Pipeline Commisioning

The critical procedure of drying offshore pipelines to allow safe transportation of natural gas is greatly aided by use of pigs of borate-crosslinked gel, uncrosslinked gel, and methanol pumped through the pipelines. A pig can be a piece of hardware or a compartment of liquid pushed through a line by pressure to clean the line, remove obstructions, dry out the line, or other functions.

A single train of mechanical pigs and gel, driven by gas or drying liquid, can be pumped through the pipeline to dewater the line as part of the commissioning process.

Some operators choose to dewater the pipelines with mechanical pigs to remove most of the water, then complete the drying by pumping a second train of pigs including foam pigs, nitrogen, and methanol.

Commissioning, the process of bringing a pipeline to a state of readiness for use, can present a challenge rivaling the challenges faced by pipeline construction teams. To allow for safe transport of dry natural gas through the completed system, operators must purge the line of all debris left during construction and all water left by subsea construction, cleaning, and hydrotesting procedures. A gas pipeline must be dried to specific levels to control:

• Hydrate formation within the pipe
• Corrosion in lines when sulfur is a component of the gas
• Formation of carbonic acid when carbon dioxide is a gas component.

Dryness defined

Pipelines that will be used to transport gas must he dry before put into service. Dryness is expressed in several different ways:

• Dew point in degrees F at 14.7 psia
• Dew point in degrees C at 14.7 psia or 1 bar absolute
• Parts per million (ppm) by volume
• Parts per million (ppm) by weight
• Pounds of water per volume (lb/MMcf) of air.

A dryness level of -35° C dew point at 14.7 psia means the same as the following: (1) -3l° dew point at 14.7 psia, (2) 222 ppm by volume, (3) 140 ppm by weight, and (4) 11.1 lb/MMcf.

One train or two?

Operators dewater pipelines by using the pumping methods listed as follows:

1. Pumping mechanical pigs to swab out most of the water, then pumping drying pigs (consisting of foam or methanol),
2. Pumping mechanical pigs to remove most of the water, then vacuum-dry the line or dry it by air convection,
3. Pumping a combination pig train to swab out water and complete drying of the line by chemical means in one trip.

Method 3 above offers many economic benefits, especially in drying long, low-temperature subsea pipelines. Use of hygroscopic fluids (fluids that readily take up and retain water) in pig trains eliminates the need for vacuum or air-convection drying to remove the last traces of moisture from pipe-wall matrices.

In vacuum drying, pressure in the line is reduced to atmospheric level (sometimes as low as 0.l atmospheric) for a time, then air is sent through the line to evaporate moisture. However, air is not effective at drying subsea lines.

Hygroscopic fluids in the pig train absorb most of the water left behind by the dewatering pigs in the train. Further, any fluid left behind by the pig train will be hygroscopic and hydrate-inhibiting in nature. The first contact with the product sent through the line will absorb the last traces of moisture remaining in the line.

The advantages of including hygroscopic compartments in the dewatering train and drying the pipeline in a single pass are:

• The pipeline can be used for delivery at an earlier time, while vacuum and air-convection drying times can require months to accomplish. Hygroscopic fluid-drying can be accomplished concurrent with the dewatering process.
• Hygroscopic-fluid drying eliminates the need for deck space required for vacuum/air-drying equipment.

Not every pipeline is a candidate for single-pass dewatering/drying. Those exceptions include:

1. Multiple tie-ins with connections that cannot be swept by the dewatering train.
2. Valve stations that allow water to drain into them.
3. Manifold sections that preclude pig access cannot be commissioned in a single pass.

It is still advantageous to include gel compartments in any dewatering train that can be used, since the gel helps reduce the amount of water that must be dried by vacuum or heated air. Disposal of hygroscopic fluids may present a challenge in some areas, but in most cases will not preclude use of such fluids.

Selection of the most appropriate commissioning method should include the following considerations: the level of dryness required, the product that will be delivered through the pipeline, and conditions in the pipeline that may affect drying methods.

Parameters

The volume of water left in the pipeline by passage of pigs is influenced by (1) coating or the lack of coating on the internal pipe (ID), (2) pipe roughness, (3) pig bypass, and (4) pig lubrication.

• Coating: Pipeline coatings may be added to leave an almost nonporous ID, except for weld ends. Coated pipelines are easiest to dewater for use, since the volume of water left by swabbing is very small. To dry most coated pipelines, operators include compartments of methanol in the pig train to contact water on the pipeline walls. Methanol dries water rapidly on contact. Operators may even dry coated pipelines by including a hydrate inhibitor in the dewatering pig train sent through the line, and allowing the gas to dry out the last remnants of water.
• Pipe roughness: In lines already in service as liquid carriers but being converted to gas pipelines, pipe roughness causes a high level of moisture retention in the pipe. The volume of water requiring removal can be twice the volume required in new pipe. The film of water left after use of foam pigs or other dewatering methods may require drying by vacuum or air convection to remove the water contained in the matrix of the pipe wall.
• Pig bypass: If a pig train is moved too slowly through the pipeline, the train will start and stop in a jerking movement. Each time a pig train stops, it may bypass fluids and gas ahead, bypass fluids or gas to the rear, or reverse directions. Fluid and gas bypass around static pigs occurs commonly at weld seams. Reversal of direction can occur during fast shutdown of train movement. When a pig reverses direction, a great volume of water can bypass the pig.
• Pig lubrication: Proper lubrication is necessary to prolong the life of the cups/discs that contact pipeline walls to wipe moisture and debris from the walls. Water or methanol based gels help lubricate pigs when small amounts of fluids bypass the pig during travel. Gels also influence the flow regime around the pig. The viscosity of these gels aids in prevention of forward and reverse bypass of gas and fluid.

Drying gels

Use of gels (included as compartments in the pig train) as sealants/lubricants has proven to be critical to the success of drying long, undersea pipelines, contributing greatly to the longevity of the pig discs/wipers. Schreurs et al. reported the successful commissioning of a 500-mile undersea line using gel pigs and mechanical pigs in a single train.

Results of extensive computer simulation and laboratory experimentation led Schreurs et al to recommend use of uncrosslinked, methanol and/or water-based gels for pipeline drying. In actual practice, the gelled methanol pigs performed best, showing minimal degradation. The water-based gel pigs performed adequately. Where environmental concerns restrict the use of methanol, or when a pipeline does not have critical dryness requirements, water-based gels can be used with only slight degradation in gel performance.

In laboratory testing, it was observed that uncrosslinked water gels and methanol gels provide significantly more friction reduction than that provided by water or methanol. High-viscosity crosslinked gels, however, provide no lubrication to the pig train and make the train difficult to start into motion. The viscosity decline on uncrosslinked gels compares favorably with that of borate-crosslinked gels. Therefore, uncrosslinked gels are the preferred choice for use in dewatering pig trains.

Operating the train

The pig train illustrated on page 70 includes a displacement fluid supply, a dewatering train, and a valve for water flow rate control. Flow rate helps determine the speed of pig train travel.

On page 72, a typical pig train that was placed in sequence to cleanse pipe walls, push debris ahead of the train, lubricate the pigs, dry the walls (drying by hygroscopic fluid such as methanol), and help prevent bypass of driving gas ahead of the train. Escape of gas ahead of the train can cause formation of hydrates, which can completely block the pipeline. A very heavy methanol gel can be designed to help prevent bypass of propellant gas, but leave minimal residue on the pipeline walls.

To control pig train operation, operators modulate the supply gas rate, hold a constant pressure, and control the water discharge through the valve at the downstream end of the line. A rapid valve-opening schedule results in high hydraulic transient pressures that may cause rapid velocity changes. An opening schedule that is too slow would cause start-stop or jerky pig movement, which would result in forward pig bypass.

During startup, the train must be accelerated rapidly enough to avoid start-stop pig motion. During shutdown, the train must be decelerated to a stop as quickly as possible without causing a pig reversal. Single-pass dewatering/drying of subsea pipelines offers economy of time and resources when conditions allow. When single-pass drying is precluded by conditions or pipeline arrangement, inclusions of gel compartments in any pig train used reduces subsequent drying time required.

References

Azevedo, L.F.A., Braga, A.M.B., Nieckele, A.O., Naccache, M. F., Gomes, M.G.F.M.: "Simple hydrodynamic models for the prediction of pig motion in pipelines," Pipeline Pigging Conference, Houston, February, 1995.

Schreurs, S., Falck, C., Hamid, S., Burman, P., Maribu, J., Ashwell, C.: "Development of Gel Systems for Pipeline Dewatering and Drying Applications," OTC 007577, Offshore Technology Conference, Houston, TX, May 2-4, 1994

Hamid, S., Falck, C., Wissing, M., and Grotberg, I.: "Dynamic Characteristics of a Pipeline Dewatering Train," OTC 007578, Offshore Technology Conference, Houston May 2-4, 1994.

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