POSTED 06.03.17

EASL’s team of engineers and scientists have a range of experience concerning all things structural integrity, bringing with them interests and research on a wide variety of issues. Here, Recent PhD Graduate Teddy Mbwadzawo discusses the focus of some of his research and potential improvements to Rapid Gas Decompression in the future.

 

Whilst doing my PhD at the University of Sheffield l published a paper exploring the use of numerical modelling on the potentially catastrophic issue known as Rapid Gas Decompression (RGD) within elastomer seals.

Using a sequentially coupled structural-mass diffusion FEA model, the numerical model has the ability to evaluate the deformation of an O-ring due to the non-uniform pressure exerted by the rapidly expanding gasses during this event.

In this blog I shall be looking at RGD, the issues it raises and current methods of research. I will also look into possible ways to improve the numerical method to further provide a cost-effective and efficient method to research and analyse this industry-wide concern.

What is Rapid Gas Decompression?

RGD, also known as explosive decompression, is an operational condition during which applied system pressure is quickly released, resulting in the expansion of absorbed gas damaging elastomer seals. Failure of elastomer seals due to RGD can be explained in three stages:

  1. In Applications where high-pressure fluctuations are experienced, gas in contact with the elastomer seal is absorbed into the material through permeation until the material is fully saturated.
  2. As the name suggests, the external pressure surrounding the elastomer seal suddenly drops and this leads to a pressure gradient between the absorbed compressed gas and the external surrounding gas.
  3. This pressure gradient causes the absorbed compressed gas to expand at a faster rate than it can naturally diffuse through the elastomer material. This results in the rapidly expanding gas exerting tensile stresses within the elastomer material and if these stresses are higher than the strength of the material failure occurs.

Failure of elastomer seals due to this event can be disastrous in the oil and gas industries and other high-pressure gas applications. The failure of such a small component can bring oilfield machinery or plants to a standstill leading to financial, safety and environmental costs.

For this reason, it is a major concern for engineers and designers, leading to a great deal of research in the attempt to prevent such failure.

Researching Rapid Gas Decompression

Experimental methods for researching RGD have shed a lot of light into the associated failure mechanisms. These experiments still play a major part in the research of RGD, however, on their own do not provide a full understanding about the structural response of the elastomer seal.

As you can imagine, setting up a physical experiment to replicate the conditions that occur can be potentially dangerous if conducted without proper safety protocols and equipment, not to mention incurring a large time and financial cost.

This is where numerical modelling using Finite Element Analysis (FEA) comes in. An FEA model allows an engineer to analyse and visualize the interaction between the elastomer seal and the rapidly expanding gas during RGD.

An FEA model allows the user to accurately evaluate stresses and strains generated during RGD. With this knowledge, specifically resistant seals can be developed with the ability to withstand the high stresses and strains generated.

One issue with numerical modelling of RGD is that it is a multi-physics problem which involves simulating the interaction between the elastomer material and the rapidly expanding gas.

A few FEA models have been developed to model RGD either by modelling the fluid ingress into the elastomer seals or by estimating the magnitude of tensile stresses exerted onto the elastomer seal due to the rapidly expanding gas.

There is still more work that can be done to develop a numerical model that can be used to investigate the local behaviour leading to crack initiation and propagation during RGD.

Finite Element Analysis, Numerical Method in Rapid Gas Decompression Research

The model I created was sequentially coupled, meaning that the diffusion of gasses through the elastomer seal during RGD was modelled first. Then results from the mass diffusion analysis were used in the second model to evaluate the structural response of the elastomer seal.

Results from this numerical model conformed to results from RGD experiments and results from analysing specimens of fractured elastomer seals.

However, due to the complex nature of the RGD process, improvements can always be made to this FEA model. One of the improvements could be changing the FEA model from a sequentially coupled model to a co-simulation.

During a co-simulation, data is exchanged between the two models at every increment, meaning that the results from the mass diffusion model are used to update the structural model and vice-versa at each increment.

This is important because during the RGD process the structural response of the elastomer seal will affect the diffusion of gasses within the elastomer seal and the diffusion behaviour of the absorbed gas will in turn affect the structural response of the elastomer seal. This phenomenon is not completely captured by a sequentially coupled model.

Another improvement would be to consider including strain rate dependency. This is because RGD is an explosive process and elastomer materials are highly depended on strain rates. Therefore, including strain rate dependent material properties could yield some interesting results on the structural response of the elastomer seal during RGD.

These improvements can all be done using FEA, however, experimental data would be required to calibrate the material model of the elastomer seal.

There is another interesting idea which might seem a bit far-fetched but l believe is achievable especially if the right tools and knowledge are available.

It is stated in literature that fracture in elastomer seals exposed to RGD conditions initiates and propagates from voids inherently contained within the elastomer seal.

The inclusion of voids in the FEA model could yield some interesting results especially the local behaviour leading to crack initiation and propagation.

Due to the differences between the size of the elastomer seal and the voids it will be costly to try and model both the elastomer seal and the voids on the same scale. However, a way around this problem would be to use a multi-scale FEA model. The multi-scale numerical model can then be used to investigate the effect of distributed voids contained within the elastomer seals.

Summary

RGD failure mechanisms have been researched extensively using both experimental and numerical methods. However, more work can be done to improve the existing numerical models to further understand the local behaviour leading to crack initiation and propagation in elastomer seals during RGD.

The numerical models can be used to complement experimental results allowing engineers to design RGD resistant seals at lower costs and low turnaround times.

 

If you’d like to find out more about our expert structural integrity knowledge, take a look at our services page.

Get in touch