Over the past 40 to 50 years it has become standard industry practice to apply both internal and external pipeline coatings.

The concept of internally lining gas pipelines – known as internal flow coating – was first developed in the 1950s to counter the adverse effects on pipeline capacity, operation and pumping costs caused by the rough internal surface of steel pipes and the build-up of deposits and corrosion products.

Externally, fusion-bond epoxy (FBE) coating systems were introduced in the 1960s to form a barrier between the metal surface of the pipe and the surrounding environment.

Since then, 3M has supplied more than 170,000 km of internal flow coating and more than 135,000 km of FBE for pipeline projects worldwide.

Increased flow of gas

Data are readily available to illustrate that a smoother pipe surface leads to increased flow capacity. A 2002 study on the economic benefits of internally coated pipe, completed by Rafael Zamorano, demonstrates that the capacity increase of a coated section of pipeline is considerably greater at high pressure than an uncoated section. Zamorano’s study used the 530 km Atacama Gas Pipeline in South America to articulate his findings. Similar studies have also been conducted elsewhere, demonstrating a 14–21 per cent increase in pipeline capacity for internally-coated pipelines.

While these studies indicate a significant increase in flow capacity when internal flow coating is used, it is generally accepted that even a one per cent improvement in throughput provides the financial justification for internal coating.

Corrosion protection

A major issue with uncoated pipelines is corrosion. When uncoated pipelines are flooded with seawater, the extent of corrosion can be considerable. A report completed in 2006 by John Grover of BJ Process & Pipeline Services noted that an estimated 157,000 kg of corrosion debris was removed from a 161 km section of 36 inch diameter pipe with a wall thickness of 14.3 mm, only three months after total immersion in tropical sea water.

As increasingly stringent regulations governing the disposal of scale and corrosion debris into the environment are being applied, it has become industry practice to blast clean the pipe and apply an internal flow coating prior to the pipe being delivered to site. This prevents corrosion from reforming, eliminating the need for additional pre-commissioning work.

Faster commissioning and simplified inspection

Internally coated pipework will also dry faster than uncoated pipe after hydrostatic testing, which means an easier and faster commissioning of the line. Testing and any robotic inspection procedures are also greatly simplified by the improved mobility of the equipment travelling down an internally coated pipe.

In 2005, Statoil reported that it made the decision to apply an internal epoxy coating – 3M Scotchkote Epoxy Coating EP2306 HF – to the Langeled Gas Pipeline, in the North Sea, in order to increase transport capacity and reduce pig wear.

Reduced energy costs in pumping and compressor stations

Internal flow coating can also make a significant difference in reducing pumping or compression costs over the lifetime of the pipeline. These reduced energy costs can provide a financial payback within three to five years of service. It may also be possible to achieve further savings by reducing the number of compressor stations, or compressor size and capacity.

The Zamorano’s 2002 study concluded that fuel gas costs for the compressor stations situated along the 1,200 km length of the Atacama Gas Pipeline, were 26.9 per cent lower on the coated section of pipeline than on the uncoated section.

External pipe coating protection: a history of success

The main methods of protecting underground pipelines from corrosion are external coatings and cathodic protection.

J. Alan Kehr, a leading American expert on pipeline coatings, states that external pipe coatings are “intended to form a continuous film of electrical insulating material over the metallic surface to be protected. The function of such a coating is to isolate the metal from direct contact with the electrolyte, interposing a high electrical resistance so that electrochemical reactions cannot occur.”

Single-layer FBE has proven performance in both onshore underground and offshore subsea environments. The product is an effective corrosion-resistant barrier for line pipe, field joints, fittings and bends. Since its first use in New Mexico in 1960, FBE coating has remained the external pipe coating of choice in North America.

Since their introduction in 1991, dual-layer powder coatings have also gained popularity for use with pipelines operating at a high temperature. A relatively low-cost solution, dual-layer FBE offers the performance and installation characteristics of single-layer FBE, while also providing superior damage resistance with only a minimal reduction in flexibility.

The 1980s saw the introduction of three-layer polyethylene or polypropylene coatings in Europe. These coatings comprise a FBE primary coating, a polyolefin-adhesive or tie-layer and a polyolefin topcoat, and are based on earlier single-layer and two-layer systems. The thick layer of polyolefin, which provides a high level of damage resistance, is deemed to facilitate installation under conditions of harsh environments or inexperienced handling in the factory or onsite.

Cost effective pipe protection

Pipeline coating technology is constantly evolving to provide better protection as the oil and gas industry moves to explore and produce from ever more difficult reserves and demanding environments.

External coatings for pipelines are standard practice, and internal coatings can provide economic benefits, as has been well-documented by thorough research on a number of different installations.