*Author for correspondence Indian Journal of Science and Technology, Vol 12(43), DOI: 10.17485/ijst/2019/v12i43/144730, November 2019 ISSN (Print) : 0974-6846 ISSN (Online) : 0974-5645 The Impact of Hydrostatic Pressure Test on the Interstitial Strength of Mild-Steel Pipeline Material Adjogbe A.S. 1 , Okoronkwo C.A. 2 , Igbokwe J.O. 2 , Ezurike O.B 3, * and Oguoma O.N. 2 1 Project and Program Division, Niger Delta Development Commission, Port Harcourt, Nigeria; saaadjogbe@yahoo.com 2 Department of Mechanical Engineering, Federal University of Technology Owerri, Nigeria; chukwunenyeanthonyokoronkwo@gmail.com, johnkwe678@yahoo.com, wuchioguoma@yahoo.com 3 Department of Mechanical/Mechatronics Engineering, Alex Ekwueme Federal University, Ndufu-Alike, Nigeria; ben4oke2000@yahoo.com Abstract Objectives: This study examined the effect of hydrostatic test practice on the mechanical properties of steel pipeline used in the oil and gas industry. Method/analysis: The method involves subjecting a 76.2 mm (3-inch) and 101 mm (4-inch) pipeline spools to predetermined maximum allowable operating pressures and designed hydrostatic pressures at designated data points. Thereafter, the spools were cut and samples prepared for experimental tests and analysis. Findings: The results showed that there is a significant change in the mechanical properties like fatigue strength and ultimate tensile strength. The result also showed a progressive increase in the fatigue strength from the control specimens to the hydrostatic pressure-tested specimens. The control specimens also exhibited a reduced fatigue strength compared to the maximum allowable operating pressure (MAOP)-tested specimens. Thus, since the hydrostatic pressure-tested specimens exhibited the highest fatigue strength, this could be reasonably attributed to the strain-hardening behaviour. Novelty/improvement: The hydrostatic pressure-testing procedure, at least, is not detrimental to the integrity of the pipeline. At best, it is beneficial since it increases the strain-hardening of the material. 1. Introduction Hydrostatic pressure testing is a common practice in the oil industry that is used to ascertain the structural integrity of a pipeline aſter the completion of construction and especially when a major repair is carried out in the oil and gas industry. 1 It is also used during the operational phase for the same purpose. However, the operational disturbances caused by retesting make use of less disruptive methods such as intelligent pigging preferable. In 2–3 this process, the pipeline material is subjected to pressures as high as 108% of their SMYS, 4 and the implication of this high pressure is that there may be a change in the overall properties of the pipeline material, such as the strength, creep characteristics, crack propagation properties, as well as the total service life expectancy of the pipeline installations. In view of the factors highlighted previously, the present study aims to investigate the impact of hydrostatic pressure testing on the interstitial strength of mild-steel pipeline materials. is is, therefore, carried out in order to determine the impact of hydrostatic pressure testing on the tensile strength, fatigue strength, and hardness of pipeline materials and also to establish the basis for continuity or otherwise of hydrostatic testing of pipeline systems from micro-structural impact approach. Despite the dearth of literature on previous works on hydrostatic testing, there has been a prima facie study that showed that the hydrostatic pressure testing of a pipeline material subjected to pressure as high as 1.5 times the Keywords: Hydrostatic Testing, Service Pressures, Near Point, Far Point, Mechanical Properties, Integrity