Simulations to withstand wind and ice

The oil and gas industry depends on simulation. Robert Roe explores how the technique can optimise the integrity of structures in extreme environments

It is well known that the oil and gas industry relies on advanced software tools to optimise exploration, production, transportation, refining and processing – from predictive analysis to product qualification. This all requires powerful codes which have been honed by the software companies working with the oil and gas companies to tailor the software to specific industrial applications.

However, less attention is usually given to the fact that the structures used in oil and gas exploration and production are regularly subjected to harsh, extreme environments but must be capable of performing consistently and reliably in these conditions. In order to guarantee the safety of people using the equipment, and also to prevent accidents, which can be hugely costly, both economically and ecologically, high quality simulation is needed to make sure that everything from the structure of the oil rig right down to the smallest pipe or valve has been designed with performance and reliability in mind.

Pressure vessel design

One aspect of this is simulation applied to the design of pressure vessels. This is a particular focus of NOV Elmar, which provides technical expertise, advanced equipment, and operational support to the oil and gas industry. The company specialises in the design and manufacturing of wireline products for slick-line and electric line applications.

Richard Herdman, senior design engineer, said: ‘In our particular sector of pressure vessel design, simulation is used to optimise designs and give a better understanding of stress distribution throughout non-standard product geometry.’

Herdman continued: ‘At NOV Elmar, we have used simulation to develop a new range of products to ensure well integrity for wireline intervention and which can fit through platform production floor hatches (a considerable dimensional constraint).’

Leandro Castro, co-founder of MultiMechanics, which provides computer-aided engineering (CAE) solutions, explained that finite element analysis software has been used frequently by engineers for a range of situations in the oil and gas industry.

Modelling mechanics of an iceberg

One recent case study used multi-scale technology, a key feature of MultiMechanics’ FEA software, to improve the modelling of an iceberg’s mechanical response. Castro said: ‘This data can then be incorporated into finite element analysis of iceberg-structure interactions, allowing offshore, arctic designs to be developed and evaluated with more clarity on safety margins.’ The findings of this study will be presented at the Offshore Technology Conference to be held in Copenhagen, Denmark, from 23 to 25 March. 

Castro said: ‘FEA has been successfully used to help with many aspects of the oil and gas industry, including improving risk management, reliability and safety of structures. From drilling bits, composite risers, to offshore platforms, there are multiple structures that benefit from the design flexibility provided by FEA.’

Castro continued: ‘These benefits include the ability to perform fast design exploration of non-trivial shapes and material combinations, often necessary to overcome the inherent challenging conditions of this industry.’

Simulation avoids overdesign

Both examples make use of sophisticated modelling software in order to meet the specific design requirements of the industry. As with many simulation challenges, overdesign can be extremely costly or even prohibitive. Avoiding this necessitates the use of reliable, accurate, analysis and optimisation tools at every stage of the design process.

Herdman explained that NOV Elmar uses many Altair products in its day-to-day workflows: ‘For general analysis work, we use HyperMesh for pre-processing, RADIOSS to solve, and HyperView to post-process.’ However he went on to explain that one of the advantages of using HyperMesh for FEA needs is that it can be used in conjunction with other solvers.

Herdman said: ‘Part of our business regularly uses Ansys for structural design, so HyperMesh can be used for improved mesh control which in turn leads to more reliable results. For engineering problems, which may lead to issues such as excessive geometric non-linearity, it is great to know we could use another, more appropriate solver and yet still maintain the post-processing capability we require.’

This flexibility also extends to the development of the software itself, as Herdman explained: ‘We also use a jointly developed plug-in at the post processing stage for stress linearisation.’

By working closely with Altair, NOV Elmar has fine-tuned the software to the specific needs of the oil and gas industry. Herdman said: ‘Pressure vessel design in the oil and gas sector must be carried out in accordance with relevant industrial standards and design codes. Many of these standards are quite prescriptive regarding material choices, allowable factors on material yield, and various other aspects of the product operation and design.’

He continued: ‘Appropriate use of finite element analysis is also part of such standards and, in particular, a technique called stress linearisation.  This technique generates stress outputs which are compared to geometry-based allowable stresses in the design code.’

Small details; big differences

MultiMechanics has found that users of its software have been using MultiMech software for structural design of Arctic oil rigs, as mentioned earlier. Castro said: ‘Structures used in oil and gas exploration are subjected to harsh, extreme environments that cause materials to go beyond their linear elastic limit. Complex degradation and damage mechanisms start to play an important role on the material performance.’

Castro continued: ‘For these scenarios, robust simulations that can take into account the sources of non-linearity are necessary to obtain reasonably accurate results. Sometimes the sources of non-linear behaviour are found at the microstructural level, such as material defects and micro-cracking.’

‘These small details make a significant difference to failure prediction, but accounting for them directly in a single mesh requires a level of refinement that is computationally unfeasible, even with HPC hardware,’ Castro explained.

This is where multi-scale technology can be of most benefit, as the use of many smaller models to complete the same tasks as that of a single larger model reduces the computational complexity, reducing the time to derive meaningful results from the simulation data. Castro said: ‘It has become very popular, the employment of multi-scale technology, as a way to tackle the design complexities without the expected computational overhead, by substituting an over-refined single FE model by multiple, interconnected coarser FE models representing different length scales. The computational gains of such approach can reduce computational time from hours to minutes, without loss of accuracy or flexibility.’

Composite materials

Another aspect of FEA software that, according to Castro has gained significant traction in the oil and gas sector is the design and analysis of composite structures. Castro said: ‘Composites can be tailored to specific applications, and have properties that are very attractive to this industry, such as: high specific strength; high stiffness; great fatigue performance; high thermal insulation; and corrosion resistance.’

As exploration and production of oil and gas moves into deeper water, weight, cost, and reliability of components that are sensitive to the depth of the water become increasingly important. ‘A great example of such an application in this industry is carbon-epoxy composite riser joints, designed for production and drilling operations in deep waters’ said Castro.

He continued: ‘They can be 50 per cent and 30 per cent lighter than steel and titanium risers, respectively.’ Despite their advantages, composites fail very differently than metals due to the complex structures involved. This necessitates a much deeper understanding of the material structure and the interactions on a micro-structural level.

Castro said: ‘There are multiple competing micro-scale damage mechanisms such as fibre braking, resin cracking, and fibre-resin deboning. There is also a wide range of material design possibilities. All of these micro-level aspects affect the macro-structural performance, and can be efficiently modelled by using this multi-scale technology, leading to accurate predictions, and improved designs.’

The study that MultiMechanics was involved in was preliminary and, as such, is ongoing. Its team is responsible for looking at various ways to improve the efficiency and resolution of the simulation ‘especially by increasing the resolution of the ice microstructure,’ Castro commented.

‘Future studies will apply the same multi-scale approach not just to the ice, but also to the structure, to explore how advanced materials, such as composites, respond to iceberg interaction, and the effect of microstructural design variables on structural performance,’ Castro concluded.

One aspect that particularly pleased MultiMechanics was the performance gain derived from employing multi-scale technology. Castro said: ‘There is a significant performance gain by modelling these microstructural details through a multi-scale approach, instead of a single scale one.’

The study showed that single-scale modelling of large structures becomes less efficient as the complexity of the model is increased. Castro said: ‘For example, a model with 638,000 degrees of freedom, and even without including crack initiation or propagation, still required a computational effort of approximately 14 hours for 200 solution steps on a 16 core, 2.6 GHz workstation.’

The same experiment was modelled with a multi-scale approach, where global and local finite element models were coupled and analysed simultaneously, while crack initiation and propagation was modelled at the micro-scale level. Castro said: ‘This multi-scale simulation had better results, and a total of two million degrees of freedom, yet it took only 18 minutes to run using MultiMech on the same machine. Since the modelling time was reduced significantly, future studies will be able to look into even more complex models and structures.’

Castro went on to explain that the software is parallelised effectively to scale with the number of CPU cores available, so a HPC infrastructure ‘would provide proportionally faster responses, especially for larger models. However, HPC is not required, as demonstrated by the case study.’

Much of the simulation and modelling at NOV Elmar takes place on HPC resources, as the complexity of simulation is often beyond the scope of traditional workstations. Herdman explained that all resources are in-house at the NOV datacentre in Goodyear, Arizona.

Servers consist of HP SL230 and SL250 that range from 64GB to 128GB of memory and total over 600 cores. The current scheduler is Moab/Torque with a custom web submission portal for HyperWorks and Abaqus. Ansys jobs are submitted through RSM directly to the scheduler. There is also a Virtual Desktop Infrastructure (VDI) for engineering applications that is powered by Citrix Xendesktop and Nvidia GPUs. Shared storage is provided to compute and VDI nodes via a Netapp HA filer pair.

Herdman remarked that one of the benefits of using Altair software is performance of its meshing software HyperMesh, especially for hexahedral elements. Herdman said: ‘HyperMesh is second to none regarding the development of efficient meshes.’ 

Herdman continued: ‘This has three main benefits. Firstly, by investing at the post processing stage we have reduced lead times so that approximately 20 per cent of our analysis is due to solving.  Secondly, hex meshing also removes the requirement for seeking mesh independence solutions, which would be required for tetrahedral element meshes. Thirdly, our in-house modelling experience has also shown that hexahedral elements produce much more reliable and accurate results for pressure vessel design than even second-order tetrahedral elements of a similar size.’

Herdman also stressed that it seems unlikely that NOV Elmar will need to use a different software package in the foreseeable future. Herdman said: ‘The use of Altair’s other software, such as RADIOSS, and HyperView seems a very natural solution, as we would now only ever use HyperMesh to pre-process.

Herdman also explained that NOV Elmar prefers Altair software, such as HyperMesh as opposed to a more specialist platform tailored to oil and gas because of the support provided by Altair. Herdman said: ‘To some extent, the jointly developed plug-in has given NOV Elmar a level of industry-specific customisation, but there are no business needs to use anything specialist.’

Herdman continued: ‘It was principally due to Altair’s willingness to provide free training in HyperMesh and extended use of trial software that NOV in the UK began to use the HyperWorks. That was 18 months ago and now we have two UK sites using Altair products, including NOV Elmar.  We have received regular training and Altair personnel are always on hand to support as required.’

Castro said: ‘MultiMechanics works closely with users to add requested features and optimise code performance. The software has quickly evolved and will continue to do so over the future. The fast computations presented in the study weren’t possible just a few years ago, and more breakthroughs in performance will continue to be made in the years to come.’

Although simulation is already complex, there is still room from improvement, from specialised plug-ins for niche applications to parallelising code so that it can deliver results faster, or increasing the resolution of FEA to include the microstructural effects there is still much to be done to make oil and gas industry as safe and efficient as possible.

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