This week we are going to look at a particular DP function, when using the Kongsberg system. Although it is a fairly simple point it is one that can still cause some confusion, even for some experienced operators.
That function is changing the ships heading while the center of rotation (COR) is is offset from the vessels center of gravity (CG).
When using dynamic positioning, the operator has the ability to alter the point at which the vessel rotates around. Normally this point is located at the Center of Gravity. When a heading change is carried out the vessel will spin “on a dime”, staying relatively in the same position.
The operator may choose to change the center of rotation for a number of reasons related to the operation currently underway. Some examples of changing the COR:
- to the position of a crane wire that is connected subsea, so that a change in heading doesn’t pull laterally on the load.
- to the location of the diving bell.
- so that a rotation does not move the dive bell closer to a subsea structure.
- to the roberts hoop, so that a change in heading does not pull the diver into danger.
- to the tip of a pipe lay stinger so that you can adjust to the best weather heading, without changing the location of the tip in relation to the survey line.
- off the vessels bow centered on a spar, so that the vessel rotates around that spar while maintaining a constant distance (weathervaning)
Confusion can arise when it comes to setting the speed of rotation. When the COR is located at the vessels CG, the operator selects a Rate of Turn, shown in Degrees/minute.
But as soon as the rotation point is offset from the CG, the speed that the heading changes is governed by the vessels speed setting. This is because there is now a surge & sway component in the move. If you have your Rate of Turn set to 10deg/min, but your Speed Setpoint is still at 1.0kts for example, & you input a heading change while the COR is offset, the vessel will respond much faster to a greater speed than may be desired!
Alternately, if your rate of turn is set properly, but your speed set point is zero, it will not be possible to change heading when the rotation point is set to a position other than the CG.
This is described in the Kongsberg manual as follows:
When the vessel rotation point is at a position other than the center of gravity of the vessel, the actual rate of turn may be less than the speed specified. This is because the speed of movement of the CG is limited in proportion to the required vessel speed & the distance of the rotation point from the center of gravity.
Although this is a simple point to an experienced operator, it may not be so obvious to a trainee. You may also find yourself having to explain DP principles to the client on the bridge, so it helps to review the basics.
[Image: Copyright 2010 Robert Almeida Photography]