1. Introduction

Horizontal Directional Drilling has become a widely used technique for installing sections of underground pipeline in areas that would otherwise cause a variety of problems. Whilst it was initially used to overcome major natural barriers such as large river crossings and very steep or collapsing slopes, it has now become cost effective to use on some shore line crossings as well as rail, road and creek crossings.

However, sharp edges associated with the hole geometry, rocks falling into the HDD, and high pulling forces can combine to create an environment where there is a significant likelihood of coating damage. This is usually mitigated by providing the pipeline within the HDD with an abrasion resistant coating - but damage can still occur. As pipelines typically have design lives in excess of 40 years, and HDD are usually installed in areas where coating maintenance and pipe repair are impossible, some assurance is required that the corrosion protection coating is in a satisfactory condition. The cost of failure in some of these areas, in terms of pollution costs, the results of ignition, loss of product, penalty for loss of supply, injury, public relations, and replacement of the entire HDD, can be immeasurable.

As the use of HDD has become more common and there are more HDD installers in the market, prices have become more competitive, but it appears to be at the expense of quality. Occasionally there is little attention given to hole cleaning, and smaller machines are used that do not have the extensive mud preparation and circulation equipment of larger machines. Smaller operators occasionally do not have the experience of the mud properties and the velocity required to clean the hole. Better specifications are required that provide for real time quality monitoring prior to the pipe being pulled in.

Specifiers may require the use of HDD in areas that are unsuitable without a casing, may provide inadequate geotechnical data, and may accept the lowest tender which may not use the appropriate equipment or casing if necessary for the environment. They may also select the abrasion resistant coating based on vendor information and cost rather than on a proper investigation, performance requirements and a risk analysis.

A common test method is required that has a high level of reliability in identifying coating defects, at the time of HDD installation, that are not acceptable for the long term integrity of the pipeline.

2. Test method development

2.1 Early HDD - no testing

HDDs were not used in the Australian area until the early 1990s. The Kutubu Pipeline, which was constructed from 1991, had two major river crossings. In 1990 they were designed as open cut river banks and a bottom-towed pipe. Because the rivers frequently carry high flood flows with associated high water velocity, the construction difficulty associated with the open cut technique resulted in the contractor electing to construct the crossings using HDD. The technique was new to Australia and so it was decided to leave coating quality issues to the overseas specialist HDD operators engaged by the contractor. The pipe was provided with a double coat of extruded high density polyethylene over mastic (HDPE/M), and then a 25 mm thick coating of spiral wire reinforced concrete.

Unfortunately the substructure of the area was limestone which contained caverns of varying size. When the drill goes through a cavern it can hit the rock on the other side and then move in any direction across the rock surface for a short distance before it bites into the rock. The result is that the hole may not be concentric so that when the pipe is pulled in it may bear directly on the edge of the surface, causing damage. This can remove long strips of the corrosion coating, and in this case remove the spiral wire-reinforced concrete coating as well. The accumulated disbonded concrete coating formed a plug in the HDD, jamming the pipe. The problem was resolved after removing the pipe with a steel casing over the full length of the HDD. The pipe, with insulating spacers, was pulled into the casing.

2.2 DCVG

Prior to 1994 some HDDs were tested using the Direct Current Voltage Gradient (DCVG) coating defect survey technique, but this system has decreasing sensitivity with depth and is of limited value for deep drills. Also, as most HDD are installed because of site access difficulties, the site is often not particularly accessible for DCVG surveys.

2.3 On-potential swing

The on-potential swing test method was used on pipelines in Australia from 1994 to 1997, including the Moomba to Sydney ethane pipeline where the results were reported by Ian Fotheringham1. This test method involves applying a cathodic protection (CP) current to the pipe, before it is tied-in to the rest of the pipeline, so that its potential is moved in the negative direction by at least one volt. The associated CP current is measured. The criterion used for this test was that the current requirement should not exceed 1µA/m2 of pipe area. This seemed an appropriate figure as most new pipelines at the time drew an average CP current of about that value, so it was not demanding more than the mean value of the entire pipeline, and it was a hard metric number.

2.4 On-potential

The Osborne Lateral, constructed in 1998, included an 850 m HDD under the saline Port Adelaide River in Adelaide. As the pipeline was in a saline environment of very low resistivity, it was not possible to move its potential in the negative direction by at least one volt without imposing extremely high current densities. To provide an immediate solution for the HDD in the saline environment, the 1µA/m2 of pipe area criterion was applied at an on-potential of 900mV Cu/CuSO4 on-potential. However this was not suitable for general use as in non-saline environments the current density would vary with soil resistivity.

2.5 Polarisation change

The testing methodology was reviewed in 1999 when the Longford to Sydney Eastern Gas Pipeline was in its design phase. After the review it was concluded that despite its limitations, a test method that used the CP current density of the HDD pipe as a criterion appeared to be the best available at the time.

In developing the test it was necessary to determine the potential at which the test current should be measured. It was decided to use polarisation change, the difference in potential between the natural potential and the off-potential, as this prevented soil resistivity and natural potential from affecting the result. The initially used current density criterion of 1µA/m2 for the ethane pipeline was simply changed to 1µA/m2/100mV for the polarisation change test criterion.

Figure I shows polarisation change test results from an HDD. Polarisation was carried out for 30 minutes but there was little change in the current output after 5 minutes. However the off-potentials continued to polarise with -908, -966 and -998 mVCu/CuSO4 measured at 10 minute intervals.

This test method is the most widely used test method in Australia at present, however it does have some limitations which require resolution.

3. Test method limitations and necessary improvements

3.1 Large defects in high resistivity environments

The use of a CP current density criterion for the HDD pipe may be reasonable if the current is drawn by a large number of small defects. However experience has shown that most current is drawn by a small number of large defects. Therefore on a very long HDD, all of the current may just go to one large defect. If the defect is in a low resistivity stratum then it should be readily cathodically protected. However if it is in a high resistivity stratum, such as rock, then the resistance to earth of the defect may be too high to allow sufficient current to flow to protect it. The pipe would then not be protected and may corrode.

This can be addressed by using the data collected during the existing polarisation change test to also calculate coating resistance in terms of ‘current density per square metre per 100mV IR drop’. Thus instead of dividing the current density by the polarisation change, it is divided by the difference in the on-potential and the off-potential. This provides a parameter which is more sensitive to coating defects in high resistivity strata, and also caters for artificially less negative natural potentials.

If appropriate criteria are developed for both tests, and an HDD passes both tests, there is a higher probability of the coating being satisfactory than if only one is met.

3.2 Cathodic protection shielding

The test does not indicate the presence of areas of possible shielding of the CP. CP shielding occurs where there is a crevice between the pipe steel and another object so that there is a high electrical resistance to the flow of CP current along the crevice. Very high rates of corrosion, of up to 0.5 mm/year, can occur in such crevices. Tighter crevices tend to result in higher corrosion rates. There are typically two causes of crevices:

• Coatings that have become disbonded as a result of mechanical damage during installation of the pipe. This can result in a crevice between the steel and an insulating layer of coating which can result in CP shielding. The formation of such crevices has been found to be a particular problem on some dual layer FBE coated HDD (See Figures II to IV).
• The rock or stones that cause gouging of the coating to expose the steel are likely to be in contact with the steel when the pulling-in of the pipe is completed. This will result in a crevice between the gouging surface and the pipe steel, creating a crevice and shielding CP.

There are no field tests presently available that can detect the presence of shielded surfaces, so it would appear that the only way to address this issue is to use coatings that are not prone to forming crevices and are resistant to gouging. Whilst considerable testing has been carried out on HDD coatings to measure their resistance to gouging, there does not appear to have been much work carried out on failure modes that can result in the formation of crevices. Additional work is required in this area.

At another location a gouge of less than 25 per cent of coating thickness caused a fracturing of the bond between the FBE and the steel. While this did not expose the steel to the environment it shows the sensitivity of the bond to propagating crack mechanical failure, similar to the way in which a 3 mm diameter stone chip on 400 micron FBE can cause a 100 mm diameter loss of adhesion.

3.3 Void annular space

The test will not detect coating defects in areas where there is no electrolyte in the annular space of the HDD. This can occur in HDD that are above the water table. In such HDD there can be condensation on exposed pipe steel surfaces which can cause a high rate of corrosion that can not be mitigated by CP. Such void annular spaces can occur when:

• The annular space is not filled with mud, or when
• The mud in the annular space leaks out at rock fissures or into a dry porous environment.

The solution in such cases may include:

• Topping the mud up as a maintenance operation.
• Using a cement based grout seal the drill.
• Using a cement based grout to fill the annular space, providing there are no stress implications.
• Providing the drill with a steel casing.

The implications of the alternative solutions need careful evaluation.

3.4 Current density criterion

There is an amount of field data that indicates that the 1.0 µA/m2/100mV criterion used for acceptance of HDD coating is inadequate:

Ethane criterion

The criterion used on the Moomba to Sydney ethane pipeline was 1µA/m2. It was simply changed to 1µA/m2/100mV for the polarisation change test criterion without detailed evaluation of ethane pipeline test conditions. Subsequent evaluation revealed that the ethane pipeline testing was carried out at a mean polarisation change of 840mV.1 Thus the mean criterion used on the ethane pipeline was equivalent to 0.125µA/m2/100mV, not the 1µA/m2/100mV used in the present criterion.

Intact Coating

The ethane pipeline test results1, summarised in Figure V, indicate that the coating CP requirements covered a wide range of values. This may indicate that intact coating draws an insignificant current, and that any current over say 0.001µA/m2 reflects an amount of coating damage.

Excavation Results

Figure VI presents test results of a number of coating tests on HDD. The HDD with current densities in excess of 1µA/m2/100mV polarisation change, and the SEA Gas HDD, have been investigated and the damage measured and rectified. The results are:

a) 16.3 µA/m2/100mV (ethane) [Test failure result was 29 µA/m2 which calculates to 16.3 µA/m2/100mV]: Most of the pipes were found to be deformed with extensive damage to the coating.

b) 8.0 µA/m2/100mV (EGP): Coating was abraded from two raised circumferential welds.

c) 5.0 µA/m2/100mV (EGP): Coating was scraped along five pipes where they made contact with a buried steel casing.

d) 1.4 µA/m2/100mV (TGP): There was multiple scratching through the coating back to the steel over a 2 m length of pipe.

e) 1.4 µA/m2/100mV (TGP): Two scratches back to the steel.

f) 1.0 µA/m2/100mV (Ethane) [Test failure result was 9 µA/m2 which calculates to 1.0 µA/m2/100mV]: The coating on several pipes was severely abraded.

g) <1.0 µA/m2/100mV (SEA Gas): Three defects of 2 per cent, 3 per cent and 5.9 per cent IR respectively. (Another recent pipeline had a defect >20 per cent IR and passed the polarisation change test.)

New Generation Coatings

Commissioning the CP on pipelines with new generation coatings results in mean pipeline current densities of the order of 0.5µA/m2 or less. This is for the entire pipeline including all buried valves and fittings, and is typically for a polarisation change of at least 300 mV. That is equivalent to a current density of 0.17µA/m2/100mV polarisation change. A relatively short section of pipeline should be expected to perform better than this, especially as the section of pipeline involved often can not be repaired or maintained over the entire pipeline life and thus particular care is required.

Summary

The presently used criterion of 1.0µA/m2/100mV is likely to allow defects in pipe that are impossible to repair due to their location, and that would not be accepted for pipe installed in an open cut trench. This should be addressed by developing a lower criterion.

4. Conclusions

The present system for accepting coating condition on HDD pipe is flawed and can result in allowing coating defects that are not consistent with the long term integrity requirements of pipe that can not be readily maintained or replaced. To provide such security work is required in the following areas:

• Evaluate the use of the coating resistance test, in addition to the polarisation change test, for testing coating condition.
• Evaluate coatings used in HDD for their resistance to loss of adhesion leaving crevices between the coating and the steel that can result in cathodic protection shielding.
• Evaluate grouting options to ensure that electrolyte surrounds the pipe in HDD that are above the water table.
• Evaluate the current density criterion to be used in the polarisation change test and the coating resistance test.

5. References 1 I. Fotheringham & P.Grace, Corrosion & Materials, 22, 3, pp5-8 (June 1997).