Sea State Module

Generate sea state and extremal inputs to Metocean engineering and design studies

 

The Sea State Module is designed to generate probability and extremal statistics for use as Metocean design criteria in engineering studies. The user can select from a variety of output formats, including scatter diagrams, joint occurrence tables, and extremes including recurrence interval (return period) tables and graphics.  The user has full control to select directional bins, required output fields, and the desired output interval.  Output display options include plots, EXCEL tables and ASCII listings of sea state quantities.

 

The integration of Sea State Module is depicted in the following Figure. If probability and or extremal statistics are desired for individual sea and swell components, then the user should first run the Wave Systems Module on observed or modeled wave spectra data.  This produces a set of wave partitions for the input data set.  An optional next step is to run the Spectral Fit Module to compute standard parametric fits (i.e., JONSWAP, Gaussian, etc.) to the wave partition domains.  The partition fit results are saved along with the wave partitions data, resulting in two different representations of the sea conditions (observed and fitted).  The Sea State Module uses these results to produce a specified description of the sea conditions represented by the data.  The diagram below depicts the various data processing options.

 

 

 

Integration of Sea State Module

 

 

Extremal Analysis

Included in the Sea State Module is an extreme value analysis for wave height and wind speed using a custom Peaks-over-Threshold (PoT) routine and selected extreme analysis tools from WAFO (Wave Analysis for Fatigue and Oceanography), a MATLAB toolbox with routines for statistical analysis of random waves and random loads.

 

There are generally two accepted procedures in performing an extreme value analysis, the block maxima method and the PoT method. Both methods are used in the analysis.

 

Below is a step by step explanation of the methodology used to determine the extreme values.

 

1. The wave height and wind speed time series data was divided into 1 year blocks.

2. The annual maximum was extracted for each block.

3. Parameter estimation for the GEV, Gumbel, and Weibull distributions are computed using the Maximum Likelihood (ML) method.

4. For PoT analysis wave heights and wind speeds above the user defined threshold are extracted.

5. Parameter estimation for the GPD (General Pareto Distribution) are calculated using the Maximum Product of Spacings method.

6. Return values were calculated for each distribution for 5, 10, 25, 50, 100, and 500-yr return periods.

7. If selected by the user, 95% confidence intervals are calculated for each distribution using the profile log likelihood method.

 

For directional analysis, the data was binned by mean wave/wind direction into sectors defined by the user in the Sea State Parameters Table and steps 1-7 above were performed on each bin. The analysis is only performed if at least 1% of the dataset falls within the bin.

 

 

Using the Sea State Module

The Sea State Module is activated by selecting Sea State from the Module pull-down menu.

 

NOTE:  Wave Systems need to be run with the Extract Windsea option in order for the Sea State Module to compute wind-sea statistics.

 

The user is also given the option to run the Spectral Fit Module if desired.  Spectral Fit is only required if the user desires to have the Sea State results computed from fitted data.

 

Once the required Modules have been run the Sea State Parameters Table opens to allow the user to specify how the data are to be processed.  Here the user can specify the type of data to use (input data or fitted), and various properties of the output.

 

Processing commences once the user selects RUN on the Sea State Parameters Table.  Sea State will use Wave Systems data if available and, if selected, Spectral Fit data to set up a Sea History Table that provides a time series of sea state specification data. A variety of statistical joint occurrence tables and climatology plots are then computed from these data.  When Sea State processing is finished a special sea state layout page in Data View opens to provide the user access to all Sea State Module products.  Each of these products is described below.

 

Sea State Tables

The Sea State statistical tables are accessed from the Sea State options in the Table pull-down menu in Data View.  For each table type the view options are:

 

  • View Table – A separate text editor window opens to view table contents
  • Export to ASCII File – A text file is saved to disk with table contents (user provides filename and location)
  • Export to Microsoft Excel File – An Excel file is saved to disk with the table contents (user provides filename and location)  The Excel file automatically opens for user inspection.

 

The various table types are listed and described below.

 

Sea State Tables

Table

Description

Joint Occurrence

Joint occurrence (%) of wind sea and swell systems

Hs-Tp Scatter

System occurrence (%) over Hs and Tp bins

Hs-Tp-Dm Scatter

System occurrence (%) over Hs and Tp bins in directional sectors

Hs-Dm Scatter

System occurrence (%) over Hs and Dm bins

Tp-Dm Scatter

System occurrence (%) over Tp and Dm bins

 

 

Joint Occurrence Table

Based on information in the Sea History Table, the Sea State Module will produce a Joint Occurrence Table for windsea and existing swell systems.  As the example shown below depicts, this table provides the occurrence frequency (%) of both solitary and joint system events.  Calms are also counted. This table is only available if Wave Systems processing has been performed on the data.

 

 

Example Joint Occurrence Table (%)

Conditions

Primary Swell

Primary and Secondary Swell

No Swell

Total

Wind Sea

22.3

38.6

14.6

75.5

No Wind Sea

12.4

12.1

0

24.5

Total

34.7

50.7

14.6

100

 

 

Scatter Tables

Scatter Tables summarize the sea state history by depicting the distribution of wave system occurrence (%) over the domains of significant wave height (Hs), peak period (Tp) and mean direction (Dm), if available.  Scatter Tables are computed separately for each wave system.  For example, if there are three wave systems in the Sea State Table, three sets of scatter tables will be produced.  Scatter Tables are normalized by one of the following approaches as selected in the Sea State Parameters Table:

 

  • Component – Scatter Tables provide occurrence percentages relative to individual wave components (Windsea, swell1, etc.).  This is only available if Wave Systems processing has been performed on the data.
  • Total – Scatter Tables provide occurrence percentages relative to all components present.  Calm conditions are included in the computation.
  • None – Scatter Tables provide the number of observations

 

An example Wind Sea Hs-Tp Scatter Table appears in below.  In this case the ‘Component’ normalization was selected.  Note that occurrences for each system are presented relative to the total occurrence of that system, provided at the top of the table.  All occurrences include a counting of calm (system absent) time periods.  The Hs and Tp bin widths and Dm directional sectors are all user-specified in the Sea State Parameters Table.

 

Example Wind Sea Hs-Tp Scatter Table

Wind Sea Occurrence: 52 %

Tp (s)

Hs (m)

3-4

4-5

5-6

6-7

7-8

8-9

9-10

10-11

11-12

12-13

Total

0-0.5

3.4

7.3

8.6

9.6

5.9

2.1

1.2

0.1

 

 

38.2

0.5-1.0

0.8

9.4

12.1

12.5

11.3

6.5

3.8

0.6

0.04

0.02

57.2

1.0-1.5

 

0.2

0.5

0.7

0.7

1.3

0.7

0.2

0.04

0.02

4.5

1.5-2.0

 

 

 

 

 

 

0.1

0.2

 

 

0.1

Total

4.2

16.9

21.3

22.8

17.9

9.9

5.7

1.1

0.08

0.04

100

 

Extreme Value Analysis Tables

The Extreme Value Analysis tables are accessed from the Extremes options in the Table pull-down menu in Data View.  There are two different types of output tables:

 

Peaks Table

A listing of the extracted wind speed and wave height peaks sorted in descending order, giving the date, time, and magnitude of each.

 

Recurrence Interval Table

The recurrence interval tables provide the 5, 10, 25, 50, 100, and 500-yr wave height and wind speed return values for each distribution. Tables are also generated for each directional bin and the lower and upper confidence bounds are provided if available.

 

Sea State Plots

Sea State plots are displayed in a special Data View layout page. See the Plot Library for a description of available plots.

 

REFERENCES

Brodtkorb, P.A., Johannesson, P., Lindgren, G., Rychlik, I., Ryden, J. and Sjo, E. (2000). "WAFO - a Matlab toolbox for analysis of random waves and loads", Proceedings of the 10th International Offshore and Polar Engineering conference, Seattle, Vol III, pp. 343-350.

 

 

Adjustable parameters in the Sea State Parameters Table allow the user to select processing options for the Sea State Module.

 

 

 

The Sea State Parameters Table is divided into 3 main sections:

 

1. Commands (Left column of buttons)

2. Parameters (center and right column of inputs)

3. Interactive Display (bottom center and bottom right)

 

The features in each of these sections are described below.

 

1.  Commands

The commands appearing on the left side of the Sea State Parameters Table control the saving and use of selected parameters.  The following commands and options are available:

 

Named Settings – Select from a list of pre-existing parameter settings.  Parameter values will automatically update.  To see the results of these new parameters in the plot, click Update Plot.

 

Update – Update the displayed parameters to the current named parameter set.

 

Save New – Save the displayed parameters to a new named parameter set.  User will be prompted to provide a name for the new parameter set.

 

Reset – Reset displayed parameters to saved values

 

Delete – Delete the current named parameter set

 

Run – Process the data set using the current named parameter set

 

Cancel – Exit the Sea State Parameter Table without processing any data

 

2.  Parameters

The processing parameters allow the user to customize the resulting tables and plots.

Scatter Tables

 

These parameters control the contents of the Scatter Tables that are generated by the Sea State Module:

 

Scatter table data to use – Select data to use for generating the scatter tables.  Options are Data (input data) or Fit (fitted data). The Spectral Fit Module must be run to use fit data in the tables.

 

Scatter table normalization – Select how you would like the Scatter Table data normalized:

 

  • Total – Scatter Tables provide occurrence percentages relative to all components present
  • None – Scatter Tables provide the number of observations
  • Component – Scatter Tables provide occurrence percentages relative to individual wave components (Windsea, swell1, etc.). A maximum of 3 wave systems are output.

 

Wave Period Parameter – Select either Peak Period or Zero-Crossing Period for use in Sea State tables and plots

 

Significant wave height increment – Select significant wave height (m) increment to use in computing the wave height bins.

 

Wave period increment (s) - Select wave period (s) increment to use in computing the wave period bins.  Note: wave periods are computed with a 3-point parabolic fit to the 1D energy-frequency spectrum.

 

Number of directional sectors – Select the number of wave direction bins to use in the Scatter Plots.

 

Extremes

These parameters are used in the extremal analysis of wave height and wind speed (if present).

 

Apply Wind Speed Threshold – Apply a wind speed threshold for the wind Peaks over Threshold analysis. The user can specify this value as a constant or as a standard deviation multiplier. Data above this threshold will be identified as a peak and used in the extremal analysis.

 

Wind Speed Threshold – Select the value of the constant wind speed threshold or the standard deviation multiplier.

 

Apply Wave Height Threshold – Apply a wave height threshold for the wave Peaks over Threshold analysis. The user can specify this value as a constant or as a standard deviation multiplier. Data above this threshold will be identified as a peak and used in the extremal analysis.

 

Wave Height Threshold – Select the value of the constant wave height threshold or the standard deviation multiplier.

 

Peak Lag (hrs) – In the Peaks Over Threshold Analysis for wind speed and bulk wave height, this value indicates the minimum time lag in hours between successive events.

 

Compute Confidence Intervals – Check this box to compute 95% confidence intervals using the profile log-likelihood method

 

3.  Interactive Display – Scatter Table Directional Sectors

The plot in the bottom of the center column of the Sea State Parameter Table displays the results of the directional bin selections.  This display updates each time the directional bin characteristics are modified in the parameters to the left.  There are no user plot controls.

 

The plot in the bottom of the right column of the Sea State Parameter Table displays the resulting peaks found from the user defined Extremes parameters. Use the Update Plot button after changing parameters to see how the resultant peaks have changed. The number of peaks found is displayed in the legend. Right click on the title to switch between significant wave height and wind speed plots.