Other Sea State Parameters Sample Clauses

Other Sea State Parameters. In severe to extreme sea states, the influence of swell is usually negligible compared to the wind sea, whereas in small to moderate seaways, the influence of swell may be significant. Figure 3 shows that for the significant wave heights relevant in the assessment of manoeuvrability in adverse conditions, the influence of swell is small, therefore, a unimodal spectrum can be assumed for simplicity. For the open sea, it is feasible to assume a situation of a ship weather-vaning for a prolonged time until the storm finishes, i.e. a developed storm situation is relevant for the assessment of the weather-vaning ability in the open sea. For a developed sea state, the Xxxxxxxxxxxxx spectrum (also referred to as a two- parameter Xxxxxxx-Xxxxxxxxx spectrum or a two-parameter ITTC spectrum) is generally recommended, 4 h2  204  S  5 s exp   , 5 T4 T44 p  p  (4) where Tp , s, is the wave period corresponding to the modal wave frequency p , rad/s, and  , rad/s, is the wave frequency. For coastal waters, following from the practice and confirmed by the interviews of ship masters undertaken by SHOPERA, the assumed scenario is that ship masters do not remain near the coast in a growing storm until it escalates, but either search for shelter or leave to the open sea. Therefore, the feasible scenario to be applied with the steering and propulsion criteria is a developing storm. For a developing storm a general recommendation is to use the JONSWAP spectrum with the peak parameter of 3.3, i.e. as used in the 2013 Interim Guidelines for the evaluation of ship manoeuvrability in coastal areas, g2  4  S  exp  p  a , 5  4    (5) where a  exp    2 222  ,  0.07 if    and  0.09 if    ,  5 / 4 , and the  p p  p p peak parameter  3.3 . As a realistic assumption, directional spreading of wave energy with respect to mean wave direction is recommended; as the spreading function, cos2-spreading was used in SHOPERA. An assessment of steering and propulsion abilities in short-crested waves with cos2-wave energy spreading in comparison with assessment in long-crested irregular waves for a VLCC tanker and 14000 TEU container ship showed that assuming long-crested seaway makes assessment results negligibly to moderately conservative, thus performing assessment in long-crested waves is also acceptable, as it is more practicable for designers. The range of characteristic wave periods (for clarity, peak wave period Tp will be used) use...