Medium-Voltage Drives Shine Offshore

Less numerous than their ever-present low-voltage "cousins," medium-voltage (MV) ac drives are out there in growing numbers, doing heavy-duty motor control in diverse, rugged applications. They span industries from mining, sugar-cane processing, and liquefied-gas handling stations to fluid-power system test stands, fan/pump flow optimization in power-generating plants, and control of mobile off...

By Frank J. Bartos, P.E. May 1, 2006
  • Variable-frequencydrives (VFDs)

  • Medium-voltage (MV) ac drives

  • Variable-speed improves energy efficiency

  • Electric drives for harsh application

  • Offshore gas/oil engineering

MV equipment onboard

Less numerous than their ever-present low-voltage “cousins,” medium-voltage (MV) ac drives are out there in growing numbers, doing heavy-duty motor control in diverse, rugged applications. They span industries from mining, sugar-cane processing, and liquefied-gas handling stations to fluid-power system test stands, fan/pump flow optimization in power-generating plants, and control of mobile offshore oil/gas drilling rigs. A common theme in all this is variable-speed control of motors to match the connected load—thereby enabling large energy savings plus more efficient power conversion at higher supply voltage. MV operation allows higher power output or lower current usage at the same power.

Deep-water oil/gas drilling platforms represent a particularly severe working environment for electrical equipment, including motor drives. Wind and wave action, salt-water spray, and remote locations make reliability and maintenance of equipment critical issues. A novel application of this type in further explored here.

Working in open seas

One of the most modern oil drilling rigs built in the last few years, named Stena Don, started operation in the North Sea in early 2002—tasked with opening new oil fields as well as maintaining existing ones. Designed to work in water depths up to 500 m (1,640 ft), Stena Don measures just under 100-m high from the keel to the top of its tower and has a platform area of 67 x 72 m (220 x 236 ft).

Robust construction of the drilling rig—a semi-submersible mobile platform—reportedly meets top international requirements for safety and environmental protection, making it usable worldwide. Norwegian classification company Det Norske Veritas (DNV) has designated Stena Don as Category DP III, which meets the highest requirements of three Dynamic Positioning classes defined by DNV. Contractor for building Stena Don was the Scottish firm Stena Drilling Ltd., a wholly owned subsidiary of Stena AB of Sweden. The company is a notable independent drilling contractor with worldwide projects, claiming pioneering work for leading-edge technologies and innovations in the drilling world.

No anchors needed

One of Stena Don ‘s most noteworthy features as an automated drilling platform is that it does not require any form of anchor apparatus. The entire deck construction, including drilling tower, drill equipment, and helicopter pad, rest on two connected pontoons, each roughly 43-m high and 95-m long. Mounted under each pontoon, three azimuth thrusters keep the platform on planned position—even under extreme weather conditions—by means of a dynamic, satellite-based positioning system.

Each thruster is driven by a variable-speed, high-voltage motor fed by a Siemens Simovert MV frequency converter or variable-frequency drive (VFD). “The six azimuth thruster drives had to be able to fix the platform in its desired location to the nearest meter without anchorage, and also to maneuver the platform from one operating location to another as quickly as possible,” says Björn Rasch, MV drives product manager at Siemens A&D Large Drives Division. “All drives on the platform adopted every potential means of saving energy.” (Numerous low-voltage VFDs and motors also are used to handle various other functions on the drilling platform.)

Siemens AG supplied equipment for the full electro-technical outfitting of Stena Don . This includes drive engineering, automation and process management engineering, electrical energy and power distribution system, and the platform’s telecommunications equipment. (See “equipment onboard” table for key MV-technology elements supplied.) Project management, coordination, and engineering functions were handled by Siemens’ Oil & Gas Business Department in Oslo, Norway.

Moreover, the drilling rig’s entire electrical power supply (from generation to distribution) required automation, allowing continual adjustment of generated power to the needs of specific equipment. Appropriate synchronization devices ensure optimal load distribution among multiple diesel generators; powered equipment incorporate automatic surveillance and protection functions. In addition, equipment delivered meets the high demands of climatic and external conditions, explains Rasch.

Greatest challenge

The six azimuth thrusters that hold the drilling platform in place, without the need for anchors, and drive it from one operation site to another, posed the greatest challenge from a drive technology viewpoint. “Both tasks require an immense amount of dynamics and flexibility from the drives, which today can only be fulfilled by variable-speed electric drives,” states Rasch. A satellite-based “Dynamic Positioning System” (DPS) is used for navigating and positioning Stena Don. DPS provides signals required for precise control of the MV drives and helps to move the rig rapidly between drilling sites. Platform transit speed is 8 knots.

Each thruster drive system consists of three main elements: 3.3 MW variable-speed high-voltage motor, Simovert MV variable-frequency drive, and Geafol cast-resin converter transformer.

  • Motors are of vertical design, water-cooled, three-phase ac asynchronous 6-pole machines running on 4.16 kV input, with protection category IP54 and ship classification from DNV.

  • Simovert MV converters (VFDs) rated at 4,000 kVA/4.16 kV are also water-cooled units and come with IP54 protection. To save installation space, the VFD’s heat exchange subsystem has redundant construction and is separately located.

  • Main input for the drives comes from Geafol power converter transformers rated 4,200 kVA, 11 kV/2 x 2.2 kV, having protection category IP23 (protective casing).

  • VFDs and converter transformers incorporate design for shipping with certification from DNV.

  • Nine diesel generators provide the drilling platform’s power supply of 11 kV at 60 Hz.

Siemens’ power management system provides automatic control and monitoring of Stena Don ‘s power generation and distribution network. It includes protection measures against events such as power surges, over-voltage, or short-circuits. Comprehensive control and monitoring enhances the potential for energy saving from use of variable-speed drives.

Variable-speed, vendor selection

As Stena Don was a newly built rig, this application represented a new project for MV drives. Key to realizing energy savings inherent in VFDs was variable-speed control of the thruster propellers. This departs from traditional power-wasting design, where hydraulic control of propeller-blade pitch changes the rig’s speed, while the motors continue to run at constant speed.

Variable-speed control of the thrusters with MV drives allows matching propeller speed to specific conditions. For example: run all six thrusters to obtain maximum transit speed for moving between drilling sites or to hold position in severe weather, or just “idle” with one or two thrusters in calm seas. Stena Don ‘s MV drives and controls enable those and other scenarios. MV drives typically save up to 50% the power of constant-speed thrusters—depending on the application—suggests Rasch.

Basis for competitive selection of Siemens MV drives was complete delivery and experience with drilling platform projects. “Siemens offered complete delivery with all technology needed. The customer did not wish to split technology between vendors,” he says. Completeness extends to diesel generators and the overall automation system. Siemens also claims extensive system integrator experience in offshore platforms and related applications.

MV drive specifics

The power section of Simovert MV drives employs high-voltage insulated-gate bipolar transistor (HV-IGBT) modules as power-switching devices. They allow VFDs to operate without the need for additional power circuits. The result is a MV drive with simple, modular construction that saves space and promotes ease of use and maintenance. “The MV drive proved itself to be tremendously advantageous for use on the open sea,” says Rasch.

Simovert MV drives use high-performance field-oriented control (FOC)—also known as flux-vector or transvector control—originated by Siemens. Using closed-loop FOC together with HV-IGBT modules in a three-level inverter circuit arrangement ensures the generation of sinusoidal motor currents without the need for a sine output filter. “This leads to reduced motor losses and minimizes motor torque fluctuations, which substantially lower stressing of the entire power transmission line,” he continues.

In addition, HV-IGBT modules offer the possibility to create intelligent crossbar switches, which guarantee optimal protection of the VFD and motor, explains Rasch. “The modules serve to limit or switch off short-circuit currents without the need to reduce power.”

Technology, user benefits

Besides the benefit of energy savings and efficiency through variable-speed operation, Siemens says its Simovert MV drives offer either air-cooling or water-cooling to suit specific application needs or customer choice. Implementing redundant cooling designs maximizes ease of service and offers extremely high plant availability, according to the company. High level of synchronization between VFDs and other Siemens components also benefits the tough operational conditions encountered on oil-drilling platforms.

As for the user, MV drives (and other Siemens equipment supplied) met the platform’s special requirements: space constraints, system functionality and availability, low maintenance, and ease of operation. Compact design of Simovert MV drives enabled optimal use of available installation spaces. In particular, it eliminated extra components, such as special output filters, because of the degree of synchronization between motors and VFDs. In addition, optimal coordination between the navigation system and drive controls reportedly maintains platform position within one meter, even in severe weather.

New generation MV drives

Even very capable automation products undergo evolution. In 2006, the highly successful Simovert MV line is being migrated to the next-generation medium-voltage drives—Sinamics GM150—explains Rasch. GM150 incorporates advantages of its predecessor in a newer design, along with Siemens’ new Sinamics drive control platform. “A hallmark of Sinamics control is a unified look-and-feel for users of variable-frequency drives from 230 to 7,200 V supply and power ratings from 0.12 kW up to 30 MW,” adds Rasch.

In the low-voltage sector, Sinamics controls already count thousands of units working in the field. In the oil and gas industry, Siemens has awarded several projects with new Sinamics MV drives for thruster applications, compressors, and pipeline pumps.

With the potential for energy savings and efficient power conversion that medium-voltage drives offer, implementations are bound to increase on land and sea.

MV equipment onboard

For the Stena Don project, Siemens provided these key medium-voltage elements:

Diesel generators—9 units rated 4,375 kVA;

Power distribution—3 units rated 5,000 kVA;

Transformers—6 units rated 800 kVA;

HV switchboards—3 units 8DC11, 11 kV;

Thruster drives
—6 Geafol converter transformers (double-tier) 4.2 MVA;
—6 Simovert MV drive converters (VFDs) 4.0 MVA;
—6 H-modul high-voltage motors with 3.3 MW shaft output;

Drilling drives—3 Geafol converter transformers (double-tier) 3.0 MVA; and

Automation system—including process management, power management, dynamic positioning, and telecommunication systems.