Mammoet's super PTCs go to work in the USA

In 2009 Mammoet initiated a development programme to create the next generation of super heavy lift cranes using the platform, twin ring, containerised concept (PTC). Since then, Mammoet has engineered, designed, fabricated and tested the PTC 140 DS and PTC 200 DS versions. Final testing was completed in March 2012 where maximum chart capacity was determined as 3,200 tonnes. The PTC 140 DS set a world record by making a test lift of 3,500 tonnes on main boom plus jib configuration.

To ensure that these cranes conformed to the highest international safety standards, all engineering designs were verified by Lloyd’s Register of London, England. In addition, the cranes were designed for maximum reliability with 100% redundancy for the electrical and hydraulic systems. A specialized testing company based in the USA also conducted strain gauge testing of the booms to ensure that the cranes would be in compliance with SAE J-987, ASME B-30.5 and OSHA 1926.1433. The cranes were designed for the most severe climatic conditions of -40°C and up to +55 °C.

First to Nucor
Nucor Corporation, one of the largest steel producers in the USA, made a decision to build a new DRI plant in Louisiana. This would be one of the largest industrial projects in Louisiana history. One of the greatest challenges on the project was to erect a 109 metre structural steel reactor tower in the safest possible manner. Nucor concluded that the safest method would be to erect the tower in large modules using a super heavy lift crane to lift each module into place.

After conducting a strategic technical study of available cranes, Nucor concluded that Mammoet’s new PTC 140 DS would be the best solution. A further benefit emerged when it was determined that the PTC could also lift all major process vessels into place from the same crane location.

The project construction site was selected to be adjacent to the Mississippi River to allow for water transportation of raw materials into the plant and finished products out of the facility. To prevent river flooding, a series of embankment levees were constructed over the years by the U.S. Army Corps of Engineers. According to Mike Cook, Mammoet project manager, “By far the most challenging aspect of the Nucor Project was the engineering and construction of the heavy haul ramps and steel bridge spans that were required to transport all major process vessels from barges on the Mississippi River to the plant site. Maintaining the stability of the levee was paramount and required extensive engineering and Federal permits to ensure that all levee crossings could be accomplished safely.”

This was further complicated by the historic low water levels in the Mississippi River due to drought in the central sections of the USA. All challenges, however, were resolved successfully.

Full potential
To achieve maximum lifting capabilities, the Nucor PTC was configured with 94 m of main boom and 61 m of luffing jib. With this set up, the heaviest lift was the 980 tonne reactor at 54 m radius. The maximum structured steel tower lift weighed 556 tonnes at a radius of 103 m. The crane as configured was capable of making a lift at a maximum radius of 141 m.

A constant challenge for any super heavy lift crane is the ground bearing pressure that the crane typically exerts on its foundation. To do this, Mammoet designed the PTC 140 DS to be supported on a circular configuration of load spreader mats. The outside diameter of these load spreader mats is 46 m. With this design feature, the PTC normally has a maximum ground bearing pressure of 20 tonnes per square metre. With this low ground bearing pressure, the crane can be placed in most locations without needing a piled foundation.

Second for Texas
Gulf Marine Fabricators (GMF), a major offshore fabrication contractor, was awarded a contract to construct a large spar for a new offshore platform going to the Gulf of Mexico. A spar is the cylindrical section of an offshore platform that both supports the top side structure plus acts as a vertical floatation chamber, which keeps the top side floating above water.

GMF conducted a worldwide search of all available lifting devices that would be capable of accomplishing the spar fabrication. The results of this study concluded that there were only two cranes that could accomplish the task. The Mammoet PTC 140 DS was one of them. After careful evaluation and review, GMF selected the Mammoet crane primarily because it was significantly faster than the other crane and operated with a cable drum hoist system where the other crane had a strand jack system that required replacement of the strands after a certain number of lifts.

The enormous size and weight of the spar section meant that GMF developed a plan to fabricate the structure in 17 separate modular sections. After each section was fabricated in the laydown yard the PTC 140 DS would place the module in a graving dock that would ultimately be flooded with water to allow the spar to be floated to its final destination in the Gulf of Mexico. When fabrication is completed, during 2013, the Spar will weigh 12,363 tonnes, have a diameter of 26 m and will be 185 m long.

The biggest lift by the PTC 140 DS will be the double sections A & B that will weigh 2,412 tonnes and be lifted at a radius of 47 m.

Doing it right
Even though both PTC 140 DS cranes were engineered and fabricated to the most stringent international standards, the erection, operation and dismantling must also be conducted with equal care and attention. To accomplish these operations safely and effectively, Mammoet developed detailed erection plans and procedures to ensure that all complex electrical and hydraulic systems are installed correctly. After they are installed, a sophisticated operational check out procedure is then followed to complete the final quality assurance phase.

Once the crane becomes operational, the Mammoet field teams have been specifically trained to safely operate all systems of the crane. Since most systems have 100% redundancy, the systems automatically swing from one system to the other on a daily basis to ensure that both systems remain fully operational.