What is the strength and stress of oil and gas steel pipelines

1. Strength

Deformation: The deformation of the material (steel pipe) is divided into two types: elastic deformation (the original shape can be restored when the external force is removed) and plastic deformation (the original shape cannot be restored when the external force is removed, and the shape changes).

Strength: The ability of a material to resist deformation and fracture under external force is called strength (MPa).

Yield strength and tensile strength: Under the action of external force, the material undergoes elastic deformation, and when the external force continues to increase to a certain value, it will enter plastic deformation. The critical point strength between elastic and plastic deformation is the yield strength. The value of the stress before the material breaks under tension is called the tensile strength (strength limit). Yield strength is the resistance to initial deformation and tensile strength is the resistance to deformation. All countries in the world widely adopt the minimum yield strength value of the pipe specified in the standard to calculate the strength of the pipe.

Using high-strength steel for large-diameter and high-pressure pipelines can reduce the pipeline's wall thickness and dead weight, make the manufacturing and welding process easier, and reduce the cost of pipeline laying.

Yield ratio: The ratio of yield strength to tensile strength is the yield ratio. The smaller the yield ratio, the greater the deformation capacity of the steel pipe after yielding and before the final fracture. Considering the safety of the pipeline, the yield ratio of the steel pipe should be as small as possible.

Steel grade: Steel grade is expressed by the minimum value of tensile yield strength.

The steel grades specified in American Standard API SPEC 5L-2018 "Pipeline Steel Pipe Specifications" are A25, A, B, X42, X46, X52, X56, X60, X65, X70 and X80. GB/T 9711-2017 "Steel Pipes for Pipeline Transmission Systems in the Oil and Gas Industry" adopts a steel grade notation similar to API SPEC 5L, replacing "X" with "L", and converting the yield strength to the legal unit of measurement, MPa (MPa). For example, the first line of the West-East Gas Pipeline uses X70 (L485) steel-grade pipes, and the minimum yield strength of the pipes is 485 MPa.

2. Stress

Stress: When an object (steel pipe) is deformed due to external factors (pressure, humidity and temperature changes, etc.), an internal force that interacts with each part of the object is generated to resist the effect of this external factor and try to make the object from deformation. The condition returns to the condition before deformation.

Circumferential stress: To help understand, we assume that 10 children hold hands and form a circle (simulated pipeline), and there is a force in this circle that pushes the children from the inside out to try to rush out of the circle (simulated internal pressure), to maintain the circle Integrity, the child resists this force by grabbing the left and right children's hands hard, which is the hoop stress (see the figure below). The hoop stress is equal to the pipe diameter times the pressure divided by twice the wall thickness. According to the current code, the pipeline strength calculation formula only considers the hoop stress of the pipeline.

Allowable stress: The allowable stress value is used to judge the pipeline's safety. It is usually related to the minimum yield strength of the material and the welding coefficient and design coefficient of the pipeline. The working pressure of the pipeline is not allowed to exceed the allowable stress.