Bosch Rexroth IndraDrive ML: Universal Inverter for Marine and Offshore Oil & Gas Applications

Bosch Rexroth IndraDrive ML — Modular Drive for Marine and Offshore Oil & Gas Facilities

Offshore platforms, drilling rigs and support vessels operate under the harshest conditions imaginable: salt-laden air, constant vibration, temperature swings from –25 to +55 °C and persistent humidity above 95 %. Any drive failure on a continental-shelf platform means more than downtime — it poses a direct threat to crew safety and the surrounding environment. It is precisely for these challenges that Bosch Rexroth developed the IndraDrive ML inverter family — a modular, high-power variable frequency drive system engineered to withstand the most demanding marine and industrial conditions.

What Is IndraDrive ML and Why Does It Matter

IndraDrive ML is a series of universal variable frequency drives (VFDs) designed to control electric motors ranging from 110 kW to 500 kW per individual module. By connecting up to eight units in parallel, the system delivers a combined output of up to 4 MW. This makes it possible to deploy a single, standardised drive platform across a wide spectrum of tasks — from rotating anchor winches and shipboard cranes to controlling drilling mud pumps and compressor stations.

Unlike conventional industrial frequency converters, IndraDrive ML is purpose-built for marine and oil-and-gas environments. Every module undergoes type-approval testing to the standards of DNV GL, the American Bureau of Shipping (ABS) and Lloyd's Register (LR), confirming reliable operation under elevated vibration, extreme temperatures and electromagnetic interference.

IndraDrive ML Technical Specifications

Key parameters that define IndraDrive ML capabilities for marine and offshore applications:

• Single-module power range: 110–500 kW
• Maximum power in parallel configuration: up to 4 MW (8 modules)
• Supply voltage: 3-phase AC 380–500 V or 525–690 V / 50–60 Hz
• Supported networks: TN, TT and IT (isolated neutral — the standard for marine installations)
• Cooling: liquid or air (depending on configuration)
• Regeneration: braking energy returned to the grid or to battery energy storage systems
• Built-in safety: certified Safety on Board technology (STO, SS1, SLS per EN 61800-5-2)
• On-board controller: Motion Logic for autonomous multi-axis synchronisation
• Certifications: CE, UL, CSA, DNV GL, ABS, LR

The extended 525–690 V voltage range is a critical advantage for offshore platforms, where 690 V is the prevailing grid standard. Higher voltage means lower cable current, smaller conductor cross-sections and reduced energy losses over the long cable runs typical of platforms — from generator to drive.

Why 690 V Is the Standard for Marine Installations

Ships and platforms predominantly use 690 V electrical networks (IT systems with isolated neutral). There are several reasons for this:

1. Lower current. At the same power level, a 690 V network carries 1.8 times less current than a 380 V network. Lower current means thinner cables, less weight and reduced losses.
2. Long cable runs. On large vessels and platforms the distance from the switchboard to the motor can exceed 200 m. At 690 V, voltage drop along the cable stays below critical thresholds.
3. IT network. An isolated neutral keeps the system energised even after a first earth fault — the motor continues to run while the crew locates and repairs the fault.

IndraDrive ML supports both ranges — 380–500 V for onshore industrial facilities and 525–690 V for offshore platforms. Switching between ranges requires no hardware modifications, only parameter reconfiguration.

Modular Architecture: From 110 kW to 4 MW

The defining design feature of IndraDrive ML is its modular construction. A single Power Supply Unit (PSU) combines with one or more Inverter Units to match the application:

• Single-motor drive: 1 PSU + 1 inverter → up to 500 kW
• Multi-axis system: 1 PSU + multiple inverters sharing a common DC bus
• Parallel connection: up to 8 inverters on one motor → total output up to 4 MW

The common DC bus allows braking energy from one motor to accelerate another. On a shipboard crane, for example, energy released while lowering cargo is fed back into the bus and reused to raise the boom. According to Bosch Rexroth, regenerative braking reduces overall electrical consumption by 20–30 % compared with resistor braking.

Marine Applications of IndraDrive ML

The IndraDrive ML frequency converter is deployed on vessels and platforms to power a variety of drive systems:

• Anchor and mooring winches — loads up to 2 MW, handling shock and dynamic loads
• Shipboard cranes — synchronised hoist, slew and luff via Motion Logic
• Pipe-laying reels — precision positioning during subsea pipeline installation
• Drawworks and rotary tables — torque regulation during well drilling
• Pumps and compressors — fuel-consumption optimisation on platforms (see also: VFD setup for compressors)
• Ventilation systems — maintaining climate control in enclosed platform spaces (VFD for ventilation)

DNV GL, ABS and Lloyd's Register Certification

Equipment destined for marine service must hold a type approval from a classification society. IndraDrive ML has completed the full test programme under three schemes:

• DNV GL (Det Norske Veritas — Germanischer Lloyd): vibration testing (IEC 60068-2-6), shock (IEC 60068-2-27), 95 % humidity without condensation, electromagnetic compatibility (IEC 61000-4 series)
• ABS (American Bureau of Shipping): temperature range –25…+55 °C, inclination resistance ±22.5° and roll ±7.5°
• LR (Lloyd's Register): design and manufacturing-process verification against marine standards

Triple certification (DNV GL + ABS + LR) means IndraDrive ML can be installed on vessels and platforms registered in virtually any jurisdiction — from Norway to South Korea.

Marine VFD Comparison: IndraDrive ML vs ABB ACS880 vs Siemens SINAMICS

Several manufacturers compete in the marine and offshore VFD market. A comparison of the key parameters:

ABB ACS880 offers a wider power range at the low end (from 0.55 kW) and more marine certifications, making it a more versatile choice for shipyards. Siemens SINAMICS G150 delivers higher single-module power (up to 800 kW) but is limited to air cooling. IndraDrive ML from Bosch Rexroth stands out with its built-in Motion Logic controller, which enables complex multi-axis synchronisation algorithms without an external PLC — a critical advantage for shipboard cranes and pipe-laying systems.

To help choose between these solutions, see common questions when selecting a frequency converter, and consider using soft starters for auxiliary machinery.

Regenerative Braking and Energy Savings

On offshore platforms, generator fuel costs represent a major share of operating expenditure. Every kilowatt-hour returned to the grid instead of being dissipated across braking resistors translates directly into diesel-fuel savings.

IndraDrive ML supports two regeneration modes:

1. Grid feedback: braking energy is fed back into the vessel's electrical network through the PSU's active front-end rectifier. The generator automatically reduces output by a corresponding amount.
2. Battery storage: for platforms equipped with energy storage systems, IndraDrive ML can charge industrial batteries. This is particularly relevant for three-phase electrical systems where peak loads alternate with idle periods.

According to Bosch Rexroth calculations, regenerative braking on a 1.5 MW anchor winch returns approximately 120–180 MWh to the grid per year of operation. At an on-platform generation cost of roughly $0.30/kWh, this yields annual savings of $36,000–$54,000 on a single drive alone.

VFD Requirements for Oil & Gas Facilities

Frequency converters on oil-and-gas platforms must meet additional requirements related to hazardous-area classification:

• ATEX (Europe): Directive 2014/34/EU for equipment controlling motors in explosive atmospheres (Zone 1, Zone 2)
• IECEx (international): a global certification scheme for equipment in explosive atmospheres, recognised in over 80 countries
• VFD placement: the frequency converter is typically installed in a safe area (clean room), with motor cables routed to an explosion-proof motor in the hazardous zone
• Cable length: at 690 V, maximum motor-cable length without additional output filters is up to 300 m (depending on cable type and PWM switching frequency)

IndraDrive ML is not itself explosion-proof — it is installed in clean rooms or enclosed electrical switchgear rooms. However, its design accounts for electromagnetic compatibility (EMC) in accordance with IEC 61800-3, preventing the generation of interference that could affect the platform's safety systems.

Safety on Board: Built-In Functional Safety

IndraDrive ML features a built-in, certified Safety on Board system compliant with EN 61800-5-2, delivering safety integrity up to SIL 3 / PL e. Key functions include:

• STO (Safe Torque Off): safely removes torque without disconnecting power
• SS1 (Safe Stop 1): controlled deceleration followed by STO
• SLS (Safely-Limited Speed): restricts maximum speed for safe maintenance operations
• SOS (Safe Operating Stop): holds position while monitoring deviation

For shipboard cranes and winches these functions are critical: SLS limits hoist speed as the hook approaches the upper limit, while STO instantly halts the drive when an emergency stop is triggered.