MECHANICAL DAMAGE TO PIGS DUE TO REDUCTION IN DIAMETER OF PIPELINES AND PIG BYPASS LUBE OIL PIPELINE

We often encounter damaged pigs after a pigging operation. During construction of a 30”NB liquid pipeline 12 pigs were damaged.
The main damages seen

i. Bending of cup mounting Plate or impact marks on disc / plate / cups ii. Bending or Buckling of main body pipe iii. Fasteners loosening during pig run
Reasons of damage
i. Diameter reduction due to damage or debris ii. Partially closed valves iii. Pilferage attempts

Twelve damaged pigs were found. The matter was further complicated as the damage occurred in a pipeline segment, which was pigged successfully. The total operation lasted 45 days. We ran a caliper Pig and discovered that when the tie in occurred between the two individual sections there was an alignment mismatch. To align the pipes a massive mechanical force was applied which buckled the pipe.
A lube oil pipeline was constructed, hydrotested and commissioned. During all stages pigs regularly bypassed and stalled in the pipeline, however with insertion of a following pig, a pig would be removed and matter remained unsolved till the operations started.
During a detailed operation check it was found that when the second pig was launched it would stall, when the third pig was launched the first pig would be recovered, however two pigs would always remain in the pipeline and pressure would not build up. As there were a number of vertical expansion loops it was assumed that the loops were causing the problem.
A pig with transmitter was launched and tracked. The launcher was functioning perfectly, the movement was as desired, crossing all the vertical loops without a problem. At an intermediate tap off, an impact was felt / heard in the transmitter and the branch topper flow tee. On close examination of the flow tee design we discovered that the slot area of flow tee which was greater than the length of single tee but less than the length of two tees. The problem was solved by running two bi-directional pig with water and air first and the defective flow tees cut and new tees installed.
The line today runs with only one batch pig.