Integrity Management of Wet Sour Gas Clad Subsea Pipelines

Introduction

Pipelines are essential in the oil and gas and petrochemical industry, as well as other critical industries such as water transportation. To ensure the quality of longstanding pipelines, inspection and quality management during the design, construction, and operation stages are of prime importance. During construction, pipe manufacturing and field fabrication represent the major cornerstone of a construction quality assurance program. Many international publications, including codes, standards, procedures, and technical reports have laid the groundwork for this. During fabrication, welding inspection, by way of nondestructive testing (NDT), is vital and performed regularly. NDT methods are selected based on design rules, technological limitations, and the importance of the pipelines to customers and operators. During the operation stage, inspection programs, along with corrosion control and monitoring techniques, are implemented to maintain the integrity of the operation lifecycle.

This article reviews an inspection and corrosion control assessment that was conducted on 13 subsea flowlines. These flowlines transport the wet sour gas feed from two offshore fields to a downstream gas treating and NGL recovery facility located onshore through two 36-inch trunk lines. The scope of the study includes evaluating the flowlines’ associated inspection programs and identifying all available NDT technology solutions. The requirements of the inspection programs rely on the continually changing flowline baseline records, the review of corrosion damage mechanisms, monitoring records, and obtained inspection results.

Figure 1 illustrates a schematic distribution for the 13 flowlines. Two tie-in platforms gather the sour gas feed to two trunk lines to supply the field gas to the onshore gas plant. All of the flowlines are 12 inches in size and made of API 5L grade X65 with a wall thickness of 0.5 inches. The flowlines are internally cladded with Inconel Alloy 625 with a thickness of 0.12 inches. The average length of the flowlines is 3.4 km.