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Cranes on barges
The vessel
Working with cranes on barges? This page has what you need to know about the vessel.
A vessel is any craft designed for transportation on water. The vessels that cranes are usually used on are barges.
Using a land-based mobile crane on a vessel such as a barge can change the technical requirements and conditions for this type of crane. It may also change the equipment type to a floating crane. Further approvals and/or compliance with standards, regulations, and insurance details may be needed. Before using a land-based mobile crane on a vessel, a review of the regulations, standards and class rules should be carried out.
Vessel movement
Vessels may move in these ways:
- heave
- heel
- pitch
- roll
- surge
- sway
- yaw.
Definitions of these types of movements can be found in the glossary.
Such movements are the result of:
- wave action
- wind action
- ballast configuration of the vessel.
The actual movements and their amount depend on:
- the condition of the vessel
- free floating (during transport)
- anchoring
- mooring
- beaching the vessel.
Crane and vessel combination
The ‘crane and vessel combination’ consists of:
- the floating body
- crane and crane supports
- intended loadings.
All of these factors are important for the safe operation, transport and installation of the crane on the vessel.
It is important to review and approve each individual case of crane and vessel combination by persons with knowledge and experience in naval architecture or marine engineering.
This analysis should consider:
- the forces and loadings that will happen during lifting operations, transport and installation of the crane on the vessel
- environmental circumstances (such as wind, tide and water flow forces).
Class rules and local regulations
Before loading the crane, an inspection of the vessel should be conducted. Marine surveys are completed at specific intervals and may be required for insurance purposes. These surveys will depend on class rules and local regulations. Vessel classification differs worldwide.
Vessels will have a class approved stability booklet produced as part of the marine engineering design during ship building. This booklet provides the limitations on the vessel loading.
Where crane operations on vessels need to be confirmed to be within the original class approved books, organisations need to consult with a specialist such as:
- a naval architect
- a marine engineer
- local regulators for requirements.
This consultation may find out if extra class approval is needed for the operations.
Vessel stability
Stability is the ability of the crane and vessel combination to handle a changing loading condition without too much heeling or turning over. Stability also refers to its ability to get back on an even keel or keep the floating complex stable on a certain inclination.
Lightweight, deadweight and displacement
The vessel’s own weight and the distribution of the weights on the vessel are important for figuring out the vessel’s stability. The vessel’s weight is composed of:
- lightweight
- deadweight
- displacement
- buoyancy.
Lightweight
Lightweight is the weight of the unrigged vessel without any of these elements:
- gear
- fuel oil
- water
- containers
- crew
- provisions.
Examples of what causes changes in the lightweight changes are:
- when the vessel is fitted with optional equipment
- replacing engines, winches or other fixed components.
Deadweight
This is the term for all the weights the crew takes on board during sailing. Deadweight includes:
- equipment
- fuel oil
- water
- crane
- crane supports
- load (cargo)
- crew
- provisions.
Displacement
Displacement is the term for the vessel’s total weight. That means displacement = lightweight + deadweight.
During sailing, the vessel’s displacement changes constantly. For example, as a result of fuel consumption.
Buoyancy
Buoyancy is the force acting upwards on the vessel created by the displaced amount of water. A vessel floating on the water will displace an amount of water equal to the weight of the vessel (displacement).
Centre of Gravity
The centre of gravity (CoG) combines all weight in a single point. It includes all weights on board, including the vessel’s own weight (lightweight). The total weight of the vessel (displacement), including deadweight (such as gear and cargo) can be replaced by one total weight, located in the center of gravity.
For most vessels, the CoG is just above the waterline. The centre of gravity can change as follows:
- cargo and gear on deck pull the centre of gravity up
- installation of new equipment on deck or in the wheelhouse pulls the centre of gravity up.
The results may be:
- a high centre of gravity makes the vessel roll more slowly and can be a danger signal
- vessels may become unstable if the centre of gravity is positioned too high.
Centre of gravity
Centre of buoyancy
All parts of the hull under the waterline contribute to the vessel’s buoyancy. The total buoyancy can be merged in one single point called the centre of buoyancy. This is indicated by the letter B as shown in the image below:
Centre of buoyancy
The centre of buoyancy (B) is not fixed. It changes all the time depending on the vessel’s draft, heel and trim. The centre of buoyancy moves when the vessel heels.
When the vessel is upright and not tilted, the centre of gravity is in the vessel’s centre line. In a straight line below is the centre of buoyancy and the vessel is in balance.
If the vessel is heeled, the buoyancy centre moves immediately off to the side of the vessel.
If the gear and cargo are stowed away safely, there is no weight on board that can move during the roll. So the centre of gravity remains in the same position.
Metacentric height GM
Metacentric height (GM) the distance between the centre of gravity and its metacentre. The metacentric height determines the vessel’s ability to get back on an even keel.
Under a light heeling, the vertical line of buoyancy intersects with the vessel’s centre line. This point is called the metacentre which is indicated by the letter M.
The distance between the centre of gravity and metacentre is called the metacentric height GM.
The GM value is a measure of the vessel’s stability under small heeling, also called initial stability. The higher the GM value, the better the vessel’s initial stability and the harder it is to get the vessel to heel.
The following three situations may arise:
- GM > 0 the vessel is stable
- GM = 0 the vessel is labile
- GM 0 the vessel is unstable.
Righting arm
When the vessel suffers a heel, the centre of gravity and the centre of buoyancy are no longer on the same vertical line above one another.
The vessel is brought out of balance.
Shown in the image below, there is a distance between the vertical line of the vessel’s weight through the centre of gravity, and the vertical line through the vessel's buoyancy:
Righting arm
The horizontal distance between the two lines is called the righting arm. The size of the righting arm is crucial to whether the vessel can straighten up and get back on an even keel. The greater the righting arm, the better the ability of the vessel is to get back on an even keel.
The image below shows how the crew can influence the size of the righting arm depending on how the vessel is loaded:
Size of the righting arm
For smaller heel angles (up to 10º) the righting arm can be calculated using the following formula:
Mr = D * GM * sin (Ø)
Mr = righting Moment
Ø = heel angle ( º )
D = Displacement (Ton)
GM = Metacentric Height (m)
Eccentrically loaded vessels
A crane on a vessel can have an eccentricity to its centre of gravity position. Calculations to reduce the centre of gravity GT to point G results in a load moment ML as shown below:
ML = GT * e * cos(Ø)
K = keel
GT = total weight (floating device & crane & load)
G = total weight (floating device & crane & load) related to vessel centre line
B = centre of buoyancy
M = metacenter
GM = metacentric height
GZ = righting arm
Ø = heel angle
(º) “e” = eccentricity
Float stability is given if: ML GZ * B
Float stability
Vessel stability in different situations
The vessel must be the adequate size (width and length) and capacity to support the weight of the crane and suspended load.
The maximum allowed vessel list and/or trim during crane operation should be:
- lesser of 5 degrees, or
- the maximum allowed by the crane manufacturer or qualified person.
It is common for the vessel and crane combination to be limited by the crane list values from the stability assessment, rather than the combined stability of the crane and vessel. There are several situations that may need special attention to stability of the vessel, explained below.
Lifting a load onto the vessel
Lifting a load onto a floating vessel (for example, from a quay) will change the height and horizontal position of the vessel’s centre of gravity at the same time. This results in vessel heel and/or trim if not compensated. Any extra horizontal movement of the load will also change the CoG.
Lifting a load from the vessel
Lifting a load from a vessel will change the displacement of the vessel. This results in the vessel lifting up and possibly heeling. This change may restrict the ability to place the load back to the previous position.
Sudden release of the load
A sudden release of a load could result in extraordinary heeling to the backside, causing a potential falling back of the boom system. Sudden release of the load may be the consequence of lifting gear breaking or slipping.
Vessel freeboard
The freeboard of the vessel will vary in relation to the loading of the vessel and the position of the load lifted. A vessel has a large righting moment as a result of its cross-section. But this cross-section shape also means that when the freeboard is lost, the righting moment is dramatically reduced instantly. This is why an allowance for a freeboard safety margin should be made.
When engineering lifts using a land-based crane on a vessel, consider the most unfavourable crane position in relation to heel and list of the vessel. In that position, figure out the remaining freeboard.
The recommended minimum allowable freeboard is between 15% to 25% of the vessel moulded depth, with a minimum of 0.30 – 0.40m which increases with vessel length overall. These figures depend on which standard is adopted. Check minimum freeboard guidance through local and international guidance and class rules.