Measurement reduces rudder hysteresis
The BIMS control unit
German company Becker Marine Systems has introduced BIMS (Becker intelligent monitoring system) which it says overcomes delays and improves dynamic positioning (DP) and autopilot (AP) systems.
Input parameters used by most DP and AP systems are, according to Becker, subject to inertia which results in over-high rudder angle corrections being applied, leading to unnecessarily high rudder forces and over-correction when turning. This effect results in hysteresis, or rudder flipping, causing the steering gear to work continuously to maintain a set course or position.
To improve manoeuvrability under computerised DP + AP support systems, Becker identified a need for real-time measurement of rudder forces, allowing the generated lift and resistance to be determined, and manoeuvring hysteresis to be eliminated or significantly reduced. The company has therefore applied a force measurement arrangement for full spade rudders, capable of determine rudder lift, drag and an interface for processing the sensor signal and transferring force data to the system.
Becker believes that the improved performance in manoeuvrability will lead to more reliable and safer operation in DP, and lower fuel consumption and reduced emissions. It claims that its system will enable reduced power in DP operation, allowing ships to use just one engine or even a shaft generator to maintain station.
The BIMS control unit calculates each force component from four sensors and transform these into rudder coordinates. Data validation is performed by defined allowable deviation from navigation system results as well as BIMS internally by considering all input values and checking for plausibility. BIMS can display rudder forces, in three modes: lateral force and drag values; a graph showing force application over time; and optimal rudder angle.
In AP mode, fuel savings of about 2%-3 % with a corresponding reduction in emissions are expected using BIMS. In DP mode, Becker expects savings to be significantly higher. In standby operation, using one of two main engines, savings of around 50% in fuel and emissions can be expected, something which the company says is particularly relevant to offshore vessels operating in ECAs. Other expected benefits are reduced rudder bearing wear, longer maintenance intervals and better vessel availability.
The company summarises the potential advantages of BIMS as:
- Reliable and safe dynamic positioning/manoeuvring operations;
- Less rudder motion in DP and AP mode;
- Energy savings through improved efficiency;
- Elimination of aft tunnel thrusters on a case-by-case basis;
- Emission reduction;
- ‘Quieter’ rudder system;
- Reduced manoeuvring response times;
- Significantly less rudder motion; and
- Reduced waste and maintenance costs
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