Dynamic Simulation
Dynamic simulation is now an established step in the engineering process to design, upgrade and de-bottleneck plant equipment. Xodus Group is at the forefront of dynamic simulation technology. To this end we have the largest privately held token system for Aspen software in the UK and have over 100 man-years of simulation experience. In addition to standard model building and case studies, Xodus hold expertise in the advanced use of Hysys®, including the development of custom interfaces and objects. Xodus personnel have presented several papers on extending the capabilities of Hysys® dynamics.

Compressor surge analysis. Click image to enlarge.		Slug mitigation option screening. Click image to enlarge.

What is simulation?
A dynamic simulation is a virtual representation of existing or future plant. Changes can be made quickly to this model - both in terms of equipment and operating conditions - to rapidly and cost-effectively evaluate various operating scenarios. A steady state simulation will give a snapshot of a process, producing an idealised view of flow rates, pressures and temperatures. Dynamic models take into account time, leading to the evaluation of transient conditions, i.e. slugging, start up and shut down, which are typically when plant limitations can be reached.

In some circumstances operating personnel can limit the fidelity of a simulation. Knowledge of the data requirements to populate the model, knowledge of the simulation package to build the model and astute interpretation of the model output is crucial to delivering the optimum solution. All Xodus personnel engaged on dynamic simulation projects are engineers with design and ops support experience - using the simulation as a tool to reach an engineering solution.

Business benefits
The primary benefits of simulation lie in the potential cost savings which can be achieved in terms of the following examples:
› Ratification of a design prior to purchase of equipment – rapid evaluation of different feed GOR’s if a comprehensive well test program was not carried out or if confidence is low in respect of reservoir analysis, perhaps due to analysing old fluid samples
› Confidence in control system performance prior to installation and commissioning - mistakes in control system design could lead to sub optimal operation, an overrun shutdown or additional shutdown to install additional instrumentation
› Optimisation of separator-level control experiencing slugging conditions - offline evaluation of various control strategies
› Retuning of compressor antisurge and performance controllers to optimise production following the introduction of additional gas to the process
› Minimizing off specification oil sent to slops tanks due to slugging or start up conditions
› Development of an operator training system to maximise process understanding and subsequent process uptime.
› Development of real-time pipeline models for XPLORE pipeline monitoring systems.

Experience
Xodus personnel have a broad range of experience in simulation. Previous projects include the following:
› Slug control study on oil train level control
› Produced water design stage control review
› Compressor control optimisation – utilising CCC controllers
› Compressor control reconfiguration for LP ops – utilising CCC controllers
› Development of loadsharing scheme for export compressors
› Slug control study on reciprocating compressor control
› Control solution for two platforms exporting oil to a common manifold
› Development of operator training systems.

Specialist Experience
Xodus personnel have specialist experience which differentiates them from casual simulation users - in particular their industry-leading experience in linking Hysys® to external applications. As well as using Aspen toolkits to link Hysys® to pipeline simulators (such as OLGA® and PROSPER GAP) and Excel, Xodus has developed several custom links to control systems, leveraging the COM and OPC communication protocols. We recently completed an industry first of using a dynamic simulation at the heart of an allocation system, reading in-plant data to the model then calculating and publishing allocation reports to a company intranet.

Linking Hysys® to control systems and SCADA graphics allows Xodus to provide cost effective and realistic operator training. Utilising the experience of each division within the Xodus Group provides the engineering knowledge of subsea, flow assurance and topsides, allowing us to provide world-class simulation, whilst our Technology Division enables us to deliver this software on a state of the art software platform.

Advanced Control
The Xodus Technology Division offers advanced control capabilities which encompass the design, development and commissioning of single loops to multi-variable control systems for applications such as slug mitigation, slug suppression, gas compression control and NGL optimisation. Xodus takes a unique approach to advanced control which involves reviewing the entire process from an engineering standpoint to ensure that changing a loop response does not adversely affect the downstream processes. This holistic approach to control is complemented by our skills in dynamic modelling of flowlines and topsides processes.

Slug Control
Xodus has developed a number of slug control solutions ranging from cascade control through to non-linear error squared control and into custom designed algorithms. Our approach is to look at each system individually and through a combination of experience and modelling, determine the optimum solution for the system. Our flow assurance experience allows accurate determination of the slugging type and slugging source, which ultimately determines the optimum solution, whether it be a change in fluid phase rates (i.e. changing gas lift rates), a change in choke or riser valve position, feed forward control based on early slug detection, primary separator control or a combination of each.

Compressor Control
Xodus personnel have carried out a large number of compression optimisation studies both in the North Sea and abroad. Studies range from improving control of a single electric motor driver compressor with a DCS based flow vs. dP antisurge algorithm to rationalising three parallel trains of three stage compression with 19 CCC antisurge and performance controllers to designing and commissioning a loadsharing speed controller for turbine driven export compressors. Suboptimal compressor operation can lead to excessive recycle valve wear, more frequent maintenance intervals, more frequent trips (the costs of which can be exacerbated by loss of gas lift), increased fuel consumption and subsequent emissions.

NGL Optimisation
Although the scale of success of onshore multivariable controllers and process optimizers has not been replicated offshore, the NGL plant offers the closest template to onshore operations, allowing successful implementation of onshore techniques and software. As Xodus is software company independent, we will recommend the optimal solution for each plant following a review of the current process capabilities. Daily market analysis of NGL and oil and gas costs can determine whether to produce more NGLs or increase gas lift to enhance oil recovery.

For more information contact:
Steve Hamilton, Director – Technology Division
Tel: +44 (0)1224 628307 Email: