U disertaciji su obrađeni spektri valova, učestalosti pojavljivanja različitih stanja mora te njihove ekstremne vrijednosti za akvatorij Jadranskog mora. Poznavanje ovih podataka je od presudne važnosti za određivanje pomorstvenosti plovnih objekata te za analizu sigurnosti različitih objekata morske tehnike u Jadranu. Podaci dugogodišnje baze podataka, ostvarene kombinacijom numeričkih rezultata i mjerenja satelitskom altimetrijom, statistički su analizirani s ciljem određivanja učestalosti pojavljivanja, međusobne ovisnosti glavnih parametara vjetra i valova, te izvedenih ekstremnih vrijednosti za povratna razdoblja od 10, 20, 50 i 100 godina. Analizirane su nesigurnosti rezultata obzirom na odabir teorijskih distribucija vjerojatnosti i metoda njihove prilagodbe empirijskim podacima. Izrađen je novi, detaljan, atlas valova Jadranskog mora, prikazan u obliku parametara združene Weibullove razdiobe značajnih valnih visina i logaritamsko-normalne razdiobe valnih perioda. Predložena je poboljšana jednoparametarska spektralna formulacija valova Jadranskog mora, dobivena optimiranjem parametara JONSWAP spektra. Praktični primjer primjene rezultata prikazan je na proračunu graničnih valnih visina za putnički brod.
|Abstract (english)|| |
The doctoral thesis was aimed at producing an improved spectral and statistical analysis of wind-waves in the Adriatic Sea in a form applicable for typical seakeeeping and maritime transport applications. The analyses were done primarily based on a long-term WorldWaves database of wind and wave parameters which constitutes of 22.5 years of numerical wave model re-analysis (by the WAM wave model run at the European Center for Medium range Weather Forecasting) calibrated with altimetry measurements from available satellite missions. Given that the database combines the advantages of both numerical modelling and satellite measurements, it is considered the best currently available data source for systematic offshore wave data in the Adriatic Sea.
In the introductory part an overview of wave climate features is given, and the main data acquisition techniques as well as the available data sources are listed. Based on these considerations the later analyses were conducted for 39 locations specifically, regionally for north, central and south Adriatic, for the Adriatic basin as a whole, and where appropriate, separately for waves generated by bora and sirocco winds.
Long-term statistical analysis performed included the calculation of joint significant wave height and wave peak period, and joint significant wave height and wind speed distributions. In line with the standard recommended procedures, the three-parameter-Weibull and log-normal theoretical distribution were fitted to WorldWavess data for the marginal distribution of significant wave height and conditional distribution of wave peak period, while the two-parameter-Weibull distribution was used for conditional wind speed distribution. The distribution parameters are presented in a tabular form for each location separately, regionally and for the entire wave basin. Such data, representing original contribution of the present thesis, can be used as an input for mooring and fatigue analysis for offshore structures. Based on the developed theoretical distributions, extreme values of significant wave height were estimated for long return periods (up to 100 year). The extreme value can be applied for limit state calculations. Simple quadratic wind-speed-to-wave-height relation were also derived in order to allow fast estimates of wave heights for sirocco and bora winds. And finally, the database data was analysed for visible climate trends. Although a yearly increase of half a centimetre of average significant wave height has been found, the number of extreme storm
events has varied largely through the years and firm conclusions about influence of global warming on frequency and magnitude of extreme sea states have not been reached.
In addition to making long-term statistical analysis, an uncertainty analysis of the long-term prediction methods was also performed. The methods vary in selection of the underlying data to be used from the analysis, the respective choice of the candidate distribution and the mathematical fitting method of the distribution parameters. The following methods were compared: the initial distribution, the annual maximum and the peak-over-threshold method with their respective candidate distributions (3-parameter-Weibull, Gumbel and Exponential) and the mathematical fitting techniques: the least mean square (LSM), method of moments (MoM) and the maximum likelihood estimate (MLE). The cumbersomeness of the MoM for three parameter fitting and the inapplicability of MLE were observed and commented. As for the choice of the method, the peak-over-threshold showed the best overall behaviour. Depending on the choice of the method and fitting technique, significant variations in the results were observed.
Another scientific contribution of the work is the improved spectral description of short-term sea states based on the World Waves database. The Tabain spectrum was used as a reference because it is generally accepted for marine technology problems in the Adriatic Sea. It was found that the existing Tabain relation for modal frequency dependence on significant wave height can be improved, but that the general JONSWAP formulation provides better results for definition of spectrum. Moreover, unlike the Tabain spectrum, the latter is a standard wave input definition form in most of the modern seakeeping and CFD tools. The numerical constants were optimized and a single parameter JONSWAP spectrum form was developed specifically for the Adriatic Sea, named JONSWAP-Adriatic wave spectrum. The parameters of the spectrum are given for the entire basin and regionally and also separately for bora and sirocco winds.
Finally, an application example was given of the newly derived wave spectrum based on a seakeeping limit criteria analysis of a liner passenger ship in the Adriatic. The problem was set up to show the methodology of an estimate of how much time the ship would not be able to operate at full speed based on a chosen limit criteria.