DEVELOPING AN EXCITING PROJECT IN ATHENS

Proud to announce another exciting international project. Now the project has been executed in Athens, Greece.

We have collaborated with “Armos SA” in the new “Agia Sofia Stadium” project for the AEK sports club in Athens, performing the GRC (Glass fiber Reinforced Concrete) design verification and the fixings design and calculations.

To validate and improve the panel’s design, the SDEA advanced engineering team created a FEA (Finite Element Analysis) model starting from the base drawings. Several load cases are then performed over that model based on the relevant building normative, such as wind or earthquake forces, with its corresponding safety factors. On the final step, the FEA results are assessed and compared with the limits allowed by the standards to ensure that the design is safe and therefore, validated to move to the manufacturing process.

GRC is a composite material that is used extensively worldwide, both as a functional and decorative construction material in building and civil engineering. The high-strength embedded glass fiber properties combined with ones from the concrete matrix result in a much more resistant and lighter material. This allows the GRC to stand above other solutions or materials when it comes to manufacture complex facades with intricate designs, as it is able to be easily precast in any shape or texture we want by using a mold, offering almost limitless personalization possibilities. In addition to its structural performance, the precast method offers low execution times, which translates into lower manufacturing costs. On top of that, it can also be designed to add an extra insulation layer (both acoustic and thermal) to any type of building, having an impact in energy savings.

More new international projects are coming. Keep an eye on our news in LinkedIn

Thermal Stress Analysis Of Dissimilar Welding Joints using FEA





Finite Element to foreseen residual stress from the welding process

Most welding process involve local heating, therefore temperature distribution is not uniform and structural thermal stresses beyond the material yield [strongly dependent on the temperature] show up, which can create issues on the final design related to fatigue or even geometry distortions.

This local temperature field is enough to create metallurgical and phase changes. As the pool solidifies and shrinks, it begins to exert stresses on the surrounding weld metal and heat affected zones. When it first solidifies and the weld metal is hot, yield is very low and exerts little stresses. As it cools to ambient temperature, this stresses in the weld area increase and eventually reach the yield point leading to a tensile stress state prone to giving problems during the equipment operational life.

As it cools to ambient temperature, this stresses in the weld area increase and eventually reach the yield point leading to a tensile stress state prone to giving problems during the equipment operational life.

Often pressure vessel suppliers are asked by their clients on how the residual stresses would influence the expected structural performance. Those residual stresses, that in tension may lead to high local stress in weld regions of low notch toughness and, as a result, may initiate brittle cracks that can be propagated by low overall stresses that are normally present. Also, notice that residual stresses can be reduced using thermal or mechanical relief methods, but not always post weld treatments are performed for different reasons.

Residual Stress and Temperature contours

SDEA engineering team used a Finite Element Analysis [FEA] approach to foresee the residual stresses from a dissimilar weld between the tubular plate and the shell [see figure 1]. SA-516 Gr 70 against SA-965 F304 will be welded with ER 309L Mo.

The welding procedure includes the geometry detail of each pass and a two-staged coupled simulation involving a thermal stage and a posterior mechanical step that use the temperature field as an input from the previous model. This can be done because changes in the mechanical state do not cause a change in the thermal state, but the opposite doesn’t apply; so that in this study, computation of the temperature history during welding and subsequent cooling is completed first and then this temperature field is applied to the mechanical model as a body force to perform the residual stress analysis.

Heat input during welding is modelled by a distributed heat flux applying on individual element based data provided by the client for each pass. The amount of heat input is obtained from the next expression, that depends on electrode speed, energy applied and a welding efficiency factor.


Principal stress and contours after weld is finished

Is important to highlight the fact that the reduction in yield for temperatures above 700-800 °C will govern the melting transition. Image above shows the expansion coefficient used for the both materials to be welded, notice the different thermal properties are the ones responsible for different heat diffusion and different thermal strain response.

As expected, the high temperature in each pass relaxes the stresses as yield drops with temperature. Weld metal melts and gets to flow state where stresses are relieved; this effect is clear and can be seen in the images around.

As expected tensile stresses appear on the weld surface. Next plot represents the Von Mises stress on the weld surface for the top and bottom side:

The model was also assessed for protection against ratcheting. Ratcheting is a behavior in which plastic deformation accumulates due to, in this case, thermal stress. To evaluate the model against ratcheting, as well as in the previous point from this document, an elastic-plastic analysis was used, according to the guidelines provided by “ASME BPVC.VIII.2-2017” Part 5, point 5.5.7.

For this analysis, same model from plastic collapse was used. In addition to its boundary conditions, three thermal cycles were calculated, as required in the normative, by setting the following temperature profiles on the interior faces:

Summary:

  • Weld residual stresses were calculated using a non-linear FEA coupled thermo-mechanical analysis. As expected, tensile stresses appear at ambient temperature on the weld surface close to the material yield [in the range of 400 MPa].
  • Stresses on the model were calculated using a non-linear FEA coupled thermo-mechanical analysis under operating loads, achieving convergence according to “ASME BPVC.VIII.2-2017 Part5” and thus satisfying the elastic plastic protection criteria
  • Elastic-plastic ratcheting assessment was performed after three thermal cycles, according to “ASME BPVC.VIII.2-2017 Part5”. Two main dimensions on the shell were measured during the cycles, showing no permanent deformation due to the effect of the thermal loads. This condition satisfies the elastic plastic ratcheting assessment criteria as stipulated by the standard.

If you are interested in stress analysis please contact us.

Keep your curiosity in good shape.  

CFD in HVAC for railway applications

CFD simulation for HVAC studies

When travelling, arriving to your destination on time is not the only concern. Comfort also plays a very important role on the travelling experience and may have an impact on your evaluation about the trip.

Railway transportation is one of the most used systems in the world, from high-speed trains allowing people to travel long distances quickly to underground railway facilitates mass transit for people movement inside a big city.

What all these types of trains have in common is their need for a Heating, Ventilation and Air Conditioning [HVAC] system to ensure environmental comfort inside the vehicle. In order to regulate these conditions, there is a strict normative related to comfort for both the passenger cars and driving cabins.

These regulations, specified in Standards such as EN-13129 or EN-14813, determine the comfort parameters inside the vehicles related to air temperature, humidity, air renovation and surface temperature in a wide range of locations, with the final objective of guaranteeing the maximum passenger comfort during the travel.

Computational Fluid Dynamics [CFD] is a powerful tool that can help assessment in the design phase of HVAC systems, since it can simulate the flow and temperature distribution inside the carriages and any environmental and input conditions even before the system is built. With these previous evaluations, it can be easier to determine optimal flow distribution, impulsion parameters, flow rates, poorly optimized areas and so on. CFD also counts with the advantage of not being restrained by any physical components, so data can be analysed in any spot of the geometry, giving even more additional information that can be useful to the designer than the parameters asked by the Standards.

Both EN-13129 and EN-14813 stablish the probing coordinates and sections during test conditions; so in the CFD model these locations can be fully monitored during the calculation to ensure the convergence and stability, and later on, these data can be compared to the requirements asked in the Standards.

HVAC systems have to meet an exhaustive process of evaluation in terms of thermal homogeneity in a large amount of different locations: seats, aisles, bathrooms, windows, walls, etc. in order to provide a comfortable feeling to the passengers along the ride. Air velocity is also regulated in order to prevent that any area is exposed to any jet stream or, on the contrary, be improperly conditioned. All these requirements involve a deep understanding of fluid dynamics and air behaviour.

SDEA Solutions provides a fully customized service using its knowledge on CFD HVAC modelling, simulation and expertise on Standards to fulfil any client’s request in HVAC systems assessment. Our services include the whole process of designing the geometry from drawings or modification of a given one to the creation of a high quality model to be simulated and a complete analysis of the results, giving the client the precise data they need to know in order to better understand and improve their system, with complete and exhaustive reports made to ensure that every condition and result is well interpreted.

TLD Tuned Liquid Dampers

FEA for TLD Tuned Liquid Dampers

One of the greatest challenges when it comes to design steel stack structures such as chimneys, is to deal with oscillating loads as dynamic wind forces and seismic spectrum, as commented in this article. These loads threaten the design with coupling the excitation with its natural frequency, leading the structure to failure through resonance due to the high stresses produced by the oscillation amplitude.

Two main strategies can be followed to address this issue: either increasing the natural mode of the structure or damping its response to the excitation.

The objective of the first way (natural mode improvement) is to run away from the dangerous resonance frequencies. 

Figure 1 - Vortex induced oscillating loads on a cylindrical profile due to the wind action

This can be achieved by geometrical changes on the design such as plate thickness enhancement or a gain of chimney’s diameter, or by the addition of new structural elements as stiffeners or supports. Although this method is conceptually simple and does not require an extra designing effort other than adding some more material, is has other limitations. Usually, geometrical changes as wider diameter or the addition of new elements are restricted by other features of the whole project. 
 

Figure 2 - TLD pools coupled oscillation

Neighbour structures limiting the available volume or chimney’s exhaust properties are examples of these limitations. Other inconvenience for this solution, and the one who uses to have the last word, is the cost-efficiency. An increment on plate’s thickness involve a not negligible increment on the project’s expense moving it away further than the structure’s natural mode.

The other path to follow in order to guard the structure against oscillations is dramatically increasing the structure’s damping by the addition of a damping system. 

Even though there are different methods to achieve this, this article will be focused on the tuned liquid dampers (TLD).

The mission of the TLD is to dissipate the energy absorbed by the structure from oscillating loads, through a mass-spring-damper equivalent system that moves desynchronized with the chimney at its natural frequency. 

It is, in essence, a series of pools connected to the structure that moves against its pendulum movement. The phenomenon responsible of the energy mitigation effect inside the pools is called sloshing, and is defined as the movement of liquid inside another object. An example of sloshing familiar to almost everyone can be seen every time you walk with a soup plate. 

The human gait frequency (acting as the excitation) is close to the plate mode, so each step amplifies the soup’s wave motion, finally causing it to ruin your menu by splitting out of the rim. 

Figure 3 - Amplitude vs Frequency graph comparing a chimney with and without damper

On the other hand, if you are carrying a cup of coffee at a standard walking speed, despite it creates waves on the surface, the excitation frequency remains lower that the coffee’s natural mode, and as long as you don’t speed up, you breakfast will be safe.

Figure 4 - Plate & cup first oscillating mode shapes

The parameters that define geometrically a TLD, controlling its behaviour, are its interior [Rint] and exterior radius [Rext], the number of pools in which the TLD is divided, and the liquid height [H] that controls the liquid volume and thus the liquid mass. This liquid mass is divided in two distinguishable masses: the impulsive mass, the one that moves creating the sloshing phenomenon, and the convective mass, that has no relative movement with the TLD. Note that despite it has its influence on the sloshing frequency, the amount of convective mass also acts as dead load on the chimney, moving its natural frequency, so it has to be taken into consideration when tuning the assembly.

Figure 5 - TLD main dimensions and water mass distribution

The interior radius is used as the start point to calculate the previously mentioned variables [as it is more or less set by the chimneys geometry itself] along with the chimneys natural frequency and modal mass, acting as the focus result for the tune and as a reference for the total liquid mass respectively. Then, using the linear wave theory and the Graham & Rodríguez equations, the first group of parameters is obtained. Because of the influence of the TLD’s convective mass on the base chimney’s natural mode, an interpolation process has to be performed in order to obtain a fine tuning for the liquid damper.

Figure 6 - TLD sloshing mode shape at tuned frequency

To optimize this method’s accuracy and asses its results, SDEA’s advanced engineering team assembled a coupled FEA modal analysis plus harmonic response computational model. A fluid-structure interaction model along with a fluid free surface boundary condition is needed to capture the sloshing effect on the TLD’s pools. The next image shows the pools from a TLD where it can be appreciated that the liquid from all pools move in the same direction when they get coupled with the chimney’s response to external excitation.

Although this solution for the problem introduced in this article has a bigger technical load than enhancing the structure plate’s thickness, the cost-efficiency and adaptability of the damper installation compensate these cons by relieving the projects budget and providing certain design flexibility to overcome hypothetical installation issues.

STADT latest generation solution, STADT Lean Propulsion ®

STADT

The Norwegian company STADT has for many years been a frontrunner in development of electric propulsion for ships. The company’s latest generation solution, STADT Lean Propulsion ®   have already been awarded for its revolutionary design with a set  uniqueness’s : very high efficiency, STEALTH-silence, low signature,  reduced weight, high reliability and low maintenance cost  for shipowners.

To meet new requirements STADT now release a new feature in

Lean Propulsion ,  The  VariAC .      

This option give customers the possibility to take increased effect /
advantage of variable speed from Diesel or LNG-gensets  fitted with
variable speed-control.   

STADT VariAC  operates
on an AC grid with variable frequency in a range of 45-65 Hz,  and will
give ship operators more efficient and precise control of power-load and ship-speed
via a  seamless regulation of  propeller speed (RPM) and Pitch in
three different power-modes – Low, Medium  and High-power  – very scalable for propulsion systems in the range 1 to more than
 50 MW pr propeller.

The first version of STADT   VariAC
  is designed
and developed  in cooperation with PON –

Et bilde som inneholder overvåke, skjerm, innendørs, skjermbilde

Automatisk generert beskrivelse

Caterpillar, with the utilization of
Caterpillars and MaK products.

By the new VariAC  feature,  STADT have increased the Lean Propulsion® system benefits in a set areas, and will secure

  • Overall
    lower fuel consumption and emission, especially on low and medium power
  • Lower
    noise level during operation
  • Less
    Lube oil consumption
  • Less
    wear and tear on genset with lower maintenance cost
  • Higher
    Propulsion efficiency

“In cooperation with leading LNG and
Diesel-generator vendors, we see that we with the STADT VariAC  can utilize  latest generation
motor-technology  to even lower operational cost for our customers” says
Hallvard Slettevoll, ceo at  STADT AS.

STADT AS

Moljevegen 50

N 6083 GJERDSVIKA

Norway

                                                                                              
www.stadt.no