Gemma Church investigates how simulation software models the equipment used in the oil and gas industry
There are many challenges when modelling the complex vessels and machinery used in the oil and gas industry. From the sheer range of equipment, to managing environmental risks, to optimising operational performance within a huge range of application areas, all these requirements must be met and must comply with the tight regulations that the industry demands.
Modelling and simulation is now an integral part of the oil and gas industry’s engineering toolkit, to produce such robust equipment. Simulations are used to model anything from drills bits to an entire offshore platform, and to evaluate design alternatives, answer operational questions, provide extensive scenario analysis, and troubleshoot equipment in the field.
Such pieces equipment must avoid catastrophic failures if they malfunction, are used incorrectly, or cause damage to the complex systems they operate within. David Kan, VP of sales in the Southwest USA at Comsol, said: ‘Failure of offshore products has been well-documented by news sources, so oil companies have seen an urgent need to design better systems. Designers use simulations to create and test virtual prototypes of either a single part or a complex assembly. This includes everything from reservoir navigation, to the precise placement of new wells, to corrosion prevention in existing wells.’
The different application areas add another layer of complexity, as Livio Furlan, chief technical officer for EnginSoft’s structural and oil and gas competence centre, said: ‘This in turn governs the varied design scopes required. So to cope with the different characteristics of any machine, it is essential to know the background detail.’
Then there are the financial considerations, as Alex Read, director of business development, oil and gas at CD-adapco said: ‘In this time of low oil prices, cost reduction is front and centre. Simulation is playing a key role to reduce spending on physical testing, deliver projects on time and in budget, and reduce operational expenses by delivering designs that are right the first time.’
Now factor in the extreme and harsh environments – such as deep waters – in which the oil and gas industry operates. Furlan said: ‘The real challenge here is not necessarily the software chosen to simulate any phenomenon, but the need to make sure that the final design of technically and economically safe machinery is sufficient to operate in anticipated critical conditions.’
The need for 3D
In the past, 1D software tools were used to provide global analysis to design offshore equipment. Read added: ‘Historically, naval architects and offshore engineers have relied on potential flow codes (simple CFD) and physical testing in their design process. While both these methods have advantages, they also leave significant uncertainty in the design.’
For example, potential flow codes do not account for viscous effects, overturning, or breaking waves, and wave basin tests are scaled by the Froude number rather than the Reynolds number. Read added: ‘Consequently, when designers try to estimate the platform’s tendency for or vortex induced motion VIM – motions caused by vortex shedding from platform legs when a strong current is present – this is a viscous effect, where the behaviour changes as a function of the Reynolds number; they’re often highly conservative, resulting in significant additional cost.’
This is where multiphysics simulations can pick up the simulation slack to create models close to the complexities of the real world, as Kan added: ‘Users can couple fluid flow with heat transfer, structural mechanics, electrical signals, and other physical phenomena, allowing them to benefit fully from virtual prototyping by solving systems of equations representing coupled physics effects, as they would occur in nature.’
The design stage can be quite lengthy, and needs to be optimised. One example of such optimisation is the new MapleSim CAD Toolbox, which allows for easy model preparation through CAD model import. Paul Goossens, VP for engineering solutions at Maplesoft, said: ‘Customers have designs in a CAD model early on, to see if the components will fit together. We take advantage of the work already done to give a dramatic increase in productivity and reduce design timings from one day to one hour.’
This need to improve turnaround times is a clear business requirement, as Read said: ‘When geometries are complex, engineers spend significant amounts of time fixing CAD and building high quality meshes. When the physics are complicated, for example multiphase flow, the mesh quality requirements are more stringent. STAR-CCM+’s ability to build a high quality mesh, combined with a robust multiphase solver, again saves significant time and effort.’
Virtually prototyping platforms
Offshore platforms are moving further away from the seashore, which means they must survive in deeper waters and increasingly severe conditions. These vessels need to withstand a huge range of indirect and direct effects, such as rogue waves, extreme weather conditions, and even earthquakes over an extended period of time. You cannot physically prototype an entire offshore platform, so virtual prototyping is used.
Virtual prototyping evaluates the concepts and designs against system requirements using numerical simulations, before putting those concepts and designs into production.
One key application area is the simulation of semi-submersible platforms, also known as floating platform storage and offloading (FPSO) vessels. FPSOs were developed as a cost-effective solution to fixed offshore processing facilities and pipelines, as the oil and gas industry moves its operations to deep water locations.
FPSOs are floating vessels and can be either a conversion of an oil tanker, or a vessel built specifically for the application. They need to withstand severe conditions to maintain the platform’s structural integrity, production capabilities, living quarters and personnel. Arnaud Ribadeau Dumas, solution experience director at Dassault Systèmes, said: ‘Multiphysics simulation, as part of large vessels and platforms engineering, helps to reduce development time and cost, improve quality and power innovation.’
Dumas added: ‘4D simulation enables the user to simulate all aspects of the construction phase, optimise resources, detect potential problems, and complete the construction ahead of schedule thanks to such virtual modelling techniques.’
Offloading the product for transportation to another vessel is an integral aspect of FPSO operation, and it is a delicate exercise. Fouad El Khaldi, director of strategy and diversification at the ESI Group, said: ‘Numerous configurations are possible and increasingly larger facilities are now being built, requiring a holistic view and significant rich assessment, containment and optimisation.’
The relative motions of the two vessels during offload were simulated, as well as the resulting wave motions, using rigid body models to predict the kinematic motions. Full Finite Element Models (FEMs) were also used to investigate dynamic stresses in different components of the vessels.
Rigid body models with mass and inertia properties can predict global motions and accelerations of the rigid bodies, and the reaction forces between these rigid bodies. The simulations also revealed the extent of the offloader linkage motion, which is useful information to ensure those motions remain within the design limits for a prescribed sea-state.
The everyday wear and tear on FPSOs must also be modelled based on the results of hydrodynamic analysis. Such strength and fatigue verifications of the mooring components are achieved using FEMs, taking into account the manufacturing process, under extreme load cases – nearing the chain minimum breaking load.
A FPSO has various systems for handling and separating the different hydrocarbons, as well as mooring systems and a system for the dynamic positioning. These have to be designed against severe sea states because, should the weather and sea conditions exceed the design operating conditions, the vessel has to be free to leave the site in order to avoid possible damage to the structures, and to the pressure pipelines. These pipelines, through the wellheads located on the seabed, transfer the hydrocarbons from the oil pool to the FPSO. Similarly, when the oil field is exhausted, the FPSO has to be disconnected to be relocated in a new oilfield.
Therefore it is necessary for the FPSO to be equipped with a disconnectable transfer system (DTS). One of the main components of a DTS is the multibore quick connector disconnector coupler (QCDC), which contains multiple production and injection lines and valves.
Designing a multibore QCDC is a complex engineering task involving advanced knowledge in designing pressure vessels, as well as structural systems. For example, the riser’s pressures are in the range of 520 bar, and resulting buoy axial load is in the range of 20,000 kN. A variety of design standards also have to be taken into consideration, since they apply to the different components of the system.
The connector has to be designed against normal operating conditions, extreme operating conditions, offshore pressure test conditions, and hydrostatic test conditions. A fatigue analysis must be performed as well, to evaluate the impact of the variations of the axial load transferred by the risers and by the mooring lines.
FEM models were largely used with shell-type models for the structural components – upper and lower part of the connector, upper spool connecting the QCDs to the rotary table, and lower spool connecting the QCDC to the riser buoy – and full 3D models for pressure components.
This type of connector is unique, and trial and error procedures cannot be applied. Efficiency and risk have to be assessed upfront, during the design phase. Shop tests are applicable at the end, but they are just a means of confirming that the design was correct. In other words, only a simulation-based approach can efficiently lead to the correct sizing of the structure and its components, as well as evaluating the different what-if scenarios to deliver the required robustness.
Conditions from normal up to the extreme must also be considered in simulating the dynamic response of a semi-submersible platform and its on-deck facilities. Key features such as the water, the moving floor, and the finite element structure of the platform, cranes and anchor cables, must all be included in the simulation.
For example, the deflection of a crane boom, as well as the fluctuating stresses induced in the crane structure by the motion of the semi-submersible platform, have been taken into account in Figure 1.
The results show the influence of a flexible mooring system on the dynamic response of the semi-submersible platforms. Compared to an unmoored platform, the flexible anchor cables cause a moderate increase in the pitch and heave of the platform, and a significant increase in the surge of the platform. The calculation of fluctuating stresses induced in the structural members of the cranes, and the forces transmitted through the crane mounting points, can help in the design of on-board equipment to ensure that it better withstands loads seen in-service.
Atkins also presented work recently on how STAR-CCM+ was used to investigate a semi submersible platform to understand whether exhaust gases from process equipment and gas turbines will interact with the helideck – which is a potential safety hazard – as well as ensuring there is adequate ventilation of the topside to minimise explosion risks in the event of a leak. The software allowed Atkins to incorporate complex topside geometries, combined with detailed and accurate physics modelling, to reduce the uncertainties and potential errors in the analysis.
A recent study from ESI on a free-fall lifeboat investigated how pressure forms on the surface of the lifeboat when it is dropped into the sea. El Khaldi said: ‘The lifeboat example gives you an idea of the different factors that must be taken into account: different angles of entry into the water; the number of people inside; the shock of impact; all these things need to be simulated.’
These pressures directly affect the design of the lifeboat. In the virtual prototype, the lifeboat was dropped into the water at the correct angle and the pressures on the surface were compared to measured values. The lifeboat trajectory, occupant acceleration, structural pressures and force predictions were extracted from the simulation and exploited to improve the design.
Data management is one of the overriding challenges for simulation software to address when modelling the vast range of equipment used in the oil and gas industry. Detailed 3D analysis requires access to additional computational resources, good data analytics, and the ability to process and store large amounts of data.
The input, output, and transfer of data across networks of computers is often an issue and the software needs to operate on shared and parallel computers, as Ahmad Haidari, global industry director of process, energy and power at Ansys, said: ‘The ability to streamline and optimise use of the software performance on high performance computing enables users to include all relevant geometrical and physical details, making simulations as close to real life as possible.’
Haidari added: ‘At the same time there is a need for continued improvement in data analytics and decision post-processing, to help reduce the need to store results generated from all the numerical studies.’
In other words, it is the extremity of data and not the extremity of environmental conditions that the simulation software must master to meet the demands of designing and testing equipment for the oil and gas industry.