The dials for ‘Wind Sea’ and ‘Swell’ are turned to represent the forecasted directions. The Vessel dial is turned to represent the required vessel direction, or to optimize the vessel direction to find the best heading. The reading window on the vessel dial now presents the allowable wave heights for wind and swell, ready for comparison to the forecast.
The weather forecast, presented four times every day, is used on board vessels to plan operations. Operations may be performed if there is a sufficient weather-window.
Determination of the weather-window without Nereon Compass
For most operations, this is determined solely by checking the ‘Hs’ column for ‘Total Sea’. Total Hs should be below an allowable limit. More advanced decision-making involves going back and forth between forecast and a limits table. First, the ‘Tp’ or ‘Tz’ for column for ‘Total Sea’ is read from the forecast. This is used to look up the ‘Hs’ limit from the limits table. This is then compared to the value for ‘Hs’ for ‘Total Sea’. These methods vary from simple to demanding. In any case, only the total sea state is evaluated.
Hazards of using ‘Total Sea’ forecast
The ‘Total Sea’ state blurs together Wind Sea and Swell wave systems. It obfuscates the individual directions of Wind Sea and Swell. It muddles their individual periods. Every experienced seafarer has been surprised by high roll motions when the Total Sea state is a steep head sea. In most cases, the forecast was not wrong. It was just interpreted incorrectly by looking at Total Sea only.
Using Wind Sea and Swell components
The ‘Wind Waves’ and ‘Swell’ sections of the forecast provide individual ‘Dir’, ‘H’ and ‘T’ columns. These can be easily interpreted using the Nereon Compass. The Nereon Compass has a dial for Wind Sea direction and for Swell Direction. The vessel direction is also set on the compass, so that the relative directions of the sea states become obvious.
No more surprises of beam seas Swell hiding in the total sea state!
A Nereon Compass Vendor, provides the WindSea and Swell dials presenting the allowable wave heights for the offshoreoperations for which the Nereon Compass is to be used.
Straightforward cases
In many cases, the limits are dependent upon a motion requirement. For instance, maximum allowable vertical downward motion at a chute. Or wave clearance of an object protruding outward from deck to prevent wave impact.For these cases, the Nereon Compass vendor is presented with a detailed guide. The vendor is responsible the quality of the hydrodynamic model used to create an RAO.
Complex cases
In some cases, the limits to workability are complex. Time-domain simulation is required to determine the wave limits for the dials.The Nereon Compass vendor is presented with high-level guidelines. The vendor is responsible for the quality of the time-domain simulations and their representativeness.
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