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Protection of pipe bores from corrosion

24 March 2016

The extent of the damage caused by corrosion costs the oil and gas industry large amounts every year and there is an increasing need to manage and minimise this particular problem as pipelines and associated equipment are fitted into increasingly inaccessible and hostile areas around the globe. Here, Alan Robinson of Arc Energy Resources looks at some of the major protection solutions for pipeline interiors.

Stock image
Stock image

Even when corrosion rates are predicted by calculation, there are unexpected factors which can still exacerbate the problem – changes in the composition of the product flowing, reductions in flow rate through shutdowns, additions of well-injected water, souring of wells or mechanical damage.

The control of corrosion on the pipe exterior can be achieved through coatings and cathodic protection.  Whilst an important element in the overall protection of a pipeline, it is not a consideration for this article, which will instead concentrate on the control of corrosion in the pipe bore.

Oil and gas process fluids can carry a variety of corrosive impurities such as free water, carbon dioxide and hydrogen sulphide.  The effect of these products will differ dependent upon factors such as pipe geometry and attitude (horizontal or vertical) flow rate and fluid composition. In addition the corrosion mechanisms are diverse and include galvanic, erosion-based, microbiological, stress cracking, crevice corrosion, CO2 corrosion and hydrogen embrittlement.

The use of carbon steel pipe in combination with inhibitors is without doubt the first option to be considered by the designer as the initial cost is the lowest.  However, the choice does rely on a comprehensive understanding of the likely corrosion performance of the medium within the various geometries of the linepipe.  It is also essential that operational procedures ensure that chemical supply is maintained.  Use is made of a corrosion allowance which allows for a calculated level of pipe wall thinning as a result of expected corrosion.

Where carbon steel pipe is selected, it is frequently specified that the seal areas of associated equipment such as valves and flanges are weld overlay clad in order to provide protection from localised corrosion. Here, the selection of corrosion resistant alloy (CRA) is dependent upon the aggressiveness of the medium.  Generally the selection will be 316L (AWS A5.9 ER316L) or Alloy 625 (AWS A5.14 ERNiCrMo-3).

Where it is predicted that the pipe will be transporting fluids which are excessively corrosive (guidelines set out in NACE MR01-75/ISO15156), then alternative means of protecting the pipe bore are needed.  In these circumstances, the options are:
•  Solid CRA pipe
•  Plastic lined pipe
•  Mechanically lined (expanded) pipe
•  Co-extruded bonded pipe
•  Rolled and welded pipe
•  Weld overlay clad pipe

This article will not consider solid CRA pipe nor the use of plastic lined pipe for water injection pipes.

For both the lined or clad pipe, the CRA thickness will be 2-5mm and generally will not be considered as part of the design strength criteria.  The cost of this choice of protection increases the price of the pipe significantly – value greater than 10x the cost of carbon steel (dependent upon protection method) can be expected because the overall cost will include both the additional cost of the application of the lining plus the extra cost of joining the pipes using a CRA consumable.  However, if the linepipe is located in deepwater with very limited access for inspection and potential replacement then the increase in asset cost is unavoidable.

Currently there are no other options. The benefit to the operator of reduced corrosion issues, less inspection reduced downtimes, savings in investment in chemical and chemical injection equipment and overall increase in confidence are attractive factors.

Mechanically lined pipe

A CRA liner in the form of a pipe is pushed into the bore of a carbon steel pipe and is then expanded within the steel pipe. On release of the expansion forces, there remains an interference fit between the pipe and liner.  The gap at the two ends of the pipe is then seal welded to ensure no leak path between pipe and liner.

Benefits:
•  Reduce cost on solid CRA or metallurgically (weld overlay clad or roll clad) bonded pipe. The cost of the product is quoted as being 30-50% less than the cost of the equivalent metallurgically bonded pipe.
•  Production times are said to be less than other forms of lined pipe.
•  Said to be not susceptible to hydrogen induced stress cracking.
•  Is available in 12m random lengths.
•  Can be reeled.
•  Smoother bore than weld overlay clad products.

Potential disadvantages:
•  There is limited use in fields which operate at high temperatures. There can be significantly differing coefficients of thermal expansion between the carbon steel pipe and the CRA liner and this can affect the fit and can also cause stress at the seal weld.
•  There have been concerns regarding the ability of the assembly to withstand lateral buckling as a result of axial forces within the pipeline. In these circumstances there is a risk of buckling of the liner.
•  Where induction bending of the pipe is necessary, it is not possible to use mechanically expanded pipe.
•  Where non-standard lengths of pipe are required, then special measures are needed to cut back the liner and re-weld the ends.
•  There are reported limitations on minimum order size.

Coextruded pipe

Two variants are available.  One option is to follow the route of the mechanically lined pipe and then to extrude the lined pipe at elevated temperature thereby producing a solid phase metallurgically bonded pipe.  The second option is to weld overlay clad the bore of a forged base material and then extrude the combined welded product in order to form, again a lined pipe with metallurgical bonding.

Benefits:
•  Full metallurgical bond.
•  Smoother bore surface than with weld overlay clad products.
•  Can be used for induction bends.
•  Can be cut to shorter lengths with little additional work.
•  Can be reeled.
•  Is available in 12m random lengths.

Potential disadvantages:
•  There are reported limitations on minimum order size.
•  For the 2nd option, lack of uniformity of clad thickness.

Rolled and welded pipe

The pipe is formed by pressing plate into cylindrical form and then welding the longitudinal seam to produce the pipe.  The plate used has been clad by roll cladding a sheet of CRA material to carbon steel plate.  In this case the bulk of the longitudinal seam is welded from the outside using a carbon manganese  steel filler whilst the seam inside the bore is back clad using a CRA consumable which matches the bulk of the CRA bore cladding.  The join between the carbon steel plate and the CRA liner is solid phase. It is a fully metallurgically bonded pipe. 
The bond is checked using ultrasonic techniques, generally to ASTM A578 or a derivative of this standard.  This can permit some fairly healthy areas of lack of bond – the acceptance standard will be either indications confined to within a 25mm or 75mm circumscribed area plus a loss of backwall echo.  A band along each edge of the plate will be checked to a tighter UT standard and will be supplemented by dye penetrant examination in order to ensure that no pipe/clad defects will affect the interpretation of the eventual longitudinal seam and the pipe to pipe butt weld.

Benefits:
•  Full metallurgical bond.
•  Smoother bore surface than with weld overlay clad products.
•  Can be used for induction bends.
•  Can be cut to shorter lengths with little additional work.
•  Can be reeled.

Potential disadvantages:
•  There are reported limitations on minimum order size.
•  The pipe length is limited by the initial plate size and the pipe forming capabilities.  In order to achieve 12m lengths, it may be necessary to double joint two 6m lengths.

Weld overlay clad pipe
Standard carbon steel pipe can be weld overlay clad using a wide range of CRA consumables.  Where 300 series stainless steels have been selected, a buffer layer using an over-alloyed consumable (e.g. ER309L or ER 309LMo) can be used in order to accommodate the change of composition as a result of the inevitable dilution with the base pipe. When using 309LMo as a consumable and the effects of dilution are taken into consideration, the resulting layer of CRA will meet 316L composition.  When Alloy 625 is used (as it is in the vast majority of clad pipelines), the usual acceptance standard for chemical composition is either 5% Fe or 10% Fe.
 
Whilst this measure does not give an absolute indication of corrosion resistance, it is a good indicator of the quality control mechanism in place during welding – a high level of iron means that the weld has penetrated too deeply into the base metal causing the higher dilution. It is not a direct measure of corrosion resistance.

Pre-production qualification testing will include corrosion testing to ASTM G48 practice A at 50°C for Alloy 625 and ASTM A262 Practice E for 316L.

Cladding is invariably undertaken with the gas tungsten arc process using hot wire addition to the wire consumable.  The organisations which have majored on cladding of multi lengths of pipe will also use multi wire and multi head units.

Post welding NDT for weld overlay clad pipe comprises visual inspection using video cameras plus ultrasonic, dye penetrant inspection.  Acceptance standards are extremely tight (UT, Ø1.5mm flat bottomed hole; DP, Ø1.6mm indication).  Testing also includes PMI, clad thickness measurement and in some cases, laser surface recording.

Benefits:
•  Incredibly flexible in terms of pipe diameter, length and batch size – a batch of one or one hundred pipes can be produced with minimal increase in preparation.
•  Wide range of CRA products can be deposited.
•  Is available in 12m random lengths.

Potential disadvantages
•  The process is relatively slow.
•  The bore profile is not smooth.
•  It is vulnerable to through thickness defects.  The cladding process has to be perfect.  Any through thickness void will be the focal point for on-going corrosion.

Corrosion monitoring

Intrusive methods - Corrosion performance within pipe is monitored by means of intrusive devices such as smart PIGs which are intelligent sensing devices which are introduced into a pipeline and usually carried by the product along the length of the pipe.  The devices are able to measure and transmit dimensional data regarding the pipe bore and will highlight heavily corroded areas.

In line – Ring pair corrosion monitoring equipment such as that provided by Teledyne is specified when high resolution, real-time metal loss measurement on the full pipeline diameter is required.

Summary

Corrosion continues to challenge the oil and gas industry.  The methods of protecting the bores of pipeline and associated equipment are listed above. The control of corrosion requires a combination of calculation/prediction of likely product content, design of the pipeline, protection of the pipe bore and monitoring of the actual corrosion activity. With a comprehensive combination of these disciplines, successful control of corrosion can be achieved.


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