Active Heave Compensation
Understand the control. Separate peak power from energy over the duty.
Two numbers. Two different design questions.
The existing 100 t inline-AHC example gives both. Peak actuator power describes a rate; battery energy accumulates over the stated ten-hour duty.
How fast must work be delivered?
The highest mechanical power in the stated cycle. It is not an installed supply rating or an average draw from the battery.
How much energy does the duty use?
The ideal net energy after the declared drive and regeneration efficiencies. It is not a specified battery capacity.
100 t payload in air · 1 m sinusoidal amplitude · 10 s period · 10 h / 3,600 cycles. Inline gas-supported architecture: R = 10, maximum stroke 4 m, γ = 1.4.
Efficiency convention: 90% battery-to-actuator drive efficiency; 50% of available negative mechanical work returned to the battery.
Drive draw minus recovered energy.
The original calculation integrates 130.2 MJ of positive work and 130.2 MJ of available return work over ten hours. Apply each efficiency to its own path.
130.2 MJ ÷ 0.90 ÷ 3.6
130.2 MJ × 0.50 ÷ 3.6
79.6 MJ over ten hours
Energy-path values are rounded to one decimal place. The full calculation and efficiency definitions remain below.
Read the sign of power before adding the work.
Red marks drive work. Navy marks mechanical work available for regeneration; only the declared fraction returns to the battery.
Original three-line worked summary
The energy budget, in three lines
- The case. 100 t payload in air, sinusoidal zeta equals 1 metre at a period T of 10 seconds; inline AHC with gas ratio R equals 10, maximum stroke S max equals 4 metres, and the adiabatic nitrogen exponent gamma equals 1.4. Here drive efficiency eta drive equals 0.90 is battery-to-actuator drive efficiency and regeneration efficiency eta regen equals 0.50 is the fraction of negative mechanical work returned to the battery.
- The physics. Over ten hours, positive drive work and available return work are each 130.2 MJ (36.16 kJ of each per cycle). Peak mechanical actuator power is 11.6 kW.
- The answer. battery energy equals drive work divided by drive efficiency, minus regeneration efficiency multiplied by return work; the result is 79.6 megajoules, equal to 22.1 kilowatt-hours.
This is an ideal screening calculation with an explicit gas and efficiency convention. Friction, hydraulic losses, real-gas behaviour and full pump/motor/battery maps refine it — that project model is part of a CONSTELLATION study.
This idealised case excludes friction, auxiliaries, real-gas behaviour and detailed pump, motor and battery maps. Project sizing also needs the actual duty and motion spectrum, losses, control demands, reserve margin and operating limits.
Read the full model and six original equations ↓Active heave compensation (AHC) is a method of reducing the vertical motion of a payload suspended from a crane or winch during offshore operations. Unlike passive heave compensation, which relies on mechanical spring-damper systems, active heave compensation uses powered hydraulic or electric actuators controlled by sensors and algorithms to actively counteract wave-induced motion.
AHC systems are commonly used in subsea construction, pipe laying, ROV deployment, deep-water drilling, and heavy lift operations in challenging sea states.
How does active heave compensation work?
An active heave compensator consists of at least:
- An actuator, which may be a linear (e.g. cylinder) or rotary (e.g. winch) actuator, with position measurement.
- A motion reference unit (MRU), which may be placed on the AHC in case of an inline AHC or on the vessel in case of an integrated AHC.
- Some form of manipulation of the actuator position, that is sufficiently fast to be able to follow vessel motion (e.g. hydraulic motor).
When the active mode is on, the controller commands the actuator to reduce payload motion relative to a fixed reference. Residual motion always remains — sensing and control delay, bandwidth, stroke, force and power limits and the coupled payload dynamics bound what the loop can remove.
Compensation performance is case- and frequency-dependent: the achieved residual motion depends on the input signal, the payload, the stroke and force available, and the control limits. Quote a compensation ratio only together with how and where it was measured.
The main types of AHC
There are many types of active heave compensator. The main ones, and the duty each suits, are compared in the table below:
| AHC type | Integration | Project checks |
|---|---|---|
| Electric rotary (winch) | Winch drive | Torque, speed, thermal duty, power and regeneration |
| Hydraulic rotary (winch) | Winch drive | Force, flow, HPU/accumulator capacity, heat and control bandwidth |
| Deck-based sheave | Existing crane/load path | Deck integration, reeving, stroke, force and fail state |
| Topside inline | Above-hook load path | Headroom, suspended mass, stroke, power and controls |
| Subsea inline | Subsea load path | Depth rating, local sensing/control, energy, retrieval and maintenance |
No architecture is selected from payload mass alone. Compare the complete force–stroke–speed envelope, duty cycle, power source, installation and failure-state basis.
Active vs Passive Heave Compensation
Active, adaptive passive and passive equipment have different control, power and integration arrangements. Assess the required response, operating limits, failure behaviour and cost on the same project basis.
Read the full active vs passive comparison →Passive, adaptive-passive and active architectures have different force, stroke, bandwidth, power and integration envelopes. Compare them on the same project motion and load basis; no performance percentage transfers from one system or lift to another.
How much energy is consumed?
This idealised example considers a 100 t payload compensated in air by an inline AHC. The actuator follows a sinusoidal displacement with amplitude 1 m and period 10 s for ten hours.
- Motion: S(t) = ζ cos(ωt), with ζ = 1 m, T = 10 s and ω = 2π/T. Therefore Ṡ(t) = −ζω sin(ωt).
- Gas spring: gas-to-oil ratio R = 10, maximum stroke Smax = 4 m and γ = 1.4, the ideal adiabatic-nitrogen assumption used for this transparent screen.
- Energy convention: ηdrive = 0.90 is battery-to-actuator drive efficiency; ηregen = 0.50 is the fraction of negative mechanical work returned to the battery.
- Duration: τ = 10 h = 3,600 cycles. Friction, auxiliaries, real-gas behaviour and detailed pump, motor and battery maps are excluded.
Define mechanical actuator power as P(t) = F(t)Ṡ(t). Positive power is drive work; negative power is energy available for regeneration. The two must be integrated separately:
Relative to the mid-stroke support force, the idealised gas-spring actuator force is:
with:
Numerical integration gives 36.16 kJ of positive drive work and 36.16 kJ of available return work per cycle. Across 3,600 cycles:
The battery balance uses the declared efficiencies on their respective energy paths:
Peak mechanical actuator power is 11.6 kW. The result is an ideal screening value, not battery sizing: project work also includes friction, hydraulic and electrical losses, auxiliaries, control duty, real-gas behaviour, reserve margin and the actual irregular motion spectrum.
When to choose active or passive compensation
Evaluate powered correction
Where the assessed passive options cannot meet a stated residual-motion, load-response or positioning requirement.
- Define the reference and required response for each phase.
- Check sensing, bandwidth and force–stroke–speed limits.
- Assess energy supply, loss of power and recovery.
Evaluate passive response
Where a gas spring and damping, with adjustment if needed, can satisfy the operation’s motion and load criteria.
- Check payload, hydrodynamics, tuning and available travel.
- Assess changing conditions and lift phases.
- Compare installation, maintenance and failure behaviour.
An available HPU or a long duty is not a selection rule. Payload mass alone does not select the architecture either. Use the complete analysed operation and equipment limits.
Related Products
- ANTARES Adaptive PHC — Advanced passive heave compensator with multiple operating modes and automatic damping control
- RIGEL Basic PHC — Cost-effective passive heave compensator for straightforward operations
Further Reading
Active heave compensation — frequently asked
What is active heave compensation?
How does an active heave compensator work?
How efficient is active heave compensation?
How much energy does an AHC consume?
When should I choose active over passive compensation?
Evaluating active compensation for your operation?
AHC, passive damping and adaptive-passive control each suit different force, stroke, motion, power and integration envelopes. Send the project target, sea state and constraints and we will compare the architectures on the same basis.
