Passive Heave Compensation

Basic concept

Heave means vertical motion, in our context vertical motion of the crane hook, caused by waves. Passive heave compensation can be thought of as a spring-mass-damper system with the objective of reducing wave induced motion below the compensator. The below simplified sketch illustrates our scenario: 

The crane hook motion follows the sinusoidal given by  \( \zeta \cos(\omega t) \), the PHC has stiffness  \( k \), the water has mass density \( \rho_w \), while the payload has basic properties \( \rho, m, A_\perp \), respectively for payload mass density, mass and area perpendicular to the heave motion.

  1. Shock absorbers, like Polaris, are unmatched when it comes to shock absorption, but they are useless for other applications as the spring component is effectively missing.
  2. Basic passive heave compensators, like Rigel, has generally a lower gas to oil ratio than an adaptive PHC which gives lower performance.
  3. Adaptive passive heave compensators, like Antares, are the best allrounders, they can do almost everything reasonably well, except for motion compensation in air or subsea motion compensation under unfavorable conditions
  4. Topside active heave compensators, cannot do subsea applications as they not designed to be submerged.
  5. Active heave compensators are not well suited for “fast” applications such as shock absorption and gives generally worse performance than a passive heave compensator when operated in passive mode due to higher weight and friction.

What is the relative cost between the products?

Product Relative Cost
Shock absorber 1x
Basic PHC 2–3x
Adaptive PHC 3–6x
Topside AHC 10–20x
Subsea AHC 15–30x
Legend
Lowest cost
Low-medium cost
Medium-high cost
Highest cost

The relative cost is relative to a shock absorber cost (with similar capacity/stroke) and can be considered a rough indicator.

How do the products rank in terms of reliability?

  1. Shock absorber
  2. Passive heave compensator
  3. Adaptive passive heave compensator
  4. Topside active heave compensator
  5. Subsea active heave compensator
Where shock absorber is the most reliable.