HEAVE COMPENSATION – AN OUTLINE

A brief summary of heave compensation (passive and active), the main methods and notes on what to consider when renting or buying a system

Heave compensation is a subject that S&A has considerable expertise in so we’ve compiled this handy reference page to help outline the important technical issues, methods and purchasing points.

The Basics

Ships, as much as we hate to admit it, move around a lot which is a bit inconvenient when you’re trying to do something that requires everything to be still and calm. The way ship movement is quantified is to define the six degrees of motion which are: roll, pitch, yaw, heave, sway and surge.

It is important to understand at an early stage that motion compensation as a term tends to imply compensation/ reduction/ elimination of vessel movement in all or almost all degrees of motion; not just heave. Systems required for true ‘motion compensation’ are not discussed in this article but suffice as to say they are a degree of complexity higher than heave compensation and as such are an entirely different conversation altogether!

Heave compensation, alternatively referred to as swell compensation, is the increasingly common method used by marine lifting systems to reduce or eliminate the effect of vessel heave on a suspended load.

To better understand heave compensation and the need for it in the offshore lifting industry, one must first understand that when there is no heave compensation, the suspended load heaves up and down concurrently with the heave motion of the vessel. When the vessel raises up due to a wave, the load on the end of the system moves up by the same amount. The same is true for when the vessel lowers down due to a wave.

This heave motion is an issue when the lifting system, such as a crane or winch, is being used to transport loads during operations. For example: ship-to-ship transfer, subsea positioning activities, cargo between vessels and static locations or when the load is required to be still for a specific operation such as subsea interconnection work using ROVs.

In short, a lifting system with heave compensation is expected to keep the suspended load steady while the ship heaves up and down.

Types of Heave Compensation

Heave compensation has two main subdivisions:

Passive Heave Compensation (PHC) – ‘reactive’ system that requires no additional power and in general features no instrumentation
Active Heave Compensation (AHC) – ‘proactive’ system that uses instrumentation and additional power

Typical Passive Heave Compensation (PHC) systems are ‘shock absorber’ type systems such as in-line cylinder/ sheave systems or hook mounted pre-pressurised cylinder based systems. They are generally a simple closed loop hydraulic system with little or no requirement for an electronic control system.

Active Heave Compensation (AHC) implies that the compensation system proactively predicts the vessel motions using instrumentation (primarily a Motion Reference Unit or MRU). Based on commands from the control system, a system is operated to raise or lower the load so as to reduce or eliminate the vessel heave motion.

Typical AHC systems are either winch based or cylinder/ sheave based arrangements. There are also methods that changes the geometry of the over-boarding sheave support (crane boom, A frame, etc.) however this is less common as a lifting system. A heave compensation approach gaining popularity is the personnel transfer system also known as ‘walk-to-work’ but that is a different discussion entirely!

The most common heave compensation analogy is to imagine PHC as simple cart springs on a pickup truck and AHC as the high tech active ride suspension system on a Mercedes! They both kind-of do the same job but one is significantly more complex than the other.

Note: There are also hybrid systems that make use of elements of both PHC and AHC to achieve a specific aim. For example, high tonnage AHC systems often have both an active and passive element so that the passive components do the ‘heavy lifting’ and the active components provide the accurate small movements. This is to help reduce the high power requirements inherent in high tonnage AHC.

Speeds and Whatnot

Waves are not linear so in general heave compensation is modeled using sine waves. The impact of this is that the speed and acceleration of the load can be higher than expected if the engineers are more familiar with standard lifting systems. As an example, a 10Te SWL AHC system with a performance specification of +/- 2.5m wave (5m peak to trough) over a 10 second period, can be illustrated as follows:

This graph shows that for a 10Te SWL AHC winch the maximum speed can be as high as 1.5m/s and the power peaks at 0.2MW or 200kW (with some assumptions for safety factor, losses, etc.)

The Good Points and the Bad Points

There are significant differences between PHC and AHC the biggest of which is that PHC is significantly cheaper than AHC! As with everything, there are very good reasons for this…

POWER: The biggest difference between PHC and AHC is that AHC requires power while PHC does not. AHC, because of the sinusoidal nature of wave motion, requires speed and acceleration to be far higher than typical lifting operations which in turn requires a lot more power than an equivalently rated non-AHC lifting system.

Regenerated POWER: As much of a problem as ‘where do you get the power from?’ is ‘what do we do with the power the system is generating when the load is dropping?’. Everyone is aware that to lift a load requires energy; what a lot of people tend to forget that when lowering a load energy is generated in the lifting system that needs to be dealt with. The energy generated by an AHC system can be almost as much as the energy used to provide the lifting part of the cycle so it’s not going to be insignificant. The positive point about regenerated energy however is that if you store this ‘spare’ energy you can use it when you have to pull the load back up. An effective regenerated energy management system could reduce the power required from the vessel by as much as 50%.

Accuracy: PHC systems can be used to ‘dampen’ the effects of heave however they are not as accurate as AHC systems for most operations. Arguably, the difference between PHC and AHC in terms of accuracy can be as large as a degree of magnitude between the two.

Operability: Most PHC systems require a specific pressure in a hydraulic accumulator system that is calculated on a ‘load by load’ basis. This means that new calculations are required for each differing load scenario. One ‘spring’ does not fit all loads! PHC cannot typically be used where the load changes such as subsea coring activities where the deployed weight is not the same as the recovered weight. By contrast, an AHC system can lift any load at any time and that load can be one thing one minute and something else later in the operation. PHC or systems that have a PHC element require weight on the hook whereas pure AHC systems can work as well with an empty hook as a heavily loaded hook.

Complexity: AHC systems are considered the most accurate solution for controlling a load however they do require substantially more system complexity to achieve full operation. Complexity means more design and expensive components which combine to make for a far more expensive arrangement.

Installation considerations: For both PHC and AHC it is important to understand that a lifting system must be designed to take account of heave compensation; heave compensation cannot simply be added*. Typical areas to be considered include:

Increased fatigue loading in structures
Wire routing as motion compensation significantly reduces the fatigue life of wire rope
Sheave design (low friction bearings, heat dissipation, etc.)
Power consumption and control or dissipation
Cooling

* The exception to this is hook mounted heave compensators which are outlined below.

Cylinder Based Heave Compensation

A typical cylinder based heave compensation system consists of one or more cylinders and two sets of wire rope sheaves. One set of sheaves is statically mounted on a support frame and the others are attached to the cylinder arrangement. A wire from a basic non heave compensated winch wire is fed from the winch drum, around both sets of sheaves then to an over-boarding arrangement. The vessel motion is compensated by the two sets of sheaves moving closer together or further apart as dictated by the cylinder system. Changes in the distance between the sheaves lengthens or shortens the deployed wire rope.

As the heave compensator is between the winch and the overboarding point this style of arrangement is frequently referred to as ‘in-line’ heave compensation.

For a PHC system the cylinder is expected to operate like a simple spring so a hydraulic accumulator arrangement will form part of the arrangement. The ‘spring constant’ is dictated by the accumulator pre-charge pressure which cane be changed by injecting or removing nitrogen.

For an AHC system the basic cylinder and sheave arrangement would also include a control system and a hydraulic power unit (HPU) to provide the fluid power. The HPU may also include an accumulator bank for regenerated energy storage.

Winch Based Heave Compensation

Almost all winch based heave compensators are AHC although a lot of high tonnage winches have multiple motors with some AHC and some effectively working as PHC. It is very rare to find a 100% PHC winch (we have yet to see one!).

A typical winch based heave compensation system consists of a single drum winch*, a control system and a power unit to rotate the drum. The vessel motion is compensated by the rotation of the winch drum that is dictated by the control system and instrumentation.

* Traction/ storage winch AHC systems are rare as it is difficult to synchronise the two winches. If a storage and traction winch are not in synchronisation you tend to lose the ‘capstan’ effect around the traction winch. The way around this is to put a cylinder based heave compensator in-line with the pair of winches so the traction/ storage winch arrangement is maintained and the AHC is performed independently.

Hook Based Heave Compensation

The alternative to an in-line or winch based heave compensator is to have a device that is connected to the hook that provides a short amount of additional travel for the lifting arrangement. The vast majority of hook based heave compensators are PHC systems as it is difficult to arrange for power to be made available to the hook to drive anything, especially if that hook is to be deployed subsea!

A typical hook mounted heave compensation system consists of a hydraulic cylinder with one end connected to the lift systems hook and the other to the load. The hydraulic cylinders local hydraulic arrangement will include a pre-charged accumulator that provides the ‘spring’ in the system.

Imagine an old spring based scale with 5Kg of bananas hanging from it. The scale is calibrated from 0Kg to 10Kg so the scale is sitting in the middle with the bananas hanging from the end. If you where to lifting the scale quickly for a short period the bananas would stay still and the load on the scale will increase. The load will continue to increase until the max load due to how much and how fast you lifted the scale is achieved and the bananas rise up with the scale, the load peaking at say 7Kg. The opposite happens when you suddenly lower the scale i.e. the load will go down from 5Kg to say 3Kg before the bananas start to drop. In theory if you where to time it just right, the bananas would stay still as you move the scale up and down. A hook mounted PHC system works in identically the same way. The accumulator provides the spring constant which is based on a set of calculations that will allow the load to sit in the middle of the cylinders travel when in a static condition.

It can take a lot of skill and tweaking to get a hook mounted PHC to perform well. This being said, it is a very popular technique due to its simplicity, cheap cost and the sky is the limit in terms of maximum load you could design a system to take.

Electric AHC Vrs Hydraulic AHC

We believe it is fair to say that hydraulic AHC is more popular than electric AHC. This is mainly due to it being ‘easier’ to store regenerated energy in a hydraulic accumulator than the electrical equivalent such as batteries or capacitors. But, accumulators are large, time consuming to set up and the use of hydraulic oil in general is becoming more of an issue where pollution from leaks can lead to environmental headaches.

The primary means of managing regenerated energy within an electric AHC system is to ‘burn off’ the power using cooled resistors. As you’re effectively throwing that energy away it is a very wasteful approach, especially for high tonnage AHC systems that can regenerate large amounts of energy. You can push the energy back into the vessel’s power generation system but that is also fraught with problems as regenerated energy is dirty energy so requires significant filtering and adaptation. Regenerated energy is also ‘peaky’ so you need a vessel distribution system that is robust enough to take big spikes in power at random intervals otherwise you’ll have generators tripping which is no good for anyone.

This is mainly why you tend to find electric AHC winches up to a rating of around 15Te but nothing above that. Throwing away 200kW/ 250kW of power is about as much waste as most people will tolerate.

But all is not lost for electric AHC! With the advent of mass produced electric drive systems (thanks car industry!), components like ultra-capacitors are getting a lot more development which leads to cost effective and highly compact solutions for energy storage. Give it another 5 years and we believe electric AHC will overtake hydraulic AHC.

Rigging Considerations

An often overlooked consideration for heave compensation is the routing of the wire. Unless you have a hook mounted heave compensator, the wire rope is going to have to travel from the winch to a point over the side of the vessel which means it has to travel around sheaves. If a wire is routed through a sheave and then paid out, pulled back, paid out, pulled back, etc. the wire will suffer wire fatigue which will eventually lead to the wire failing. The more sheaves, the worse the situation gets. Winch and in-line heave compensation, by their very nature, imply a lot of paying out and pulling in so heave compensation willreduce the life of a wire rope.

One way to reduce the impact of heave compensation on the life of the wire rope is to ensure the sheave is correctly sized as, for example, DNV recommend a 20:1 D:d ratio* for heave compensated systems which is higher than it would be for a non heave compensated sheave. Another way is to try to minimise the sheaves in the routing or to try to get the heave compensator as close to, or as directly in line with, the overboarding point as practical.

And then there is the ‘multifall/ multipart’ problem. When a lifting arrangement has more than one fall or part of rigging, the relationship between the position of the hook and the length paid out/ in by the lift winch halves. For every 1m of wire paid out or in, the hook moves 1/2m. This ratio halves again for each additional part or fall of rigging. If you consider that most heave compensation systems function through lengthening or retracting the lift wire, they have to work twice as hard for each part or fall of the rigging. In general, a heave compensation system is of no real practical use on any system with more than 2 part/ fall rigging. Multifall/ multipart rigging arrangements with more than 2 falls/ parts tend to be compensated through the use of hook mounted PHC systems however PHC can never be as accurate or adaptable as AHC.

The final significant challenge is the impact of a naturally occurring catenary in the lift wire of deep water operations. As has been found on 2000m+ subsea operations, loaded lift wires rarely sit perfectly vertical in the water column which creates a section of ‘slack’ in the wire. This creates problems with vessel based heave compensation systems as when they pull in or pay out the wire a percentage of the movement intended for the hook is lost in the slack of the wire. The ramifications of this can be sudden ‘jerky’ motion at the hook, shock loading through the system, confusion in the AHC control system and poor synchronization. Our only suggestion is the use of a hook mounted system as that compensates at the load rather than the ship but as above, hook mounted systems are very limited operationally.

* D:d ratio (diameter of the sheave, D: diameter of the wire rope, d)

Fibre Rope

One holy grail of subsea lifting is to be able to use fibre rope as fibre rope is effectively weightless when used subsea. This impacts massively on a systems capacity in really deep water as the lift rope can end up weighing more than the load on the end of the hook.

The main reason why fibre rope has yet to break through into the mainstream is that fibre rope tends to extend when loaded so creates all sorts or problems. For example, heave compensators rely on the rope being a set length for accuracy. Any system that uses lengthening and shortening of the lift rope will have issues if that rope is never the same length twice!

In addition to this major problem, fibre ropes also have a tendency to ‘work harden’ and be more succeptable to fatigue issues.

Use of fibre ropes with heave compensation is not impossible and major companies are working on various arrangements to limit the issues although there is still some way to go!

Renting and Buying Guidelines

Putting theory to one side, the practical realities of purchasing a heave compensator tends to require the buyer to focus on a few simple points:

Performance: Anyone buying or renting a heave compensator needs to specify two key things: the size of the wave and the period of the wave. In general, most heave compensators are sized based on being capable of dealing with a sinusoidal wave of +/- 2.5m (5m peak to trough) across a 10 second period. If your requirements differ from this you need to be clear with the supplier as any change away from the typical specification will significantly impact on the power requirements and also the stroke if it is a cylinder system.

Accuracy: Rather than ask about whether you require PHC or AHC, it is perhaps better to ask ‘what level of accuracy do you want?’. If you need a consistent +/- 100mm accuracy across a wide range of loads then you need AHC. If accuracy isn’t as important and it is only for a one-off load then PHC probably suits your operation better. Whether you go for AHC or PHC massively impacts on the power and price so it is an important consideration from the very start of the process.

Power: This only applies to AHC! If you have power generation limitations on the vessel you need to clearly state them as it could lead to the requirement for PHC elements in the AHC system which will again impact on the price.

Cost: A typical hook mounted PHC system can be as little as a few hundred $/£/Euro per day to rent. It is unusual to find rental AHC systems but they are out there and they tend to be in the region of a few thousand $/£/Euro per day to rent. Rental AHC units will be for relatively small tonnages (up to around 30Te) whereas rental PHC systems can be multiple hundreds of Te. Most AHC systems are bought and many buyers are shocked by the price. As an example, a low tonnage system (5Te) would be in the region of two or three hundred thousand $/£/Euro for a very basic arrangement, probably without a HPU. The high price is due to two things: the cost required for the powered elements and the cost required for the control system. An MRU alone can be $/£/Euro 10,000.00. The steelwork cost is no more expensive than a normal non heave compensated lifting device (e.g. winch) but it is the shear power and step change in control that pushes the price up so far.

Wire route: Always consider where you plan on putting the heave compensator before you make any decision on what system you want! For example: if you have little space then you can’t really have an in-line system as you need to respect the fleeting angle of the winch the wire is coming from. If you want to put the heave compensation on a crane then fleeting angle is even more of an issue as the heave compensator will have to be a good distance along the boom so will impact on the load capacity of the crane.

S&A has a questionaire that we tend to pass to potential clients. We would advise anyone considering buying or renting a heave compensator to first fill out this document in order to ensure they can answer the questions a supplier may have.

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