During inspection and maintenance operations, pipeline operators may encounter pipes showing incomplete records. However, data such as pipe tensile properties and toughness are essential to perform a realistic pipeline fitness for service analysis.
These situations most often occur with older pipelines, in a period where line pipe quality control and quality assessment were not as stringent as today. In order to avoid a cut and a replacement of the pipe, introducing transit interruption and high expenses for the operator, a methodology for determining mechanical properties has been developed.
The methodology described in this paper relies on data obtained from many tests performed on this specific type of line. The study and analysis of these database information led to working out correlations between parameters measured on field, and missing recorded mechanical properties.
The first data that can be obtained quite easily is the chemical composition of steel, which can be analyzed in a laboratory from samples directly removed from parent material of the line pipe.
Using the result of the previous analyses, the following correlations have been determined from the database information, and have been compared to correlations given in international standards (API 579, BS7910,…):
- Charpy V energy measured at 0°C versus Charpy U energy pleasured at 20°C,
- Charpy V energy versus sulfur content,
- Fracture toughness versus Charpy V energy,
- Fracture toughness versus sulfur content.
A practical experiment of these results have been performed, as chemical composition analysis from samples were made on 5 removed test pieces issued from line pipes. These tests aimed at comparing the results given by correlations with the mechanical properties of the line pipes, and validating the feasibility of this methodology on the field.
At the same time, database information was also used to check the theoretical behavior of parent metal regarding to the design temperature, by using a relation between steel toughness value, and its transition temperature at 28 J, issued from toughness transition curves.