Bollard Pull Calculation for Marine Pilots

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Pilot’s Tug Assist Tool – PTAT

Tug power and number of tugs needed in variable conditions of wind, current and waves is in most cases an assessment made by pilots based on their professional experience. However, assessments might be subject to scrutiny of lawyers if something goes wrong.

They will use tools to calculate what really is needed with respect to tug power and number of tugs. They have furthermore the advantage of time. But a pilot does not have so much time in a real-world scenario.

For a pilot, if tugs are needed, it is hard to calculate the required tug power just before or during ship manoeuvring. Furthermore, more extreme the weather conditions become, less accurate are the assessments; and higher the risk of too little tug power.

A handy and simple tool to determine a minimum time what is really needed as tug assistance, is the Pilot’s Tug Assist Tool(PTAT) which calculates in an approximate way the total required tug power for ships in various conditions of wind, current and waves. This tool can be loaded as an app on the smart phone.

The tool is based on the calculations and graphs as explained in chapter 5 of  the book “Tug Use In Port”, written by Captain Henk  Hensen FNI; first published in 1997 by The Nautical Institute, London, UK, with a 3rd edition published by The ABR Company, UK, in 2018 (www.tugandosv.com). In this book is also explained why a safety margin of 20% is included in the calculations.

The program has been tested for more than two years and it has been observed that it works in a satisfactorily way. The various possibilities of the PTAT are also addressed in the book.

BPC Section

Much of the information can also be viewed by selecting the “ⓘ” symbol of each section on the app.

Results window

Calculations for required tug power in case of cross winds:

Ship Height: The value to be written in this box is critical.

It is the height from sea level to the average maximum height of the vessel, or in case of deck the average maximum height of cargo loaded on deck, including deckhouse. It can be difficult to assess the sideways wind area. With container vessels it is rather easy. When you keep in mind that the height of one container is about 2.60m, then it is easy to calculate the total height of the containers on deck. Between the lowest container and the main deck is also about 2 meters space. Height of container can furthermore be used to assess other heights as well, such as the height of main deck above water.

Wind force: Another important box in the same section.

As wind does not blow at constant speed, the highest wind speeds are important. Therefore, it is recommended to use the estimated wind force in gusts. Wind speeds given in Beaufort scale are average wind speeds during a 10 minutes period and therefore too low and not suitable for calculation of required bollard pull.

Calculation of forces created by cross waves

Although jetties, terminals and harbour basins are usually located well sheltered from waves, certain terminals and jetties may be located such that they can be under influence of waves. Wave action may become important when wave conditions exceed certain threshold values. Wave forces are essentially dynamic in nature and it is important to understand the nature of wave loading and vessel wave motion response and when a more rigorous dynamic analysis may be required.

Web link to free download the mobile application “Bollard Pull Calculation For Marine Pilots”:

https://play.google.com/store/apps/details?id=com.lti.bollardpullcalculator

https://apps.apple.com/tr/app/bollard-pull-calculator/id1495335297?l=tr

Sea News Feature, February 20

Baibhav Mishra
Author: Baibhav Mishra