CRM Engineering

Design validation through FEA

Charles McCreary — Mon, 17 Dec 2007 17:25:13 +0000

This particular client often uses our services to validate design modifications to plant equipment. Their equipment is used to transport large sheets of glass from the production line to the shipping bays. Failure of the equipment during a lift operation can result in severe injury or death due to glass shards or the payload.

We first calculate the appropriate loads and safety factors using appropriate codes (ASME in this case). We then evaluate the design as far as possible using tradiational structural analysis methods. Once the member sizes are appropriate, we perform a final validation of the structural design using finite element analysis (FEA) methods.

Random response analysis of avionics tray

Charles McCreary — Fri, 14 Dec 2007 16:40:14 +0000

A client approached us with a difficult electronics packaging problem. Their customer required a commercial, off-the-shelf (COTS) board to be included in a device that must pass the MilSpec random response envelope for a particular aircraft. Since the COTS board could not survive the vibration unless the enclosure provided shock and vibration isolation, the objective of the analysis was to specify the required properties for the enclosure supports as well as to ensure that the enclosure itself had sufficient fatigue life to survive the vibration spectrum.

Machine Design - Fatigue

Charles McCreary — Fri, 14 Dec 2007 03:09:16 +0000

A client was experiencing failure in a cantilevered shaft which supported a heavy spool that was subject to frequent rapid starts and stops. The initial design incorporated a a 3″ diameter shaft that was supported by two bearings with a drive gear halfway between the bearings. A larger, 5″ diameter shaft was fillet welded onto the 3″ shaft just outboard of the outer bearing.

The massive spool was accelerated from rest to 60 rpm in a fraction of a second and braked several times an hour, 24/7. Thus the number of stress reversals seen in a year required a design in which the peak stress is well below the fatigue limit. The shaft would break at the fillet weld within a few weeks of service at the fillet weld (no surprise).

Examining the problem from first principles (beam theory and stress concentration factors) showed that the expected fatigue life was measured in hours. A detailed finite element analysis of the system corroborated the diagnosis.

Our recommendations:

Machine the shaft in one piece with a generous shoulder fillet. This eliminates the fillet weld and greatly reduces the stress concentration at the shaft diameter transition

Specify a generous root fillet in the gear keyway to reduce the peak stress

Use a material with a fatigue limit much higher than the mild steel used in the original design

Hanger Flanges

Charles McCreary — Thu, 13 Dec 2007 02:47:38 +0000

A client recently engaged our services to design hanger flanges to satisfy a prescribed load combination for each size. A hanger flange is a joint in a pipeline whose primary purpose is to react the axial load below the hanger flange into some structure (hence the term hanger). The requirements were that the design satisfy ASME BPV Section VIII - Div. 2 (primarily that the linearized stresses are within the allowables).

InjectSafe Safety Valve

Charles McCreary — Wed, 09 May 2007 22:23:17 +0000

Assume you wanted to deliver something via capillary tubing from topside to the producing zone(s) and there is (will be) a tubing retrievable sub-surface safety valve. You need a mechanism to bypass the safety valve without compromising it’s function. BJ-Services/Dyna Coil division has developed a technology that allows an operator to inject chemicals past the safety valve into the production zone without compromising the safety valve’s function.

Assisting BJ-Services in the design and qualification of a new size, CRM Engineering Services provided design and analysis support. As with any safety critical device, considerable thought must go into designing load cases for every conceivable operational event. Finite Element Models were built and analyzed for a number of load cases. As usual, there is a trade-off between the time required to build the model and the time required to solve the model. Fortunately, we have the computational infrastructure to handle very large, non-linear models and since computer time is relatively cheap, we usually choose to build very fine, detailed finite element models. Furthermore, we perform convergence studies on the models to verify that the discretization is sufficiently fine that we can have confidence in the predicted results. Thus the emphasis is on turn-around time, the time from when we are initially engaged to the delivery of the final report.

Hurricanes and Offshore Structures

Charles McCreary — Mon, 16 Apr 2007 23:21:49 +0000

It’s difficult to comprehend the magnitude of the waves developed during a major hurricane. The hurricanes that wreaked havoc on the US Gulf Coast left plenty of evidence in their wake of their destructive potential. Offshore structures received considerable damage from the waves and wind. Older platforms which set cellar deck elevations based on older API guidelines were hard hit by the extreme wave heights exhibited by Ivan, Katrina, and Rita.

The damage seen in the picture on the right is from hurricane Rita in which a wave topped the cellar deck. As can be seen in the picture, quite a few large beams have been bent. Current codes are a little unclear on assessing residual strength of deformed members, so we had to improvise.

Our analysis procedure needed a method to compare the results from the FEA model of the deformed beam to an equivalent FEA model of an undeformed beam. The uniform load that will produce an extreme fiber stress equal to is applied across the top flange of both the undeformed and deformed beam. The peak stress in the deformed beam is calculated and compared with the peak stress obtained from the undeformed beam:

The ratio should then be a direct measure of the reduction in capacity of the beam.

New site

Charles McCreary — Mon, 16 Apr 2007 21:37:42 +0000

For the past seven years, we have been using a relatively static site. We found that updating the site was a task in the “important but not urgent” quadrant. Thus our web presence was indeed rather static. We decided to change course and implement a site based on Wordpress. We currently use Wordpress on our sister site and have found that updating the site with relevant and useful information is much less burdensome.

For those of you who have used our javascript based unit converter, we have decided to retire it and instead implement a Turbogears based unit converter that will be easier to maintain and extend.