Plastic fantastic

Polymer-lined pipe has been around since the 1990s, but its potential has yet to be fully tapped, according to those behind JIPs to extend use of this technology. Mostafa Tantawi and David Whittle explain.

Swagelinings enior development engineer machining thepolymer lining to accept connector technology. Photos from Swagelining.

The UK oil and gas industry is in the midst of some troublesome times. The new, lower price of Brent oil, averaging US$45-60/bbl during the last six months, has seen drastic measures taken to make cost savings and reduce operational spend.

Operators are being challenged to investigate new ways of implementing cost reductions to oil and gas producing assets. Key to addressing these challenges is innovation, and investment into research and development must continue to help maximize the recovery of remaining hydrocarbon reserves.

Extensive research and development has gone into developing pipeline polymer lining solutions, which have the potential to offer considerable cost savings as well as significant operational and technical benefits. These developments have been driven by both industry and consultancies, in an effort to achieve wider understanding of the significant benefits that polymers bring.

Lined pipelines and the traditional alternatives

Pipelines typically form a major proportion of the development cost of a subsea project, particularly for long subsea tie-backs. The costs, which include procurement, fabrication and installation, are mainly driven by the pipeline material selected and method of installation.

A significant part of the cost of maintaining a subsea pipeline is incurred attempting to combat internal corrosion. Carbon steel is traditionally selected for the fabrication of risers and flowlines, with protection from corrosion and erosion afforded by the “thickening” of the pipe, through the addition of a corrosion allowance and introduction of a corrosion inhibitor into the product.

However, for ambient water injection service, the application of a polymer lining system is now becoming the benchmark. Where water injection service requirements are more extreme and in hot sour hydrocarbon service environments, corrosion resistant allow (CRA) lining and cladding, or even a solid CRA, is commonly specified to handle the corrosive nature of the transported product. This method, however, has significant implications on the procurement, scheduling and installation costs of a subsea pipeline.

installation of polymer liner into steel string.

Polymer lining

Butt fusion welding of polymer pipe.

Polymer lining technology was introduced to the oil and gas subsea sector in the mid-1990s. Installation is achieved by pulling an extruded polymer pipe through a reduction die to temporarily reduce its diameter. While in this reduced state, the pipe continues to be pulled through the constructed carbon steel carrier (outer) pipe, before being released and reinstated to its initial size so that it fits tightly to the host. Lengths of liner up to 1500m have been installed in single operations.

Following the completion of this insertion process, bespoke flangeless connector technology is used to join together lined pipe sections at the field joints.

Polymer-lined subsea pipelines are traditionally installed by reel lay, J-lay and in bundles. Existing connector technology restrictions have excluded the application by S-lay until now, but new developments are allowing this to be considered as an option.

Indicative example of steel temperature for external 5LPP system vs. 15mm PVDF internal liner (based on case study conducted by Xodus Group Ltd. Inlet fluid temperature = 60°C) Graphs from Xodus.

Technical overview

The technical benefits of plastic lined pipelines are vast. Polymers are corrosion resistant, which immediately negates the need for corrosion inhibitors throughout the life of the pipeline or the requirement of a CRA derived pipeline.

Installation of polymer liner into steel string.

Polymer liners are currently favored in the majority of subsea water injection pipelines in the North Sea due to the cost effective corrosion resistance and reliability, with a significant increase in the number of operators recognizing the technology in recent years.

Polymer lining can also allow for potential unprocessed (production) water re-injection service, which saves on the requirement for topside de-aeration equipment. With an increased need for hotter water injection temperatures, different types of higher performing polymer materials are now available to fulfil varying service requirements.

Until now, high performance grades of polyethylene, predominantly polyethylene (PE) 100 have been used for the vast majority of lining applications. PE 100 is suitable for use in water injection service up to 60°C, where long lifetimes can be expected in the relatively benign conditions of treated seawater for example.

At higher temperatures, up to 85°C, and where an option for produced water re-injection may be required, a more chemically resistant polyethylene such as PE-RT (polyethylene of raised temperature resistance) can be used. Polyethylene materials may be considered for hydrocarbon applications but as service environments change as temperature increases, above 50°C the properties of ‘engineered polymers’ such as polyamides and polyvinylidene difluoride (PVDF) may be preferential.

The technology’s internal pipeline corrosion protection has great potential for usage in hydrocarbon pipelines using engineered polymers. It is recognized that the use of polymer liners in subsea production pipelines has yet to be fully developed and that there are challenges in this arena, which must be resolved before polymer lining can be considered in every service application, but these are now being seriously considered by operators worldwide.

Theoretically, in hydrocarbon service, the threat of liner collapse exists, due to the permeated pressure gas build-up in the annular gap between the polymer liner and the inner wall of the carbon steel pipeline. Additionally, the issue of potential liner swelling when in contact with hydrocarbons needs to be considered.

Work to address this is currently being carried out by Swagelining (a UK-based specialist in the design and installation of polymer linings), which has recently embarked upon a joint industry project (JIP) with The Welding Institute (TWI) and Saudi Aramco. This JIP will examine the extent of corrosion incurred in a variety of polymer lined pipelines when subjected to a hot sour hydrocarbon environment.

Swagelining is also currently carrying out technical qualification programs with operators, identifying how polyethylene performs with higher temperature injection water and considering the effects of chemicals used for enhanced oil recovery (EOR).

Using polymer liners in hydrocarbon pipelines can deliver significant technical advantages. Not only are they fully corrosion resistant, polymer liners are also relatively smooth compared to steel and metallurgic alloys. The low roughness of polymers minimizes the pressure drop across the pipeline, which is maintained along the lifetime of the pipeline, unlike steel/CRA roughness which degrades due to erosion and corrosion.

The elastic nature of polymers is also more tolerant to both fluid and particle erosion. These fluids often contain debris and deposits, which can accumulate on the inner wall of a pipeline, causing flow restrictions and occasionally, blockages. It is envisaged that the adhesion between fluid deposits, for example wax and hydrates, and polymer material is lower than that of steel, which decreases this blockage risk and reduces pigging frequency requirements.

An additional advantage for regular hydrocarbon service is that polymers have good heat insulation properties and add significantly to the thermal performance of a pipeline system. For example, a layer of polymer between 10-15mm will have a significant effect on the pipeline overall heat transfer coefficient, greatly reducing the outer insulation requirement of the pipeline. While a 12in carbon steel buried pipeline with a 5LPP insulation system can typically have a U-value of 3 W/m2K, a 15mm PVDF liner will result in a U-value around 4 W/m2K. This can provide substantial cost saving, as the cost of outer insulation is often higher when compared to the cost of carbon steel.

The insulating impact of the inner polymer liner on the inside of the pipe will also result in a reduced steel temperature, which will also enhance the mechanical behavior of the pipeline, minimizing thermal expansion and propensity for global buckling. An indicative example of the reduction in steel temperature when utilizing a PVDF polymer liner is presented in the figure below.

A typical procurement and fabrication cost comparison between polymer liners and alternative corrosion resistant methods (includes indicative pipeline insulation). Graphs from Xodus.

Economics advantages

The use of polymer liners in subsea pipelines can result in substantial cost reduction for both capital expenditure (capex) and operational expenditure (opex). The cost of procurement and fabrication of a PVDF lined pipeline for service application to 130oC is about 50% cheaper compared with the equivalent CRA clad pipeline. In low temperature applications (for example <60oC), where a PE liner can be used, the cost of procurement and fabrication can be 80% cheaper than the equivalent clad pipeline and 40% cheaper than carbon steel pipeline with corrosion allowance. (Figures presented are representative of a 12im X65 pipeline and are indicative only).

Further opex reductions will be achieved by negating the need for corrosion inhibitors, frequent pigging and energy (pumping) costs. These savings are measured across the total life of a pipeline, and adopting at the early stages of a project means that IRM costs can be drastically reduced throughout the average life span of a pipeline.

Risk reduction

Corrosion is a critical problem that can lead to major pipeline failure, and in some cases total shutdown, if not monitored and managed properly. This impacts the operator’s business with down time, loss of supply and the significant cost of remedial work.

Further to the impact on an operator’s schedule and bottom line however, is the dramatic consequences which can result from pipeline failure. Polymer lining technology stops corrosion being incurred from the outset, which is one of its most important benefits.

Polymer lining technology performs to the stringent health and safety standards that the oil and gas industry demands, allowing pipelines to be constructed and installed within a low risk environment and affording lifetime protection.

Conclusion

The future seems bright for the usage of polymer linings for affordable internal corrosion protection. With the average cost of installation and maintaining a pipeline accounting for around 35% of a typical subsea tie-back project, it is little wonder that such efforts are going into research and development activity and cost saving measures.

The ongoing work being undertaken by operators, and the interest being shown by the industry indicates that the message is getting across and polymer lining is being considered as a serious alternative.

Is polymer lining technology finally shaping up to its promise?

Mostafa Tantawi is a senior pipeline engineer at Xodus Subsea. He has over four years’ experience in subsea engineering and pipelines mechanical design. He has an MSc in Subsea Engineering.

David Whittle has over 40 years’ experience in the pipeline industry, in various engineering capacities in the UK and internationally for organizations including British Gas and Subsea 7.

Whittle joined Swagelining in 2009 at the inception of the company as business development director. He works with new and existing clients to determine future industry requirements for polymer lining technology across a range of projects in subsea and onshore pipeline systems.