Pipeline Material Grade Selection

General PrincipleThe steels applied in the offshore oil and gas industry vary from carbon steels (taken from American Petroleum Institute standards- Grade B to Grade X 70 and higher) to exotic steels (i.e. duplex). The following factors are to be considered in the selection of material grades:

• Cost;
• Resistance to corrosion effects;
• Weight requirement;
• Weldability

The higher the grade of steel (up to exotic steels) the more expensive per volume (weight). However, as the cost of producing high grade steels has reduced, the general trend in the industry is to use these steel of higher grades.

Fabrication, Installation, and Operating Cost Considerations

The choice of material grade used for the pipelines will have cost implications on:

• Fabrication of pipeline;
• Installation;
• Operation.

Fabrication
The cost of steels increases for the higher grades. However, the increase in grade may permit a reduction of pipeline wall thickness. This results in the overall reduction of fabrication cost when using a high grade steel compared with a lower grade steel.

Fabrication

Installation
It is difficult to weld high grade steels, and consequently lay rate is lower compared to laying the lower grade steels. However, should the pipeline be laid in very deep water and a vessel is laying at its maximum lay tension, then the use of high grade steel may be more suitable, as the reduction in pipe weight would result in lower lay tension. In general, from an installation aspect, the lower grade steel pipelines cost less to install.

Pipeline Installation

Operation
Depending on the product being transported in the pipeline, the pipeline may be subjected to:
– Corrosion (internal)
– Internal erosion;
– H2S induced corrosion.
Designing for no corrosion defect may be performed by either material selection or modifying operation procedures (i.e. through use of chemical corrosion inhibitors).

Material Grade OptimizationOptimization of material grade is rigorously applied today based on experience gained from the past 20 years of pipeline design, and the technical advances in linepipe manufacturing and welding. The optimization is based on minimization of fabrication and installation cost while meeting operating requirements. As the selection of material grade will have a significant impact on the operating life of the pipeline, the operator is normally involved in the final selection of material grade.

Source:Bai, Yong and Bai, Qiang. Subsea Pipelines And Risers. USA: Elsevier Inc. 2005.