Environmental Forces for Dynamic Positioning: Ships vs. Semi-Submersibles
Arun Kr Dev
School of Marine Science and Technology, University of Newcastle Upon Tyne, Newcastle Upon Tyne, NE1 7RU, UK, Newcastle University Marine International (NUMI) – Singapore.
The dynamic positioning (DP) is a fast growing technology, which started exactly 50 years back in USA. In last two decades, a tremendous growth has been noted in DP because of rapid development in both hardware and software associated with dynamic positioning. Dynamic positioning is primarily used in ships; mainly offshore support vessels like AHTSVs, PSVs, AHTs, pipe laying vessels, diving support vessels, etc. due to their operational requirements. However, synonymous with drill ships, DP is now being attempted in many deepwater semi-submersibles deploying 6 – 8 thrusters. One of the main design parameters of DP capability for either ships or semi-submersibles is assessing environmental forces quite accurately before selecting the thrusters’ power for the ship or the semi-submersible.
Of the three main environmental forces (wave, wind and current), the horizontal wave drift force is quite important. Calculation of this force is mainly done using 3-D radiation-diffraction theory under potential theory or sometimes empirically using wave drift force coefficients. While this method is quite accurate in predicting wave drift force for a ship-shaped body, the same may not be true for a semi-submersible where the slender column structure especially in low frequency (long waves) will cause the drift force to be influenced by viscous effects. In irregular waves, it is not only the wave drift force but also the low frequency part also plays an important role. Surprisingly the viscous effects also influence this low frequency part. A serious design deficiency will result in wave drift force prediction if these viscous effects are not considered for column stabilized semi-submersible type floating structures.
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