Engineers at Selantic Industrier performed a drop test simulation with Accupak/VE Mechanical Event Simulation software to determine the limits of a protection net for a new ELF off-shore oil platform currently under development in the North Sea. The finished structure will stand on three legs that can move up and down. With the legs retracted, the platform can float into position; then the legs can be extended, first to touch the seabed and then to raise the platform above the ocean surface. Selantic's engineers decided to study the problem by using engineering software to simulate a steel container landing in the net and bouncing back. They chose Accupak/VE, mechanical event simulation software for virtual prototyping from Algor Inc. of Pittsburgh. The simulation showed that the net would be stretched permanently under the loading used in the model. Selantic expects to conduct small-scale prototype tests to confirm the simulation results. This testing will replace full-scale testing that would have been necessary without the event simulation. This translates into a decrease in the time and cost of materials needed during physical prototype testing.
The contractors building an oil rig off the coast of Britain realized that they had to protect oil carrying pipes from falling heavy objects. It's not that they expect anything to fall, but they have to take the possibility into account. Failure to do so could prove disastrous, because the rig's design puts the pipes, or oil risers, in the legs of the platform; cranes to lift shipping containers will operate over two of the legs. One slipped container could lay an oil slick on the North Sea and cost the oil company, ELF Exploration, a significant amount of inventory.
The contractors, Technip Geoproduction of France and McDermott of the United Kingdom, came up with an unusual solution. They would mount safety nets on broad steel rings above the legs where the cranes would operate. Technip and McDermott hired engineers at Selantic Industrier of Agotnes, Norway, to work out the details.
Three Movable Legs
The contractors are building ELF a jack-up rig, which is currently under construction off Britain's east coast. The finished structure will stand on three legs that can move up and down. With the legs retracted, the platform can float into position; then the legs can be extended, first to touch the seabed and then to raise the platform above the ocean surface.
The cranes directly above the legs will handle containers carrying supplies daily between the platform and ships. The nets will prevent falling freight from hitting the risers, which transport oil from the ocean floor.
The engineers at Selantic needed to come up with a design that would restrict the net's deflection to certain limits and ensure that the net and its supporting structure would withstand stresses created upon impact.
In the beginning of the project, Selantic engineers tried rough hand calculations to determine the best approach for the net design, but rough numbers could not yield solid conclusions. It was clear that performing the necessary calculations manually would have been impossible.
A Bouncing Container
Selantic's engineers decided to study the problem by using engineering software to simulate a steel container landing in the net and bouncing back. They chose Accupak/VE, mechanical event simulation software for virtual prototyping from Algor Inc. of Pittsburgh.
The original model interpreted the net for finite element analysis, and represented the falling object as nodal forces acting directly on the net. Selantic's engineers were dissatisfied with this method for two reasons: Calculating the correct loading was complicated and time-consuming, and they wanted to simulate the springboard effect that occurs when a falling object deflects off the net.
Modeling the container and applying known physical properties, such as its dimensions and mass along with the height from which it falls, enabled the engineers to realistically simulate the interaction of the container and the net.
To set up the event simulation, a container was added to the existing protection net model, which is made of truss elements with three degrees of freedom.
Engineers placed a model of a 5,000-kg steel container measuring 2x2 meters approximately 12.6 meters above the net. The net had three sides, each 16 meters long, and was terminated in each of the three corners with fully constrained boundary conditions. The simulation included a specification of gravity acceleration for the container, according to latitude and height above sea level.
Preparations also included designating contact elements, to simulate dynamic motion, between the surfaces of the container and the net to enable complete interaction, including the transfer of inertia from one object to the other.
Engineers specified the material properties of steel for the container and of aramid, a synthetic fiber, for the ropes of the net.
The problem that needed solving involved the maximum deflection of the net during the unlikely event that a crane might accidentally drop a container. Calculations focused on that part of the simulation.
The initial deflections exceeded Technip's failure criteria of 2.6 meters, the distance to critical objects underneath the net. Engineers performed several variations of the analysis by dropping the container at the center, at one corner, and along the edge of the net, to confirm the results before conferring with the client.
After the first set of analyses, Selantic determined that the net design represented in the model would fail under the extreme loading of a falling shipping container.
Technip revised the requirements and asked Selantic for a modified set of analyses. A new net design replaced aramid with high-performance HMPE (high molecular polyethylene) rope. The fiber is lightweight and ounce for ounce is ten times stronger than steel.
In addition, the redesign added six termination points distributed along the three sides of the net. Three of the points are adjustable in order to control the net's tension.
The modified set of analyses revealed much more reasonable deflections throughout the net and satisfactory material dimensions. Although stresses showed up higher than expected at the termination points, they were within the acceptable range.
What the Simulation Showed
The simulation showed that the net would be stretched permanently under the loading used in the model—that is, 5,000 kg dropped from 12.6 meters. As it turned out, this was not a concern because each net will be replaced after a single accident. To let the clients visualize the net behavior, Selantic created.avi files from the simulation, converted them to VHS form at, and then presented the videotapes to the contractors.
Selantic expects to conduct small-scale prototype tests to confirm the simulation results. This testing will replace full-scale testing that would have been necessary without the event simulation. This translates into a decrease in the time and cost of materials needed in the course of physical prototype testing.
Physical prototype testing will begin early this year, and the platform is due to become operational next year.