INVESTIGATION ON THE EFFECTS OF SEAWATER ON MECHANICAL PROPERTIES OF API 5L X70 STEEL PIPELINE WELDMENT.

Abstract

Corrosion has been classified as a major threat to structures in marine environments leading to pitting, surface defects and subsequent failures. There is still a significant use of oil and gas pipeline which remains the only transportation medium for oil and gas products. These pipelines are classified according to their strength, chemical composition and the areas of application. There is a need to investigate the mechanical properties of welded pipeline operating in seawater. In this research, tensile, fracture, compression and hardness properties of API X 70 steel pipeline was investigated in natural seawater using a developed corrosion rig equipped with a chiller. The rig was developed using locally sourced materials. Tests were carried out using tensile, charpy, compression and hardness samples that were extracted from parent, weld and heat affected zones (HAZ). The fracture surfaces of the sample were analysed using scanning electron microscope (SEM). The coefficient of performance of the chiller was determined to be 3.58. In air, the mean yield strengths for parent and welded materials were found to be 560 Mpa and 578 Mpa respectively while the corresponding tensile strengths were determined to be 634 Mpa and 674 Mpa respectively. In seawater, the mean yield strength for parent and weld materials were obtained to be 428 Mpa and 416 Mpa and the corresponding tensile strengths were determined to be 512 Mpa and 529 Mpa respectively. Charpy energy values were higher in parent than in HAZ and with weld materials having the lowest energy. The absorbed energies in the specimens were relatively lower in seawater than those tested in air. The effect of seawater was also applicable to the reduction in compressive stresses of the material. The hardness value of the HAZ decreases from the region closer to the weld towards the parent material. The highest hardness of 239 HV value was obtained in weld, while the minimum harness of 196 HV was obtained in parent material. Fracture surfaces of the specimens showed the combination of brittle and ductile failure mechanisms. The effect of seawater corrosion reduced the tensile properties of parent and weld materials by factors of 1.2 and 1.3 respectively. Environmental reduction factor of approximately 2.0 was obtained in charpy parent, weld and HAZ materials. Seawater also reduced the compressive properties of parent and weld materials by factors of 1.7 and 1.8 respectively. Consequently, the study provided a novel approach of establishing mechanical properties of API 5L X70 steel plates at parent, weldment and heat affected zones in air and seawater for optimum pridiction of corrosion failure of pipelines and their prevention strategy. Design parameters were also established for the development of a new laboratory-scale corrosion rig for the API 5L X70 steel plates.