Page 52 - Pipelines and Risers

Wall-thickness and Material Grade Selection 25

The 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. See Chapter 19.
It is clear that the selection of steel grade forms a critical element of the design.
2.2.2 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.
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.
Operation
Depending on the product being transported in the pipeline, the pipeline may be subjected to:
- Corrosion (internal)
- Internal erosion;
- H;?S 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).

2.2.3 Material Grade Optimization
Optimization 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