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General material selection guidelines for oil and gas industry pipelines.

This article presents a few tips on pipeline material selection methodology based on a rational approach drawn from oil and gas industry practices and field experiences. There will be no dwelling on any specific selections since the pipe material chosen purely depends on the operating envelope that includes such parameters as type of fluid, fluid compositions, flow, pressure and temperatures. The subject is vast and every selection is determined case by case. However, this is mainly a look at some general guidelines.

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 Threats

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.

Materials Options

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.

Recommendations

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.

Conclusion

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:knalli47@yahoo.com
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.
Author:Nalli, K.
Publication:Pipeline & Gas Journal
Geographic Code:1USA
Date:Mar 1, 2009
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