the stage of the storm, etc. MEASUREMENT TECHNIQUES Buoys The Waverider buoy determines the vertical sea surface displacement by integrating acceleration measurements. The buoy moves horizontally as well as vertically, so it cannot give a precise value of the vertical surface displacement at a fixed horizontal position. Laser instruments The laser instruments determine the vertical position of the sea surface by measuring the time from when a light pulse is emitted to when it is detected after Figure 1. Map of Flemish banks with indication of buoy stations. Figure 2. Typical from the Zeebrugge prototype
منابع مشابه
Modelling extreme values of processes observed at irregular time step . Application to significant wave height
The distribution of extremes such as flood peaks, maximum wave height or minimum daily returns over annual or other time intervals is of common interest to many disciplines including the natural and social sciences. This work is motivated by the analysis of extreme values from times series of significant wave heights observed in North Atlantic. One of these time series exhibits missing data (bu...
متن کاملReliability and robustness of rainfall compound distribution model based on weather pattern sub-sampling
A new probabilistic model for daily rainfall, named MEWP (Multi Exponential Weather Pattern) distribution, has been introduced in Garavaglia et al. (2010). This model provides estimates of extreme rainfall quantiles using a mixture of exponential distributions. Each exponential distribution applies to a specific sub-sample of rainfall observations, corresponding to one of eight typical atmosphe...
متن کاملThe wind and wave atlas of the Mediterranean Sea – the calibration phase
Within the WW-Medatlas project, sponsored by the Italian, French and Greek Navies, an extensive atlas of the wind and wave conditions in the Mediterranean Sea has been completed. The atlas is based on the information derived from the archive of the European Centre for MediumRange Weather Forecasts, UK, then calibrated on the base of the data available from the ERS1-2 and Topex satellites. The c...
متن کاملEstimation of storm peak and intra-storm directional-seasonal design conditions in the North Sea
Specification of realistic environmental design conditions for marine structures is of fundamental importance to their reliability over time. Design conditions for extreme waves and storm severities are typically estimated by extreme value analysis of time series of measured or hindcast significant wave height, HS. This analysis is complicated by two effects. Firstly, HS exhibits temporal depen...
متن کاملCharacterizing Dangerous Waves for Ocean Wave Energy Converter Survivability
Ocean Wave Energy Converters (OWECs) operating on the water surface are subject to storms and other extreme events. In particular, high and steep waves, especially breaking waves, are likely the most dangerous to OWECs. A method for quantifying the breaking severity of waves is presented and applied to wave data from Coastal Data Information Program station 139. The data are wave height and len...
متن کامل