Environmental Condition - Waves

Articles on this page:

• How is the automatic frequency range for SEA files calculated?
• Turbine has deflected / floated beyond calculated wave train
• How can I model breaking waves in Bladed?

Problem

How is the automatic frequency range for SEA files calculated?

Solution

Frequency is denoted by f, the wave spectrum by S(f) and the permissible fractional energy loss by p. An intermediate value of the area under the wave spectrum can be computed:

An upper and lower limit are determined by excluding the area A from the upper and lower frequency range of the wave spectrum respectively. The lower limit is found by increasing in small frequency steps along f starting at zero, until the area under S(f) to the left of current position is equal to A. This sets the lower limit. For the upper limit, the process is repeated in reverse by decreasing in small frequency steps along the f axis starting at the highest frequency value.

Keywords

SEA file; Frequency range; Spectrum; Wave

Problem

It's not unusual to get this error with an offshore turbine. The message reads "Turbine has deflected..." for a bottom-fixed turbine or "...floated..." for a floating turbine.

Solution

Bladed sets a default wave-train extent based on a heuristic calculation, which does not always result in a large enough value. You can set your own value for the wave-train extent using the "Non-standard options" window in Project Info (for an introduction to this functionality, refer to the Project Info options document).

Steps:

• Go to File -> Project Info
• Tick Turbine calculations (dtbladed.exe)
• Click Define... to open the "Non-standard options" window
• Enter the following text in the Special data textbox (taken from the Project Info options document):
  Copy

    MSTART EXTRA  
    WaveExtent 150.0 ** (example, 150 metres) Extent of wave train in metres (upstream and downstream)  
    MEND
  

  
  

The custom wave train will extend by at least this distance, both upstream and downstream. This value will only be applied, in either direction, if its absolute size in that direction is greater than the default wave train extent calculated by dtbladed.

If this doesn't solve the problem, it's probable that the simulation is going unstable. This may be fixed by adjusting integrator settings for instance.

Keywords

Waves; Wave train; Offshore

Problem

Bladed doesn't have a built-in breaking wave model. However, offshore turbine modelling may require breaking waves to be considered.

Also, Bladed may sometimes exit with an error message of ERROR: Wave height greater than breaking wave height. What to do in this situation?

Solution

Breaking waves cannot be modelled explicitly in Bladed. In any case, accurate modelling of breaking wave is not straightforward and all practicable engineering models are only approximations.

Normally, when a breaking wave is present, we advise to use the Constrained Wave option, choosing the "Stream Function" option, not the linear wave:

Next, adjust the height of the constrained wave by trial and error until you have the steepest wave that just falls below breaking steepness. The stream function theory can give some unrealistic results when operating too close to its limits. Therefore we advise that you carry out a visual inspection of the surface elevation time history, as well as the water particle kinematics outputs. Any problems with the model in this marginal situation will usually be obvious from the results, in which case you should reduce the wave height accordingly.

The stream function wave will provide a partial solution to the wave loading, but an additional load time history may be applied as well in order to approximate the additional loading from the breaking wave. Guidance can be found in Annexes B and C of the offshore standard IEC 61400-3-1 (2019 edition). Specifically, Section B-4 contains a general discussion of breaking waves and C-4 describes an empirical formula for calculating the additional load due to wave impact forces (slap and slam loading) which can be applied to Bladed using the Point Loading function, timed to coincide with the arrival of the constrained wave peak.

Keywords

Breaking wave; Breaker; Steepness; Stream function; Slap load; Slam load