Modeling in the deep

September 1, 2015

An approach to modeling and simulation combining subsea electrical and fluid systems can facilitate a better understanding of the interaction of long-distance tiebacks supplied by step-outs. ABS’ Milton Korn explains.

Subsea pipeline. Images from ABS.

The great distances and difficulties in placing equipment and systems associated with subsea production exert extraordinary pressures on equipment/system installation and commissioning to “get it right” the first time.

The cost of rework and lost production can be compounded by environmental and economic extremes, which pose major challenges as evolving subsea concepts transition into the field. Failed installation can be fixed, but a failed design places additional impediments in the way of successful operation.

Several years ago, ABS started developing simulation capabilities as part of its technology program. The resulting modeling and simulation proficiencies provide insight into equipment/system design and operation, including subsea power systems.

Applying simulation in the electrical and thermal fluid domain provides a means of exploring aspects of equipment and system that were not possible previously – until a system had been installed and commissioned. These simulation capabilities can be used for subsea operations as well as operations on marine and offshore assets that deploy sophisticated and complicated power systems that were not imagined a few short years ago.

Advanced power systems

Voltage variation along the length of an unloaded transmission line at fixed frequency.

Subsea power system requirements are challenging the functional limits of traditional equipment and systems. The velocity and magnitude of change to equipment and systems have raised concerns about the reliability of power systems that incorporate these new technologies and the adequacy of the analysis techniques used to predict equipment and system performance.

ABS models are being developed to study equipment and system performance under a variety of normal and fault operating conditions. These same models will also allow engineers to study the optimal coupling point of energy storage systems. Optimizing the coupling point of energy storage systems offers the opportunity to reduce capex by providing the potential to justify reduction in the size and number of onboard generators. Opex and emissions can be reduced using this approach because it provides the opportunity to run the minimal set generators at their lowest cost/emissions point.

 The dependence of transmission line voltage on both time, position along the transmission line and frequency.

Subsea power systems research

Pumping stations and compression stations are located along the tieback to achieve the desired flow for the piping system. Linear step-outs supplied from shore are coupled to the tieback at multiple points along its length to deliver electrical energy at the required rate and appropriate quality for pumping and compression stations.

Subsea modeling gives engineers a way to individually and collectively evaluate the operation of the electrical power systems, tieback and the pumping/compression stations during normal operation scenarios such as startup, acceleration, production and shutdown as well as during transient events. This approach also allows for examination of anticipated system performance during fault events and allows for correction and optimization prior to deployment. Correction prior to deployment provides the opportunity to reduce the risk of catastrophic failures such as an environmental incident, personal injury, damage to equipment or loss of production.

The ABS Modeling and Simulation team explored the transient nature of energizing subsea transmission systems, including capacitive charging current, the Ferranti effect and the dependence of transmission line voltage on both time and position along the line. Working from the basic performance criteria of the coupled electrical and fluid system, engineers used the characteristics of the fluid being transported (specific gravity, viscosity and temperature) to size the tieback to achieve the required mass flow rate with acceptable flow velocity, maximum pressure and pressure drop. The hydraulic power required to achieve the desired flow rate was calculated and with it an approximation of the overall electrical power required.

Distributing the pumping stations and hydraulic power along the length of the tieback facilitates maintenance of fluid parameters within the parametric bounds of maximum pressure, pressure drop and fluid velocity. Using a cost function, the selection process also can be optimized for capex and opex.

If the pumping system is to be designed for N+1 operational criteria, further iterations of the process must be performed to facilitate locating and sizing pumping stations so sufficient energy is present for the fluid to bypass a pump station and remain within the parametric bounds of maximum pressure, pressure drop and fluid velocity.

Fixing required hydraulic power at various points along the tieback allows pumps, motors and drives to be selected and defines the electrical power required at each pumping station as well as the overall electrical power required from the step-out.

The length of the step-out and the unit cost of the cable itself make it unlikely that a uniform cable will be selected along the step-out length. At the source (shore) end, the cable will be sized to deliver the full electrical load of the system. Downstream of the first pumping station, the cable will be tapered to supply the remaining pumping load during both normal and N+1 operations.

Step-out cable development cannot take place in a vacuum. Engineers must consider the influence of the other elements of the subsea pumping system. Cable design must take into account the desired nominal operational values of voltage and frequency as well as behavior during the transient energizing period, the capacitive charging current and the operating loads.

Transient response and resonance at the receiving end of transmission line during switching events.

Subsea pumping system operation

During operation, power quality at each tap point is dependent on the total system load as well as the specific loading of individual pumping stations. Pumping stations interact with one another. Variations in discharge pressure of upstream units influence the suction pressure of downstream units. Separating pump stations with subsea pipeline segments of varying lengths complicates pumping station control where hydraulic signal propagation speed is limited to the speed of sound in the pipeline fluid. Overall pipeline flow may be set by a master or supervisory control system. At each pump station, local controls work with the master set point to achieve the desired flow while working with pump specific limits for suction and discharge pressure control.

The transfer of pumping/compression load from one station to another can impact not only the total system load, but can result in the potential excursion of power quality outside acceptable values. The effects of excursions can be mitigated by installing equipment that can operate in a diverse power quality environment or by using line compensation equipment to actively stabilize power quality.

Moving from concept to reality

The ABS Modeling and Simulation project, which incorporates model-based design, is part of ongoing multiyear research targeting subsea power systems, power systems associated with dynamic positioning vessels, and other innovative technologies that are rapidly being rationalized and introduced into the offshore sector.

The results of ongoing ABS efforts can be used to optimize the subsea pumping system cost function and deliver insight into the complexities of maintaining control of both the hydraulic and electrical domain to provide reliable pumping system operation.

Refined models include more of the hydraulic, thermal and electrical characteristics of the coupled systems to increase simulation fidelity. ABS hopes to further these efforts through the formation of a joint industry project that will focus on practical simulation technology for marine and offshore applications.

Milton Korn is ABS Managing Senior Principal Engineer. Korn leads the Electrical & Controls Group within the Offshore Technology Division and manages research on advanced techniques for structural health monitoring and wireless subsea sensor networks. Korn holds a BE in electrical engineering and a BS in computer science/mathematics from the SUNY Maritime College. He received his MS in electrical engineering from the Polytechnic Institute of New York.

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