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New technology effective in reducing top-of-the-line corrosion rate.

In wet, sour, or carbon dioxide-containing multiphase gas production or gathering lines, top-of-the-line corrosion (TLC) can be a significant problem that may not be readily identified by conventional monitoring methods. While corrosion inhibitors are used to combat this problem, it is difficult to achieve effective wetting and inhibitor transfer to the upper portion of the internal pipe wall. T.D. Williamson, Inc. (TDW) worked with a European oil and gas company to develop a special pipeline pig called the V-Jet[TM].

The V-Jet uses a Venturi Effect to spray liquids (containing inhibitors) from the bottom of the pipe to the top. TDW recently teamed with another production operator to apply the technology to specific top-of-line corrosion problems. Ongoing tests reveal that it effectively transfers corrosion inhibitor from liquids in the lower portion of the pipeline to the upper surfaces of the pipe wall and significantly reduces corrosion rates (by an order of magnitude).

The V-Jet is now used by the operator to enhance corrosion control as part of the normal de-watering process. This article describes characteristics of the V-Jet and best practices for mitigation and monitoring of TLC.

TLC And V-Jet Technology

Injected corrosion inhibitor chemicals that are water-soluble typically are transferred to the inner pipe wall by direct contact with water in the pipeline. The majority of the corrosion chemical remains in solution in water pooled along the bottom portion of a pipeline.

In some wet multiphase systems, conditions exist that allow water vapor to condense on the upper internal portions of the pipe wall. Condensed water on inner pipe wall surfaces in certain acid gas environments can be highly corrosive. Often, corrosion inhibitors do not have sufficient volatility to transfer into the vapor phase and ultimately to the upper portions of the pipe wall. This can leave the upper portion of the internal pipe wall unprotected.

By applying the Bernoulli (or Venturi) Effect, the V-Jet uses the energy of bypass flow for the work needed to redeploy and redistribute residual inhibitor chemicals throughout the pipe run. Reusing or effectively transferring corrosion chemicals in pooled accumulations along the bottom of the pipeline has been viewed as an alternative to "chemical batching" and an effective step in solving the problem of TLC.

V-Jets can be used effectively in several ways but are particularly effective for multiphase gas pipelines that use the process of continuous injection as the primary means of introducing inhibitor chemicals into the line. They are also effective as the trailing pig in a chemical batching train.

Operator Benefits

Increase overall pipeline integrity and reliability.

--Through better control of TLC (10 to 2 o'clock) when continuously injecting corrosion inhibitor.

--Transfer of corrosion inhibitor to the upper portion of the pipe in multiphase systems.

Reduce production downtime, special equipment and additional chemical handling associated with periodic batching of corrosion inhibitors.

--Typically four-eight hours of downtime per line, every two to four weeks

--Reduced use and handling of chemicals

Chemical Treatment

Standard methods for controlling corrosion in multiphase gas lines are continuous injection and/or batch treatment with neutralizer/inhibitor. Often, a combination of both methods of corrosion control is used.

Continuous Injection

An injection nozzle, inserted into a line at or near the wellhead or just downstream of a compressor/pumping station, injects/pumps corrosion inhibitor into the line under high pressure, vaporizing it where it mixes with the gas and/or liquids in the line.

Corrosion inhibitors used in these applications are typically water soluble. The chemical goes into solution and travels with liquid water on the bottom of the pipeline. Direct contact between the pipe wall and water is the primary mechanism for inhibitor transfer. Gravity effects, low flow rates, and stratified flow make transfer of the corrosion inhibitor to the top of the pipe improbable at many locations.

As water vapor and other gases travel along the pipeline, they are cooled. The cooling allows water vapor to condense on the upper portions of the internal pipe wall. The condensed water often contains no or insignificant quantities of corrosion inhibitor and does not provide any corrosion protection. In the presence of certain acid gases, corrosion rates can be very high in the upper portions of the line.

Batching

Batch application of corrosion chemicals to address TLC requires sending a solid slug of liquid that contains corrosion inhibitor between two pigs. In order to be effective the pigs must ensure that a solid slug or column remains intact and contacts the upper portion of the inner pipe wall. The process is expensive, typically requires special equipment, and a production shutdown.

It is difficult to ensure that a solid slug or column remains fully intact between the two batching pigs. The result of not maintaining the solid slug throughout an entire pig run is unprotected upper portions of the pipe wall Batch or chemical slug applications last a finite time even at areas of effective application.

How Does The V-Jet Work?

Bypass is a process often used in pipeline pigging and describes the venting of differential pressure and movement of product through a pig. It is a common feature in many cleaning pig designs to prevent solids from building up in front of the pig by suspending them in bypass flow turbulence. Bypass is also often used to help reduce speed excursions experienced when pigging a gas pipeline.

Bypass flow and differential pressure are the only dynamics required for the V-Jet to do its job. There are no moving parts or pressure vessels to fill and charge. In its patented design and process, it simply allows the higher pressure to flow through its body and spray head. Bypass flow acts as the accelerant to transfer and vaporize fluid while creating a low pressure area in the spray nozzle as it passes through.

This drop in pressure produces a vacuum at the front inlet ports that draw in pooled inhibitor fluid to be recycled and redeployed. As the pig moves forward, residual inhibitor fluid that has pooled on the bottom of the pipe is siphoned through the front inlets and sprayed onto the top area of the inner pipe wall.

By employing a counterweight system to ensure proper orientation of the spray head, the V-Jet projects the inhibitor spray upward at approximately 45[degrees] from parallel, while fanning it out across the top 120[degrees] perpendicular, to the centerline of the pipe (reference figures 1-4).

[FIGURES 1-4 OMITTED]

The rate of liquid being sprayed is directly related to the differential pressure across the pig, the viscosity of the fluid, and its specific weight. For liquids that have physical properties similar to water, each nozzle will deliver approximately three quarts per minute at 15 psi. For large-diameter pipelines, multiple nozzles are used accordingly to increase the delivery and coverage area.

V-Jet Testing

In a test fixture the V-Jet is shown delivering a dense spray at a 45[degrees] projection angle. This prototype had only one spray nozzle, the production models have two or more.

When launching the V-Jet with inhibitor chemicals, in launcher barrel, the area between the front and rear cups will fill with these chemicals and act as a temporary reservoir. This stored liquid is directly jetted thru the nozzle(s) and then repeatedly sucked back into the front inlets of the spray head as the pig advances through the pipe.

The V-Jet in transparent pipe showing the spray pattern of a single nozzle at 15 psi differential pressure.

V-Jet being received in the proper orientation after a 20-mile run offshore platform-to-platform.

Recent Field Trials

A gas-gathering system containing multiphase fluids was experiencing TLC. Conventional monitoring techniques such as corrosion coupons and probes did not detect the problem. Typically, corrosion coupons and probes are installed in the lower portions of a line in the liquid contact areas. The true extent of the corrosion problem was identified by an inline inspection.

Inspection results showed that the line was in excellent condition along the bottom of the pipeline for the entire length, but had damage at select locations on the upper portion. The corrosion management program included continuous injection of a water-soluble corrosion inhibitor and frequent maintenance pigging for water removal. The chemical was very effective at controlling corrosion in the water wet portions of the line.

After discovery of the corrosion in the upper portions of the gathering line, it became imperative to modify existing corrosion-control practices. A combined approach using continuous corrosion injection and batch application between pigs was employed. Special coupons were added to the top of the line at the 12 o'clock position. The initial target for the program was to limit the TLC rate to less than 2 mils per year (mpy).

The operator elected to test the effectiveness of the V-Jet as an alternative to batch application of chemicals. Prior to employing the pig, corrosion rates had registered as high as 5 mpy at some of the TLC coupon locations. Corrosion coupons have now confirmed effective corrosion control in all portions of the line.

After fine-tuning the corrosion management processes, which included the implementation of regular V-Jet runs, corrosion rates have dropped to around 0.2 mpy (vs. a 2 mpy objective). Periodic batch chemical treatment is no longer necessary to control TLC due to the pig's effectiveness. In fact, the V-Jet has also been found to be very effective at dewatering, to the extent that it is now the only pig run during weekly dewatering runs--and batch inhibitor treatments have been eliminated altogether.

An effective ongoing corrosion management system must include a careful evaluation of all the factors that come into play for each unique pipeline or unique segment of a pipeline.

A few examples of these factors include:

* Liquids, solids and other contaminants present in the product stream (corrosive agents);

* Product contaminants that limit inhibitor effect (neutralization/de-activation);

* Deposits preventing inhibitor from coating the pipe surface; and

* Corrosion inhibitor transfer/redistribution mechanisms.

Early pigging work to prepare for running the V-Jet included cleaning runs to minimize the deposits that would prevent inhibitor from contacting the pipe surface. Various pigs were used--each particularly suited to different types of contaminants (such as black powder or iron sulfide or sand) while others were better at removing deposits such as scale, etc.

As part of an evaluation program, inhibitor batching runs were made to introduce a "slug" of inhibitor solution--both with the V-Jet at the front of the slug (with a batching/foam pig at the back) and with the V-Jet at the back (with a batching/foam pig at the front of the slug). It was found that the V-Jet is more effective at the back since there is a higher pressure differential across it, resulting in higher jetting intensity. Corrosion rates were reduced through this V-Jet/batching method.

Alternatively, the V-Jet was run as a dewatering pig, which also served to effectively redistribute the inhibitor-containing liquids to the top of the pipe. This was found to be most effective (and more efficient in terms of operating costs), presumably due to the increasingly dense vapor cloud that forms in front of the V-Jet as it splashes through and jets the liquids from the bottom of the pipe to the top.

To ensure that a vapor cloud of inhibitor is formed at the beginning of the run, it is recommended that the pig launcher be filled with inhibitor solution, behind the V-Jet pig, before launching. This will cause the inhibitor solution to begin to be jetted through the pig and into the line ahead of it before it leaves the launcher. The drive cups on the V-Jet pig will tend to "press" the inhibitor against the pipe wall as the pig passes by, rather than to wipe it off completely, as discs might tend to do.

Field Results

Pipe Diameter: 10" Std Wt

Product: Multiphase Natural Gas and Condensate

Flow Rate: 5.3 ft/sec

Length of Pipeline: 7 miles

With the exception of the "stand-alone" pig runs, which were exclusive to the V-Jet, all subsequent runs involving it and other types of pigs followed standard practice and procedures. The pipeline was pigged twice a week, Mondays and Fridays, and prior to the batch run which was done in three-week intervals. Coupons were pulled and replaced at four-week intervals. Inhibitor injection rates were left at 2 gal/day for batch testing and increased to 4 gal/day when the V-Jet was run as a stand-alone dewatering pig.
Average Corrosion Rates in
Mills Per Year (MPY)

 Top of Line
 Coupon
No Batching, Injection Only >4.00
Batching & V-Jet (avg. of 2 chemicals) 0.42
Dewatering Only w/V-Jet
(4 gal/day injection) 0.24

Based on measurements of the coupon farthest from the
inhibitor injector.


Top-line corrosion coupons before (Fig. 5) and after (Fig. 6) running V-Jet regularly as a dewatering pig only (no batching).

[FIGURES 5-6 OMITTED]

Bottom-line corrosion coupons at the end of the line also showed improvement due to better inhibitor distribution throughout the length of the line (Figs 7 and 8).

[FIGURES 7-8 OMITTED]

Results of the field trial provide evidence proving that the method of re-spraying residual inhibitor fluid from the bottom of the pipe to the top of the pipe wall does work. In these tests performed by the operator, the V-Jet worked better than standard batch treatments for TLC.

Preparing The Pipeline To Run The V-Jet

Piggability

The first order of business is to ensure that the pipeline will allow the passage of a pig. A few things to consider are:

* Ensure that valves have full, concentric bores and are fully open;

* Bend radii are large enough (3D min);

* Near-full-size tees are barred and any wyes are piggable;

* Bottom-line corrosion coupons must be retracted; and,

* Top-line coupons (and holders) are flush with the top of the pipe ID.

Any cleaning pigs to be used--and the V-Jet--must also be properly configured for these features.

Cleanliness

Generally speaking, to ensure the effectiveness of corrosion-inhibitor application, the pipeline internal surface must be clean enough for the chemical to contact the pipe wall. Contaminants that can prevent effectiveness include scale, iron sulfate (powder) or anything else that attaches itself to the internal surface of the pipe. The V-Jet will also work best when these contaminants are removed.

Not only will the pipe surface be most ready to be treated, but the absence of powders or other debris will ensure that the pig's jetting function is at its optimum performance. Jetting efficiency is proportional to the differential pressure across, and velocity and volume of bypass flow going through the nozzle. To keep this flow velocity high, the bypass portion of the nozzle is a relatively narrow passage. For this reason, a filter or screen is included near the inlet portion of this flow circuit to keep larger particulate from clogging the narrow passage.

In the presence of large amounts of particulates in the pipeline, this filter can become contaminated, resulting in reduced volume of flow. This can, in turn, reduce the suction capacity of the fluid redistribution (spraying) circuit. Of course, larger particulates, scale or sludge can also reduce the flow capacity of this suction circuit and jetting portion of the nozzle.

Conclusions

1. TLC can be a difficult problem that may require special corrosion-mitigation and monitoring techniques. Typical applications of corrosion coupons in the lower portions of lines will not effectively detect and measure TLC. Special coupon installations in the top of the line are effective at monitoring this type of corrosion.

2. Continuous injection of corrosion inhibitors alone did not effectively control TLC in the multiphase gas gathering line. The most effective corrosion control was achieved when a combination of continuous injection and batch inhibition or the V-Jet were utilized.

3. The V-Jet was highly effective at controlling TLC in the multiphase gathering line. Corrosion rates fell by an order of magnitude from > 4 mpy to 0.2-0.42 mpy. The pig effectively redeployed existing pools of inhibitor-containing water from the bottom of the line to upper portions of the pipe wall. The V-Jet was also effective at dewatering the pipeline. The positive results from the pig allowed the operator to discontinue the batch inhibition program and simply run the V-Jet during normally scheduled dewatering runs.

By Eric N. Freeman, P.E., Engineering Manager, T.D. Williamson, Inc., Pigging Products Division, Tulsa, OK and George C. Williamson, P.E., BP America Production Co., North American Gas SPU, PSIM Manager & Engineering Authority
COPYRIGHT 2007 Oildom Publishing Company of Texas, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 2007 Gale, Cengage Learning. All rights reserved.

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Title Annotation:Inhibitor Dispersal Pig Tested; T.D. Williamson Inc. develops V-Jet
Author:Freeman, Eric N.; Williamson, George C.
Publication:Pipeline & Gas Journal
Date:Jan 1, 2007
Words:2737
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