Relative Risk Assessment of Cable-Supported Structures

Throughout the Midwest, there are a myriad of underground petroleum and gas pipelines connecting the source of the product (refineries) to distribution sites (terminals) via pump stations. Originally constructed in the 1920s to 1940s, there are isolated sections of 150 mm (6 in.), 200 mm (8 in.), 250 mm (10 in.) and 300 mm (12 in.) diameter pipelines carried overhead by cable-supported structures where the underground pipeline crosses a waterway. This work is based on the structural assessment of 52 pipeline crossings for a confidential client. The purpose of the project is to inspect the structural components comprising the support structures to determine the current condition and identify areas for future maintenance to ensure the safe operation of the overall pipeline distribution system. These steel structures consist of tower frames constructed of welded pipe tubes with vertical legs in the front and diagonal legs in the rear. Each pair of front and rear legs is connected with horizontal struts to each other and to the opposite leg on each side. The legs of the towers are supported on concrete foundations, connected through steel base plates on embedded anchor bolts. The two towers, located on each bank of the waterway, support two main catenary cables that are hung across the span with backstay cables anchored into the ground. Suspender cables hang vertically from the main cables, terminating at pipe hangers that directly support the pipeline. The structural components were inspected to determine the remaining useful life expectancy of each structure. With an accurate inspection and rating, the client can effectively manage their total inventory of 82 cable-supported structures located throughout a 14,500 km (9,000 mi) pipeline distribution system as part of an overall pipeline system-wide risk management plan.