A construction megatrend is modularisation; rather than building large infrastructure such as a power station or a bridge in situ, which can take years, big chunks are assembled offsite, transported to site, and bolted together. This has the advantage of speeding up construction, as well as allowing components to be built in factory conditions, rather than in the dark, at height, in rain or snow.
One of the biggest enablers of modular construction methods is the SPMT, self-propelled modular trailer, originally developed in the early 1980s by German trailer manufacturer Scheuerle and Dutch heavy lift contractor Mammoet.
Essentially big powered dollies, they support a bed of steel latticework with rows of wheel axles. Each double-wheel axle is propelled, swivelled and raised or lowered by hydraulic pressure.
The smallest Scheuerle unit consists of four axle lines of four wheels each, and the maximum lifting capacity of the latest unit that size is 70t per axle line (280t). Each unit measures 2.43m wide, and weighs 24t, so they are thin enough to fit into a shipping container and light enough to be carried by truck. But that’s just the start. Individual SPMTs can be linked laterally and longitudinally, and diesel-powered power packs attached. Together, they form vast moving synchronised platforms that transport turbines, chemical process vessels, offshore structures, buildings and more. The machines’ electronics funnel data from sensors and controls to a single operator control.
Although SPMTs must be assembled, just as cranes would, that process can take place offsite, which is a key advantage, explains senior project manager Mitchell Smith at UK heavy lift firm Osprey. “If you have a large open construction site, you might very well use a crane, and build a bridge close up in sections. You haven’t got to worry about possessions. But when rail or highways are involved, there are strict start and finish times – there’s only a set amount of time to do the job. SPMTs can be waiting to take an object out or put it in. With a crane, you need to rig it, which can take 12 hours, and you still need to get the bridge from the compound where it is built to installation, so you need SPMTs.”
Of course, SPMTs can also lift far heavier loads than all except the world’s biggest cranes (which occupy ground the size of a football field and take months to erect). For example, SPMTs were used in the installation in November 2020 of the one-piece, 4,260t Gipsy Patch Lane railway bridge near Bristol. That involved 144 axles – six 24-axle trailers with six power packs. A report at the time said it was the heaviest lift of its kind ever undertaken in the UK.
Often, Osprey now only uses SPMTs to install bridges. The company loads the structures on top of supporting steelwork to their final height, which is typically about 5m high, and then drives them into position. Then, using only the stroke of the SPMTs’ hydraulic pistons (+/- 300mm), the structure is lowered on to its supports, and the SPMT drives away.
Test lifts are essential. Even then, changes in conditions may affect the way SPMTs function, and it takes experienced operators to handle those situations. At Gipsy Patch Lane, for example, a change in conditions after the test lift meant doing further work, adapting the surface on which the SPMTs needed to run.
To help avoid such issues, Smith at Osprey says that it calculates the load going into each wheel and forwards that information on to contractor to prepare the ground. He adds that in cases where the ground surface needs reinforcement, steel plates or a trackway may be laid down. In extreme cases, the company even deploys bridging units that can span 20-25m gaps.
The speed of such operations belies the advance preparation required, which is measured in months and years. “It all comes down to planning and engineering; looking at jobs two years in advance before anyone arrives on site. If you plan early enough, you can influence the design of the bridge and the design of the site,” he adds. Osprey uses 2D Autocad to make plan, elevation and detailed drawings, fabrication drawings and so-called ‘swept path drawings’ that track the course of the loaded SPMT through the site.
Mammoet SPMT expert Ludo Mous points out that the very flexibility of the machines makes them vulnerable to user mistakes.“Every one has several valves that have to be connected, and if you miss one, and start lowering the combination, things can go wrong.”
“In our training, all of our guys are taught to connect valves in a certain way. You start at one corner, I start at the other, and we go through the combination connecting the valves. Half-way across we meet, and I start checking his work, and he mine, and when we come out, we know the valves have been set. The most important part of the SPMT is the suspension, because if you make a mistake there, that can cause an accident.”
To mitigate that risk, Mammoet, which owns just under 5,000 SPMT axle lines in total, runs its own four-level staff training programme for operating SPMTs. Betweenwork and lessons, it might take operators a full year to become competent; perhaps two to perform a job without supervision. This training will help them cope with a machine so complex that the controller operating manual alone runs to 100 pages. (Osprey offers a similar training scheme for staff).
As the loads and axle lines increase, even the simplest calculations – of stability, either in terms of tipping or overloading the structure – can go wrong, says Mous. “The boundaries are not clearly set up. For a crane, the standard limitation is 70% of the chart, although of course there are some country-by-country differences. With SPMTs, there is nothing; I can load up to 100%.” Helping here is the best practice guidance published by industry association ESTA, available via www.is.gd/geludi, which among other things classifies jobs by the amount of engineering planning required.
Mechanically speaking, SPMTs have changed little in the past 40 years; but their electronics definitely have. The first model was completely analogue, and featured a 52-pin connector plug. “We had a word for when the axles at a certain moment would shudder, and then we knew we had to tap a bad connection somewhere,” recalls the Mammoet operations director. The first-generation digital system, which had a main computer the size of a large television, required users to use a painstaking coordinate system to program steering. Some more advanced elements were brought into the far more sophisticated second-generation system, such as automatically recognising when another unit was attached. He adds that Mammoet is currently working on a new-generation touch-screen control system with manufacturer Scheuerle: “That is the disadvantage of electronics; in 2021 we need to get spare parts for a system that was produced starting in 1994.”
Also in testing is a zero-emission power pack that runs on batteries, and Mammoet is also exploring the possibility of hydrogen-powered generation. Still, construction must be practical if nothing else. Concludes Mous: “In the end, running on old-fashioned diesel for us is a really important thing, especially if you end up countries like Nigeria and Russia, where the diesel is not the same quality as what you have in the UK or Holland.”
This article first appeared in the Spring 2021 issue of Engineering Designer, the magazine for the Institution of Engineering Designers: www.ied.org.uk