Since extent of the damage caused by corrosion costs the oil and gas industries many millions of dollars annually, there is an increasing need to manage and minimise this corrosion activity.
Even when corrosion rates are predicted by calculation, there are unexpected factors that can exacerbate problems, such as changes in the composition of the product, reductions in flowrate through shutdowns, additions of well-injected water, souring of wells or mechanical damage.
Corrosion control on the outside of the pipe is usually achieved by means of coatings and cathodic protection. However, a major challenge to pipeline design engineers is in the control of corrosion on pipe bores. The process fluids can carry a variety of corrosive impurities – such as free water, carbon dioxide and hydrogen sulphide. The effects of these products will differ depending on factors such as pipe geometry and attitude, flow rate and fluid composition.
One option for mitigating this corrosion is using pipe made entirely of a corrosion resistant alloy (CRA), such as Alloy 625. However, in many cases this is prohibitively expensive. A much cheaper and more readily available option is to use carbon steel pipe, which has been weld overlay clad.
Weld overlay cladding is a welding process that provides protection for products that are to be used in aggressive environments – such as pipelines, valves, flanges and specialist fabrications – by welding a corrosion resistant protective layer to areas at risk of corrosion and wear. The versatile weld overlay process is a practical combination of readily available base materials, coated with a suitable long-lasting corrosion resistant alloy to protect vulnerable areas. This provides the benefit of cost savings as well as a reduction in lead time.
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 CRA is dependent upon the aggressiveness of the medium.
Where a pipe will be transporting fluids that are excessively corrosive (guidelines set out in NACE MR01-75/ISO15156), the bore of the pipe will also be clad.
Generally, the CRA thickness will be 2 - 5mm and will not be considered part of the design strength criteria. This protection option greatly increases the price of the pipe– value greater than 10 times the cost of carbon steel can be expected as 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.
Weld overlay clad pipe
Standard carbon steel pipe can be weld overlay clad using a range of CRA consumables. Where 300 series stainless steels have been selected, a buffer layer using an over-alloyed consumable (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 in the vast majority of clad pipelines), the usual acceptance standard for chemical composition is either 5% or 10% Fe. While 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. It is not a direct measure of corrosion resistance.
Corrosion performance in pipes is monitored by means of intrusive devices such as smart pigs, which are intelligent sensing devices that 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.
Ring pair corrosion monitoring equipment – such as that provided by Teledyne and manufactured by Arc Energy Resources – is specified when high resolution, real-time metal loss measurement on the full pipeline diameter is required.
Arc Energy Resources’ weld overlay cladding services are used by clients where service conditions require corrosion resistant properties. The company’s weld overlay and inlay service offers a reliable and high quality product. The scope of supply includes flanges, elbows, tees, reducers, pipes, dished ends, valves, vessel strakes, heat exchanger tubesheets, fabricated sub-assemblies and various other geometries. Cladding can be carried out on components as small as 20 mm and as large as 4 m in diameter.
Arc Energy Resources also maintains a radiographic quality fabrication facility and segregated stainless steel workshop, allowing it to project manage more complex structures such as pressure vessels and spools (both clad and unclad). The company also offers SME U and U2 Stamps, so that it can provide a full range of services in compliance with the ASME code.
Arc Energy Resources’ machine shop provides CNC milling, turning and drilling to tight tolerances. It specialises in flanges, connectors and other subsea components.
In-house NDT incorporates visual inspection, dye penetrant inspection, ultrasonic testing, magnetic particle inspection, radiographic testing, dimensional inspection and CNC co-ordinate measurement. This, along with in-house procurement, design-for-manufacture, project management and heat treatment facilities, allows Arc Energy Resources to provide a complete one-stop-shop for all cladding, fabrication and machining requirements.
Read the article online at: https://www.worldpipelines.com/product-news/17102016/arc-energy-resources-combatting-corrosion/