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Bollard Pull

What is Bollard Pull?

Bollard pull can be defined as a measure of pulling power of the vessel, like horsepower rating of conventional vehicle engines. Therefore it is defined as the thrust developed by the propulsion system of any vessel when it has moves with zero speed in the forward direction.
Bollards are short, stocky structures generally built out of metal, used on piers or docks to assist mooring of the vessel to port, by anchoring the tow lines. So, many people are generally familiar with bollards as they can be found in major cities to prevent car-borne incidents in public places.

bollard pull
fig.1

The Bollard pull is similar in principle to the drawer pull used on tractors, trucks, trailers-haulers, etc. to measure their ability to pull heavy load or trailers. Generally, it is required for vessels such as tugs and is recorded in KN or in others units like tonnes. Every tug or vessel that is required to pull an additional load other than its weight, requires Bollard Pull certifications from one of the internationally accepted societies like American Bureau of Shipping, Indian Register of Shipping and Bureau VERITAS etc. which give certificates on successful completions of Bollard pull test.

Formula for calculating the Bollard Pull

For any physical process, there is always some energy lost to the surrounding, which can reduce the expected output. In the bollard pull test, some amount of propulsion energy is lost in the form of heat, friction or tension losses. Thus, an efficiency factor is introduced to give a more accurate and actual pulling force of the tug as given:

Actual pulling force (Pf) = (Bollard pull Force in KN) x (Efficiency Factor)

Therefore the efficiency ranges between 0 to 1. If it comes out 1, considered it 100% efficient and if 0, consider it as complete loss of power.

How many tonnes can a tugboat pull?

Friction arises between the hull of the vessel and the surrounding water, so the current moves in a direction opposite to the expected direction of motion. Wave resistance arises due to the oscillation set up by the waves, or slamming effects created at the bow. This significantly affects the movement of the vessel, hence we must overcome this resistance.
When the wind force is against the sail-area of the vessel, it can hamper forward movement due to hull resistance. This hull resistance is calculated as.

Resistive force(R) = ½ x (density of water) x (wetted surface area of the hull) x (velocity of the vessel)2

Now, any tug pulling a load behind it must think about the thrust that requires to maneuver not only the towed object but also to overcome its own resistive forces. Thus, the bollard pull value are often given as:
Therefore the force available on Bollard Pull can be given as = (Bollard pull measured) – (Force utilised by the tug)
Here, we can observe the importance of maintaining a zero based on the speed during the experiment. At such conditions, the employed by the tug to beat its own resistance amounts to zero. Thus, everything of the bollard pull force can be utilized for towing any vessels linked to the tug.

How do you test a bollard?

To measure the pulling capacity of a vessel, a bollard pull test is employed to classify tugs on their towing ability. Experimentally, the test is administered by employing a bollard to anchor a vessel to the pier. Then, without slack within the connecting link, the vessel is required to propel itself at maximum thrust. Since, the vessel is technically moored, it’s zero forward speed, and the tension built up with in the connecting towline indicates the entire developed by the vessel propellers. To carry out the bollard test, ideal conditions are required to be able to accurately gauge the pulling power.

Some of these conditions include:

  • Water without tides or any disturbances,
  • Sufficient clearance on all three directions of motions
  • Even draft of the vessel along its length,
  • Unobstructed propeller wake and stream.

Manufacturers of such vessels are able to market their craft by alerting the horsepower ratings mentioned for a particular vessel. Bollard pull is an internationally recognized means to measures the thrust developed by the propellers and nozzle system on vessels. Most tugs and support vessels lately are required to posses these certifications to work.

Procedure of measurement

This methodology is useful for occurrence ships styles and smaller shipyards. It restricted in exactness variety of boundary conditions ought to be discovered to get reliable results. Summarizing the below needs, sensible bollard pull trials ought to be conducted in a deep water seaport, ideally not at the mouth of a watercourse, on a peaceful day with hardly any traffic.

  • The ships ought to be in undisturbed water, current or strong winds would falsify the measurements.
  • The statics force that intends to maneuver the ship forward must only be generated by the friction between the propeller discharge race and the surrounding water. If the ship were too on the brink of a wall, the measurement would be falsified.
  • The ships should be in problem. If there have been any aerodynamic lift, the measure would be falsified. A similar holds true for propeller walk.
  • Water salinity should have a well-defined price, because it influences the particular weights of the water and thereby the mass rapt by the propeller per unit of time.
fig.2
  • The pure mathematical towline must have a well-defined value. Ideally one would expect it to be specifically horizontal and straight. This is not possible essentially, because ;
    1. the line falls into a centenary because of its weight,
    2. the two fixed points of the line, being the bollard on shore and the ships towing hook or cleat, may not have the same height on top of water.
  • Condition should be static. The engine power, the heading of the ships, the conditions of the propeller discharge race and the tension in the towing line must have settled to a relentless or neat-constant value for a reliable measurement.
  • One condition to look out for is that the formation of a brief circuit in propeller discharge race. If part of the discharge race is sucked back into the propeller, potency decreases sharply. This might occur because of an endeavor that’s performed in too shallow water or too brink of a wall

The illustrations of figure-2 of error influences during a sensible bollard pull trial. Note the distinction in elevation of the ends of the line (the bollard is more than the ships towing hook). Moreover, there is the partial tangency in propeller discharge current, the uneven trim of the ship and the short length of the tow line. All of these factors contributed to measure error.

Fig.3

Importance of Bollard Pull

It is similar to HP used on road vehicles, waterborne vessels even have a typical measure unit of power that may be universally understood. Not like standard road transport wherever the torque and speed relations match, tugs and such vessels that have to compel to haul huge masses on the water are needed to control in low-speed condition. Thus, whereas their speed could seem to be extremely low, the total torque and power developed by their thrusters and propellers would be important. Through a study of ways want to live actual power, the conception of bollard pull was developed.

The reason bollard pull records the force at zero speed within the forward direction is to make that the tug or support craft can hold a ship steady beneath its own power once the towed ship exerts its own thrust in the reverse direction. In ports, to guide incoming vessels, tugs are unit deployed to ships in navigating.

And forthcoming field of use of bollard pull is the testing of ‘Anchor Handing Tug Supply Vessels (ATHS)’used to tow huge semi-submersible structures like oil rings. These usually ought to be new locations supported the sub-surface oil deposits. Rather than building rigs attached physically to the sea bed, fashionable technology has enabled rigs to stay afloat because of part floating columns. Such rigs are called semi-submersibles and area unit ordinarily found altogether major oil-producing regions around the world. To stay the rig in constant locations rather than floating away, significantly heavy anchors weigh it down on to the seafloor. Once a replacement preparations in needed, AHTS vessels tow these huge rigs using a connector towline.

Since these structure area units usually a lot of larger than standard ships, the actuation superior skill of those tugs is of utmost importance. Additionally, their speed and performance beneath such masses should even be recorded for the aim of supply improvement.

Conclusions

Generally the values of bollard pull for medium-sized tugs employed in port operations will vary between 500 to 600 KN. A specialized Anchor Handling Island Offshore currently has the very best recorded bollard pull. It can do nearly 6700 KN of bollard pull beneath general check conditions. Set to be commissioned in 2020, it’ll serve as a twin purpose AHTS and deep water installation craft. The bollard pull of any vessel is extremely important to be ready to classify its load-pulling and bearing condition. Generally, it’s a measure of what proportion external load the tug can pull. It also indicates whether a vessel can keep any larger and heavier ship or tanker during a steady condition. It is essential to conduct bollard pull tests on tugs and ancillary support vessels, as it provide the operator and manufacturer with an idea of the capability of the vessels. Bollard certifications provide a seal of quality on the propulsions of the vessels, similar to other standardizations offered on conventional items.

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