PIPELINE CLEANING (ROUTINE PIGGING)

 

Pigging in the context of pipelines refers to the practice of using devices known as “pigs” to perform various maintenance operations. This is done without stopping the flow of the product in the pipeline.

These operations include but are not limited to cleaning and inspecting the pipeline.

 

This is accomplished by inserting the pig into a “pig launcher” (or “launching station”) — an oversized section in the pipeline, reducing to the normal diameter. The launching station is then closed and the pressure-driven flow of the product in the pipeline is used to push the pig along down the pipe until it reaches the receiving trap — the “pig catcher” (or “receiving station”).

 

Pigging has been used for many years to clean large diameter pipelines in the oil industry. Today, however, the use of smaller diameter pigging systems is now increasing in many continuous and batch process plants as plant operators search for increased efficiencies and reduced costs.

 

Pigging can be used for almost any section of the transfer process between, for example, blending, storage or filling systems. Pigging systems are already installed in industries handling products as diverse as lubricating oils, paints, chemicals, toiletries, cosmetics and foodstuffs.

 

Pigs are used in lube oil or paint blending to clean the pipes to avoid cross-contamination, and to empty the pipes into the product tanks (or sometimes to send a component back to its tank). Usually pigging is done at the beginning and at the end of each batch, but sometimes it is done in the midst of a batch, such as when producing a premix that will be used as an intermediate component.

 

Pigs are also used in oil and gas pipelines to clean the pipes. There are also “smart pigs” used to inspect pipelines for the purpose of preventing leaks, which can be explosive and dangerous to the environment. They usually do not interrupt production, though some product can be lost when the pig is extracted. They can also be used to separate different products in a multiproduct pipeline.

 

If the pipeline contains butterfly valves, or reduced port ball valves, the pipeline cannot be pigged. Full port (or full bore) ball valves cause no problems because the inside diameter of the ball opening is the same as that of the pipe

 

ROUTINE PIGGING, simply refers to a systematic programm carefully followed for the periodic clean of the production pipeline using pigs. This program when followed as outlined, enhance the follow of product and eliminate restriction which could result increase in pipeline pressure.

 

MADEL OIL AND GAS LIMITED is made strong personnel who have managed, supervise and operated this method of pipeline cleaning for over 15 years working with multinationals and indigenous companies alike. Considering the wealth of experience of MOG team, we have the network of pipelines both on land and swamp at our finger tip.

 

MOG experience shows that some pipelines have not been pigged for a very long time, or strong put, have not been pigged from the day of pipe laying completion. Hence to embark on such routine cleaning operation requires skill and techniques. MOG team member have managed and supervised challenging project of this nature successfully in the past.

 

This was done by the introduction of specialised chemicals that keeps the debris or sand in a loose form to ensure that compact sand does not occur hence leading to heavy load of debris in front of the pig  resulting to high back pressure and pig stucking.

 

MOG Team have managed and supervised several of routine pigging operation using both product and auxillary pumping support for cases where the product flow rate can’t propel the pig at the desired pig velocity. Hence pipeline cleaning by use of pigging operation has been our regular routine. You can count on MOG anytime.

 

PIPELINE HYDROTESTING

 

On completion of the cleaning, gauging and filling operations, the pipeline shall then be filled to eradicate all air bubbles, and be tested to the Acergy/Company specified test pressures (Design pressure x 1.4) and held for 24 hours. This operation shall be performed using a high pressure pump spread located on the Support Vessel. Water shall be sourced from the sea water and pre filtered through100 mesh size filters and chemically dosed.

 


Initial Pressurization

 

Pipeline section shall be topped up with water and all air vented from the high elevation point. Pressurization shall commence at the rate of 1 bar/min from atmospheric pressure until 35 barg is reached where upon pressurization shall cease. The pressurizing pumps shall be shut down and a 15-minute hold implemented. During this period, an air entrapment calculation shall be carried out. A pressure volume graph shall be produced using figures obtained during pressurization.

Should the air content exceed the maximum allowable of 0.2%, the pipeline section shall be depressurized and all air vented. Re-pressurization shall recommence until acceptable air content is achieved.

 

Final Pressurization

 

On attaining a successful air entrapment, pressurization shall continue as before to 80% of test pressure at the same rate (1 barg/minute, and held for not less than 1 hour. Thereafter, at 0.5 bar/min up to 95% of test pressure and held for 30 minutes and then finally to the test pressure. The following data shall be recorded at half-hour intervals during pressurization

 

  • Line pressure from dead weight or pressure gauge or Digital Pressure indicator
  • Ambient temperature
  • Volume of water injected

 

Stabilization and Hold Period

 

On completion of pressurization, the line shall be allowed to stabilize. During this period, re-pressurization may be carried out should the pressure drop below test pressure and the volume of water logged.

All instrumentation shall be certified fit for purpose prior to testing.

 

On completion of a satisfactory stabilization period of 3 hours or as may be requested by Acergy, the pressure shall be held for a 24 hour period. At this point, NO

further pressurization is to take place during this 24-hours period, unless attributed to temperature or environment effects etc.

The following data shall be recorded at every 15 minutes for the stabilization and the hold periods.

 

  • Time
  • Dead weight/ Digital Pressure indicator / Gauge Pressures
  • Ambient Temperatures & Subsea Temperatures

 

Pressure drops not attributed to ambient temperature variations shall be agreed with the Acergy Representative on site may attract re-pressurization.

 

Depressurization

On test acceptance after completing a satisfactory hold period, as agreed with the Acergy Rep on site, depressurization shall commence at not more than 1 bar/min. The pressure recorder chart shall not be removed until the pressure is at ambient.

 

 

 

PIPELINE DEWATERING AND DRYING

 

Dewatering operation is an activity in the precommissioning of a new pipeline, depending on the use. If the pipeline is used for gas, then it must be dewatered. This operation follows a simple sequence of running bidirectional pigs on a given pipeline after hydrotesting operation. Couple of bidirectional pigs are run to carry out bulk dewatering, after which a pig train is run to carry out the final dewater operation. Once the final dewater is accomplished, the pipeline will be ready for the drying operation.

The typical dewatering sequence of operation is as shown below.

 

BULK DEWATERING

 

1st Bidirectional Pig

2nd Bidirectional Pig

3rd Bidirectional Pig

 

FINAL DEWATERING

 

1ST High sealing Bidirectional Pig

2ND High Sealing Bidirectional Pig

3RD High Sealing Bidirectional Pig

 

The drying operation starts once the final dewatering is accomplished. The drying process involves the used super dry compressed air run couple of foam pigs through the pipeline, ranging from medium density foam pigs to soft foam pigs. These foam pigs will be run continuously until a dew point of -20oC, then the pipeline will be considered dry and ready for commissioning through the use of nitrogen. The typical drying sequence of operation is as shown below

 

 

DRYING PIG RUN SEQUENCE

 

1ST Medium Density Foam Pig

2ND Medium Density Foam Pig

3RD Medium Density Foam Pig

4TH Medium Density Foam Pig

 

1ST Soft Foam Pig

2ND Soft Foam Pig

3RD Soft Foam Pig

4TH Soft Foam Pig

 

NITROGEN/HELIUM LEAK TESTING

MADEL OIL AND GAS LIMITED is an indigenous company that is foremost in providing Commissioning Services, having developed and introduced many of the specialist techniques, which are now regularly used during the commissioning of process plant and pipelines.

 

MOG has in the past been involved in leak testing operations using the soap bubble test method, but has recently purchase a nitrogen/helium mixing pump and a portable mass spectrometer. We have also trained our personnel accordingly to meet up with the demand for Nitrogen/Helium Leak Testing operation in the nation.

 

Nitrogen/Helium Leak Detection was first introduced in the late 1970’s and has been improved and refined continuously. Nitrogen/Helium Leak Detection is achieved by pressurizing the system or component to be tested to their design pressure with a test medium comprising of 99% Nitrogen and 1% Helium.

 

To obtain 1% Helium in Nitrogen gas mixture, the liquid Nitrogen is pumped from the Nitrogen storage tanks at the rate of 1000 scf/min.  And then mixed with the Helium which is pumped by the Helium boost pump at the rate of 10 scf/min supplied from a quad of Helium cylinders.

 

To carry out the leak testing, all flanges are taped so that any gas leaking past the gaskets or seals is collected in the space between the outer edge of the gasket and the tape. Leak detection is performed by puncturing the tape with the remote probe. This in turn is connected to the portable Helium mass spectrometer. Hence, any Helium gas that has collected beneath the tape is sampled, detected and quantified.

 

Finally, a fully documented report is prepared covering all aspects of the leak testing process, highlighting the extent of the system undergoing the test and listing all leaking joints provides an invaluable record for future shutdown / maintenance planning.

 

Nitrogen/Helium leak testing offers the advantages of reducing the oxygen content of a process plant; systems are tested close to their operating pressure; all leaks are detected and repair before usage; and an inert nitrogen blanket can be left in the system prior to kick off. The system is tested at or close to its working pressure with a gas

 

closely simulating “live” conditions.  This also allows instruments to be calibrated or checked and operators to gain familiarity with the system.

The purpose of Nitrogen/Helium Leak Testing is to render production equipment and piping systems safe for the introduction of hydrocarbon gases and liquids and to create an auditable leak testing report to demonstrate the integrity of all such systems.

 

Outlined below are the different stages involved in Nitrogen/Helium leak testing

 

 

All potentially leaking joints are taped with coloured adhesive tape to avoid leaking test gas from being blown away from the leaking joint and to aid system identification. A remote probe is then used to sample the gas beneath the tape.

 

The sample probe is attached to a helium mass spectrometer, which is used to quantify the concentration of helium beneath the tape and establish the leak rate from that joint.

 

The nitrogen and helium mix is used as it closely simulates the hydrocarbon gas that will be present in the system once it is commissioned.

 

System Preparation

System Pressurization

Nitrogen gas is pumped into the system or plant as it may be, using a nitrogen converter unit. The system is pressurized at a rate agreed upon by both ENL and the client, often at low rate of 100scf/min for the initial period. The system is often pressurized up to 1barg and a walk is taken round the system to check for audible leaks or major pressure drop, should any leaks be observed, the system will be depressurized and repair work done. Upon no circumstance will repair work be allowed with system under pressure.

 

The system is then gradually taken to 25%, 50%, 75% and finally the predetermined test pressure.

 

 

Stabilization and Test Period

 

Upon reaching the test pressure, a predetermined and agreed stabilization time is observed to allow the system pressure to stabilize. Usually a pressure drop is observed due to the nitrogen gas cooling off; the system is then topped up once again to the test pressure. The stabilization period is often about 15 – 30mins, upon satisfactory stabilization the testing period will commence.

 

Leak Identification