JIP Prepares for Final Shallow-Water Test of Subsea Power System
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A joint industry project (JIP) comprising ABB, Equinor, Total, and Chevron, is developing technologies for subsea power transmission, distribution, and conversion. The output will form a critical part of future advanced subsea-field developments. Begun in 2013, the project reached a major milestone in late 2017 when the first full-scale prototype of the variable-speed drive (VSD) passed a shallow-water test (Fig. 1). Final preparations are now underway for a 3,000-hour test of the complete subsea power system with two VSDs in a parallel configuration combined with subsea switchgears and controls. The complete paper highlights elements of the technical development and an overview of the primary building blocks of the system, and presents in detail some of the challenges in developing, designing, and testing the control system.
A subsea power transmission and distribution system will enable an entire oil or gas production system to be placed directly on the seabed, allowing expansion of development to deeper and more-remote locations while yielding cost and safety benefits from reducing significantly, or even eliminating, the need for topside installation.
The JIP is developing three products for the system:
- Subsea VSD
- Subsea medium-voltage (MV) switchgear
- Subsea control and low-voltage (LV) distribution
Providing a technical solution that is realistic, possible to engineer, tolerant of extreme environments, and reliable in its performance presents a significant challenge. The equipment—the MV switchgear, control and LV distribution, and the VSDs—must be able to run without intervention for many years. The equipment is qualified for water depths to 3000 m and will have capacity of up to 100 MW with a transmission distance of up to 600 km.
The primary focus thus far has been to qualify the basic building blocks to serve the typical voltage and power ratings for subsea processing. All project-qualification activities follow the recommendations and technology readiness level (TRL) stages defined in DNV Recommended Practice (RP)-A203, applicable for components, equipment, and assemblies in hydrocarbon exploration and exploitation offshore. This RP provides a systematic approach to ensure that the technology will function reliably within specified limits, and it provides a common understanding and terminology of technology status and risk management. Other important aspects of the RP include the ability to identify required design changes at an early stage and to improve confidence in the new technology by close interactions and traceable documentation.
To ensure compact and reliable solutions, oil-filled pressure-compensated tanks are used for enclosure of the VSD and switchgear. All components are tested extensively under the full pressure they will experience at the target water depth. A high-level objective of the project is to design the equipment to minimize production downtime and the number of retrievals.
Target Requirements and Project Objectives
The base case for the JIP is a subsea power system with a subsea switchgear feeding four VSD loads at a water depth of 3000 m. In general, existing requirements applicable for topside systems and equipment apply, as well as API Standard 17F for subsea production control systems. Equinor’s technical and professional requirement TR3025 was developed in parallel with the project and specifies requirements applicable for subsea systems and equipment (e.g., system-design margins and equipment-immunity levels).
While the JIP has developed packaging technologies to enable robust and cost-efficient power distribution and conversion for subsea, the final goal of the project is to instill confidence that the developed products are ready for use. Before taking the steps into commercial pilots, the JIP partners will make sure all relevant risks are addressed and mitigated sufficiently by the technology-qualification program. The subsea industry has adopted a TRL system for a common understanding of the development stages in the qualification process, and the degree of testing required to reach each stage. Compliance initially entailed breaking down the overall subsea power system into separate manageable technology parts and classifying these in terms of novelty. The project is currently progressing from API ratings of TRL 3 to TRL 4 with full-scale prototypes that include a four-feeder subsea switchgear, two MV subsea power drives with input transformers, and a subsea-control module with the subsea protection relay. Each prototype has undergone tests at various assembly stages, and all are now prepared for launch into shallow waters for a final 3,000-hour high-power demonstration. The objective is to test and prove the thermal properties and the marinization of the equipment. The final TRL assessment will be made by JIP subject experts before the commercial launch scheduled for the second half of 2019.
Subsea Electronic Module Qualification
The subsea control system is an integral part of the overall system and consists of main assemblies for power distribution, conversion, auxiliary supply, and control for a high-reliability, ruggedized subsea electrical power supply for longer distances and deeper waters. The power-delivery system is dependent on an advanced automation and control framework that will create the ability to control, monitor, and protect the entire system.
The control system for the JIP is based on established control solutions and products, but needed to be upgraded and modified significantly to be qualified for use in the subsea environment. It also needed a completely new enclosure design and an advanced penetrator solution. While some existing subsea electronic modules are used today for subsea well-control applications, the control system designed here will offer more-advanced functionality. The controller will be significantly more powerful than existing subsea controllers, and the amount of additional equipment, communication, and wiring will be much greater. Additionally, the new control topology will make use of large-scale fiber-optic signal distribution between subsea units to enable digitalization and will provide an important building block for eventually realizing a complete subsea factory.
The challenge for the control solution is to validate that the control design can fulfill the environmental requirements required by the oil industry in a range of industry standards and to qualify these units for use. Software that enables all systems to work together will require significant development and validation.
The JIP followed a stepwise development process from the concept stage to system testing. The framework for the subsea control and protection system is a powerful and advanced controller developed for subsea use. The development work began with an evaluation of whether the existing controllers could be deployed with only minor modifications and some ruggedization, but it became clear quickly that this would not provide a satisfactory or long-term reliable solution. Modular reuse of a standard power electric controller was adopted instead, with a new, updated printed circuit-board-assembly form factor and a new electronic layout design. The complete paper provides a detailed discussion of several rounds of environmental development testing and the resulting design updates for single electronic boards and assembly design concepts, in which multiple boards were combined to create a complete controller.
The results and experience obtained during the environmental testing of different electronic boards established the foundation for a set of general rules and guidelines for design, layout, mechanical mounting, and support of electronic boards for subsea use. These guidelines served as central input into the development of the final prototypes and product. The controller has been subjected to testing beyond the requirements without any issues.
Subsea VSD and Switchgear
The subsea VSD is the key equipment developed to control the speed and the torque of the subsea motors for seawater injection, boosting, and compression applications. Pump motors are primarily of the induction type, but permanent magnet types are also foreseen in special cases.
The subsea VSD features a completely pressure-compensated design. The enclosure is oil-filled, with the hydrostatic pressure of the seawater acting on the oil to maintain ambient pressure inside the enclosure. All power components are cooled by the natural convection of the oil, transferring heat to the surrounding sea. The oil also serves as electrical insulation and meets the specifications for the integrated multiwinding transformers.
A final 3,000-hour shallow-water test in a sheltered harbor is planned to be competed by the end of 2019. Two prototype versions of the VSD will be operated in a parallel configuration at full-rated current, in a so-called power-in-the-loop setup that also includes the subsea switchgear prototype. Different load conditions allowing variable output frequencies will also be tested in the same setup. The main difference between the two prototype VSDs lies in different cooling designs of the tank walls. The thermal performance of the VSDs at various loading conditions will be investigated thoroughly by measuring how the temperature distributes throughout the units. There will first be a thermal run-to-stability test at nominal power, and, after checking operating point sets and overload conditions, both drives will run at steady and cyclic loads for a longer period. Parts of the test will be application-specific. These will cover various redundant communications, black-start sequences, verification of ride-through (energy storage), protection-setting adjustments, fast-load shedding, and operation of the drives with one or more cells out of service. The prototype equipment and the electrical infrastructure will be controlled using the ABB Ability Control System 800xA, which provides a single interface to the entire system.
JIP Prepares for Final Shallow-Water Test of Subsea Power System
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