Operations on a vessel

Working with cranes on barges? This page has what you need to know when operating on the vessel, including: mooring, ballasting and vessel arrangement.

Moving a crane on and off a vessel

Crane work can involve a land-based crane on a vessel that requires the crane to be transported on and off the vessel. For this type of operation, a detailed lift plan should include a method statement and calculated steps to load and off-load the crane. It is recommended to raise or lower the floating vessel along its longitudinal centre line without a load on the hook with as low centre of gravity height as possible.

A crane can be loaded by its own propulsion using its wheels or tracks (Ro-Ro), or can be lifted on and off by a second crane.

When doing this, consider the points below.

Roll-on/Roll-off (Ro-Ro)

Ro-Ro operations may need:

• a load-in/out plan to address ballast and Ro-Ro ramp calculations
• quay height assessment
• ballasting tidal marks
• mooring arrangements.

The incline of the floating unit may be increased during Ro-Ro, so make sure to prevent sliding. All deck surfaces should be above water. The entire bottom of the floating unit should be submerged.

A pre-set ballast with Ro-Ro is a simpler operation but a live ballast can generally accommodate a larger crane mass. It is preferred to Ro-Ro cranes longitudinal to the vessel with less trim, rather than broadside with list where the vessel is longer than wider.

Consider the structural loading (including point loading from stiff tracks and Ro-Ro ramps) in the vessel structural assessment.

To prevent structural or stability issues, take into consideration the use of:

• fenders
• moorings
• degrees of freedom of the different elements.

Ro-Ro operation

Lift-on/Lift-off

A crane can be lifted on and off a vessel by a second crane. If lifting the crane on board, an assessment of the quay strength at the lift location should be included. Often larger crawler cranes are lifted on to vessels in smaller sub-assemblies. The location of the sub-assemblies and their structural impact on the strength of the vessel or the quay also need to be considered.

Breaching the vessel

Cranes are not always driven onto a floating vessel. There are cases when a crane drives onto a beached vessel via ramps and then waits for the next high tide before re-entering the water. In these cases, a survey of the grounding area should be conducted. This is to make sure the resting area of the vessel is free of stones and debris. A beached vessel can have higher structural loads on external vessel plates and cause concentrated load paths through internal frames and bulkheads.

Tidal movement

Do not neglect tidal influences that might occur to ports and inland water. It's particularly important to consider tidal movement when loading and off-loading from a vessel. Loading and un-loading may need a special time slot to account for the tide. As these operations last a certain period of time, this needs to be observed as well.

Check if tidal movements take place at the location of the vessel operation. If so, take the effects of the tidal movement into account for:

• the load out plan
• designing mooring arrangements
• connecting walkways to the vessel.

Mooring of a vessel

During lifting operations, it is recommended to secure the floating vessel. A vessel can be secured to any variety of specially constructed areas (such as piers and quays). Or a vessel may be secured by using anchoring arrangements (such as spuds and spudwells), or a combination of both.

Mooring lines

Mooring is often done using thick ropes called mooring lines or hawsers. The lines are fixed to deck fittings on the vessel at one end and to fittings such as bollards, rings, and cleats on the other end.

Mooring requires cooperation between people on a pier and on a vessel. Once a mooring line is attached to a bollard, it is pulled tight. Large vessels generally tighten their mooring lines using heavy machinery called mooring winches or capstans.

The heaviest vessels may need more than a dozen mooring lines. Small vessels can generally be moored by four to six mooring lines.

When designing mooring arrangements, the allowed movement of the vessel should be checked against the planned operations. Lifting operations will have different movement restrictions than Ro-Ro operations will do. The outcome of this check will influence the choice for a mooring line material.

Mooring line material

Nylon

Mooring lines are usually made from manila rope or a synthetic material such as nylon.

Nylon is easy to work with and lasts for years, but it is very elastic. This elasticity has advantages and disadvantages.

The main advantage is that during an event, such as a high wind or the close passing of another vessel, stress can be spread across several lines.

But if a highly stressed nylon line breaks, it may part catastrophically. This causes snapback, which can fatally injure bystanders. A blow from a heavy mooring line carries much force and can inflict severe injuries or even sever limbs.

Steel

Mooring lines made from materials such as steel, Dyneema® and Kevlar® have much less elasticity. But such lines do not float on water, and they do tend to sink. They are relatively more expensive than other sorts of line.

Wire rope

Some vessels use steel wire rope for one or more of their mooring lines. Wire rope is hard to handle and maintain. There is also risk associated with using wire rope on a vessel's stern near its propeller.

Combinations

Mooring lines and hawsers may also be made by combining wire rope and synthetic line. Such lines are more elastic and easier to handle than wire rope, but they are not as elastic as pure synthetic line.

Mooring of a vessel during loading/off-loading and lifting operations

Loading/off-loading and lifting will normally be considered weather restricted operations. Take into account the following weather conditions for the loading/off- loading and lifting operation:

• the forecast reliability for the area
• the tidal movements in the area
• the duration of the operation, including a suitable contingency period for the exposure of the worksite
• the time required for any operations before or after the loading or lifting operation, including vessel movements and moorings, ballasting, system testing, final positioning, and initial sea fastening
• currents during and following the operation (if applicable)
• the wind area of the cargo and the vessel.

When designing mooring arrangements for vessels during loading/off- loading operations, make an allowance for draft adjustment of the vessel as a result of increasing or decreasing loading of the vessel during the operation. This is a point of attention when short mooring lines between the quay and vessel are used.

Safe working load

Mooring involves a range of parts, including:

• mooring lines
• wires
• shackles
• mooring furniture
• bollards
• chocks
• cleats
• clench plates
• winch to deck anchoring arrangements
• spuds
• spudwells
• other steel components.

All of these parts should be designed to the certified safe working load where applicable. Or to give a factor of safety of not less than 3.0 on the breaking load for moorings.

Where a safe working load can't be determined (for example, for bollards or chocks), then its adequacy should be documented by design calculations offset against the maximum expected loads.

Ballasting of a vessel

All calculations and engineering standards used in this guide assume the vessel used in the engineered operation is “dry’’. This means no bilge water or free-floating fluids in holds, tanks or bilges. Free fluid movement in the holds or tanks will have a negative impact on the stability of a floating vessel.

So it is important to check the vessel condition before starting any operation on deck. This is to establish the dry or filled state of the tanks, holds and bilges to find out the starting situation of any operation.

During planning, consider the source of the water (freshwater or saltwater ballasting) and environmental policy for the discharge of the ballast water during operation.

The ballast system should be checked to make sure that:

• the system is operative
• it is still the same as during planning (often changes over time)
• the operator is familiar with the system.

Pre-ballasting

Sometimes loading of a vessel can lead to a heel or trim angle. These effects can be reduced by pre-ballasting the vessel. This ballasting should be carried out per an engineered ballasting plan, based on the vessels’ as built drawings. Care should be taken that tanks used to hold the ballast water are completely filled, to avoid free surface effect. If pre-ballasting is incorporated, ballasting during lifting operations may not be necessary.

Ballasting during lifting operations

In some cases, lifting of a load with a crane on a vessel can lead to unwanted heel or trim angles. This is as a result of the load moment introduced to the system.

It is possible to use ballasting as a way to correct the unwanted heel or trim angles of the system. But this can only be done based on a step-by step engineered lifting procedure. In this case, the movements of the crane and the required ballasting are aligned with each other.

A correct timing of all interacting activities is important. Ballasting is a time-consuming activity in comparison with the operational speed of a land-based mobile crane. So, during lift planning, consideration should be given to the operational movements that may impact the balance of the vessel (such as hoisting, slewing, booming up or down).

Crane stands on vessels

Before positioning a land-based mobile crane on a vessel, check the vessel’s structural integrity. Calculate the static loadings to the floating vessel from the crane, including:

• erection
• assembly
• lifting loads while in operation, or travelling with a load.
• check for admissible deck loadings.

These should also take into consideration deck loadings as a result of the placement of cargo or loads to be lifted on the vessel deck and ballast conditions.

Position of the crane stand

The position of the crane stand on the vessel deck is determined by these factors:

• the preferred position for the execution of the designed lifting operation
• the location of trusses and bulkheads in the vessel
• the stability requirements for the ‘crane and vessel combination’.

The most suitable position of the crane on the vessel should only be decided after making an engineered lift plan. The plan should take the above factors into consideration.

If the crane can't be positioned centric on the vessel deck (preferred location), it is recommended to appropriately pre-ballast the floating vessel, compensating the eccentric position of the crane to keep additional vessel inclination to a minimum.

Strength of the vessel and crane stand

A check on the bending strength of the vessel in the most unfavourable load case should be performed as part of the overall engineered lift plan. Attention should be paid in the pre-ballasted bow/stern lift cases. The structural strength should be calculated by a verified, acceptable engineering method.

Local and global structural assessments are necessary to assess areas such as:

• local deck plate
• frames
• complete vessel bending strength.

Crane load distribution

The deck carrying capacity of a vessel may not be capable of supporting the bearing pressure directly from the crane. So it may be necessary to distribute the bearing pressure. Making sure the crane doesn't harm the boat is easier when the weight is spread between the crane tracks, outriggers and the boat deck.

Care should be taken not to create an extra ‘sliding surface’ between the crane and the vessel deck when using steel load spreaders. The deck carrying capacity of most vessels varies between 10 t/m2 – 30 t/m2.

Track pressures are much higher. Distribution needs to travel into frames and bulk heads via longitudinal load spreaders. Load distribution is part of the ‘crane and vessel combination’. The extra weight from the use of load spreaders should be taken into account.

Example of load spreading

Vessel arrangement

The vessel arrangement should be designed so that:

• there is enough room for personnel to safely access and egress the crane and crane components as needed for the operation
• all crane components can be transported on board of the vessel, next to the crane before lifting into position, or
• all necessary lifts of crane parts can be done safely from the shore onto the vessel. Special attention should be given to the handover of taglines from shore to vessel
• the vessel area should allow space to lay the boom down during heavy winds or storms
• the handling of crane parts can be executed without the need to lift over persons or personnel accommodations.