Pipeline Commissioning

By definition, commission is a request to someone to perform a task (duty or mission). The pipeline mission is to transport products safely, without failure or leak during the design life. Commissioning (or startup) is to introducing the first product in the pipeline system after the new system is installed. Prior to commissioning, the pipeline system needs to be checked for cleanness, structure strength, leak proof, etc. These actions are called pre-commissioning which include;
  • Debris removing, cleaning, gauging, and flooding (watering)
  • Hydrotesting and leak testing
  • Dewatering and drying
After installation, pipeline should be checked for internally cleanness and free from debris such as welding rods, tools, etc. After debris-removal pig runs, a wire-brush cleaning pig should run to remove more stubborn debris such as mill scale, weld bead slag, etc. After cleaning the line, the pipeline should be checked for the pipe ID reduction due to dent or flattening (increased ovality), by using a guage pig. The guage pigs are fitted with aluminum plate of which diameter is typically 95% of the minimum pipe ID. Now the pipeline is ready for hydrotesting and should be filled with filtered water with biocide or corrosion inhibitor (for a long flood time). Prior to water pumping, a pig is placed in front of the water to ensure removal of all the air in the line. To save offshore operation cost, the above steps could be performed simultaneously using a series of pigs (pig train) while flooding the line.

Each pipeline system, such as pipe segments, jumpers and PLETs, are hydrotested at factory or confirmed by structural integrity test (SIT) or factory acceptance test (FAT). However, the overall pipeline system, after completion of transportation and connections, should be checked for its structural integrity (hydrotest) and leak proof (leak test). The hydrotest pressure is set to be no less than 1.25 times of the maximum allowable operating pressure (MAOP) or no more than 90% of the pipe SMYS, for at least 8-hour holding time. The gas riser needs to be hydrotested for at least 1.5 times of the MAOP. The leak test can be done with 1.1 times of the MAOP, for at least 1-hour holding time.

After successful hydrotesting or leak testing, pipeline needs to be dewatered before introducing the oil or gas. Dewatering pigs (or pig train) is used to displace water efficiently. Air drying or vacuum drying is required for dry gas pipelines, but not required for wet gas or oil pipelines. If pipeline is dewatered using a nitrogen gas, there is no need to dry the pipeline.

During commissioning, pigs (pig train) are located in front of the first produced gas or oil, to remove remaining air in the line and ensure that the line is fully filled with the product.

Commissioning of a Multi-products Pipeline

Multi-product pipeline commissioning
Source: http://petrofed.winwinhosting.net/upload/15-18June11/13.pdf

  • Ensure the pipeline is ready in all respects for the commissioning.
  • Ensure adequate product (HSD) is available for the commissioning (At least more than line-fill of a Pipeline section).
  • The pipeline or a section of the pipeline is pressurized with air (2 to 3 Kg/Sq-cm)
  • Pump a water column of about 500 kLto 1000 kL
  • Launch a Bi-Di Pig followed by 1000 kLto 2000 kL water
Multi-product pipeline commissioning
Source: http://petrofed.winwinhosting.net/upload/15-18June11/13.pdf

  • Launch second Bi-Di-Pig behind 1000 KL to 2000 KL water
  • Pump HSD at minimum flow
  • Track the movement of Pig
  • Vent the air at receiving end and maintain back pressure 3 to 4 Kg/Sq-cm
  • Monitor receipt of water at Receiving end –it may be disposed directly in to a protected pit or in a designated tank.
  • Receive Ist Bi-Di Pig
  • Monitor the quantity and quality of water at receiving end
  • Receive second Bi-Di Pig
  • Once diesel or diesel mixed with water arrives, switch over to
    designated HSD Tank.
  • Observe flow and pressure, once these become stable, the operation may be made normal and the pipeline may be declared as commissioned and handed over to Operation.
Commissioning of an ATF Pipeline

ATF pipeline commissioning
Source: http://petrofed.winwinhosting.net/upload/15-18June11/13.pdf

The procedure is similar to multi-products pipeline except the following
  • Commissioning liquid is used SKO instead of HSD
ATF is not used because it is likely to get contaminated with water and dirt particles.

Commissioning of Naphtha Pipeline

Naphtha pipeline commissioning
Source: http://petrofed.winwinhosting.net/upload/15-18June11/13.pdf

The procedure is similar to multi-products pipeline except the following
  • Air from entire Pipeline is displaced with Nitrogen column using a Bi-Di Pig
  • Commissioning liquid is used Naphtha instead of HSD 
Commissioning of LPG Pipeline

LPG pipeline commissioning
Source: http://petrofed.winwinhosting.net/upload/15-18June11/13.pdf
 The procedure is similar to multi-products pipeline except the following
  • Air from entire Pipeline is displaced with Nitrogen column using a Bi-Di Pig
  • Commissioning liquid is used LPG instead of HSD 
Commissioning of a Gas Pipeline

A gas pipeline is treated as Tank. However, before taking gas in the Pipeline, it is to be made moisture and oxygen free.

Commissioning Steps:
  • Drying –Purging of super dry air
Compressed Super dry air is purged using air compressors with accessories viz. Moisture Separator, Oil Separator and Dryer. The air shall be supplied in the pipeline at (-) 20 deg. C dew point. Super dry air with dew point (-) 20 deg. C will have sufficient capacity to absorb water vapour to the extent of 30 % of the desired capacity.

  • Vacuum Drying
The process utilizes high capacity vacuum to reduce pressure within the Pipeline to a level from 760 Torr to 40 Torr. At this pressure (40 Torr), any water within the pipeline will start boiling and vaporizing.

Air left inside the Pipeline is subjected to a vacuum of 40 Torr, the water vapor will expand approx. 18.8 times, which will be displaced using high capacity vacuum booster pumps (rated capacity 5000 CuM/Hr).

When the vacuum of 7.6-10 Torr is achieved, it is confirmed that whole pipeline system has been dried to required level.

  • Nitrogen Purging
At vacuum level 7.6 Torr and dew point (-) 20 deg.C, the oxygen content inside the Pipeline is 0.20%.

To further dilute the oxygen content, nitrogen purging is done almost 13 times more than pipeline volume. This will reduce oxygen content to 0.015%, which is considered negligible.

Now the Pipeline system is ready to receive Gas.

Introduction to Offshore Pipelines & Risers - Jaeyoung Lee

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