Question

Assume that you are an engineer for a natural gas transmission company. Your company recently purchased a small, interstate gas transmission pipeline from another company. You are in charge of assessing the pipeline integrity, which means ensuring that it will meet mandatory safety factors against both yield and fracture when operated at normal service pressures. You have the following information about the pipeline materials, dimensions, and corrosion history. Federal regulations require that your pipeline have a safety factor of 2.0 against yield and 2.5 against fracture. Material: API X65 steel • Yield strength = 65 ksi; Fracture toughness (KIC) = 280 ksi*in1/2 Pipe Dimensions: Outer diameter = 28.000 in; Inner diamter = 26.750 in Maximum Flaw Size: 0.1 in (ultrasonic inspection at time of installation) Geometry Factor for Fracture Toughness Calculations: ()1/2 Corrosion History: • 8 years unprotected with estimated corrosion rate of 0.048 in/yr • 2 years under cathodic protection with estimated corrosion rate of 0.005 in/yr Hint: The hoop stress (circumferential tensile stress) in a pressurized pipe or cylindrical pressure vessel is given by: hoop = (r*p)/t where hoop = hoop stress r = (0.5)*(inner diameter) p = internal pressure t = pipe wall thickness Show all calculations (with units!) in order to receive credit for your answers. Question #1: Given the information above, will your pipeline meet required safety factors for yield and fracture if pressurized to a service pressure of 600 psi? Question #2: How would your answer change if you discovered that, during the two years that your pipe had been under cathodic protection, the polarity was reversed? Assume that reversed polarity cathodic protection will result in a corrosion rate two times that of the unprotected condition. Question #3: If your answer to question #1 or #2 was that the pipe did not meet required safety factors for operation at 600 psi, when in its service life did it fail to do so? This is important since it is not legal to sell a pressurized natural gas transmission or distribution system which is not safe to operate.

Answer 1