Ship Heeling Moments

Heeling moments will be noted when the mooring line is not acting at the centre line of the ship, the control and mitigation of the generated heeling moments has to be achieved by changing headings and winch tension. The thrust of the ship has a minor influence in the magnitude of the external force, but it will affect the shape of the catenary and the direction that the external forces are acting over the ship. Situations where the vessel is dragged aft/sideways during anchor handling operations are often reported, meaning that the pull of the ship is less than the drag effects introduced by the hawser. In such situations the heading of the vessel is clearly difficult to control and the risk of the ship experiencing higher heeling moments

The towline force has to be counteracted by the tug and this is done by the thruster forces or by the drag forces or by a combination of both. This means that with the towline force, two other forces are acting on the tug, the drag force and the thruster forces. The thruster forces can act in the same direction as the towline, counteracting the heeling moment, or in the opposite direction, thereby increasing the heeling moment. The horizontal forces are projected in a vertical plane as shown in Figure 2. The towline force and the drag and thruster forces cause a heeling moment which has to be counteracted by the transverse stability moment of the tug. The transverse stability moment consists of two forces in opposite direction: the downward vertical acting weight force and the upward vertical acting buoyancy force. The lever of this moment is called GZ (Figure 3). Traditional tug criteria should provide for sufficnet stability to prevent capsize in case of girting when the towline exerts maximum pull perpendicularly to the vessel’s centerline. Such situation can develop rapidly, and tugs should be designed with sufficient stability to survive such events. Since the towline will in most cases act horizontally or upwards, a verricial component of the towline force is often disregarded. Tug masters must be alert to the danger of capsizing which can occur when the tow wire/rope reaches a large angle to the centre line of the tug and the quick release cannot be activated and girting occurs (see page 36). If girting is experienced tugs with towing hooks forward of the propeller system can find it difficult to recover. The terms mean the same thing and refer to the situation when a vessel, usually a tug, is towed broadside by a towline and is unable to manoeuvre out of this position. This phenomenon is known to all tug masters. It is the most prevalent