Advancement in Internal Field Joint Coating Systems

K B Singh,
Consultant - Coatings and CP Seal for Life India Pvt Ltd
For water injection or multiphase oil and gas pipelines, internal corrosion is a dominant factor contributing to failures and leaks in pipelines. Pitting corrosion along the bottom of the pipeline is the primary corrosion mechanism leading to failures in uncoated carbon steel pipelines. The pitting corrosion occurs due to presence of different elements in the water ie, O2, CO2, H2S, bacteria, chlorides, scale, or suspended solids. This paper - Advancement in Internal Field Joint Coating Systems for Off-shore and On-shore Pipelines by K B Singh, Consultant - Coatings and CP, Seal for Life India Pvt Ltd, Part of Berry Plastics Engineered Division, USA, explains about the types of internal coatings and highlights crucial elements associated with it. The article also talks about the innovative technology Cylindrical Corrosion Barrier Sleeves for preventing corrosion at the internal field welds.

In the past, mitigating internal corrosion of carbon steel pipelines was essentially impossible. The internal lining of carbon steel pipes could be accomplished at pipe coating mills but as the pipeline was welded - onshore or offshore, the welding of the pipe joints typically caused significant damage to the internal coating as well as creating significant corrosion cells in the heat effected zone of the unprotected internal girth weld area. For obvious reasons, it was deemed uneconomical to coat the internal portions of the pipe despite knowing that corrosion would ultimately occur internally. In case of critical lines, the only choice the owners had were to lay pipelines with higher quality alloy steels such as duplex steel or to use clad lined carbon steel pipelines - both the options being extremely expensive.

Innovations in internal coatings - fusion bond or liquid epoxies and application technologies coupled with innovations in internal girth weld technologies have enabled the owners to design and successfully lay internally coated carbon steel pipelines both off-shore and on-shore.

Internal epoxy lining are of two types:
- Liquid Epoxy Coatings
- Fusion Bond Epoxy Coatings


Liquid Epoxy Coatings: For internal lining of pipelines carrying water or other corrosive fluids, Liquid Epoxy Coatings is used. Here two components are crucial solventless high build epoxies comprising of resin and hardener mixed in a volumetric proportion as recommended by the manufacturer and sprayed on to the internal surface of the pipeline.

The Liquid Epoxy Coatings are generally made through the reaction of phenols with acetone or formaldehyde. Those reactants are then further reacted with epichlorohydrin. The resultant materials are diglycidyl ethers of what are called bisphenol A epoxies, bisphenol F epoxies, or phenolic novolac epoxies . These resins are then cross-linked via polymerisation reactions with various curing agents or blends of curing agents (Hardeners). During application of the Liquid Epoxy Coating - Part A resin, which has a high viscosity and specific gravity, is heated to a temperature of 80~900C and is then mixed with the Part B hardener in a mixing manifold of the spray machine and sprayed at high pressure of 4000~5000 psi through special designed spray guns. The mixed epoxy in atomised form gets deposited on the internal surface of the pipe and goes through a curing cycle - gel, touch to dry and hard dry, fully cured. The curing time of the epoxy - touch to dry can vary from 20 minutes to 2 hours depending on the make or mixing ratio or environmental conditions. Full curing of the liquid epoxy could vary from 2 days to 7 days again depending on the conditions mentioned above. In one single spray, maximum thickness of the wet epoxy which can be sprayed, varies from 400~1000 micron (depending on the formulation of the epoxy).

Fusion Bonded Epoxy Coatings:
Commonly referred to as FBE Coating, these are epoxy-based powder coating systems that are widely used to protect internal surfaces of steel pipes used in oil and gas fields for various applications - raw water, high temperature or high pressure injection water, formation water, down hole tubing, piping connections, valves, etc. from corrosion.

FBE coatings are thermoset polymer coatings. The name fusion-bond epoxy is due to resin cross-linking and the application method is different from Liquid Epoxy Coating. The resin and hardener components are combined in a dry powder form and remain un-reacted at normal storage conditions. At typical coating application temperatures, usually in the range of 180 to 2500C (356 to 4820F), the contents of the powder melt and transform to a liquid form. The liquid FBE film wets and flows onto the steel surface on which it is applied, and soon becomes a solid coating by chemical cross -linking, assisted by heat. This process is known as 'fusion bonding'. The chemical cross-linking reaction taking place in this case is irreversible. Once the curing takes place, the coating cannot be returned to its original form by any means. Application of further heating will not 'melt' the coating and thus it is known as a 'thermoset' coating .

FBE coatings are applied in an automated one-part process so that the mixing , and multiple-coat problems associated with liquid epoxy coatings are eliminated. The electrostatic application process for FBE provides a smooth, even coating thickness with no runs, sags, or thin spots common with applying liquid epoxy coatings.

The application of FBE powder involves spraying electro-statically charged powder on a primed or un-primed heated steel surface followed by post cure. FBE depending on the pipe or fittings to be coated can also be applied either by fluidised bed or flocking (air spray) followed by post curing.

FBE coatings are durable and provide twice the impact strength of liquid epoxies. The surface provides high abrasion resistance and resists high temperature and is resilient to meet the requirements of laying or field bending without cracking. FBE has a long-term performance history in water and sewage environments including salt water, slurries, methane and hydrogen sulfide exposure.

The applied thickness of FBE coating is generally - 350 microns. However, in case of high TDS and probability of sand particles being present in the water, thickness recommended is between 500 to 600 microns. For the above thickness, FBE powder has flexibility > 1.50 and resists any cracking during transportation, laying, cold bending or hydro-testing.

Selection of Internal Coatings
Internal coatings are selected based on series of AUTO CLAVE testing which simulate the harsh aggressive corrosive conditions in pipeline carrying high temperature or high pressure injection water or formation water and crude oil with high water content.

The autoclave test consists of placing coated panels in a small pressure vessel(autoclave) in which oil field conditions are simulated with respect to fluid, temperature and pressure. The coatings, applied to test panels at ambient temperature, are then exposed to the simulated process conditions for a period of time, after which the temperature and pressure are reduced back to ambient, noxious fluids are removed, and the coatings are inspected for performance - blistering and loss of adhesion.

Usage of Internal Coatings
Pipeline manufactures with integrated coating facilities and standalone coating companies in India and Middle East are now equipped for internal coating of carbon steel pipelines for internal corrosion control either with Liquid or Fusion Bond Epoxies; selection being based on the type of fluid in the pipeline and other technical requirements. The internal coatings can be done for pipe diameter ranging from 4" to 120".

In Saudi Arabia, Oman, Kuwait, etc for oil and gas onshore or offshore - water injection pipelines, sea water intake raw water pipelines, multiphase gas and oil, inter-field flow lines coated with FBE are being used for the last 15 years.

In India, for oil and gas off-shore pipelines, ONGC India Ltd, has already laid three - offshore water injection pipelines - internally coated with liquid epoxy in the last two years. Oil and gas exploration companies in India are also planning to use internally coated FBE pipelines for internal corrosion control.

Internal Field Weld Joint Coating
The most critical section of an internally coated pipeline is the internal field joint. Proper coating or isolation of this section is essential to prevent corrosion and maintain integrity of the pipeline.

There are two technologies which have evolved over the years:
Robotic Internal Field Joint Coating:
The internal coating equipment consists of self-contained robots that travel inside the pipe, find the weld and then blast clean, vacuum and coat the area. Utilising various cameras, these field joint coating robots transmit a real-time video image back to the operator, which is then used for control and inspection.

Welding operations must produce an internal weld profile that can be coated

Poor welding procedure and quality can result in damage to the parent pipe coating and a weld profile that is un-coatable. The welding operations shall be carried out in accordance with GMAW-RMD or GMAW-STT or GTAW. Whichever welding technique is used, the profile of the root run shall be spatter free , contain no sharp edges, no high/low caused by pipe ovality and the penetration shall not exceed 1.5mm. Shielded STT root pass is also very acceptable.

It is vital that the weld bead be free of spatter, IP/EP, sharp edges, rust or other potential defects/anomalies that prevent the internal field joint and cutback from being coated per the client specification (NACE visual standard RP0178 [A, B and C]). Any deviations from this requirement will lead to slowed production and unnecessary delays in the project completion date.

It is also highly recommended that the welding procedures include the use of an internal line-up clamp with well-maintained copper backing shoes that preclude penetration greater than 1.5mm and any weld spatter. If line-up clamps are used, they are to be fitted with non-metallic wheels and non -metallic shoes, and the operator's handle and steel pull cables are to be suitably padded to avoid coating damage. Fitting a drip tray on the line-up clamp is recommended where excess lubricants may leak out. Excessive lubrication of the line-up clamp and any attached equipment, such as air motors, is to be avoided because any liquids on the weld area will be cause for rejection.

Robotic equipment needs a minimum of 1.5o bend per inch of pipe diameter and can operate on any slope < 12o .

Robotic coating is a proprietary and complex operation and needs trained crew to be deployed at the site. Improper internal welding can render the robotic coating ineffective.

Cylindrical Corrosion Barrier Sleeves (CCB):
Another innovative technology which is now being increasingly used for preventing corrosion at the internal field welds is to use a cylindrical corrosion barrier at these locations. The cylindrical corrosion barrier comprises of a pup pipe section inserted in the internally coated pipe prior to welding. The 2nd internally coated pipe is then inserted over this pup pipe section and the pipes are welded over this pup piece. The pup pipe piece termed as cylindrical corrosion barrier is so designed that the internal welded section of internally coated pipelines are isolated from the corrosive fluid thereby preventing corrosion.

The cylindrical corrosion barrier comprises of a pipe pup piece (also called as internal coupling) manufactured from same grade as the main pipe. The center portion of the internal coupling has a machined recess wherein a heat resistant fabric is installed followed by metal backing ring manufactured from A-109 grade of steel. The internal and external surface of the internal coupling is coated with FBE coating to meet the technical requirements of the corrosive fluid. On the external side of the internal coupling there are two outer rings affixed into the grooves machined around the outer circumference of the ends. The O-rings are manufactured with compounds especially designed to resist the process flow being transported in the pipeline. The length of the internal coupling is designed based on internal coating cut back of 25+/-5mm.

When the above internal coupling is inserted in the coated pipe and the 2nd pipe has been positioned, the girth welding is carried out. Due to the heat resistant fabric, the heat generated during welding does not damage the internal FBE Coating of the pup piece. Due to the welding, the pup piece becomes an integral part of the pipeline. The O rings isolates the internal girth weld area in coming in contact with the corrosive fluid providing 100% ID coated pipeline.

Further, this technology unlike the robotic technology does not require high precision of the welding and independent of the contour of the internal weld . This technology can be used either with internal liquid epoxy or FBE coated pipelines.

CCB sleeves are designed to various pipeline systems as:
  • Injection and re-injection
  • Crude oil gathering
  • Highly corrosive gas piping
  • Potable & Waste Water
  • Jet Fuel transfer
  • Fire Water & Salt Water piping
Advantages of the CCB system are:
  • Light weight and easy to install
  • Attachment system acts as an internal line-up clamp while automatically setting weld gap
  • Weld time is similar to welding noncoated pipe
  • Creates a full penetration weld that can be 100% x-rayed per API & ASME specifications
  • Easily inspected and cleaned with pig devices.
  • Installs as normal welding process
  • Protects the I.D. coating from damaging weld spatter during installation
  • Isolates the girth weld area, providing a 100% I.D. coated pipeline
  • Becomes a part of the finished pipeline providing permanent corrosion protection
  • Available in standard sizes for pipe dia 2" to 42"
  • No special crew required for installation.
  • Proven technology for off-shore and on-shore pipelines.
Conclusion:
CCB is a proven patented technology for preventing corrosion of internal girth welds. With the successful application of this technology, more such internally coated pipelines to prevent internal corrosion and subsequent catastrophic failures are being planned.