Simulation and analysis offer benefits to designers in diverse industries

Written by: Paul Fanning | Published:

The use of simulation and analysis software is well known in a range of applications.

However, it can nonetheless be instructive to examine individual case studies to demonstrate the way in which such technologies can overcome major obstacles and radically enhance design.

Two UK-based examples offered by leading multiphysics engineering simulation software provider Comsol concern the design of underwater cables for the offshore oil and gas industry and the optimisation of lubricants in complex friction problems. The first of these involved JDR, which custom-designs and manufactures subsea power cables, umbilical systems and reeler packages for a broad range of applications in the oil and gas and renewable sectors.

Because umbilicals are long, they need to be strong, and are generally very heavy and difficult to handle. Thus, the physical testing of these cables is cumbersome and expensive. Tim Poole, design automation engineer, is responsible for testing and analysing products at JDR and says: "In order to understand fatigue properties and performance, a typical fatigue regime for an umbilical is to undergo 100,000 usage cycles around a sheave wheel on a large fatigue rig. At approximately 6,000 cycles per day, plus all the other required testing, it takes at least a month to complete the process and costs between $30,000 and $50,000 for all the resources involved. It is critical that we can predict the behaviour of our products to ensure they meet the requirements, so while physical testing is very important, it has its limitations. Apart from the time and cost factors, we cannot replicate conditions 100%."

Umbilicals pose a particularly complex analysis challenge, as Poole explains: "Typically they incorporate multiple layers of wire with helical geometries and multiple contact points, or they contain aramid (Kevlar) braid, a synthetic material that is very difficult to analyse because of its braided construction." JDR therefore turned to COMSOL Certified Consultant, Continuum Blue for some specialist assistance.

Dr. Mark Yeoman of Continuum Blue picks up the story. "Our starting point was a 2D cross-section of a cable, including material specifications. What was of concern was that the cable cross-section had a double-armour layered structure with 50-60 armour wires in each layer, where each layer twisted along the length in the opposite direction to the other. Building the model to reflect bend and axial load conditions with contact for the internal structures was done, but also included adding in the contact for these counterrotating armor wires. This resulted in well over 3,000 localised regions of high contact pressure along a unit length of cable, creating high stresses at every point of contact."

Continuum Blue's answer was to build a bespoke program, so that JDR could quickly and easily generate the 3D cable structure through COMSOL's Livelink for MATLAB and then build the COMSOL cable model. The MATLAB code added advanced material properties and relations from Continuum Blue's extensive materials database, and utilised these properties to help define the bespoke contact expressions and parameters that were necessary to solve the contact analysis. Everything was then imported into COMSOL Multiphysics so that it could be solved.
"The first time we adopted this approach, it worked really well," comments Poole. "The models were clear, the local stress analysis was reliable and we were able to feed the values obtained into our OrcaFlex models." JDR has now worked with Continuum Blue on developing its capabilities, and JDR can now analyze subsea cable structures with multiple internal counter-rotating structures and up to six protective armour layers with ease. From ten weeks on the original project, turn-around time is now down to two weeks and the amount of data produced has risen five-fold.

By contrast, Shell Global Solutions was seeking a reduction in lubricant viscosity – regarded as one of the key approaches to energy efficiency, and the choice of base oil is significant. There is a move towards synthetics, in which molecules are highly controlled, often by further processing of mineral base oils. 'Slippery' chemical additives, called friction modifiers, are also being used. In general, energy-efficient lubricants deliver lower friction because the oil film thickness in the contact is reduced.

Of course, if oil film thickness is reduced too much, there is the possibility of higher wear. It is therefore particularly important to be able to predict the effect of lubricant properties on the thickness of the oil film and the friction of a lubricated contact. "For lubricated contacts, such as plain journal bearings or piston rings, and pressures below 200 MPa, the Reynolds' equation can easily be solved to predict oil film thickness and friction," comments Dr Robert Ian Taylor, technology manager at Shell Research in the UK. "However, there are many important lubricated contacts, such as gear teeth or rolling element bearings, where extremely high pressures of up to 3 or 4 GPa can be generated in the lubricant."

Under such pressures, the viscosity of the lubricant increases dramatically, causing metal surfaces to deform elastically. As a consequence of these two effects, the oil film thickness is greater than otherwise expected, which is exactly why such high-pressure contacts can be successfully lubricated. The fact that high pressure promotes lubrication rather than hinders it can seem non-intuitive. The key is to account for these two effects when predicting performance.

Finite element analysis, using a multiphysics solver, is an approach for taking into consideration all of the participating properties. Says Dr Taylor: "We wanted to solve the Reynolds' equation on the contact line or surface; find the pressure in the lubricant; use that calculated pressure to calculate the elastic deformation in the underlying surface; then use the changed surface shape to recalculate the pressure distribution. However, the proprietary solver code is not so straightforward to modify and it sometimes poses a steep learning curve for newcomers." Members of the Shell team are increasingly turning to COMSOL Multiphysics for complex lubrication problems.

Over the past three years, the team has been using both systems to confirm that they obtain the same results from COMSOL Multiphysics as they do from their own solver. "It is still easy to use the solver for some problems but we use COMSOL for more difficult issues," says Dr. Taylor. "One of the fundamental advantages of COMSOL is that we can develop models without any lines of code whatsoever. This makes maintenance and modification of models much simpler. It is also much easier when we need to consider real lubricated contacts. These are rough, at the micron scale, and so a typical 3mm contact would need to be described by approximately 1,000 nodes, if we model it accurately. This would take up too much computer time and memory for the direct method used in the proprietary solver"

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