Cargo systems designer Macgregor used software modelling to ‘test’ a new design – because a physical prototype would have been too large to build. Lou Reade reports
Building a prototype is like writing the first draft of a letter: you try out early ideas, work out how to improve it, then go ahead and create the final version.
Physical prototyping allows you to visualise a component, and also allows you to test it: this could be anything from a drop test to full scale wind tunnel testing. But building a prototype is not always appropriate – as Jonas Nordström can testify. He is head of the project department at Macgregor, which designs and builds a range of cargo systems for marine applications.
“We can’t build prototypes for many of our products because they are too big,” he says.
For this reason, the company used ‘digital prototyping’ – creating digital models of its designs, and carrying out virtual testing on them. One of its latest projects proves the point: Macgregor recently won a $20m (£10m) order from the US Navy to build a key component in its ongoing ‘Seabasing’ plan: a ship-to-ship ramp – dubbed the Test Article Vehicle Transfer System (TAVTS) by the Navy – that will allow military vehicles to be moved directly from a sea-based container ship to one that can take them ashore.
“We were asked to design, manufacture and deliver this ‘prototype’ ramp,” says Nordström.
While the US Navy calls the ramp a ‘prototype’, this is in the sense of it being the first of its kind – which will be tested in service and extended if successful.
“We don’t actually build prototypes,” says Nordström. “For us, this is the real thing.”
The ramp incorporates two hinged sections. The angle between them constantly adjusts itself – thanks to a multitude of sensors, and a hydraulic cylinder – depending on the conditions of the sea. Vehicles are driven along the ramp, then onto a connecting ‘bridge’, and over to the adjacent ship. The idea is to ensure that vehicles can be driven across the ramp constantly, under varying conditions at sea.
“We needed to know the speed of the hydraulic cylinder that controlled the angle between the ramp sections,” he says. “Knowing the speed, this gives the oil flow – allowing us to determine the hydraulic power pack capacity.”
But this is a very difficult calculation. In 2D, it would have meant creating ‘snapshots’ of data. But by using the recently introduced ‘dynamic simulation’ capability of Autodesk Inventor Professional, Macgregor was able to model the behaviour of the system.
“Without software, it would be almost impossible to calculate,” says Nordström.
A digital model of the ramp was created, then the motion of the two ships was simulated – using accurate ‘ship motion’ data supplied by the US Navy. The simulation supplied a cylinder speed of almost 70mm/second.
“It doesn’t sound very quick, but it’s fast in our world,” says Nordström. “That’s a big power pack for our business.”
The ramp design will be finished over the summer. It will be built during the autumn and delivered next year.
The US Navy project was a major use of digital prototyping for Macgregor, which had earlier used the technique on a smaller – though no less important – design problem.
“We found that there were difficulties in closing a bulkhead door under certain conditions,” says Nordström. “It had originally been designed in 2D. We needed to solve the problem because the door was now in service.”
In this case, Macgregor was contracted to fix the problem. Because a ship might be anywhere in the world when a problem arises, it is in Macgregor’s interest to keep incidents like this to an absolute minimum.
The door closes in a similar way to a garage door, but with a hydraulic cylinder ‘damping’ its slow descent. Near the end of the sequence, the door falls free of the cylinder’s effect and drops ‘under gravity’. Two positioning pins – one near the top of the door, and one near the bottom – each falls into a guide plate to lock the door in place.
When originally fitted, the bulkhead doors closed perfectly. But this was when the ship was in ‘even keel’ – that is, there was no cargo. When cargo was loaded, the ship quite naturally moved slightly off the horizontal. This affected the closing sequence of the door – with the pins failing to lock into place – but not in a way that was immediately apparent.
“We originally went back to the 2D drawings and moved the door in incremental stages – to try and identify the centre of gravity,” says Nordström.
The centre of gravity was a vital piece of information, because the door falls straight down ‘under gravity’ at the end of its closing sequence.
“We formed a small team of three or four designers to work on the problem – but the answer presented itself when the new version of Inventor Professional was launched,” he says.
This latest version was the first to include dynamic simulation modelling – as would later be used on the US Navy ship-to-ship platform. In this case, a simple 3D model of the door was simulated in an environment that included the effects of gravity, the hydraulic cylinder and 2D contact joints.
“We found that the clearance on the bottom plate was only about 5mm,” says Nordström.
The answer was to redesign the upper guide so that the pin approached it at a shallower angle – in this case, 10º rather than 16º – helping the guide to ‘catch’ the door higher up.
“It was a simple solution to a complex problem,” says Nordström. “But we had to identify the problem.”
Based on the success of these two projects, Macgregor is looking to extend its use of digital prototyping further. Nordström lists some of the advantages as: checking in advance for potential collisions and interference “because all our products move kinematically”; simulating equipment operations for function control, fault finding and optimisation; producing exploded views, to help service engineers; and producing 3D models and animations, to help the sales team.
“All of our products use hydraulics – from cylinders to winches,” he says. “We currently design them in 2D AutoCAD but have started to do it in 3D Inventor. We will add electric and hydraulic installation to all our models and assemblies.”
This will be done using two modules within Inventor Professional: Tube & Piping; and Cable & Harness.
A further step will be to test Product Stream to integrate 3D design with process and production, and to integrate strength calculations into 3D design by using Ansys Professional alongside Inventor models.
In terms of specific cost savings, he says it is too early to work out any concrete savings – but believes it has delivered an advantage.
“We’ve not detailed how much we’ve saved,” he says. “If we didn’t use digital prototyping, we might solve these problems by luck, so it cuts down on a lot of uncertainty. But it definitely saves us time – and in that sense will save us money.”