Bollard Pull Calculating Keys

Key Factors for Calculating Bollard Pull in Towing Operations

In towing operations, determining the right amount of bollard pull (the pulling force a tugboat can exert) is crucial to safely move a vessel or barge. Bollard pull depends on several factors that influence how much force is needed for a successful tow. Here’s a breakdown of those key factors:

1. Vessel Resistance

The tugboat has to overcome various types of resistance to tow a vessel effectively. The two main types of resistance are:

• Hydrodynamic Resistance: This is the force exerted by water against the hull as the vessel moves through it. Larger vessels with more surface area below the waterline experience more drag, and thus need more bollard pull.
• Aerodynamic Resistance: Wind resistance affects the exposed parts of the vessel (superstructure). For larger vessels, especially in windy conditions, the tug will need additional power to counter the resistance caused by the wind.
• Wave Resistance: If the vessel is operating in rough seas, the waves create additional resistance that needs to be considered. The height and frequency of the waves add to the total force that the tug must overcome.

2. Vessel Size and Weight (Displacement)

The vessel’s displacement (the amount of water displaced by the vessel) directly impacts the bollard pull. A heavier vessel requires more pulling force because it creates more resistance in the water. For larger vessels, the required bollard pull is generally higher.

For instance, larger and heavier vessels displace more water, so the tug’s bollard pull must be able to counter this increased resistance. The size of the vessel also affects how easily it can be moved, with larger vessels being harder to tow.

3. Towing Speed

As towing speed increases, the drag forces acting on the vessel rise significantly. At higher speeds, both the water resistance and wave drag become more intense, meaning the tug will need considerably more bollard pull to maintain speed.

At lower speeds, the resistance is reduced, and less bollard pull is required. This is why towing operations often occur at slower speeds, ensuring that the tug can maintain control without excessive strain on the towing line or engines.

4. Environmental Conditions

• Wind Speed and Direction: Wind creates additional force that acts on the vessel’s exposed areas. Stronger winds, especially when coming from ahead or the side of the vessel, increase the need for higher bollard pull.
• Current and Tidal Forces: Towing against strong currents or tides significantly increases the resistance, making the vessel harder to tow. Conversely, a favorable current can help reduce the required bollard pull.
• Sea State: Rough seas with large waves also increase the resistance, as the tug has to exert more force to maintain steady movement.

In general, more extreme environmental conditions require a tug with a higher bollard pull capacity to maintain control and safety during the operation.

5. Towline Length and Arrangement

The length and angle of the towline affect how efficiently the tug can pull the vessel. A longer towline reduces the impact of the waves and makes for a smoother operation. Additionally:

• Long Towline: Reduces the forces exerted by the waves on the vessel and decreases shock loads on the tug.
• Towline Angle: A direct, straight line between the tug and the towed vessel is the most efficient arrangement. An angled or curved towline puts more strain on the tug, requiring more bollard pull.

6. Vessel Shape and Surface Area

The larger the surface area of the vessel exposed to wind (a factor known as windage area), the more bollard pull is required to move the vessel against the wind. Vessels with large superstructures or deck cargo are more affected by wind forces, requiring a tug with higher pulling capacity.

7. Safety Margin

When determining the required bollard pull, it’s essential to add a safety margin. This margin accounts for unforeseen factors such as worsening weather conditions, unexpected mechanical issues, or other operational challenges.

For example, a typical safety margin would be an additional 25-30% above the estimated bollard pull requirement. This ensures that the tug has enough reserve power to deal with emergencies or unexpected increases in resistance.

8. Load and Cargo Distribution

If the towed vessel or barge carries cargo, how that cargo is distributed plays a crucial role in the operation. Unevenly distributed weight can make the vessel harder to control, increasing the drag and resistance forces acting against the tug. A well-balanced cargo load reduces strain on the tug and ensures a smoother towing operation.

9. Industry Standards

Guidelines and standards provided by organizations such as Lloyd’s Register or DNV GL offer helpful references for estimating the bollard pull required for different types of vessels and operations. These standards are based on decades of towing experience and help ensure safety and efficiency during the operation.

Finally

When calculating the bollard pull for a towing operation, it’s essential to consider the vessel’s resistance (water, wind, and wave forces), displacement, environmental conditions, towing speed, towline arrangement, and safety margins. All these factors together determine the amount of pulling force needed to safely and efficiently complete the towing operation. Proper planning and consideration of these elements are key to a successful and safe tow.