Principal Functions Provided

• THRUST

This allows you to specify a design environment in terms of wind and current speed, and sea-state. For a selected range of environmental directions, the thrust required to balance with the design environment is determined and hence the maximum thrust required by each unit can be determined. The wave direction may be specified relative to the wind direction by giving an offset in degrees. A zero offset assumes that the wind and waves are coincident. The current direction may be specified in a similar way or may be fixed in one direction which will usually be either head on or beam on to the vessel. Rudder effects cannot be included in this option.

• ENVIRO

This allows you to specify the maximum available thrust in each unit and determine the maximum environment the vessel can operate in for a range of specified environmental directions. The wind speed and wave height are assumed to be correlated either from a default relationship or as specified by you. The current speed is fixed and the maximum wind speed and wave height can be determined. Rudder effects can be included and operated either dependently or independently.

As with the THRUST calculation, the wind directions are specified, and the wave direction is given relative to this. The current direction may also be given relative to the wind, or may be fixed in a particular direction.

• ERN

This allows you to determine the environmental regularity number ERN in accordance with the procedure defined by Det norske Veritas (References 46). The ERN number gives the percentage of time that a vessel will be able to maintain position and heading in a reference geographic location of the North Sea. The reference position is at the weather ship 'M' located at 66 North 2 East off the Norwegian coast, as shown in References 46.

The most critical vessel heading relative to the environment is considered, which has been taken as the environment beam on to the vessel. The current is fixed at 1.5 knots and the maximum wind speed and wave height that the vessel can hold station in is determined with all thrusters operating at up to 100% power, where the significant wave height is assumed to be related to the wind speed as specified in References 47. The analysis is repeated with each thruster inactive sequentially and the most effective and least effective thruster is determined. Further analysis is undertaken with each pair of thrusters sequentially made inactive and the most effective and least effective pair are therefore found. The ERN number is obtained for the five conditions, from data given in References 47. This data relates the cumulative probability distribution of observed significant wave height and wind speed at the reference location to the significant wave height and wind speed in which the vessel can hold station.