Offshore knuckle-boom crane at dusk, a single fall from the boom tip to the sea

Active Heave Compensation (AHC)

How an active system measures the vessel’s motion and drives the load against it, and when you need one instead of a passive compensator.

By Peter Wang, COO · October 2025 · Updated September 2026 · 10 min read

In brief

Active heave compensation (AHC) holds a suspended load at a fixed height relative to the seabed while the vessel heaves, rolls and pitches.
Most AHC is built into the crane or winch. Inline AHC puts the active unit in the load path below the hook.
It can remove more motion than a passive spring, but it needs power, sensing and a controller, and some motion always remains.
Most splash-zone crossings and subsea lifts are handled with passive or adaptive passive compensation. Active control is worth it when a passive unit can’t meet the residual-motion target.

Active heave compensation (AHC) keeps a suspended load steady while the vessel moves. A motion reference unit (MRU) measures the vessel’s heave, roll and pitch; a controller turns that into the motion of the crane tip; and a powered actuator, usually the crane’s winch, pays out and hauls in wire so the load holds its height relative to the seabed. AHC is used for subsea installation and landing, ROV and pipe-lay work, and other lifts where the load must stay still in a moving sea.

How active heave compensation works

Every AHC system runs the same loop, many times a second:

Measure. The MRU measures the vessel’s heave, roll and pitch. It is rarely at the crane tip, so the controller uses the surveyed lever arms to work out how the tip itself moves. See motion reference units.
Predict and command. The drive needs time to respond, so the controller filters the signal and usually predicts the motion a fraction of a second ahead. It then commands the actuator to move by the same amount in the opposite direction.
Actuate. The winch pays out wire as the crane tip rises and hauls in as it falls. In an inline system a cylinder in the load path does the same job. A position sensor on the drum or cylinder closes the loop.
Hold the reference. The target is normally earth-fixed: the load keeps its height relative to the seabed. To land, the operator lowers slowly while the AHC removes the wave motion.

Figure 1. Two places to put the active drive. In A, the crane’s winch pays out and hauls in wire on the MRU signal (solid arrows: signal and command; dashed arrow: drum-position feedback). In B, an inline unit below the hook carries its own sensing, control and battery; a gas spring carries the weight and an active cylinder strokes against the motion. In both, the payload is held at a fixed height relative to the seabed.

Types of active heave compensation

AHC systems differ in where the actuator sits in the load path and what drives it.

Architecture	How it compensates	Typical use	What to check
Winch AHC, electric	Electric motors turn the winch drum to pay out and haul in every wave.	Offshore and subsea cranes, installation vessels.	Peak drive power, energy recovery, motor heating over the duty.
Winch AHC, hydraulic	Hydraulic motors on the drum, often with accumulators that store and return energy.	Offshore cranes, A-frames and launch-and-recovery systems.	Pump flow, accumulator capacity, cooling.
Cylinder or sheave AHC	A hydraulic cylinder moves a sheave in the wire path, changing the wire length without turning the drum.	Retrofits to existing cranes and winches.	Deck space, reeving, cylinder stroke.
Inline AHC, topside	An active unit rigged below the hook, with its own sensing and power.	Cranes without built-in AHC.	Headroom below the hook, stroke, onboard energy.
Inline AHC, subsea	The inline unit works at depth, close to the payload.	Deepwater landings where the long wire’s stretch matters.	Depth rating, subsea power and control.

VEGA, ND’s active compensator, is an inline design: VEGA-T for topside use first, VEGA-S for subsea to follow.

What limits the result

An active system never removes all of the motion. What remains depends on four things:

Delay. The time from measurement to movement: sensor filtering, the controller and the drive’s response. The longer the delay, the more work the prediction has to do.
Speed. How fast the actuator can move and reverse. Short, steep waves ask the most of it.
Stroke, force and power. The actuator has to follow the whole crane-tip motion inside its stroke and wire-speed limits, and deliver the force at that speed.
The wire and the load. A long wire stretches, and a load in water gains added mass. At depth the payload can respond differently from the hook the AHC controls.

Suppliers quote performance as a compensation ratio or a residual heave for a stated wave period and amplitude. Compare figures only on the same basis: the same period, amplitude and measurement point.

Why an inline, gas-supported design needs little power

A drive that carries the whole weight of the load must deliver roughly weight × heave velocity at the fastest point of each wave. For 100 t under a crane tip heaving ±1 m every 10 s, the peak velocity is about 0.63 m/s, so the drive needs around 0.6 MW, even if much of that energy is recovered on the way down.

In a gas-supported inline design, a gas spring carries the static weight. As the rod strokes, the actuator only supplies the difference between the spring force and the weight, at most about 37 kN in the same 100 t case. Peak mechanical power falls to 11.6 kW, and ten hours of continuous compensation take a net 22.1 kWh from the battery in the ideal case (worked example below). That is why an inline active unit can run on a battery, without an umbilical.

Active, adaptive passive or passive?

All three reduce the motion that reaches the load. They differ in how they do it and what they need.

	PassiveRIGEL, CYGNUS	Adaptive passiveANTARES	ActiveVEGA, in development
What carries the load	Gas spring	Gas spring	Gas spring, with an active cylinder
How it reduces motion	A soft spring isolates the load from the crane tip; damping is set on deck.	The same, with damping and gas charge adjusted automatically through the lift.	Measures the motion and drives the load against it.
Sensing and power	No external power. RIGEL: none. CYGNUS: small battery, no umbilical.	Onboard sensing; battery, no umbilical.	MRU and controller; large battery, no umbilical.
Residual motion	Set by how well the spring and damping suit the sea state.	Kept low as load, depth and sea state change.	The lowest achievable, within the delay, speed and stroke limits.
Best fit	Most splash-zone crossings and subsea lifts.	Multi-step lifts, changing loads and depths, quick lift.	Where a residual-motion or landing target can’t be met passively.

Passive and adaptive passive units need no external power, no umbilical and no vessel-side control system, and less maintenance than an active system. Active control makes sense when a project’s residual-motion target, landing tolerance or long exposure at depth can’t be met that way. The active vs passive comparison goes through the trade-offs, and the selection guide matches a lift to a unit.

In development

VEGA, ND’s inline active compensator

VEGA combines MRU-driven active cylinder control with automatic damping control. A large onboard battery powers it, so it needs no umbilical and no external power. VEGA-T for topside use comes first, and VEGA-S for subsea follows.

Design targets: SWL 75–2,500 t, stroke 2.5–6.0 m.

See VEGA →

Worked example for a 100 t inline AHC

The case: a 100 t payload in air, compensated by an inline, gas-supported AHC that follows a sinusoidal motion of ±1 m at a 10 s period for ten hours (3,600 cycles). Gas-to-oil ratio $R = 10$ (gas volume ÷ swept oil volume), maximum stroke 4 m, ideal adiabatic nitrogen ( $\gamma = 1.4$ ); 90 % battery-to-actuator drive efficiency, and 50 % of the work returned by the gas spring recovered to the battery.

Result	Value	What it means
Peak mechanical power	11.6 kW	Sizes the drive: a rate, not an energy
Battery energy into the drive	40.2 kWh	Over 10 h; 36.2 kWh of it reaches the actuator at 90 % drive efficiency
Recovered to the battery	18.1 kWh	Half of the 36.2 kWh the gas spring gives back
Net battery energy	22.1 kWh	Sizes the battery, before friction, auxiliaries and reserve

Figure 2. One wave cycle of the worked example, drawn from the model below. In A, the actuator force peaks when the piston stops, a quarter of a cycle away from peak speed. In B, power is force × speed, so it changes sign every quarter of a cycle. In the solid areas the drive adds energy from the battery; in the hatched areas the gas spring gives the same amount back, 36.2 kJ per wave, of which 50 % is assumed recovered.

The model and equations

Motion of the actuator, with amplitude $\zeta = 1\ \text{m}$ , period $T = 10\ \text{s}$ and $\omega = 2\pi/T$ :

S(t) = \zeta\cos\omega t, \qquad \dot{S}(t) = -\zeta\omega\sin\omega t

Actuator force relative to the mid-stroke support force, for an ideal adiabatic gas spring:

F(t) = m g \left[ \left( \frac{S_0}{S_0 - S(t)} \right)^{\gamma} - 1 \right]

S_0 = S_{\max}\,(R - 0.5)

Mechanical power is $P(t) = F(t)\,\dot{S}(t)$ . Positive power is drive work; negative power is work available for recovery. Integrated separately over the ten hours:

\begin{aligned} W_{\text{drive}} &= \int \max(P, 0)\,\mathrm{d}t = 130.2\ \text{MJ} \\ W_{\text{return}} &= \int \max(-P, 0)\,\mathrm{d}t = 130.2\ \text{MJ} \end{aligned}

\begin{aligned} E_{\text{batt}} &= \frac{W_{\text{drive}}}{\eta_{\text{drive}}} - \eta_{\text{regen}}\,W_{\text{return}} \\ &= \frac{130.2}{0.90} - 0.50 \times 130.2 \\ &= 79.6\ \text{MJ} = 22.1\ \text{kWh} \end{aligned}

An ideal screening case: friction, real-gas behaviour, auxiliaries and detailed pump, motor and battery maps are left out. A CONSTELLATION lift simulation models them for a real project.

Frequently asked questions

What is active heave compensation?

Active heave compensation (AHC) keeps a suspended load at a steady height while the vessel moves. A motion reference unit measures the vessel’s motion, a controller works out how the crane tip moves, and a powered actuator, usually the winch, pays out and hauls in wire to cancel it.

What is the difference between active and passive heave compensation?

A passive compensator uses a gas spring to isolate the load from the crane-tip motion; the compensation itself needs no external power. An active compensator measures the motion and drives the load against it, which can remove more motion but needs power, sensing and a controller. An adaptive passive unit sits between the two: passive load support, adjusted automatically through the lift from an onboard battery.

Does active heave compensation remove all the motion?

No. Delay in sensing and control, the actuator’s speed, stroke and power, and the stretch of a long wire all leave some residual motion. Performance is quoted as a compensation ratio or residual heave for a stated wave period and amplitude, so compare figures on the same basis.

What does an active heave compensation system need?

A motion reference unit, a controller, an actuator with position feedback (a winch drive or a cylinder) and a power supply. Crane and winch systems take power from the vessel; an inline unit such as VEGA carries its own battery.

How much power does active heave compensation need?

If the drive carries the whole load, peak power is roughly weight × heave velocity, about 0.6 MW for 100 t under a crane tip heaving ±1 m every 10 s. A gas-supported inline design only supplies the difference from the spring force: the same case needs about 11.6 kW at peak and 22.1 kWh over ten hours in the ideal case.

When should I choose active over passive compensation?

When a passive or adaptive passive unit can’t meet the project’s residual-motion or landing target, for example a landing on a tight tolerance or a long hold at depth. Most splash-zone crossings and subsea lifts are handled passively.

Evaluating active compensation for your operation?

Send the payload, the sea state and what the load has to do at the seabed. We’ll tell you whether passive, adaptive passive or active compensation fits, and screen the lift in CONSTELLATION, our lift-simulation service.

Send your lift case →

Basis and sources

DNV-RP-N202 Heave compensating systems, DNV recommended practice, edition 2024-06. Covers the design, verification and operation of heave compensating systems.
DNV-ST-0378 Offshore and platform lifting appliances, the standard ND compensators are designed and classed to.
Worked example: ND screening model with the inputs stated above; results rounded to one decimal place.
VEGA specifications are design targets and may change during development.