Despite its notable values, deployment of subsea processing can present additional complexity. To ensure maximum return on investment, careful consideration must be given to the overall system design and operating strategy. A thorough understanding of system behavior is fundamental to establish a suitable system design and operating strategy. To achieve this, modeling of the multiphase flow in the system is performed to ensure changes in fluid behavior that can be detrimental to system operability are fully understood.

Traditionally, multiphase flow modeling implements a simplified boundary, where one component is isolated from another. This approach, however, dismisses the interdependency within the overall system and thus potentially provides unrealistic behaviors. Unlike the traditional approach, full-field integrated modeling defines a static boundary at the reservoir and arrival facility where pressure is relatively stable over long periods of production. The full system components are incorporated in one model that takes into account their interdependency, thus providing more realistic system behaviors.

Unrealistic behavior with simplified boundaries
The following example demonstrates the unrealistic behaviors observed with a simplified boundaries modeling approach. A system with a multiphase pump at the riser base anticipates riser slugging on turndown mode. Multiphase flow modeling characterizes the slugging behavior. The simplified boundaries modeling approach simulates the riser as a standalone component with outlet pressure and inlet flow rate set as inputs.

A full-field integrated modeling approach incorporates well inflow, manifold, flowline, riser and pump station into one model. Inlet and outlet pressures are set at the reservoir and topside, respectively, and the flow rate is adjusted by varying the well choke opening. The model also incorporates the pump controller where pump operability is kept at safe limits by altering the pump speed.

Though similar in maximum amplitude, different slugging frequencies are clearly observed. With the full-field integrated model, slugging is less frequent due to the effect of controller in the pump station. Suction pressure is controlled by adjusting the pump speed to respond to the disturbances entering the station. With this configuration, the discharge pressure (i.e., riser inlet pressure) is stabilized and able to dampen the instabilities entering the riser.

Observing the slugging behavior drives further design considerations such as slug catcher control, riser structure integrity and pump controller design. This highlights the importance of obtaining realistic fluid behavior before any further actions are taken.

One model, one solution
Full-field integrated modeling enables holistic system investigation. It emphasizes “one model, one solution,” evolving throughout the life-cycle phases of a field development. From conceptual front-end detail design until the life-of-field operation, the model evolves, promoting continuity and providing a wide range of information that is relevant across multidisciplinary teams. When used early in design, it enables design screening to identify the optimal field solution. During the detailed design stage, dynamic simulations using the same full-field model are performed to ensure system operability. In the life-of-field operation, it can be used to perform real-time metering and monitoring as well as forecasting as an advisory system.

Conceptual front-end design
The following example describes a conceptual design to identify the optimal solution for a greenfield development. A steady-state integrated simulation model is performed, incorporating all existing constraints and physical information such as:
• Platform arrival pressure requirement and its fluid handling and power capacity;
• Flowline velocity limit and its topography;
• Subsea tree and wellbore capacity, well trajectory, reservoir pressure and temperature; and
• Well productivity index.

The full-field modeling simulation identifies an optimum flowline size and its corresponding backpressure. The model also reveals the flowing wellhead pressure (FWHP). Relating the flowline backpressure with FWHP provides the information of total system production.

For the system with subsea processing, the effect of differential pressure due to boosting is taken into consideration, revealing the increment of achievable production as compared to natural flow. As a result, production profiles for the different options can be benchmarked.

Economic analysis is then performed to identify the most optimal configuration. Production rate is regarded as revenue, with subsea equipment and flowlines as capex and utilities like power and hydrate inhibition as opex.

It is understood that simplified boundary modeling could conclude a similar benchmark exercise, but to do so would require a significant number of simulation runs and iterations.

Detail design
At the detail design stage, system details including control and operational philosophy have been defined. The existing simulation model from the previous phase is evolved to include the process controller to investigate the system dynamic behavior.

For example, production from three subsea wells is commingled in the manifold and sent to the arrival facility via a flowline and riser with aided pressure from the subsea multiphase pump station at the riser base. Special focus on turndown operation is presented to highlight the implication of operating procedure on pump station operability.

Turndown operation investigates the holistic system responses following the shut-in of one well. Fast turn-down investigates the system response following well shut-in instantaneously, while slow turndown investigates the gradual well shut-in.

Following turndown, decreases are seen in the total flow rate, backpressure, pump suction pressure (as pump differential pressure stays about the same if not increasing), pump torque set-point and pump speed, while gas volume fraction (GVF) at the suction inlet increases.

On fast turndown it can be seen that suction pressure is expected to degrade before pump speed starts to reduce. This is due to the slow response of the pump speed controller to reduce the pump torque set-point. This dynamic behavior results in the breakout of vapor from the fluid entering the pump. Rising GVF entering the pump might be of concern as it increases the temperature rise across the pump. If it is in a rapid amplitude and period, overheating might be an issue. Fast turndown mode represents extreme scenarios such as sudden valve failure and closure.

A slower turndown provides sufficient time for the pump speed controller to react and reduce pump torque set-point continually. This results in a more stable suction pressure, more stable GVF at suction and, consequently, more stable delta temperature rise across the pump.

Full-field integrated modeling enables validation of pump speed controller performance in a holistic perspective. It takes into account the well operating strategy and the dynamic behavior of upstream flowline and downstream riser. Potential concerns following a particular transient scenario also are noted.

Life-of-field operation phase
Full-field integrated modeling can be extended over the life-of-field operation by integrating it with real-time measurements provided by the physical instrumentation. This integration allows an enhanced usage for different real-time applications such as virtual flowmetering, flow assurance and integrity monitoring, and operational advisory systems. Such a system has been successfully implemented on the Ormen Lange Field on the Norwegian Continental Shelf.

In addition, a full-field integrated online simulator for flow assurance and operational advice has recently been delivered for a subsea gas field with gas compression. The full-field online simulator is installed as an advisory tool to overcome the challenging operation that results from liquid surges in the pipeline and the knock-on effect this has on monoethylene glycol distribution and hydrate formation.

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