As the author of articles in a series in the “Heavy Lift & Project Forwarding International” magazine, Alexander Koren, project engineer at Eager.one, explains how jacks, rollers and skids fit into the transportation equation.
How can you transport a load horizontally or vertically over any distance when the load is too big, too heavy, or there is not enough space for solutions such as cranes or SPMTs? Under such circumstances, the alternatives to move the load can be:
Outsize loads, or those weighing tens of thousands of tonnes, can be moved using simple equipment that allows for accuracy within a few millimetres. Specialised companies worldwide can provide modular systems to move even the biggest and heaviest loads horizontally or vertically. But of course, small and very long and slender loads can also be moved safely in the same way.
There are a wide variety of jacks. They include strand jacks for a suspended load or to position a load over a skid track for subsequent horizontal movement, or hydraulic/mechanical jacks that are used to elevate a load. Most of the time, jacks are part of a jacking or skidding system.
Strand jacks are special hollow hydraulic cylinders that grab steel cables (strands) to pull a large/heavy load up to a certain elevation and/or over a certain distance. For vertical movement, strands are installed through the jacks, on top of dedicated support constructions. For horizontal movement, they are installed between the load and support points.
Separate hydraulic and operating/monitoring systems are required to operate strand jacks. A strong and stable foundation is required when moving a load vertically.
The layout of strand jacks is very flexible, so all kinds of configurations are possible. They can be used as a single device or with multiple devices in combination. Strand jacks are suitable for both onshore and offshore operations, in both civil and petrochemical projects.
The strands themselves can only be used for a limited number of lifting operations due to the crushing effect of the clamps that hold them. The length of the strands can vary with each lifting operation. Often, there are large reels on top of strand jack constructions to store the extra length and to bridge connections between the jacking towers.
Hydraulic/mechanical jacks are in principle hydraulic cylinders, with extra features such as a pin connection or a bottom flange to transfer the forces of the load into other parts of the jacking or skidding system.
Hydraulic/mechanical jacks normally push the load in a horizontal or vertical direction and the displacement is dictated by the stroke of the jack. To continue the movement beyond that limit, the load must be secured and the jack retracted, and then repositioned between a new support point and the load. In vertical moves, it is possible to use a tower construction and add special components to increase height. It is vital to align supports and jacks properly in order to avoid side loads in the system, which can cause the load and supports to tip over.
Hydraulic/mechanical jacks can be used in combination with skids or rollers. For instance, jack the load up from its foundation and then skid/skate it horizontally to its new position. These combined operations are especially useful for moves taking place within buildings or for installations at sites with limited room to manoeuvre.
Rollers are positioned under a load, enabling it to be moved over a level, horizontal surface. A flat, load-bearing surface is required to transport the load safely, and the rollers should be placed under designated strong points of the load. They consist of a rigid frame with wheels or rollers underneath – either freewheel or with their own means of propulsion. They can also be equipped with turntables for negotiating corners, and with pull bars to move them. In cases where turntables are absent, the load can be elevated with jacks and the rollers repositioned in any other direction.
For movement in a straight line, rollers can be positioned in low U-profiles for extra guidance and support. This method is widely used in confined areas as rollers need only minimal space for installation and operation. Simply jacking the load up a few centimetres, then positioning the rollers and additional guides or supports, allows operators to move the load in a controlled manner.
Skids (also called skid shoes) use low- friction materials and grease between two adjacent surfaces, instead of moving components such as wheels/rollers. Often, surfaces made of stainless steel plate are used with Teflon-coated skids.
Skids must be guided by parallel skid tracks (either U-shaped or I-shaped) to avoid sideways movements. The tracks should have a smooth, flat and horizontal (or slightly inclined) surface.
It is very important that the skidding surface is free from bumps, dirt such as sand or gravel, steps (which would require different skid levels) or other obstructions that might cause the skidding system to fail or stop.
Like rollers, skids should be installed under strong points of the load.
Skidding only allows loads to be moved in a straight line. To change the direction of the movement, the load must be elevated and the skid system be repositioned.
Skidding operations are executed with, for instance, push/pull units, strand jacks, hydraulic/mechanic jacks and pull lifts.
In practice, a combination of skidding and jacking is common: jacks elevate the load to install the skidding system, and subsequently the skids move the load.
The content of this article is intended to provide a general guide to the subject matter. Please contact us for specialist advice about your specific circumstances.
This article was also published in the March/April 2020 edition of HLPFI magazine. Read the online version here.
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