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Vessel types: Current and wind loads |
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Vessel types: Current and wind loads |
Current and wind drag loads are due to the relative velocity of the fluid past the vessel. They are modelled with the data on the current load and wind load pages on the vessel type data form. If the length of the vessel differs from that of the vessel type then the data will be Froude scaled accordingly.
These loads are an important source of damping when modelling vessel slow drift.
The velocity used to calculate the drag loads is the low frequency velocity of the fluid relative to the vessel. This includes any current or wind velocity and the vessel velocity due to any low frequency primary motion. The drag forces and moments due to translational motion are modelled using the standard OCIMF method. The drag forces and moments due to any vessel yaw motion are modelled using yaw rate drag load factors. For details of the calculations, see vessel theory: Current and wind loads.
| Warning: | The current and wind loads are based on theory for surface vessels and are not suitable for submerged vessels. |
The coordinates, relative to vessel axes, of the point on the vessel at which the current or wind drag loads are calculated and at which they will be applied. This need not be at the vessel origin.
The velocity used in the current drag load calculation is the current velocity at the current load origin, minus any velocity of the load origin due to low frequency primary motion of the vessel. Note that if the load origin is above the water surface then the current velocity used is that at the water surface.
The velocity used in the wind load calculation is the wind velocity, minus any velocity of the wind load origin due to low frequency primary motion of the vessel. Following the OCIMF wind load model, the wind velocity specified should be that at 10m (32.8 ft) above mean water level.
Specifies what symmetry the vessel type has below (for current load) or above (wind load) the water line, about the load origin. For $xz$ and $yz$ symmetry, OrcaFlex will derive load coefficients for extra directions generated by reflection in the given planes. For circular symmetry, you give coefficients for one direction only, from which OrcaFlex can derive coefficients for any other direction.
| Note: | The symmetry for current drag, wind drag and RAOs need not be the same, though of course the symmetry for current drag would normally be the same as that for RAOs. |
The surge, sway and heave areas and the roll, pitch and yaw area moments used to calculate the current or wind load.
Load coefficients are given for vessel surge, sway, heave, roll, pitch and yaw directions. They depend on the direction of the current or wind, and are given relative to the low frequency vessel heading.
OrcaFlex uses the given symmetry, if any, to derive coefficients for other directions and then uses linear interpolation to derive coefficients for intermediate directions.
The view coefficients button displays a graph of the coefficients that will be used – the blobs on the graph show the coefficient values you have defined plus any that OrcaFlex has derived using reflection, and the curve shows the interpolated values that will be used for intermediate directions. You should specify sufficiently many directions to define the shape of the curve and to cover the range of directions that the vessel will experience.
The yaw rate drag factors only apply to the current load, not to the wind load. They model the yaw drag moment, and any surge and sway drag forces, due to low frequency vessel yaw motion.
For a slender ship, and if the load origin has been placed at the centre of the vessel, then the surge and sway drag factors can usually be taken to be zero, and the yaw drag factor can be estimated based on the vessel length and draught.