Liquefied Natural Gas Pipeline Suspension Bridge Design

A new Liquefied Natural Gas (LNG) plant, in Equatorial Guinea, Africa required a pipeline corridor with supplementary utilities from the processing plant at about 55 m (180’) above sea level, down to the marine loading facility. The use of conventional structures and foundations on the unstable slope had substantial associated costs and risk. Various options were considered and the suspension bridge solution, avoiding the geotechnically unstable slope by providing tower foundations above the slope on stable ground and in the ocean, was selected. Buckland & Taylor Ltd. developed the final design of a 350 m (1148’) main span inclined deck, suspension bridge to carry the pipeline corridor across the unstable geotechnical slope. The configuration of the bridge was dictated by the site conditions and the allowable movements of the pipelines. The superstructure consists of steel towers approximately 60 m (197’) tall with two main cables, each composed of three 89 mm (3 1/2”) strands, supporting a 12 m (39.4’) wide truss deck system. The deck of the bridge is fully grated for safety and ease of installation of the pipes and utilities once the main bridge structure was complete. With the pipes in position, the walkway provides continuous access to the pipelines and utilities for future maintenance, as well as provides a fixed pedestrian route from the plant to the loading facility. The details of all of the components were developed to address the demanding schedule, available fabrication facilities, shipping restrictions to the remote location, flexibility in the erection sequence and ease of construction. Repetitive structural components were detailed to simplify fabrication, facilitate transportation to the site and improve the schedule during construction. The aggressive project schedule dictated that field investigations were still being undertaken while final design was being completed. Buckland & Taylor Ltd. made design changes to suit field conditions or methods of construction with minimal impact on fabrication and construction. The design process considered various methods of erecting the bridge to simplify details and improve the construction schedule. The design of the suspension bridge was initiated in August 2004, with the conceptual design complete in a month and the detailed design issued for construction in January 2005. The main foundation construction and structural steel fabrication proceeded immediately with priorities set to meet the construction schedule. With the progress of design, fabrication and construction, Buckland & Taylor Ltd. anticipates that pipes will be installed on the bridge 20 months after the start of design.