Use of linked rudder, independent rudder and separate handling of screws, together with a bow thruster
Øverst t.v. – Walking sideways
Checking rate of movement by moving linked rudders to port. Engine power kept same.
Øverst t.h. – Walking sideways bodily to port
Linked rudders to starboard, more astern power required than ahead to stop advance.
Nederst t.v. – Make sternway at angle to ship’s heading
Starboard engine moves stern across to port. Bow thruster pushes bow to port. Port engine gives sternway.
Nederst t.h. – Advance at angle to ship’s heading
Port engine and port rudder create movement of stern to starboard and ahead. Bow thruster pushes bow to starboard. Starboard engine check advance. Manoeuvring of vessels that are held back by an external force.
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Manoeuvring of vessels that are held back by an external force
This consideration is written in belated wisdom, according to the accident of Bourbon Dolphin in April 2007
At the beginning of 2014 I produced a few manuscripts as a basis of some of the University College’s external course activities addressed to national and international shipping companies.
The subjects mostly turned on lack of realization of precarious conditions and their demand for actions as exposed after the capsizing of Stevns Power and Bourbon Dolphin.
Capt. Øystein Johnsen MNI
When maneuvering a vessel constrained by external forces and exhibiting minimal or no forward velocity through the water, the propulsion system is often pushed to its maximum capacity. This scenario frequently demands the highest possible lateral force to counteract the effects of wind, waves, and current.
In this case, the vessel is restrained by a combination of 1,800 meters of chain and wire, totaling a mass of 300 tons. With wind speeds reaching 35 knots from the southwest, wave heights of approximately 6 meters, and a 3-knot current directed northeast, the vessel has drifted 840 meters eastward, deviating from the intended bearing line for the anchor.
The accompanying photograph depicts the Bourbon Dolphin deploying her final anchor, taken 37 minutes before capsizing. The visible slipstreams indicate that all thrusters were active, and the rudders were set to port. (Photo credit: Sean Dickson)
Bollard Pull and Shaft Generator Efficiency
The Bourbon Dolphin’s towing capacity was significantly reduced from 180 tons to 125 tons due to the substantial demand on the thrusters and winch. During operations involving the use of thrusters, each shaft generator powers both a forward and stern thruster, in addition to the electric motors driving the winch.
The shaft generators are designed to fully accommodate the energy requirements of the thrusters and winch. However, increasing the load on the shaft generators results in a reduction of bollard pull because the propeller pitch is automatically adjusted to maintain a consistent load on the main engines, which operate at a constant speed. As the load on the shaft generators increases, less power remains available for bollard pull.
For instance, when thrusters and winch are operating at maximum capacity, only 62% of the maximum power is available for the main propellers. Consequently, the Bourbon Dolphin’s towing capacity decreases from 180 tons to 125 tons. Therefore, the certified bollard pull of a vessel is more theoretical than practical, especially in anchor handling operations where extensive use of thrusters is anticipated.
Loss of Thruster Efficiency
When the vessel employs thrusters to maneuver the bow to starboard (SB), the following observations can be made: In the absence of forward motion, suction forms around the periphery of the SB thruster inlet. On the outlet side, the thruster’s slipstream aligns almost perpendicularly to the ship’s centerline. With increased water speed, there is a greater suction force in front of the inlet, while suction behind the inlet (in the aft section relative to the direction of travel) diminishes significantly.
The effectiveness of the thrusters is compromised by the vessel’s speed, as it reduces the inflow of water that the thrusters rely on.
On the outlet side (port side, BB), with forward speed, a slight increase in pressure occurs at the front of the outlet, but a significant suction area forms behind the outlet due to the increased current speed.
This deflection of the water flow from the thruster tunnel and along the hull reduces the thruster’s effective push, thereby diminishing the lateral force available to counteract forces pushing the vessel starboard.