Dealing in extremes – part two

A grid, drafted to the customer’s specification, was drawn on each pipe in order to record concise information and enable consistency of reporting. The coating thickness was recorded at each impact point prior to testing and photographed, then again after testing had taken place at each individual location.

Each impact point was allocated its own unique identification mark. The pipes had to be closely inspected visually for any obvious defects to the external coating, such as cuts or gouges that would potentially be detrimental to the impending impact testing taking place. This was checked and confirmed by the high voltage holiday test to ensure the integrity of the coating. Anything observed was recorded prior to testing and marked on the coating, enabling the team to factor in a distance allowance to ensure the test was not compromised or recorded as a failure. Each impact point was photographed before and after impact for visual comparison as well as technical reporting.

At each test temperature, the pipes had a specific minimum soak period, at which the data collected from the chamber, pipe substrate and surface coating temperatures was recorded and graphs produced to prove the temperatures and relative humidities over the soak and test periods. This also enabled further monitoring of the pipes and chamber throughout their return to ambient temperature.

On completion of the allotted soak period, the data collected from the chamber and pipes was scrutinised, ensuring that conditions met the required client specification. Impact testing could then be carried out at the designated points along the pipes and conducted in accordance with ISO 21809-1 Annex E at temperatures: -10, -15, -20, -25, -30, -35 and -40°C. Variable impacts were used to replicate the effect on the exterior coating during pipe handling. Therefore, a fixed impact force was used for the first five impacts, with the impact energy being increased incrementally thereafter.

Each impact point was visually inspected for any obvious evidence of cracking. If cracking was evident, the next impact was done a set minimum distance from the coating failure. This was repeated for each of the pipes under test, at each of the above temperatures.

Following completion of each individual temperature test, the chamber had to be brought back to ambient temperature so that each test pipe could be prepared for high voltage holiday detection. It was essential that the pipe samples were moisture-free prior to next stage testing to avoid the voltage tracking along the moisture.

The holiday detection was performed in accordance ISO 21809-1 Annex B, and any failures recorded and reported. Using a high voltage holiday detector (25 kV), the spark that shows a failure in the coating is clearly visible as it jumps across to the pipe substrate. Each impact point was meticulously scrutinised for failure. The determination point for the coating to have failed enough to be of concern was set at three consecutive fails along the pipe at the test temperature. Tests were repeated by rotating the pipe to the next line on the circumference of the grid, and then the chamber conditions were programmed and set ready for the next test.

How did the pipes perform under these extreme sub-zero conditions? They performed well. As for the final part of the testing not mentioned above, Exova moved on to impact testing both pipes on the seam weld at -20°C. This is the thinnest part of the coating, where you could potentially get a failure. There was a small number of fails, but nothing that met the three consecutive fails criteria in the customer’s specification. This would have meant a fail of that coating at that temperature, signalling the end of testing on the ‘one under test’.

By using specialist chambers to replicate some of the harshest environments on Earth, Exova was able to perform tests that provided the customer with valuable data and insight into tackling challenging conditions. The thorough testing methodology proved to be invaluable in terms of outweighing the potential costs of failure in the field during transportation and installation, as well as the longevity of the coating of the system used on the pipeline.

Challenging conditions

Even in controlled environmental conditions, there are risks of hypothermia setting in if people are subjected to low temperatures for too long. So extreme were the temperatures required for the pipeline testing that specific procedures had to be followed.

Temperatures down to -15°C did not create too much of a problem, but anything below this required advanced protective clothing to be worn, and the resulting margin of safety from the extreme coldness of the chamber was rated for temperatures as low as -50°C.

Additional, tried and tested, procedures also had to be implemented. One person remained outside the chamber at all times while testing was carried out. Specialist personnel from the chamber company’s team monitored reactions of personnel working in the low temperatures. As soon as temperatures went below -20°C, time allowed inside the chamber was restricted to a 15 min. window, before personnel were withdrawn and given a minimum rest period of 45 min. to recover from any effects of being subjected to extreme cold.

Everyone present was also schooled in emergency procedures, such as escalating the need to exit the test chamber, or how to evacuate anyone suffering ill effects or showing signs of distress due to the low temperatures involved.

Health and safety is paramount when working in extreme sub-zero temperatures, and procedures were reviewed and agreed by all parties who were involved prior to commencement of live testing, as well as the team responsible for safety onsite.

Written by Geoff Addicott, Exova, UK, and edited by Hannah Priestley-Eaton

Read the article online at: https://www.worldpipelines.com/special-reports/01062015/extremes-part-two/