Heave compensation

Heave compensation is a system for increasing control over an object when it's moved to and from the seabed by a ship at sea. It relies on eliminating the vessel movements, so that the object itself moves much more controlled.

A typical offshore construction task uses heave compensation as follows:

1. Module is on the vessel deck, and gets hooked onto the crane.
2. The crane lowers the module into the water, and through the splash zone.
3. The crane engages heave compensation.
4. The crane docks the module onto the subsea template, lands it on the seabed, avoids hitting the diver, whatever really.
5. The crane disconnects, disengages heave compensation and recovers the hook up to the surface.

The problem with placing items on the seabed is that the vessel you are handling the object from is moving up and down with the waves. Most of the time you can tolerate this, but when you are doing precision construction work, you do not want the tool / module or hook to go up and down 4 meters every 10 seconds (this is considered good weather in the North Sea). Imagine trying to get on an elevator doing a similar stunt.

Economically, heave compensation makes it possible to work in more severe weather than otherwise possible - called "increasing the weather window". This has a impact because a customer normally only pays for the time the ship is actually working, and not when the ship is standing by due to high seas. Heave compensation therefore reduces the economic risk of bad weather.

There exist at least two different concepts heave compensation; namely active and passive compensation.

Active Heave Compensation (AHC)

Active heave compensation is the most modern system. It relies on motion sensors stationed onboard the vessel, and based on their input, it lowers and raises the load. This is either done using gas driven cylinders (which raises or lowers the wire) or by operating the wire winch.

• Motion sensor driven - can be inaccurate and give wrong readings
• Does not pay attention to the weight of the load (if this changes)
• Driven by displacements only

Passive heave compensation

The passive system relies on a spring damper, most often in the form of a cylinder with pressure on one sides of the stroke. When the ship goes up the load in the wire increases - and the cylinder contracts - but dampened by the pressure inside the cylinder. This leads to lower load in the wire and the load remains stable.

• Must be primed to the approximate load by varying the cylinder gas pressure.
• Pays attention to the load on the wire
• Loads oscillates a bit around the mean depth

Passive heave compensation is the safest to use with light loads that have a large surface.

Problems with both systems

Unfortunately, both systems have limitations that the user has to be aware of - and therefore the systems are often combined. The crane driver must therefore be more skilled than previously, and it has become quite possible to sink ships if heave compensation is used wrongly, or if they fail¹.

• Resonance: Believe it nor not, but even a 70 mm diameter wire behaves like a soft spring at 2000 m depth. If the ship moves at the same pace as the lowest natural frequency of the load, the motion is not dampened, but magnified. A active heave compensation system will probably not notice this - but a passive system can. Both systems are not very effective in stopping resonance once it has been started. Only by paying out more wire, or by changing the vessel motion can it be stopped. It is possible to change the dampening in the heave compensation system to avoid this problem though - imagine that resonance occurs at 1200 m. We can then lower the load to 1000 m and change the dampening of the heave compensation system (new resonance depth becomes 1600 m). We the lower the load to 1400 m, and change the dampening back to the previous level. We are now beyond the resonance depth, and the next problem area will be at 2400 m (1200 m x 2).
• Fail-safe If the system fails, and the heave compensation does not move, the load will begin to oscillate with the ships movement. This is often not that bad for most items because you normally have manual control over the load. You can then just pull up the load, and be well clear of any obstacles. What becomes a problem is when you have a load with large added mass - for example when you have to move a lot of water trapped inside the object. This becomes a real killer when you have a displacement driven system as the extra water resists movement and the extra loads might break either the object, or some part of the crane.

Theory

For you dynamics people out there, the system looks like this:

~~\ Vessel  |~~~~~
   ----------
     |    |
     /    |
     \   /-\
   K /   | | C
     \    |
     |    |
   /---------\
   |  Load   |
   -----------

(For you non-dynamics people out there - ignore the illustration, we only have one wire going down to the load). It is a foundation excitation problem, where the load reacts to the oscillation movement of the vessel. The wire stiffness K is dependant on wire length and size, while the dampening C is dependant on the lifted object's size and features. Heave compensation systems modify both K and C. The response of the load is then given as:

Input oscillation is

y(t)= Y sin(&omega t)

Movement of the load then becomes:

x_p(t) = Y (k² + (c &omega)²)^1/2 sin(&omega t - &phi₁ - &alpha) / {(k - m &omega²)² + (c &omega)²}^1/2

If you remove the non-essential mathematics here you have displacement equals something divided by (k-&omega m²). Which again means that it is possible to achieve very large displacements for a certain k = m &omega², and this is when the system achieves resonance.

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1 : A little horror story for you: A ship was installing a horisontal plate using heave compensation. They were using heave compensation because the water around the plate adds to the load if the plate is moved rapidly. During this lift, the heave compensation failed - and started to pull the load up very fast. This movement was resisted by the water around the plate (hydrodynamic term: added mass). The plate now acted as an anchor for the ship, while the crane still was pulling in like mad. The crane driver finally managed to hit the big red button when the ship was at an 30 degree angle - which is pretty close to going around.