Finnish Maritime Administration WAVE STATISTICS FROM THE NORTHERN BALTIC SEA FOR PASSENGER SHIP BOW STRUCTURAL DESIGN SURVEY Kimmo Kahma, Heidi Pettersson and Laura Tuomi Finnish Institute of Marine Research January 5, 2000 18.10.1998 18:00 m 5.6 5.4 5.2 5.0 65 4.8 4,6 4.4 4.2 4.0 3.8 3.6 3.4 3.2 3.0 2.8 2.6 60' 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 55 1 \ 5 Contents 4 8451 1 Introduction 2 Wave climate in the Northern Baltic Seä3." 4 2.1 General .................................. 2.2 Treatment of missing values and the ice period .............5 2.3 Overview of wave climate in the Northern Baltic Sea .........5 3 Geographical distribution of wave heights along the route Helsinki - Stockholm 8 / 4 Measured wave statistics in the Northern Baltic Proper 9 4.1 Measured wave statistics from the Northern Baltic Proper wave buoy 9 4.2 Measured wave statistics near Bogskär ................. 13 5 Extrapolated wave statistics 18 5.1 Method of extrapolation .........................18 6 Estimated wave conditions in the Åland Sea 23 7 Duration of high wave conditions 24 8 Operational wave measurements and forecasts 25 List of Figures On the cover: Wave height and direction on the Baltic Sea on 18 October 1998 18:00 UTC. WAM-wave model hincast 1 The locations and measuring periods of the Finnish Institute of Ma- rine Research wave measurements..................... 7 2 The reduction of wave height by spreading and refraction for waves coming into the Gulf of Finland from the Northern Baltic Proper. Incident wave height is assumed to be 4m, the period 8s, and the direction 260°............................... 8 3 Significant wave height in October 1998 at the Northern Baltic Proper permanent wave buoy. Measurements and model hindcast.......24 4 Probabilities of wave conditions in different months. Wave data from Bogskär 1982 - 1985............................2-1 5 Probabilities of highest wave conditions in different months. Wave data from Bogskär 1982 - 1985......................25 List of Tables 1 The highest measured significant wave heights..............6 2 The most probable values of the maximum individual wave height at the measuring point during the wave measurements...........6 3 H8 exceeded 10% of the ice-free time .................. 8 4 Wave data from Northern Baltic Proper 59° 15.0 N 21°00.0 E. Septem- ber 1996 - September 1999. All directions................10 5 Wave data from Northern Baltic Proper 59° 15.0 N 21°00.0 E. Missing values predicted by seasonal distributions. September 1996 - Septem- ber 1999. All directions..........................11 6 Wave data from Northern Baltic Proper 59°15.0 N 21°00.0 E. Ex- ceedence probabilities. Missing values predicted by seasonal distribu- tions September 1996 - September 1999. All directions.........12 7 Wave data from Bogskär 59° 28.0 N 20°21.0 E 1982 —1986. All directions. 15 8 Wave data from Bogskär 59°28.0 N 20°21.0 E 1982 - 1986. Missing values predicted by seasonal distributions. Unit: hours per 3 years. All directions................................ 16 9 Wave data from Bogskär 59°28.0 N 20°21.0 E 1982 - 1986. Excee- dence probabilities. Missing values predicted by seasonal distribu- tions. Unit: hours per 3 years. All directions..............17 10 Wave data from Bogskär 59°28.0 N 20°21.0 E 1982 - 1986. Missing values predicted by seasonal distributions without taking into account model calculations. Unit: hours per 3 years. All directions.......19 11 Extrapolated scatter diagram, upper-limit extrapolation. Unit: hours per 3 years. All directions......................... 20 12 Extrapolated scatter diagram, upper-limit extrapolation. Unit: parts per million (ppm) All directions...................... 21 13 Extrapolated scatter diagram, upper-limit extrapolation. Unit: hours per 100 years. All directions........................22 14 Estimates of significant wave height in the Åland Sea and Bogskär . 23 3 1 Introduction The Finnish Maritime Administration has ordered from VTT Manufacturing Tech- nology a study "Passenger ship bow structural design agaist wave induced loads at the Norther Baltic". The Finnish Institute of Marine Research (FIMR) as a subcontractor calculated the wave statistics for the study. During the past three years FIMR has operated a permanent wave buoy in the Northern Baltic Proper. This new measured data, as well as wave model calcula- tions have been used to obtain more accurate statistics than have been available in the past. Previous measurements by FIMR and the Swedish Meteorological and Hydrological Institute (SMHI) during 1982... 1986 have been included in the analysis. The wind conditions and consequently wave conditions vary significantly from year to year. The existing measurements in the Northern Baltic Proper cover 8 years, but the earlier data has considerable gaps. The data still leave considerable uncertainties in the deduced statistics of extreme wave conditions. The aim here is to estimate an upper limit that is unlikely to be found too low when more data becomes available from future measurements. This conservative approach turned out to be well justified when in December 1999 significant wave height 7.4 m was measured during two different storms in the North- ern Baltic Proper. Such wave conditions have been measured only once before in the Northern Baltic Proper. Model calculations suggest that in 39 years such wave conditions have occured only four times, including the three measured events. Data after September 1999 will not be ready for statistical analysis before this report is due. However, the influence of these December storms on the probabilities of extremes have been analysed and it was found that they do not change the upper limit extrapolation. This extrapolation is the basis of the conclusions of this study. On the other hand, the most probable extrapolation that was also presented in the draft will change, and it has been removed from this final report. 2 Wave climate in the Northern Baltic Sea 2.1 General The wave climates in various parts of the Baltic Sea differ greatly. In the Northern Baltic Proper the typical wave height is roughly half of that in the North Atlantic, whereas in the Gulf of Finland the wave height is limited to roughly one half of the Northern Baltic Proper. The wave climate in the Bothnian Sea is slightly less severe than in the Northern Baltic Proper. Besides having a less severe wave climate, the Aland Sea, the Gulf of Finland, and the Quark have directionally strongly skewed statistical distributions. In other words, large waves can arrive only from certain directions, and waves tend to arrive from those directions even when the direction of the wind is different. This means that the loads for vessels that are dependent on wave direction may vary even more than the non-directional wave statistics would indicate. 4 2.2 Treatment of missing values and the ice period It is very difficult to make uninterrupted wave measurements in the open sea. In- strument or transmission failures and ice are the most important reasons for gaps in the record. Ice introduces a special problem in wave statistics, because the period of missing values caused by ice is by no means random. Especially in the Bothnian Bay the long ice season influences the wave climate. The Bay is at least partly frozen during the season when the wind climate is most severe. Annual statistics and maximum wave height give a distorted - too low - view of the wave climate of the ice-free season. When the sea there is ice-free the waves are higher than they are in an area that has the same annual wave statistics but never has ice. This applies to a smaller extent also to the other areas of the Baltic Sea, and therefore has to be taken into account to avoid inconsistencies in comparisons. There are at least four different ways to analyse wave climate in areas where there is ice during part of the year: A. Direct statistics. Only measured data is taken into account. No corrections are made to com- pensate for the uneven distribution of missing values. B. Formal statistics Wave height in the presence of ice equals zero. Missing values in other seasons are estimated from the seasonal distributions. C. Ice-free season statistics Only the part of the year when the sea would be ice-free in an average winter is taken into account when statistics are calculated. Missing values during the ice-free season are estimated from the seasonal distributions. D. Hypothetical "no ice" statistics Using wave models and wave measurements from winters when the ice period is shorter than normal, hypothetical statistics are calculated to represent the wave climate under the assumption that the sea remains ice-free throughout the year. Clearly, a different method will be the best one for different purposes. In this report we will present data using methods A and C. Note that when time, rather than a percentage of time, is used in a table, statistics of type B are identical with statistics of type C, except for classes that include zero wave height. 2.3 Overview of wave climate in the Northern Baltic Sea The Finnish Institute of Marine Research (FIMR) has made wave measurements in the Baltic Sea from 1972. The Swedish Meteorological and Hydrological Institute (SMHI) has measured waves from 1979 at Almagrundet and at some other locations, 5 all South from Almagrudet. During the years 1983 - 1986 measurements were carried out at Bogskär as a joint project by FIMR and SMHI. The periods aid the locations of the wave measurements by FIMR are shown in Figure 1. Table 1: The highest measured significant wave heights.