Retrieval of a stuck cleaning pig from a natural gas pipeline
Pakistan’s largest fertilizer complex, owned by Fatima Fertilizer Co Ltd (FFCL), sources natural gas to power its operations through a 47 km, 20 inch diameter pipeline that runs from the Mari gas field. Each month, FFCL uses a cleaning pig to remove condensate from the pipeline. In a recent operation, the cleaning tool became stuck. This article describes the events that followed to retrieve the tool.
Fatima Fertilizer Co Ltd (FFCL) is the largest fertilizer complex in Pakistan and is a fully integrated plant, capable of producing fertilizer products, two intermediate products (ammonia and nitric acid), and three final products which include urea, calcium ammonium nitrate (CAN), and nitro phosphate (NP). The plant has its own utilities and 60+ MW of power generation facilities to support the production of these chemicals. The details of plant and its facilities are summarised as follows:
The plant takes its natural gas feedstock from the Mari gas field, located at Daharki, District Ghotki, Sindh, through a 47 km long, 20 inch diameter pipeline, with a wall thickness of 7.13 mm (0.281 inches) and a maximum allowable operating pressure (MAOP) of 6.894 MPa (1,000 psig).
A reciprocating booster compressor is installed near the origin at the Mari gas field: the suction pressure at the booster compressor inlet is 3.824 MPa (554 psig), and the discharge pressure is 4.021 MPa (583 psig).
Once each month, FFCL routinely uses a mandrel-type mechanical cleaning pig for removal of condensate from the 47 km long pipeline.
Pigging operation, September 2014
The cleaning pig was launched at 13:30 on 10 September 2014 from pig launcher at the Mari site. Following the normal routine, its expected arrival time at the pig receiver near the fertilizer plant was 16:30 on the same day. However, at 17:45, the pig had not been received, and it was therefore decided that it had become stuck somewhere in the line.
The effect of this was two-fold: first was the decrease in gas pressure by around 0.2 MPa (30 psig) at the ammonia plant which caused a load decrease at the plant, and the second was an abnormal noise coming from the line near the mid-valve assembly which was noticed by a patrolling security guard. The mid-valve assembly is about 23 km from the plant and, as the FFCL personnel reached the site, the sound could be heard emanating from the above-ground section of the line near valve assembly. It was therefore suspected that the pig was stuck at this location.
The FFCL personnel discussed the best way of solving the problem, and consulted with both South Natural Gas Pipe Line (SNGPL) and the Mari Gas Co. It was decided to try to dislodge the pig by hammering the line with a wooden/brass mallet near the location of the sound at the valve assembly, but this had no effect on moving the pig. Subsequently, the downstream valve in the assembly was partially opened/vented in order to increase the pressure behind the pig and hopefully move it: this also, however, had no effect.
Next, the upstream valve in the assembly was gradually fully opened/vented and, as a result, the stuck pig was released and moved, with a sudden sound of its passage as it passed the valve assembly. It should be noted that the cleaning pig had not been equipped with any pig-tracking equipment. Nevertheless, the stuck pig was retrieved by FFCL’s engineers in less than 24 hours, and it was established that there had been no damage to the pipeline.
The following observations were made upon the tool’s arrival at the receive trap:
- A high amount of condensate was received when compared to previous cleaning-pig runs;
- The front polyurethane cup on the pig was found to be broken with about 30 per cent of the cup missing;
- Two of the connecting bolts between the cup and its support disc were found to be broken and stuck to the magnetic bars on the pig;
- The front carbon steel disc (the “÷holding plate’) was found deformed and reduced in diameter by up to 28 mm (1.2 inches);
- The holding plate’s original 1,170 mm circumference had been reduced by around 30 per cent (300 mm);
- The accumulation of iron particles on the magnetised bars on the pig was greater than normal; and,
- Analysis of the condensate showed no major difference in the concentration of iron particles compared to previous activities.
The condition of the damaged pig showed that after the front cup broke, the pig lost its drive and the front carbon steel disc holding plate (which held the cup) came directly into contact with the bottom of inner wall of the pipe, causing severe rubbing due to metal-to-metal contact. This might have resulted in scratches/dents and pitting on the inner wall of the pipe, and also may have damaged the root passes of the pipeline’s welded joints.
Pigging operations on this pipeline had been developed over recent years by the pipeline’s operator, SNGPL. During commissioning, the following sequence of pigging activities had been undertaken:
- A wire-brush scraper pig was initially launched, followed by a foam pig;
- After foam pigging, the line was blown-down, and deposits were found;
- A second wire-brush pig run was made;
- A scraper cup pig with magnetised bars was launched, and the line was blown-down again;
- Following this, excessive gas condensate was found;
- A foam pig was launched to remove the gas condensate from the line, but it became lost in the line;
- A further scraper cup pig with magnetized bars was launched to remove the foam pig; and,
- Gas was vented, and pieces of the foam pig were removed.
For subsequent pigging activities, a contract with Marathon Engineering was established and the company carried out three pig runs. FFCL then decided to perform pigging on its own. A pig-selection study was carried out by the company’s Process Engineering Department, which recommended the use of a scraper cup pig with magnetized bars, and this was purchased by FFCL.
A checklist and launching procedure was developed by FFCL, and maintained by the company’s Operations Department, supervised by the OU Operations Sub-Engineer. The pig-launching frequency was based on condensate quantity and, at present, it
Subsequent to this experience, the following recommendations have been made:
- A detailed inspection of the mid-valve assembly must be carried out;
- An internal inspection of the pipeline by intelligent pigging should be undertaken to ensure the integrity of the pipeline;
- Foam pigs should be used for cleaning the pipeline until the internal inspection can be carried out;
- The shelf life of the polyurethane scraper cups prior to launching of the pig should not be exceeded;
- Storage of new scraper cups should be in a controlled environment as recommended by the manufacturer (the maximum shelf life of polyurethane cups should not exceed 18 months);
- The correct fitting of the scraper cup on the pig’s disc prior to launching should be ensured, with all bolts and the cover plate correctly installed;
- An RF/electronic transmitter should be installed on the on pig to allow location during pigging;
- The pig’s velocity should be 4-5 metres per second, if necessary reducing the line flow to allow this speed to be maintained;
- A pig launcher and receiver unit should be installed at the mid-valve assembly;
- A speed-control pig should be employed for future cleaning; and,
- A check list, to be developed by the Operation and Maintenance Departments in the light of these observations, should be completed by the Operations and Maintenance executive engineers prior to pig launching.