Active Heave Compensation (AHC)
Active Heave Compensation Systems (AHC) use Motion Reference Units (MRUs) to measure the heave of the vessels during lifting operations offshore. A high accuracy measurement of heave from the MRUs enables to compensate the movement from the vessels by regulating the winch systems. The tensioning and the loosening of the winch cable allow to keep the lifted load motionless.
Heave is defined as the vertical displacement from a point, the point typically being the mean sea level. Heave is calculated by an MRU by filtering and a double integration of the vertical acceleration measurement. Because of the double integration, even slight errors in the measurement of the vertical acceleration, gives large errors in the heave output. Our Motion Reference Units use very high stability accelerometers to give a stable heave performance.
For active heave compensation, an accurate real-time heave output is needed. Due to the nature of heave filtering in MRUs, the heave output will always be slightly delayed with respect to the true heave. The optimal amount of filtering needed to get the best real-time heave output depends on the sea-state and the quality of the accelerometers used in the MRU.
Our MRUs automatically monitors the current sea-state, and adjusts the heave filter to be optimized real-time measurements.
Heave rate measurement
Heave is calculated by integrating accelerations in the vertical direction twice. Each integration introduces a phase shift and delay in the output signal. Many AHC applications do not need the actual heave position, but rather the heave rate (or heave velocity). Heave rate can be calculated by the AHC system from the heave output from the MRU, but by doing this, the phase shift and and delay will still be present. By having the MRU calculate the heave rate directly, only a single integration is needed, and the phase shift and delay can be lowered. All Horizon MRUs have the option of heave rate output.
AHC Applications examples:
- Offshore cranes
- LARS (Launch and Recovery Systems).