General material selection guidelines for oil and gas industry pipelines.
Types Of Pipelines
The oil and gas industry normally deals with various types of pipelines depending on the functional application of the line like flow lines, transfer lines and export lines. General descriptions of the various pipelines are in Figure 1.
Figure 1: General descriptions of pipelines in the oil and gas industry. Injection lines: Pipelines injecting water / steam / polymer / gas into the wells to improve the lift of the fluids from the wells. Flow lines: Pipelines from the well head to the nearest processing facility carrying the well Fluids. Trunk lines / Inter field lines: Pipelines between two processing facilities or from pig trap to pig trap or from block valve station to block valve station. Export lines / Loading lines: Pipelines carrying the hydrocarbons from the processing facility to the loading or export point. Transfer lines / Spur lines: Branch pipeline exiting into trunk line or export line. Gathering lines: One or more segment of pipelines forming a network and connected from the well heads to processing facilities. Disposal lines: Pipeline which disposes normally produced / separated water into disposal wells shallow or deep well disposal). Subsea pipelines: Pipelines connecting the offshore production platforms to on- shore processing facilities.
[FIGURE 2 OMITTED]
A typical oil and gas facility showing some of the industry's pipelines is shown Figure 2.
Corrosion phenomenon in the oil and gas industry--particularly in pipelines--is a major concern for many operators who normally want an uninterrupted flow of the export fluids. Some of the corrosion mechanisms which one comes across in the oil and gas industry are briefly described in Figure 3. However, the detailed information on these mechanisms can be obtained elsewhere in the published literature and case studies.
Figure 3: Some corrosion mechanisms encountered in the oil and gas industry. * C[O.sub.2] Corrosion (Sweet Corrosion)--General metal loss due to the presence of C[O.sub.2] in the process fluid * [H.sub.2]S Corrosion (Sour Corrosion)--Localized metal cracking and corrosion due to presence of [H.sub.2]S in the process fluid * Chlorides and Bicarbonates--Cracking in the metal due in the presence of stress and chlorides in the process fluid * Corrosion due to Oxygen--Oxidation and general metal loss due to the contact of metal with dissolved oxygen in the process fluid * Microbiologically induced corrosion--Corrosion induced by reducing bacteria particularly in the presence of [H.sub.2]S * Erosion (Abrasion) corrosion--Corrosion due to the fluid flow and velocity within the pipe and suspended solids in the process environments * Corrosion (External) Threats in the facilities--External atmospheric corrosion on above ground lines and corrosion due to soil contact for buried lines * Corrosion Under Insulation--External corrosion of pipeline due to water ingress under the insulating materials
Materials Selection Philosophy
The corrosion and material engineer is normally harassed by many and heckled by those in the industry who work in such functions as operations, process and maintenance. The corrosion engineer has the onerous task of selecting the appropriate materials without sacrificing the fitness of the materials for the service and at the same time not opting for exotic and expensive materials in the name of corrosion resistance. Some simple steps to follow in the selection process are listed as guidance and as a rule of thumb:
* Define the corrosion circuits or loops based on the corrosion environments.
* Identify corrosion threats addressing the corrosion mechanisms that can take place in the loop.
* Calculate the corrosion rate per year (mm per year).
* Calculate service life corrosion based on design life--total corrosion for the design life.
* Consider various materials options.
* Carry out the Life Cycle Costing (LCC)--Capex, Opex, installation cost, maintenance costs.
* Review materials selection with regard to design, operating, welding, constructability.
* Finally, select the choice materials.
The biggest challenge faced by corrosion and materials engineers is to select the proper materials which are just optimum and "fit for service" without going in for costly materials to ensure longer service life for the application. There are many options available for the corrosion and materials engineer who is obliged to analyze each option on a case-by-case basis before finally zeroing in on the choice material of construction. Some of the options at present within the oil and gas industry are mentioned in Figure 4 for reference.
Advantages And Limitations
While considering various material options, it should be borne in mind that every materials option will have advantages, disadvantages and some limitations. The corrosion engineer should exercise restraint in selecting the proper and optimum material for the application without any over-rating.
Also, it is imperative to verify the environmental conditions imposed by NICE/ISO 15156 if the process fluid conditions indicate sour applications. The advantages and limitations of material options are listed in Figure 5 for guidance.
Though the pipeline material selected purely depends on the corrosion threats based on the fluid conditions and operating parameters as indicated earlier, some typical examples of materials used for the pipelines in the oil and gas industry in the Middle East are indicated in Figure 6.
It should be clearly understood that no single material is a cure for all the ills of corrosion. Corrosion is a complex activity and a judicious approach is necessary when addressing the issue of material selection. It is equally important to note that an expensive and exotic material may not necessarily be the best choice for corrosion resistance. There may be a cheaper material that provides a more economically attractive solution for the corrosion problem.
By K. Nalli, Mott MacDonald & Co LLC, Muscat, Sultanate of Oman
K. Nalli is a mechanical engineer with the post-graduate qualification of M.Tech in production technology from the Indian Institute of Technology in Kharagpur, India. He has 28 years of experience as a metallurgist, welding, materials, and corrosion engineer in India and abroad. At present he is working as a corrosion and materials engineer with Mott MacDonald & Co LLC, a consultant in oil and gas industry in Muscat, Sultanate of Oman. He can be reached at e-mail:firstname.lastname@example.org
Figure 4: Materials options for pipelines. Metals Non Metals Metals + Lining Carbon Steel with Glass Carbon Steel with or without Reinforced internally coated corrosion allowance Epoxy (GRE) FBE Stainless Steel Polyethylene Carbon Steel with (HDPE) internal PE lining Duplex Stainless Corrosion Resistant Steel Alloy clad/lined materials Figure 5: Materials advantages and disadvantages. Material Advantages Disadvantages * Carbon Steel Cheapest option Corrosion rates (CS) Possibility of are high Opex Intelligent involves inspection Lowest Capex pigging and external pigging painting or coating Internal Additional opex corrosion thru inhibition control thru inhibition * Carbon Steel Capex slightly Susceptible for with FBE more Low Opex corrosion if FBE Internal coating is damaged corrosion Coating gets protection damaged if fluid through lining contains suspended solids * Carbon Steel Capex is Not advisable for with PE slightly more gas lines lining lining than CS+FBE collapse due to Internal depressurization corrosion Operating protection Temperature limited through lining to 65 Deg C max * Stainless No corrosion External corrosion Steel Opex is low protection thru coating Not suitable for fluid where chlorides are present Long lead time for procurement * Duplex No corrosion Susceptible to Stainless Opex is low stress cracking Steel in the operating range of 80 to 120 Deg C High Capex Long lead time for procurement * GRE No corrosion Limitation use due No operating to temperature Expenses /pressure. Medium Capex Line to be buried only No pigging possible * CS + CRA No corrosion High capex internal Can tolerate Materials clad highly wet sour Sourcing problems conditions Material Limitations * Carbon Steel High corrosion rate (CS) * Carbon Steel Coating holidays with FBE during coating / and operation * Carbon Steel Operating temp of 65 with PE Deg C maximum lining * Stainless Chlorides in fluid Steel * Duplex Operating temp Stainless Controlled Steel welding -heat input during welding * GRE Temp & Pressure Inspection limitation Aboveground lines Susceptible to damage * CS + CRA Welding difficulties internal clad Figure 6: Examples of pipeline materials used in the Middle East. Pipeline Description Materials of Construction Injection Water lines CS with CA or CS with internal FBE lining Steam Injection lines CS with CA Polymer Injection lines CS with internal PE lining Lift Gas lines DSS lines (Sour conditions) Produced Water Disposal CS with internal FBE or lines lining CS with internal PE Well Fluid lines Multiphase (Crude + (Flow lines/Bulk Gas + Water) - Wet Sour Headers/Manifolds) Conditions--CS with internal Main Oil lines/Trunk lines/ Crude Export CRA lining Lines/Dehydrated Crude CS with CA or GRE Export Gas lines Dry gas-Sweet gas--CS Wet gas-Sour gas-
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|Comment:||General material selection guidelines for oil and gas industry pipelines.|
|Publication:||Pipeline & Gas Journal|
|Date:||Mar 1, 2009|
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