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Examples : Bollard pull < 40 tons : MBL towing line = 3.0 x Bollard pull. Bollard pull > 40 and < 90 tons : MBL towing line = (3.8 - BP/50) x Bollard pull. Bollard pull > 90 tons : MBL towing wire = 2.0 x Bollard Pull. As you can see for DNV the factor 2.2 is used and for Noble Denton a factor of 2.0 multiplied by the Bollard Pull. Obviously, as one works with a weak link system, this MBL x Safety Factor calculation is applicable for the weak ink or pennant wire. Not for the main towing line. The MBL of main towing line is 10% higher. Hence, the MBL of the towing line is related to the Bollard Pull, but always a safety factor (3.0 or 2.2 or 2.0 or another factor) is used to determine the Safe Working Load of the towing line. Use of stretchers Quite often, so called stretchers are used in the towing line configuration as shock-absorbers during the towages. The MBL of the stretchers is a lot higher than the MBL of the main towing lines. Some type of stretchers are grommets of 19 -21 inch circumference (single), and have an MBL of approximately 720 metric tons, whilst the main towing lines in the same arrangement have a MBL of around 400 metric tons. Not all tug boat companies use stretchers. An average 20 inch circumference stretcher (diameter 160 mm) could be 20 to 55 meters long. Handling and storage are often criteria not to use stretchers. Life time of towing lines The life time of towing lines is commonly based on the number of towing miles the wire was operational. Roughly, a towing line should make approximately 55.000 towing miles. When a tug boat operates on its average efficiency, these 55.000 miles are reached within approximately 2 and a half years. Besides, the life time, or service life of a towing line depends on a few other things as well. Weather conditions during towing is an important factor. If rough weather occurs, the towing arrangement can be subjected to shock loading, which is a destructive phenomenon to ropes. Inspection of quality of towing lines With regular intervals, towing lines shall be checked upon its quality. Visual inspection is one. Another one is to cut of a piece of the towing line and subject this piece of rope to an extensive, internal check. Last but not least, one can carry out a break test on the rope to determine the actual breaking load. Towing Lines