Simulating Perdidos transient engineering

Increased uncertainties about operating in ultra-deepwater typically result in over-designed production systems with resulting increased costs. Prasanna Parthasarathy and Kellin Nelson of MSi Kenny and Shell's Howard Littell discuss how high-fidelity integrated field modeling prevented overdesign and reduced uncertainty for Shell's Perdido spar in the Gulf of Mexico.

The challenges of subsea production in deeper waters and marginal fields drive increasingly innovative production system designs. Perdido, the world's deepest production spar resting in 7817ft of water, is one of the foremost examples of such a development. Traditional simulators focus on specific areas such as pipeline or process simulation in their efforts to lower uncertainty. The key to preventing overdesign and reducing uncertainty in understanding these innovative production system designs is highfidelity integrated field modeling.

The simulation system MSi Kenny developed enabled Shell to gain confidence in its operation of the Perdido system and train operators well in advance of actual field startup. The ability to carry such simulators on a laptop helps to minimize maintenance costs and enable the system to be adapted after production startup for use as a surveillance tool as field conditions deviate from initial estimates.

The Shell-operated Perdido spar at Alaminos Canyon block 857 receives production from direct vertical access wells and four offset production manifolds tied back to the host via three subsea flowlines. All production is routed into five caissons driven into the seabed directly under the spar. Each caisson features a gas-liquid cylindrical cyclone (GLCC) separation unit at the mudline and three concentric top-tensioned risers. The gas flows up the outermost annulus and electrical submersible pumps (ESPs) lift the produced liquids the middle annulus.

The innermost riser primes the pump from the topsides and keeps the pump operating at its minimum rate in early life when production is not sufficient to do so. The topsides arrival facilities feature primary separation of produced gas, treatment, and compression for export, separation of oil and water, and degassing to bring the oil to export specification. Primary separation vessels are provided for each caisson before the produced fluids are combined for treatment. Oil from any of the production separators, hull tank or the export quality dry-oil tank can be used for pump priming operations, displacement operation on the regional flowline via the regional service line, or for tightline operations from the production separator for well testing. A set of recycle pumps are available on topsides with the requisite control system programming to line up any of the recirculation sources to any of the destination caissons, topsides arrival separators, or pipelines.

The field design presents several operational challenges, such as the subsea caisson separation combined with artificial lift using an ESP, multiple hydrate management strategies including blowdown for the shorter flowlines and displacement via a service line for the regional flowline, and pump startup operations using the 2 7/8in downcomer. Like all deepwater subsea production systems, hydrate and slugging management were key issues that needed to be addressed. However, the Perdido concept pushed traditional flow assurance design simulation tools beyond their operational envelope.

The Perdido team needed a fast, stable, intuitive multiphase transient simulation of the subsea kit, including multiple ESP-based mudline separation and pumping systems, along with the closed-loop controls and the topsides arrival vessels. The intent was to realistically model operational procedures and recovery from upsets with a range of fluids and process conditions throughout the life of the field.

The team set out to gain confidence in the critical operational characteristics of the field, whether using artificial lift, subsea processing, or multiple interacting tiebacks facilitated by a simulation tool that allowed natural operation of the field backed by time-accurate physics representing realistic flowstream composition effects. MSi Kenny built the customized Transient Engineering Simulator (TES) that was later tied in as the physics package for the Operator Training Simulator (OTS). The high-fidelity transient physics package modeled the entire asset from the sand face in the wellbore to primary separation and recirculation facilities on topsides using MSi Kenny's multiphase transient simulator PIMS and employed a custom-built user interface for running engineering simulations. For the OTS, CAPE Software provided the data traffic manager to push and pull data from the physics package and the various control system emulations, while gluing together the Human Machine Interface and data historians into a credible replication of the actual offshore system.

Simulator challenges
While the modeling framework included pipelines and pump models, the ESP model was specially developed for the Perdido application. Performance curves were acquired from the ESP vendor to be integrated into the engineering simulator. The data were represented in the model as a lookup table with four independent variables: specific gravity, viscosity, flow rate, and speed. The output of the tables included pump and motor behavior, as well as other parameters critical to the operation and longevity of the ESP such as thrust bearing loading.

Each ESP is driven by a variable frequency drive (VFD). The VFD manages the AC frequency to the pump to get the required flowrate while protecting the pump from resonance frequencies, high amperage, etc. Since the ESPs are integrated into the pipeline models, the active control for the pumps must also be handled by the model. The properties of the pumped fluid and its operational boundary conditions are used to calculate the pump parameters, which are then fed though the control logic to prevent damage to the pump, just like in the field.

Three concentric tubing strings in the top-tensioned risers exchange heat with each other. The cool gas rises up the outer annulus, while the oil-water mix gains some of the waste heat as it goes through the ESP. The heat transfers from the middle annulus carrying the upwardflowing pumped liquid to the outer annulus filled with gas and on through to the outer wall and insulation to the surrounding cold water.

As the fluids rise to shallower depths, warmer seawater temperatures can reverse the direction of heat flux. When liquid is pumped into the downcomer, the innermost string, it commonly enters hot at the top and loses heat to the rising gas and liquid columns in the annuli, therefore cooling down as it reaches the caisson.

A bundled heat transfer model captured these interesting transient thermal effects.

The Perdido operation poses new challenges for simulating forward and reverse flows and to seamless operator training. On an ESP shutin, the liquid flowing into the caisson can flood the gas riser if the wells are not shut down immediately. Upon restarting the ESP and production, liquid drains from the gas riser while production gas flows counter-current up the riser.

Another example is when two caissons interact. While under normal conditions caissons do not interact with each other directly, the operating procedure allows this to happen in certain cases. For example, when an ESP fails and that gas riser floods, it is possible to override the manifold valves such that the ESP in a neighboring caisson is able to draw the flooded riser column down. In this case, not only does the gas riser have to back flow liquid, the caisson, jumper, and manifold valves all have to allow backflow. At the start of this procedure, when the manifold valves are first open, the liquid column ties to the other gas riser, creating an unbalanced U-tube with purely gravity-driven flow. These types of phenomena are generally not encountered in multiphase pipeline simulations, but the ability to simulate and validate system behavior during these operating procedures was required for the Perdido system.

The Perdido facility can pump liquids from the topside facilities into the caissons and also into the regional service line. As liquids flow down the vertical pipe, different flow regimes develop. To match expected field behavior, new methods were developed for determining and handling these flow regimes. In low flow scenarios, the liquids tend to run down the sides of the pipe as a sheet. The force of gravity is exactly balanced by the drag from the wall on the outside and the gas on the inside of the annular liquid column. As the flow rate increases, the flow will tend to irregularly bridge as the layer along the walls becomes sufficiently large, ie the gravity overtakes the frictional forces.

Fluids are produced from multiple wells and commingled into several pipelines. Since the properties of the fluid within the pipeline affect the predictions from the pipeline, it was imperative that the composition of the fluids be tracked.

The separation of gas and liquid at the mudline at dynamic pressures ranging from 650psig to 1500psig results in varying bubble points for the liquid pumped up the riser, and this determines where gas breaks out in the riser. This has a significant impact on the hydraulics and slugging potential of that riser. A fit-for-purpose composition modeling scheme based on pseudo-components and mixing rules was developed so accurate bubble-point predictions were achievable at very little computational cost. The new model accurately predicts bubble points throughout the system, simulates non-equilibrium gas reabsorption into undersaturated oil, and is still was capable of running 20 times faster than real-time on a laptop PC.

In addition, the composition tracking allows a user to watch the front formed as fluids from one well or recycle point displaced fluid already in the line. Armed with this knowledge, the user can predict and plan hydrate prevention actions needed during startups and shutdowns.

Engineering simulator
The simulator has been in use since early 2009 to validate operating procedures. The simulator features several starting conditions including steady state operation at various points in field life with varying reservoir conditions, well counts, and compositions. Subsea engineers used the simulator to build other operating conditions such as packed up flowlines resulting from a shut-in, blown down lines after shut-in, and water-filled startup conditions both for developing procedures as well as for operator training.

Including closed-loop control programming enhances the utility of TES. This goes beyond the standard PID loops for controlling levels, flowrate, and pressure.

The TES includes a ramp controller for the well chokes with associated ramp rate limits, ramp controllers for the pressure setpoint of the gas riser inlet flow control assembly, VFD frequency control with the associated limits on resonance frequencies, high amperage, etc., and complicated control override logic on recirculation system where the source vessel level inputs are dynamically switched.

This has allowed engineers to practice with the operation of the logic and tune loops prior to field startup and later test the tuning in the actual control system programming using the OTS environment.

TES simulates coupled effects of thermal, pressure, and compositional transients, and allows the user to concentrate on the behavior of the various parts of the system while running the simulation in exactly the same way as the field is operated. This effectively gives an opportunity to study an extensive range of data to understand how the system operates, such as oil and water holdup changes and slug formation throughout a pipeline instead of just a pressure and temperature at the endpoints.

The ability to see how deviations develop and decay is critical to understanding how to optimize the operational procedures, as well as how to deal with unusual and potentially risky operations. Learning how to avoid degenerate circumstances helps guard against them, but even more valuable is to practice mitigations in a realistic simulation to optimize and game out responses.

A key operation is sweeping the long uphill regional flowline with cold dead oil after a shutdown, since it cannot be blown down to avoid the hydrate region. Various approaches were investigated relating timing of the sweep and flowrates employed at different stages to removal of water and gas from the line while not causing dramatic shocks to the subsea separation and ESP, which are then passed on to the topsides. The goal being to keep the system from tripping and allow it to process the swept oil as smoothly as possible.

Operator training simulator
Besides the value in providing a high-fidelity physics-based training to operators ahead of field startup on a first-of-a-kind production system, the integration of the rigorous subsea model into the operator training simulator allowed verification of the control system programming for both the topsides process control system and the ESP lift control system well before field startup. Given the high intervention costs and associated production deferment in case of failures, the value of being able to identify and resolve potential issues before startup was very high. Realistic simulations also provide the Perdido partners with a higher degree of confidence that their investment in novel subsea equipment is well protected.

Operator training occurred in multiple phases. First the basics of the Perdido development and particularly the subsea operations were presented, with special emphasis on using the ESP and subsea separation caisson. Operators also received computer-based training on using Yokogawa's control system interface.

Training also introduced unusual aspects of the system, including complex interacting flow, pressure, and level controllers on the recycle system.

Hands-on sessions at the cluster of four workstations making up the student interface to the OTS were intermixed into the classroom instruction. The trainers could affect the system behavior by pinching valves not visible to the operators to simulate a plugging flowline, for instances.

The interaction with the OTS was not scripted beforehand, an advantage to having a high-fidelity physics model over a cause-and-effects model that perhaps just pieced together steady-state calculations. Instead, field scenarios were loaded and gamed out with one or more small groups of operators running various parts of the Perdido system, such as one group simulating a shutdown of one caisson requiring blowdown of a flowline, while another group ramped up wells and dealt with changing composition and viscosity impact on the ESP.

By using innovative modeling techniques, MSi Kenny was able to deliver on the high-fidelity requirements of a subsea pipeline simulation package, and the speed, flexibility and robust operation required of an engineering simulation tool in one integrated package. The Perdido Transient Engineering Simulator, functioning as both a standalone simulator as well as the physics package of an operator training system, has provided value as a tool to validate operational procedures and for operator training prior to facility startup and will continue to provide value throughout the life of the facility. OE

The authors would like to express their appreciation for the support of their employing companies and the many people involved in this aspect of the Perdido development, as well as to the Perdido development partners, BP, Chevron and Shell, for permission to publish this article.

Kellin Nelson has worked for MSi Kenny since 2005 as a maintenance engineer and now manages the testing and implementation group. He was the MSi Kenny project manager for the Perdido Transient Engineering Simulator development. He holds a BS from Purdue University.

Prasanna Parthasarathy has worked for MSi Kenny since 2002 as a modeling engineer and now manages the software solutions group. He worked closely with the modeling team on Perdido Transient Engineering Simulator development. He holds a BS from the Indian Institute of Technology (Madras), and a master's degree from University of Illinois.

Howard Littell has worked for Shell since 1991, starting in research, then onshore facility engineering, offshore gas pipelines, and for the last six years in the deepwater projects organization. He holds a BS from Mississippi State, and an MS and PhD from Stanford, all in mechanical engineering. He joined the Great White/ Perdido project as the flow assurance lead in 2005, and served as the process system engineer for the artificial lift selection, design, implementation, and startup.

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