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Waves

Wave conditions are extremely important when determining the loads on the structure. Depending on the locations the wave conditions can varry and requires different or specefic mooring system.

 

Wave – Mooring interaction

 

Three waves related phenomena affect mooring system design. They are:

 

  1. Steady state mean drift force.

  2. Surge, sway, and yaw response at or near the predominant period of the waves.

  3. Oscillatory drift forces at or near the natural period of the spring/mass system of the moored vessel.

 

Data gathering

 

Ocean weather forcast is a specefic study. Rocket Science Inc. makes use of official data sets, and implemet them in the choose mooring system. The significant wave height is an important aspect with reference to the stresses in the mooring lines as well has the current an sifgnificant influence on the mooring system. 

Wave Analysis

 

The waves on the sea surface are not simple sinusoids. The surface appears to be composed of random waves of various lengths and periods. This surface can be describe, with some simplifications, which leads to the concept of the spectrum of ocean waves. The spectrum gives and helps predict the distribution of wave energy among different wave frequencies of wave-lengths on the sea surface. The concept of a spectrum is based on work by Joseph Fourier who showed that almost any function ζ(t) can be represented over the interval ( −T/ 2 < t < T / 2 ) as the sum of an infinite series of sine and cosine functions with harmonic wave frequencies:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

To sample the sea surface these Fourier series are used by calculating the the Fourier series that represents the sea surface the best. It requires measurements of the sea surface elevation ζ(x,y,t) everywhere in an area perhaps ten kilometers on a side for a continuous stretch of time, perhaps a couple of hours.

 

 

 

 

 

 

 

 

 

 

 

 

After digitizing and sampling this signal, the data is converted into a variance density spectrum which gives a complete description of the surface elevation of ocean waves in a statistical sense, provided that the surface elevation can be seen as a stationary, Gaussian process. This implies that all statistical characteristics of the wave field can be expressed in terms of this spectrum.

 

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