article taken from World Pipelines magazine
Through close collaboration, Barrier Ltd and its supplier, Leighs Paints, have been able to rehabilitate pipes that had been lying in storage.
The gas pipeline was due to be laid in the early part of 2001, but due to delays and challenges on site the lay programme was heavily impacted - delays are currently in the process of being resolved by the client. As a result the end customer was left with several hundred customised pipes awaiting use.
The pipes for this project were stacked and stored in a field. These conditions were hardly ideal and the pipes began to corrode, creating a build up of debris and rust. The corrosion of the pipework was a major concern, threatening further delays to the lay programme by impacting into the corrosion allowance orginally built in by the design engineers.
The pipes, highly specialised as they are made especially for high pressure pipeline projects, will transport wet gas when laid. In normal conditions, the pipes would be laid unlined, with inhibitors being used to resist corrosion. However as the pipes have been resting in an unsuitable environment, a bespoke solution was needed.
The client was faced with some decisions to make. One option was to scrap the pipes and commission new ones. This would be a costly solution, so it was decided that a coating would be required to protect the pipes while they were waiting to be laid. Initially, it was envisioned that the pipes would have to be moved to a pipe coating factory. However this option would prove to be prohibitively expensive and time consuming. Therefore the favoured solution would be for on site application to prevent any further breakdown of the internal wall thickness.
Once it had been agreed that onsite coating was the ideal way to protect the pipes from further degradation while not impacting the final use of the pipes, the search began for a suitable material to carry out the job. The client and their nominated applicator approached paint manufacturer Leighs Paints to become involved in the project.
A high-build two pack epoxy coating (Pipegard P100), developed specifically for gas pipe internals, was suggested. In terms of application, the coating can be airless sprayed using standard equipment operating around 2200 psi. The material can also be applied using specially designed spinning head techniques. The coating applied is 60% solids which makes it a cost effective solution. This material conforms to Transco CM2 qualification and has been previously used on a number of other ‘dry’ gas pipelines.
As this is unprecedented ground for both manufacturer and applicator, and considering the unique challenges of blasting and painting pipes in storage berms it was necessary to carry out a series of viability and commissioning trials. Initially, the trial took place offsite, where a number of pipes were taken to an in house painting facility. This trial was witnessed by all the parties, who were pleased to approve this process. The next stage was to conduct a trial onsite. The on site trial was subject to the conditions which the applicators would be expected to work under and was again attended by all parties. The purpose of the trial was to ensure this solution could be worked in a practical environment and finalise blasting and spraying techniques.
There were several thousand 12 metre long pipes that were stacked into safe manageable quantities. However, movement and access in and around the workplace was limited. This meant that available standard blasting and spraying equipment was not suited to this application.
A specially designed grit blasting machine was built that included a fully automated recovery system and dust extraction module. A system was approved that would operate using a reusable abrasive and offer a surface profile of around 50 microns nominal. To ensure the coating would adhere correctly, the surface needed to be free of all dust and particulate to maximise surface adhesion and the overall finish of the coating system designed and built for this project.
The coating itself was applied using a fully automated internal lining machine, fully motorised to achieve a consistent speed throughout the application and a minimum dry film thickness target of 75 microns.
The coating operation has not been without its challenges and even after the specialist application machines reached site, Mother Nature got involved. The coating work started in April due to continue until the end of October. This has coincided with one of the wettest and coldest summers in the UK for a long time.
Battling with inclement weather, which has slowed down application, the site was subject to high winds. This can effect the blast cleaning and spray patterns of coating. Combined with high humidity, this meant that the paint could not be applied as it contravened the application conditions stated by the supplier.
The delays in coating lead to the work area being fully encapsulated, using a large tarpaulin to protect against inclement weather and protect surrounding land. In what was previously a working farm field, environmental issues such as emissions and anti-contamination were taken into consideration - as once the pipes are removed the field will be returned to agricultural use.
Coating work is being carried out in accordance with the manufacturer’s recommendations - applied at temperatures above 10ºC, with the substrate at least 3ºC above the dew point. The coating manufacturer has supplied a Technical Service Representative throughout both the trials and demonstrations. These have been carried out both on and off site for pre-qualification purposes and during the actual project. This is to ensure that help and advice is on hand during this unusual project. The technicians are highly qualified ‘NACE Coating Inspectors’ and have vast experience on major projects both offshore and onshore.
Once the project has been given the green light, the coated pipes will be used to fabricate a raw gas pipeline. It is not normal practice to use a gas flow line coating for wet service conditions – they are normally only used in the ‘dry’ transmission network. To ensure that the coating is suitable for this service condition special testing was commissioned at a well know British University based test laboratory. The subsea pipeline will transport sour gas from the ‘wellhead’ to the onshore receiving station where it will undergo further processing.
While moving along the pipe, the gas will pass through the slug catcher in order to remove the bulk of the liquid. Following that, the gas will then be compressed and metered. Condensate produced in the gas, as well as that separated from the feed gas in the slug catcher / inlet separator, will be routed to the condensate stabilisation system. This is essentially a two-stage flash system with intermediate heating. The stabilised condensate is cooled in an air cooler and sent to storage.
This very challenging project has been fraught with difficulties from the start, which has meant that unusual and innovative solutions have been employed at all levels. Although the technology used for the project has been in existence for a number of years the individual requirements of this project have lead to many technologies being used outside their normal uses. The testing and trials of these ‘ad hoc’ solutions have increased the knowledge of manufacturer, applicator and customer.
A satisfactory solution has only been possible through close co-operation between all parties and their continued collaboration has resulted in the solution being implemented.