LNG Suspension Bridge

First discovered by the Portuguese in the late 15th century, Bioko Island lies off the coast of Equatorial Guinea in West Africa. It has large oil and liquid natural gas (LNG) reserves, which began being developed in the 1990s.

Bechtel Corporation was contracted for a design-build project to construct a plant to process 3.4 million metric tons of LNG per year and a marine jetty nearby for exporting the LNG.

The new plant required a pipeline corridor from the processing plant at about 55 metres above sea level, down to the loading jetty 45 metres below. However, the sloping terrain was so unstable no construction could be done on the slope. Instead, Bechtel decided to build a suspension bridge. One foundation tower is above the slope on stable ground and the other is in the ocean.

Buckland & Taylor of Vancouver was hired by Bechtel to design the suspension bridge, which has a main span of 350 metres and is one of the longest multi-pipeline suspension bridges in the world.

The elevation drop from the plant to the jetty required a 9% bridge slope. The deck height is 62 metres at the land tower and 29 metres at the marine tower. It has a deck payload width of 9.5 metres to carry cable trays, utility pipes, wastewater pipes, a sea water pipe and two 762-mm insulated LNG lines, one for vapour and one for liquid. There is also a 2-metre walkway for maintenance.

Most pipeline suspension bridges that carry natural gas use the pipe as the stiffening system for the bridge. Here, the truss provides the lateral stiffness. The truss and deck system was designed as repetitive modular units. The owner wanted the bridge components galvanized because of the corrosive ocean sea spray environment, so the components are sized to allow them to be hot dipped in a galvanizing kettle (maximum size 10 m x 40 m x 4 m). A metal grating over the deck increases the weight of the payload, but provides a stable platform for installation and maintenance.

Stiffening truss fixed to tower foundation

The edge trusses are 12 metres apart to provide lateral stiffness and to reduce deflections. This is the first suspension bridge of its size to have a stiffening truss fixed to a tower foundation. Since the LNG pipe was fully restrained at the land tower, the stiffening truss was also fixed there. This design minimized the fatigue loading at this location and reduced the potential lateral wind deformations of the bridge.

The design criteria limited the bridge’s maximum lateral deflection to 3 metres under a full wind load. The pipe was fully restrained against rotation and translation at the land tower, and left free to move longitudinally or to rotate at the marine tower. Vibration issues were considered in the design to account for the comfort and motion sickness of crew and workers. RWDI of Guelph, Ontario did local wind climate studies and a section model test.

The design and construction schedule was very tight and required delivery of a drawing package suitable for selecting fabricators and subcontractors two months after Buckland and Taylor was awarded the contract. The bridge had to be ready to receive the pipes 20 months after the award. It was completed on time in 2006.

Owner: Equatorial Guinea Train 1 SA

Client & general contractor: Bechtel Corporation

Suspension bridge design: Buckland & Taylor, Vancouver (Dr. Peter Taylor, P.Eng. Dr. Andreas Felber, P.Eng., David Queen, P.Eng., Gordon Ward Hall, P.Eng., Gil Medilek, P.Eng., Ian Walters, P.Eng., Dr. Dan Yang, P.Eng., Dr. Andrew Griezic, P.Eng.; Kam Deng, EIT; Roslyn Bubela, EIT, Dr. Tjen Tjhin, Dr. Matthias Schueller, Sergei Kuznetsov, Gilbert Hermosura, Michael Agnew and Lily Tham-Gee)

Wind engineering subconsultant: Rowan Williams Davies & Irwin, Guelph, Ont. (Derek Kelly, P.Eng., Dr. Jiming Xie, P.Eng.)

Client’s suspension bridge review engineer: Parsons Transportation