A case study by SNC-Lavalin
SNC-Lavalin’s subsidiary, Kentz Pty Ltd, has been working on the Telecommunication and Electronic (T&E) systems Engineering, Procurement and Construction (EPC) contract on the Chevron Australia-operated Gorgon Project located on Barrow Island (BWI) since 2009.
As part of the project scope, our Telecommunication & Electronic team completed the erection of the 120m Communication Main Mast (CMM). The CMM supports the permanent operations communications link from Barrow Island back to the mainland and is critical to the operation of the Gorgon LNG plant. The mast also supports other communications systems for the Gorgon Project.
The high risks and significant technical challenges that a mast of this size poses, resulted in substantial planning to ensure the safety of construction personnel. Front end engineering verified the integrity of the structure throughout the erection process, along with the propriety and bespoke build equipment. A 250 ton mobile crane erected the first 70m and the final 50m was erected with a purpose built gin pole and winching system.
In addition to the engineering, procurement and construction of a 120m CMM, a team consisting of over 200 has been executing the overall scope, including project management, engineering and integration testing of 23 T&E systems, construction, commissioning and O&M.
The pre-works process involved function testing of the gin pole and production of a 3D animated model to demonstrate that non-standard, critical lifting activities could be completed safely and as planned. The animated model was used to communicate engineered activities to riggers, ensuring the correct procedures were understood and implemented. The gin pole and crane were used at over 90% capacity at stages during the erection process.
The Barrow Island CMM is now Australia’s heaviest 120m guyed mast. The mast is located in the area of the highest ever recorded wind speed on earth of 408 km/h (113m/s) and is subject to cyclonic weather events.
To allow critical communications links to remain operational and to survive severe cyclonic weather events, the mast structure incorporates a torsional stabilization system at each of the four guy levels. The design wind speed, size and quantity of antenna located on the mast creates considerable loading that is rarely encountered for this type of structure. Under full load condition, over 1000 tonnes of force is generated at the base of the structure.
The final aspect of the mast construction involved installing four 12ft Microwave antenna and mounts.
As the mast guy wires are configured for cyclonic stability requirements, the clearance for lifting the antenna in a direct line between guy wires was limited. The sensitive nature and the sail area of the antenna resulted in lift wind speeds restricted to a maximum of 14 km/h (4m/s). Eliminating contact of the antenna with the structure was imperative as even minor damage would likely render the antenna inoperable.
Detailed lift plans and studies were completed using 3D modelling. The final method of installation, using a 450 tonne crane, incorporated a block diversion system allowing the main crane winch wire to be diverted off the mast face onto a trolley winch line. This setup allowed the complete antenna and mount assembly to be lifted in a single lift. This approach minimised exposure to weather delays, lowered the number of specialised installation personnel required and lowered personnel exposure to a high risk environment. Utilising the crane in this configuration had never been previously undertaken.
The CMM Project encountered and overcome significant challenges in engineering, supply and construction that are rarely encountered within this industry. They successfully and safely erected the 120 CMM mast utilising in excess of 30,000 man-hours with zero lost time incidents.