Capability
Heave compensation
Active and passive motion control, and the engineering judgement to know which the job needs, the full discipline, from a proven passive spring to actively controlled weight-on-bit.
When a vessel heaves, the load at the end of a wire or drill string moves with it unless something holds it steady. Heave compensation keeps tension, position or weight-on-bit constant as the platform moves and it is one of MintMech’s core specialisms. There are two main approaches: passive systems control force, while active systems control position. We design, integrate, verify and commission both, including our own Active Heave Assist, which bridges the two.
Passive – constant force
Passive heave compensation (PHC) reduces the effect of vessel motion on a suspended load without external energy or control. A gas spring, typically an air-over-oil cylinder arrangement, sits in the load path and absorbs the vessel’s vertical motion, so the load sees a far steadier force than the deck does. It is simple, robust and fail-safe: there is no control loop to lose, and it keeps working when power and signals do not. For decades it has been the standard way to protect drill strings, lifted loads and deployed equipment from the up-and-down motion of the waves.
A passive system reacts to load, not position. The gas spring is tuned so that, as the vessel heaves, the cylinder strokes to hold the line tension roughly constant, but it can only respond to the forces passing through it and it cannot anticipate the next wave. In demanding work that produces real residual error: on a geotechnical drill string a well-set passive system still leaves a compensation error of the order of several tonnes of weight-on-bit, enough to break or disturb a core in soft or layered ground. We are familiar with the full range of passive configurations in offshore use and design, integrate and evaluate ram-rig and cylinder-based PHC, with the high-pressure hydraulics and safety components to match. Our line tensioners are passive heave compensators in product form.
Active – constant position
An active heave compensation (AHC) system works independently of the force in the line. Referenced to a motion-reference unit (MRU), it actively takes up and pays out, driving a winch or cylinder in time with the vessel’s heave, to hold the load at a constant depth or position. That lets it do what a passive system cannot: hold station precisely off bottom, keep a load at constant depth through the splash zone and maintain position when there is no reaction force to work against. The cost is complexity and dependence on power and control, so an active system is specified where the duty genuinely needs position control, not as a default. See our active heave compensation winch and active constant-tension winch.
Constant force vs constant position – the trade-off
The two approaches are not better or worse; they control different things, and the right one depends on whether the job needs a steady force or a steady position.
| Passive – constant force | Active – constant position | |
|---|---|---|
| What it controls | Line tension / force on the load | Depth / position of the load |
| Reference | The force passing through the spring | Vessel motion (MRU), independent of load |
| External power / control | None – gas spring only | Yes – powered, closed-loop control |
| Fail-safe behaviour | Inherently fail-safe; no loop to lose | Needs designed-in redundancy and fallback |
| Residual error | Higher – cannot anticipate the wave | Lower – actively corrects |
| Best-fit duty | Protecting a line/load; weight-on-bit drilling where reaction exists | Off-bottom station-keeping; constant-depth deployment; tool changes |
AHA – the bridge between the two
Most drilling projects want both: the reliability of a passive system, and the precision of an active one. That is exactly what our Active Heave Assist (AHA / AWOB) delivers. Rather than replace a working passive system, AHA adds an actively controlled layer on top of it, letting the driller set a target weight-on-bit and a maximum feed rate and actively managing drill-string movement for controlled advance. It removes most of the passive system’s residual error while keeping its fail-safe behaviour and reverts to pure passive operation at any time. It is the one product that spans constant-force and constant-position control, which is why it sits at the centre of this capability. Our insight on active versus passive heave control sets out the trade-off in full.
What we bring across the whole discipline
- System design & integration – the hydraulic and gas circuit, the reeving, the control system and the structural interface engineered together so the system behaves predictably across its whole stroke, not only at one design point.
- Constant-tension systems – active and passive constant-tension equipment that keeps a line steady through payout and recovery. See active constant-tension winch and line tensioners.
- Performance verification – compensation-error and performance analysis so a systems real behaviour is known and proven before it goes to work – and before an existing system is pushed into deeper water, heavier payloads or higher sea states. Backed by our wider design verification.
- Safety & man-riding compensation – heave-compensation design for personnel-rated and man-riding duties, with the protection, redundancy and fail-safe behaviour those operations demand.
- Overload protection – instrumented load paths and overload protection integrated into the compensation system. See overload protection systems.
Active compensation can cut compensation error from the order of a few tonnes typical of a passive system to a fraction of a tonne, with proven improvements in core recovery, but the value is in matching the right approach to the duty and proving it. That judgement, across constant-force and constant-position control, is what this capability brings together.