Extreme Wave Heights

Results Presentation

One part of the fitting process used to generate the Global Wave Statistics Online data was to fit a three parameter Weibull distribution to the wave height probabilities. This fitted distribution can be used to make predictions of extreme wave heights.

The page is divided into two main parts. The top half of the page shows the results of the last requested analysis. This may be any of the following:

  1. The exceedance probability for a specified wave height. The wave height may be either a significant height, or a maximum individual wave height.

  2. The wave height (either significant or maximum) with a given probability of excedance.

  3. The return period (in years) for a specified wave height. The wave height may be either a significant height, or a maximum individual wave height.

  4. The wave height (either significant or maximum) with a given return period (in years)

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  5. The DnV Characteristic Wave Heights for Marine Operations.  These wave heights are often used as design criteria for temporary marine operations or transportation.

  6. The fitted three parameter Weibull coefficients. This is the default display when the page is first opened.

Analysis Input

The lower half of the page is a form used to enter data for an extreme analysis. The first pulldown menu is used to select the type of analysis required (one of the six types described above),

 The second menu is used to control whether significant wave height, or maximum individual wave height is to be used. This selection is only applicable to the first four analysis options.

Depending upon the analysis option selected, additional data will be required. This may be requested by additional boxes that open on the initial form, or by a second form that is displayed when you click the "Calculate Extreme" button. (Which method is used depends upon which type of browser you are using).

The following information may be requested:

  • Wave Height: This is required when calculating the excedance probability or return period. It may be either a significant height, or a maximum individual wave height, depending upon the option selected. It must be the trough to crest height, and measured in metres.

  • Probability:  This is the required excedance probability to use to calculate a wave height. It must be a value in the range 0.0 - 1.0.

  • Return Period: This is the required return period for which the wave height is to be calculated. The return period must be specified in years.

  • Storm Duration: This is required either when calculating a return period, or when calculating a maximum individual wave height. A duration in hours must be specified (the default value is 3 hours). It is used to determine the number of individual waves in a storm, and the number of independent storms in a given period.

  • Mean Wave Period: This is required when calculating maximum individual wave height. It is used to determine the number of waves that will be experienced during a storm. A mean period in seconds must be specified (the default value is 15s).

  • Duration of Operation: The DnV Characteristic Wave Height is used to provide a design wave height for short period operations such as transportation or installation. It is specified in days, and must be not less than 10 days for inshore operations, or 30 days for offshore operations. A warning will be issued for durations less than 30 days, and durations of less than 10 days will not be accepted. A warning will also be issued if the season of the year selected for the analysis is shorter than the duration of the operational period, normally indicating that the data should be selected for the whole year by using the "Return to Analysis Selection" button.

Once all the required data has been entered, clicking on the "Calculate Extremes" button will refresh the page to display the results of the requested analysis.

Global Wave Statistics Online performs checks on the selected extreme wave height analysis and considers it in the light of the amount of data (number of observations) that have gone to make up the data set. If, according to the empirical criteria from BMT's experience of making such predictions, it finds that the return period is too long, or the probability level too low, then it issues a warning message indicating the anticipated extent of reliability of the data.

A WARNING

The fitting of probability distributions to random data, followed by the use of the fitted coefficients to extrapolate data into the extreme tail of the distribution is a procedure fraught with difficulty and uncertainty.

THE AUTHORS OF THIS PROGRAM CANNOT GUARANTEE THAT SUCH EXTRAPOLATION WILL ALWAYS GIVE REASONABLE RESULTS, AND YOU ARE STRONGLY RECOMMENDED TO SEEK CORROBORATION OF THE RESULT WITH OTHER DATA.

Navigation Controls

The "Return to Analysis Selection" button at the bottom of the page will take you back to the page for selecting a new season or directional sector.

The tabs in the menu bar can be used to move through the various levels.

  • The four right hand tabs will switch between the different analyses for the current data set.

  • The "Season & Direction" tab will take you back to the page to select these parameters.

  • The "Location" tab will take you back to the location selection page.