Spectral Partitioning - Aktis Hydraulics

Partitioning of Wave Spectra

The global hindcast model gives all wave systems arriving at a location from local seas to multiple swells from distant storms. To understand and analyse these data, we first must separate these components, and this is why we need a splitter. Most design responses are sensitive to a variety of wave characteristics including wave period. From the total seastate there is usually no clear relationship between height and period (see left image below). After the use of a good splitter that identifies meaningful, natural wind wave and swell systems, the designer gets the insight he needs to complete reliable analysis.

Total SeaState

Wind Seas

Swells

The left figure above shows the unclear relationship between total wave height and wave period. The centre and right figures present the results for wind seas and combined swells respectively. They show that the relationship is clearer after the use of our partitioning software. In the right figure distinct swell systems can be identified showing that allowing more than 1 swell partition would be appropriate.

Our method

Aktis uses its own in-house splitter to separate wind seas and multiple swells. This uses a mountaineering scheme to identify all the individual peaks in a 2d spectrum. It then identifies the peak(s) that corresponds to wind seas and merge them into one partition. The rest of the peaks are identified as swells and can be stored separately or recombined into meaningful wave systems.

Aktis has developed algorithms for re-combining neighbouring partitions depending on their direction and frequency without getting a combined partition with too much directional or frequency spreading.

For each resulting partition the splitter computes the individual wave characteristics (Hm0, Tp, θ0, gamma, sigma etc…) and outputs the time series.

The image on the right presents the identification of multiple swells in the Pacific Ocean. It shows that our splitter can cope with a very complex wave spectra and offers options to recombine various swells into wave systems.

Results and Benefits

As you can see in the image on the left we can obtain continuous time series of wave parameters based on the wave spectra i.e. partitioning can be made that is consistent from one timestep to the next.

Extreme Value Analysis, can only be carried out on homogeneous populations. Where there are multiple swells, swells from different timesteps have to be grouped in a consistent way to be able to carry out meaningful analysis. Having a well- developed, consistent spectral partitioning tool makes it possible to carry out meaningful extreme analysis of partitions in an efficient and streamlined workflow without the need to make arbitrary decisions and corrections in every project.

This advanced partitioning method also allows Aktis Hydraulics to simplify and accelerate the wave modelling process for ambient conditions by treating the various partitions independently. Working with individual partitions makes it possible to propagate the waves with stationary models without losing accuracy in the results. Aktis Hydraulics has successfully used this method for local waves modelling in all ocean basins.

Partitioning of Wave Spectra

Our method

Results and Benefits

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Core expertise

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