After the BHA has been pumped down to target depth, they are pulled up to plug depth with a certain tension. If this tension is incorrect or there are issues with the line, the line can part. This can be very troublesome as trying to pull the tools and line back to the surface can cause the line to continue parting and more issues can occur.
Issues with a parting line either involve the incorrect tension level, natural wear on the line from regular use, or a combination of the two. It is possible for the tension reading devices to give incorrect outputs, which could lead to applying too much pressure on the line. This could result in parting. On the other hand, as a line wears down it is possible for this issue to occur even with the correct level of tension.
When parting occurs it is extremely important to take every action necessary to pull the line and tools to the surface successfully. The line should then be inspected and sent to an appropriate third party for further investigation. These issues highlight the importance of routine maintenance checks and keeping careful track of the state of all instruments and equipment.