Carbon steel flowlines/trunklines are the most common method used in the Oil & Gas industry to cater the requirements of transporting production fluids and hydrocarbons to designated locations, in particular for onshore fields and facilities. Produced fluids and hydrocarbons can contain high concentrations of CO2, H2S or even sulphate reducing bacteria (SRB) which can render existing carbon steel flowlines susceptible to internal corrosion causing them to eventually leak after period of time if no proper chemical inhibition mitigation methods are implemented. This is in addition to the leaks caused by external corrosion as a result of direct exposure to soil contaminated with corrosive species. 

Non-metallic rigid and lined pipes can offer an alternative corrosion mitigation method to Carbon Steels (CS), Corrosion resistant alloys (CRA) and chemical inhibition. Plastic lined flowlines have been extensively utilised in the industry as alternative means for onshore water injection systems, however track record on production fluids involving hydrocarbon transportation is yet to be established. 

NON-METALLIC RIGID FLOWLINES/TRUNKLINES

Application of rigid non-metallic pipes have been widely used in oil and gas fields compared to plastic liners. Below are the most common types used in the industry:

FIBRE-REINFORCED POLYMER (FRP)/ GLASS-REINFORCED EPOXY (GRE)

FRP and GRE are usually considered interchangeable names for the same product. FRP/GRE is an alternative to carbon steel pipes which are highly useful especially for corrosive, aggressive, and normal environments.  FRP/GRE can be affected by UV and applications with continuous exposure to sun light require the use of UV-stabilized resins for the exterior layer.

The advantages of FRP/GRE material are following:

1. External and internal corrosion are eliminated;
2. Joints are designed to ensure no leaks during design life;
3. Low weight and simple push-on couplings enable high installation rates even in less
   accessible and demanding terrain;
4. Very smooth inner pipe surfaces minimize friction and pressure loss and simplify hygienic measures;
5. High abrasion resistance;
6. Pipe length up to 16 m which reduce the number of joints required.

However, FRP/GRE material have also some disadvantages, which are:

1. Lack of methods to determine the degradation or the damage in the pipe as there are no
   NDE techniques that can detect damage, and the only means for ensuring the integrity of the pipe is through visual inspection, hydro testing, or destructive testing of removed
   sections;
2. In areas of large sand dune movements there is possibility of sand dunes migrating and
   accumulating on GRE pipes and may cause GRE lines overstressed. In such scenario it is to be noted that non-metallic pipes, which could provide an effective internal and external corrosion barrier, do not provide equivalent bending or mechanical resistance as compared to steel flowlines;
3. The pipe system is more susceptible to impact damage due to the brittle nature of the
   thermoset resin system;
4. Prone to UV embrittlement;
5. The pipe system is more susceptible to impact damage due to the brittle nature of the
   thermoset resin system;
6. GRE installations require more and careful preparation due to other joining methods,
   handling and transportation requirements and installation techniques;
7. GRE has a medium resistance to gas permeation which may pose a high health risk for
   flowlines contain high levels of H2S

REINFORCED THERMOPLASTIC PIPES (RTP)

RTP has been developed to meet very specific requirements of handling corrosive fluid such as
crude oil and sour gas at high pressures.

The advantages of RTP material is following:

1. External and Internal corrosion are eliminated;
2. Maintenance is almost not required throughout design life;
3. RTP available in the market can serve pressure up to 250 bar and temperature up to 120°C, hence can be used for pressure rating up to 2500#;
4. Minimum storage requirements;
5. Handling and transportation requirements are minimal;
6. Stringent preservation is not required;
7. Re-usable after cease of production;
8. Installation is cost-effective;
9. RTP has a high resistance to gas permeation which may which makes it a preferable option for flowlines contain high levels of H2S.

The disadvantages of RTP material are as follows:

1. There is a possibility of sand dunes migrating and accumulating on RTP and may cause
   RTP lines overstressed;
2. Limited selection of fittings and service connectors depending on RTP supplier and system pressure;
3. Specialized installation equipment and operator training required for installation
   (equipment and training generally available from RTP manufacturers);
4. Limited sizes (up to 8 inch only);
5. Limited number of vendors qualified with a wide a range of liner materials and sizes;
6. Any change in the liner material or size will require a full qualification (monogram), which
   has high cost implications;

NON-METALLIC LINED FLOWLINES

Liners are non-structural corrosion barrier plastic pipes that are inserted into a structural host pipe. The liner acts as a barrier to shield the host pipe from the corrosive fluids and potential corrosion erosion that is responsible for wall loss. Below is a description of the commonly used liners in hydrocarbon and water injection services:

HIGH DENSITY POLYETHYLENE LINERS

The main advantages of HDPE Liners are as following:

1. Internal corrosion is eliminated;
2. Significant cost savings and better availability compared to CRA;
3. Internal corrosion mitigations are not required (i.e. corrosion inhibitor, operational pigging,etc);
4. Can be used as a rehabilitation of existing flowlines which are experiencing internal
   corrosion.

The main disadvantages of HDPE Liners are as following:

1. Can be susceptible to collapse if;
   • Annulus pressure increases beyond critical collapse pressure;
   • Minimum operating pressure is not considered during design;
   • Operating procedures are non-compliant (i.e. sudden shutdown, etc.).
2. HDPE has tendency to absorb hydrocarbon liquids or gaseous such as crude oil at high
   temperature, condensate and various solvents. The effect of hydrocarbon absorption is to plasticize the HDPE and lower its physical properties such as tensile strength and Young’s modulus;
3. Another effect of hydrocarbon and chemical absorption is physical swell of the HDPE liner and increased volume, typically around two to four percent;
4. HDPE liner is prone to permeation of gas entrapped between annulus gap of steel pipe
   and HDPE liner which will be required to vent off occasionally to release any permeated
   gas and confirm that the HDPE liner is intact and there is no a physical leak found;
5. Mitigation from external corrosion still required.

PVDF LINERS

PVDF is a thermoplastic material with high resistance to most oil and gas production fluids. It has a higher temperature limit over all non-metallic materials. PVDF has been proposed for conditions beyond the PA12 range, with offshore pipelines as the most viable application because of the cost.

The main advantages of PVDF Liners are as following:

1. High resistance to ageing and environmental stress cracking;
2. Compatible with most produced or injected well fluids at high temperatures including
   alcohols, acids, chlorine solvents, aliphatic and aromatic hydrocarbons, and crude oil;
3. Suitable for high temperature applications up to 120˚C.

The main disadvantages of PVDF Liners are as following:

1. Limited resistance to strong amines, concentrated sulfuric, nitric acids, and sodium
   hydroxide (recommend for use in pH < 8.5);
2. The ageing of PVDF is prone to the loss of plasticizers added to the material. The
   plasticizers are lost due to the hydrolysis in pipes experiencing high water cuts. The loss
   of plasticizers will render the material brittle and unsuitable for service;
3. Mitigation from external corrosion still required.

POLYETHER ETHER KETONE (PEEK) LINERS

PEEK liners provide excellent wear resistance, best-in-class fatigue resistance, ease of melt
processing, high purity, and excellent chemical resistance to organics, acids and bases. These
properties make it well-suited for oil and gas application.

The main advantages of PEEK Liners are as following:

1. Higher impact resistance and ductility;
2. Can be used up to temperature of 170°C
3. PEEK materials have outstanding creep resistance and may sustain large stresses over a useful service life with little time-dependent deformation;
4. PEEK materials are not attacked by prolonged exposure to water, sea water or steam
   which makes them an ideal choice for use as liners for pipelines carrying corrosive fluid;
5. PEEK provides an effective barrier to the permeation of fluids and gasses. The solubility of fluids and gasses, the diffusion through and the permeation from PEEK polymer are up to several orders of magnitude lower than other commonly used polymers.

The main disadvantages of PEEK Liners are as following:

1. Rapid degradation to unacceptable levels at higher temperatures than 170˚C and in the
   presence of aqueous salts and completion fluids;
2. Higher cost compared to other polymers;
3. Onerous installation requirements due its high strength property;
4. Mitigation from external corrosion still required.

PA12 LINERS

PA12 is used for higher temperature service, especially for multiphase flowlines/trunklines where all polyethylene liners have a short lifetime due to swelling and softening from hydrocarbon absorption.

PA12 is resistant to swelling and softening in hydrocarbons and maintain good properties well beyond the limits of HDPE. PA12 is susceptible to swelling and softening by methanol but can easily tolerate the normal amounts of methanol used in injection fluids.

Flowlines/Trunklines containing fluid up to temperature of 55°C can be effectively protected from
corrosion using PA12 Liners. PA12 liners suffer Hydrolysis in water service as well in oil service containing high water cut, where free water phase is expected in the flowline/trunkline. Hence, PA12 liners are suitable only for low water cut (<10% BS&W) oil flowlines/trunklines in which free water phase separation is not expected. This limitation restricts use of PA12 liners for oil flowlines/trunklines.

The main advantages of PA12 Liners are as following:

1. Eliminates internal corrosion;
2. Significant cost savings and better availability compared to CRA and cladded line;
3. Internal corrosion mitigations are not required;
4. Significant savings compared to corrosion inhibitor program.

The main disadvantages of PA12 Liners are as following:

1. The design life of the PA12 liner is heavily dependent on the presence of hydrolysis, partial pressure of CO2, and temperature. Hence, the worst case operating scenario has to be accounted for during design. Any change to the conditions during operation can adversely impact the design life of the liner;
2. Mitigation from external corrosion still required.

OUR CAPABILITIES

AIE has extensive experience and track record in all aspects of internal and external flowlines material selection and design. We can guide operators through the different stages of the material selection evaluation process. Our industry experts have the right knowledge and experience to support our clients conduct a whole life cost-benefit analysis and select the optimum material for their assets and facilities that will ensure longer operability, reliability and economic running costs. For more information how we can support you, please contact us on info@aiegroup.org