TPi July 2017

Protection of pipe bores from corrosion By Arc Energy Resources Ltd, UK

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 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 to 5mm and normally 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 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 line pipe is located in deep water with very limited access for inspection and potential replacement, then the increase in asset cost is unavoidable. Currently there are no other options. The benefits to the operator of reduced corrosion issues are attractive factors, and include fewer inspections/reduced downtimes; savings in investment in chemical and chemical injection equipment; and overall increase in confidence. 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 (eg 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

The control of corrosion on the outside of the pipe is usually made by means of coatings and cathodic protection, but a major challenge to pipeline design engineers is in the control of corrosion on the pipe bores. The 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 corrosion, erosion-based, microbiological corrosion, stress corrosion cracking, crevice corrosion, CO 2 corrosion and hydrogen embrittlement. One option to mitigate this corrosion is to utilise pipe made entirely from a corrosion-resistant alloy (CRA) such as Alloy 625. However, in many cases, and particularly for larger projects, this will be prohibitively expensive. A much cheaper and more readily available option is to use carbon steel pipe that has been weld overlay clad. Weld overlay cladding is a welding process that provides protection for products that are destined for use 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 highly versatile weld overlay process provides the practical combination of readily available base materials, coated with a suitable long-lasting corrosion-resistant alloy to protect any 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 T he extent of the damage caused by corrosion costs the oil and gas industries many millions of dollars annually. There is therefore an increasing need to manage and minimise this corrosion activity as pipelines and associated equipment are fitted into increasingly inaccessible and hostile areas around the globe. Even when corrosion rates are predicted by calculation, there are unexpected factors that 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.

Figure 1: An example of pipe weld overlay cladding

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July 2017 TUBE PRODUCTS INTERNATIONAL