We are all aware of subsea pipelines that are seen in large diameters all over the world, and are being used for wet gas transportation over long distances from offshore fields to onshore facilities. For proper management and passage of uninterrupted gas supply along the chain, it's necessary to understand corrosion mechanisms that occur at the top of the line, as a result of dewing. Apart from the maintenance in production and transport of gas along the chain, this strategy would ensure long-term pipeline integrity for maintenance of the chain. The prediction of corrosion in the system is challenging, for the type of corrosion varies based on the nature of iron sulfide scales that form over the pipelines as a result of temperature ranges and condensation rates.

Corrosion inhibitors, on the inner sides of the pipe line, help in preventing internal corrosion. But it's hard to prevent corrosion on the outer sides of the pipeline. 'Spray pigs' that have the capacity to bypass flow and create a venturi effect are being used now. They help to protect the pipes against corrosion by sucking liquids containing corrosion inhibitors from the bottom of the pipe and spray them onto the top of the pipe. However, if you do not remove the corrosion causing agents, like debris and deposits from the individual pits, corrosion inhibitors will not be effective on the top or bottom of a pipe. To help remove the debris and deposits, we could use self-cleaning equipment like cleaning pigs that have sprung brush bristles that are capable of aggressively removing deposits. To remove stronger corrosion causing agents, the use of bypass flow features seen in certain upgraded pigs are used to certify good prevention and remediation against corrosion.

Keeping the system clean:

Regular Cleaning:

Prevention is always better than cure. Always ensure that your inner pipelines are regularly cleaned, for it prevents and minimizes corrosion. Cleaning is made effective with the use of pigs with the inclusion of chemicals.

Advanced Cleaning :

If you haven't cleaned the inner portions of the pipe for quite a while, you may have to use an advanced cleaning program. To begin with, you may have to use very non-aggressive pigs that give out low density foam. On doing this repeatedly, you can remove small debris and contaminants. In case of very thick deposits, a smaller pig is the tool of choice. Start with that and then later you'll be able to work up to the regular size.

Cleaning Requirements Based on Types of Contamination:

• Types Of pigs to choose from,

• Black Oxide: discs, brushes

• Ferrous debris: magnets

• Liquids: multi-lip sealing cups and/or discs

• Microbes: pit-cleaning / wear-compensating brushes, discs

• Scale: wear-compensating brushes, pit-cleaning brushes, studs

• Sand: discs, brushes

• Wax (hard / soft): blades, discs, self-cleaning / wear-compensating brushes

To ensure even more effective cleaning, its better to use chemicals in the cleaning procedures.

Applications To prevent Corrosion:

When you clean the systems, it's important that you maintain a regular cleaning regime to prevent corrosion from occurring again, maintenance pigging along with chemicals is one way to ensure a proper cleaning strategy. This cleaning regimen should involve regular removal of corrosion cells and prevent future formation of them. Depending on the type of problem , special purpose pigs are to be employed.

Bypass:

This technique is of major importance, for it increases the effectiveness of the work carried out by the pigs.

It helps in the cleaning procedure by,

• Preventing debris from piling up, that may hinder the functioning of the pig

• Regularly cleaning and clearing debris from the cleaning elements on the pig

• Reducing the amount of debris removed during the cleaning procedure.

• Prevents the debris from coming in contact with the pig

• Besides this, Bypass does allow the pig to slowdown and perform its task better in high velocity pipelines,

The areas where 'Bypass' falls back are:

• There is an occasional risk of stalling when you slow the pig down.

• You have to increase 'Bypass' gradually with experimentation.

• The 'Bypass' that you are working with must be capable of pushing a stalled pig with a foam pig or a pig with lesser bypass.

• If the 'Bypass' is too much, it may prevent the pig from being launched, especially seen in pigs with a limited kicker size.

• Finds a lot of use in high pressure pipelines.

The main purpose of using a 'Bypass' is to make the pig work slower, for a slower pig permits lesser riding up of the pig over debris. It also reduces the rate of wear and tear of the cups / discs by reducing heat built-up with in.

With the use of 'Bypass' you can be sure that you'll be able to find a lot more debris when compared to the use of a pig without 'Bypass'. Well noticing this condition, it best that you keep good provisions for debris removal.

Pigging Cycles:

As the point was already brought up earlier, its very important that the pigging procedure is carried out frequently and effectively. In the absence of pigging, the occurrence of corrosion would be high, and repetitious efforts of pigging would have to be carried out. To have a safe and sound running of the system, it's always better to have the pigging cycles at the maximum, using the right type of pig and cleaning elements, with a very good debris removal system to have a line free from corrosion.