Protecting Coatings Vital To Ensuring Pipelines' Longevity

Carl Turnage, Vice President, Ozzie's Pipeline Padder, Scottsdale, AZ

When cross-country pipelines were first built, little was known about corrosion protection and we built the best pipelines we could with the available technology. In many instances, bare pipe was laid with no coating protection. When pipelines were laid through rocky areas, the biggest worry was that large rocks would dent the pipe. Over the years, pipeline failures occurred which could not be attributed to dents in the pipe or weak pipeline welds.

We learned that the Earth's magnetism created an electrical field around the pipelines which caused cathodic corrosion. It was later discovered that an outer coating of non-conductive material placed around the pipeline would combat this electric field and prevent the pipeline corrosion. One of the first coatings used was coal tar enamel. This material was brought to the construction site in a solid form, then heated in a 'dope pot' until it became a liquid. From the dope pot, hoses ran the coal tar enamel liquid to the pipeline where it was applied to the pipe. Once the coal tar enamel was applied to the pipe, it would quickly harden, giving the pipeline a non-conductive outer coating to protect it from corrosion.

From this early crude type of pipeline coating came the modern ones used today which include poly pipeline tape, Extruded HDPE (Yellow Jacket Pipe Coating), and the current family of fusion-bonded epoxies. Many of the old hot dope lines are still in very good condition. We know that if coatings are applied correctly and buried undamaged, they can last as long as the pipelines have a use for transporting oil and gas. What we have to remember is that the coating can be damaged easily, and in places where it is damaged, it loses its effectiveness.

Pipeline Rehabilitation

Today we find ourselves rehabilitating many old pipelines because of corrosion damage. A tremendous amount of time, personnel and money is invested to keep these old pipelines operating. The pipeline companies have created new departments to monitor their pipelines, one of which is the corrosion control group. This group is continuously looking for the next weak spot caused by corrosion that needs to be repaired in order to keep the pipeline from being shut down. As these groups discover the corrosion and research its cases, they have been able to teach us what not to do during pipeline construction so the coating will not be damaged.

Coating Protection

Pipeline owning companies spend millions for the best coatings available. If the coating is damaged during the construction phase, it means the money spent to purchase the best coating available was wasted. Many measures are taken during the construction to ensure the coating is not damaged. To determine if there are holes or weak spots in the coating, a holiday detector is run over the pipeline before lowering in. This jeeping process is an excellent means of discovering coating defects that can be repaired, but this electronic eye cannot see damage that occurs once the pipe is laid in the trench and backfilled.

When no hard objects are in the excavated material, damage after the jeeping process is unlikely. Because cross-country pipelines travel long distances through various soil conditions, rocks are often encountered. Corrosion control groups have determined after the pipeline has been properly jeeped and repaired, there are three critical phases when the coating can sustain damage that may go unseen and unrepaired.

The first phase of construction that concerns the corrosion control groups is during lowering in. If the pipe is lowered in directly onto rocks, the sharp edges can immediately cut into the coating. The second phase is the period between lowering in and backfilling. Years ago, the pipeline would be lowered into the trench and days would lapse before backfilling anchored the pipe.

We now know that the pipeline will crawl in the trench because of thermal expansion. The friction caused by the pipeline movement can rub the coating off of the bottom of the pipe. Damage can also occur when objects fall in the ditch or if the trench wall caves in. All of the potential damage in this phase can be eliminated if the padding is able to keep pace with lowering in because the pipe is not allowed to move. It will be anchored and any object that may fall in the trench will not come into contact with the coating.

In the last phase, backfilling, large rocks and other hard objects in the backfill that are pushed directly onto the pipeline may damage the coating. If precautions are not taken during these three phases, coating damage may occur. The owner company must determine in which manner to protect it's coating.

Types Of Coating Protection

There are now four ways to protect coating.

A. Rockshield - this is a cushion-type of material that is wrapped and then taped around the pipeline. Because of its high construction cost, rockshield normally is only utilized on very short sections of the pipeline when the excavated material does not contain large rocks, or has to meet road-crossing specifications.

B. Polyurethane foam - this is sprayed onto the pipeline in hard-to-reach places or in wet areas where select material will turn to mush, thus not providing protection against rocks in the ditch. It also is rarely used because of its high cost.

C. Hauling in and placing rock-free material - this is the oldest type of pipeline coating protection. Dump trucks haul rock-free sand or soil to the construction site, dump it on the right of way, then loaders drop the material in the ditch around the pipeline. If a large quantity of pipe is lowered in daily, dozens of dump trucks will be needed to keep up with the construction pace.

D. Backfill separation machinery - there are three basic types of these on the market.

1. Stationary screens - a front end loader or a backhoe that is used to drop the excavated material onto a screen. The fines that drop through the screen are then picked up and placed around the pipeline by a loader. A dump truck or a loader is used to haul the screen down the right of way as the construction progresses.

2. Hopper screening machines - a hopper with a shaking screen is attached to a sideboom that carries it above the trench down the right of way. A backhoe loads it with the excavated material one bucket at a time and the fines fall into the ditch on top of the pipeline. This type of screening machine cannot determine what size of large rocks may fall back on top of the padding material.

3. Pipeline padder - a self-loading, self-propelled backfill screening machine that travels down the spoil pile on tracks, loading itself with the excavated material which is escalated to a vibrating shaking screen. The screen separates the fine material from the rocks which fall harmlessly off of the back of the machine. The fines accepted by the screen slide down to a conveyor which places it in the ditch around the pipeline. The material that is conveyed into the ditch never falls directly on the pipe. It lands first on other padding material, then rolls around the pipeline, thus recycling the existing backfill.

As all of these are effective coating protection means if done properly, capital expenditure analysis vs. longer pipeline life expectancy dictates which method is right for each particular job.

Choosing The Best Method

When building cross-country pipelines, time is money. They are built following tight construction schedules, so it is possible to bury over two miles of pipeline a day. If sections of the pipeline are lowered in, then left unpadded for extended periods of time, there is a greater likelihood that the coating will have undetected damage caused by pipe movement, unseen objects falling into the trench, and the trench wall cave-ins.

This undetected damage decreases the pipeline's operations life. If the pipeline padding cannot keep up with the project's construction pace, the lowering in crew and the crews behind padding will be delayed. The equipment and personnel in the delayed crews will not be working to their maximum potential, resulting in wasted money.

Conclusion

Our industry is changing daily. Six or seven years ago, not much was known about polyurethane foam or mobile screening machines. Today, these products have become necessary. We must continue looking for better ways to protect our pipelines and make them last longer.

This article is based on a presentation made in November 1996 to the Egyptian Corrosion Society in Alexandria, Egypt.