Free Floating
Free floating float-offs place the highest stability requirements on the barge. The barge must be able to maintain sufficient stability throughout the entire float-off operation, without relying on the seabed for stability. Typically, in this instance the barge would be fitted with floatation tanks or caissons positioned forward and aft. The size of these caissons is determined by the stability characteristics of the given cargo and barge combination. This requires detailed assessment of the barge stability characteristics and impact of the cargo buoyancy and VCG. For free floating float-offs there are two main approaches to submerging the barge:
1. Level Trim: This involves ballasting the barge evenly to maintain a level trim. This approach is typically used by large floating docks with sufficient stability and waterplane area to lift most cargo without low stability being an issue. The worst case stability stage is typically when the water level is at the height of the keel blocks, therefore the main deck is completely below the water with no portion of the cargo yet providing a waterplane area contribution. In this condition the cargo VCG is still relatively high above the waterline with no impact of cargo displacement aiding the stability.
2. Stern Down: When the above level trim approach cannot be applied, a stern trim approach is often required. This involves trimming the barge by the stern (typically to between 4 and 5 degrees) submerging the aft end of the barge and cargo while maintaining stability by keeping the forward end of the barge above the water, thus maintaining its waterplane area contribution. As the barge is trimmed and the cargo submerged, the barge VCG is lowered by the addition of ballast water, the cargo VCG is lowered, the cargo adds a component of waterplane area, and the cargo displacement reduces the cargo load acting on the barge. These benefits can be utilised until the combined barge and cargo have sufficient stability for the forward end of the barge to be lowered. Typically, the lowest stability will occur as the forward deck edge passes below the waterline. At this point all stability will come from the combined waterplane area of the cargo and caissons. The bow is then brought down until the barge is at level trim and then a slight parallel submergence towards float-off.

Water depth
The water depth required to carry out a float-off needs to be sufficient that the semi-submersible barge can submerge to allow the cargo to float free and achieve the minimum required clearance, typically 0.5m, however more may be preferable. Depending on the type of operation additional water depth requirements may be set. The water depth for a grounded float-off is critical and is dependent on the tidal range and submersion capabilities of the barge. For a free-floating float-off, sufficient water depth to maintain a clearance between the barge and seabed is required; if this is to be achieved by relying on the tide then an allowance should be made for suitable tidal windows and possible tidal cuts.

Barge selection and capability
When selecting a semi-submersible heavy lift vessel for a float-off, all of the above factors need to be considered. In some cases, the specific requirements of linked operations such as load-out, transit or mooring may need to be considered. There is a wide array of semi-submersible heavy lift vessels on the market and ensuring the correct vessel is selected is of paramount importance. In some instances, there may not be a suitable vessel available, and modifications may be required. Some key considerations should be:

• Vessel dimensions – to aid cargo stowage possibilities. In some instances, vessels require lengthening or widening to suit a particular cargo.
• Max submersion and water over deck capability – this is often defined by the height of the caissons and structural capability of the barge. The minimum support height for the cargo, cargo draught and required clearance will define the water over deck requirements for a given float-off operation. Efforts can often be made to reduce the cargo height above deck by minimising grillage heights and the cargo draught by launching midway through a build and outfit process keeping the displacement as low as possible.
• Strength – the strength of the vessel should be considered, both for longitudinal strength and deck loading capacity.
• Ballast system – The ballast pump flow rate and system will define how fast or slow a ballasting operation can take place. This may have an impact on certain float-off operations where the operational timeline becomes too long. Due to the nature of float-offs, there is significant amounts off ballast water to be added to submerge the vessel.
• Grounding capability – Some vessels are classed for grounding. This should be checked and confirmed before planning a grounded float-off operation.
• Stability capability – This critical area covers the dimensions of the floatation tanks or caissons, and if they are suitable for the float-off requirements, often this requires a full feasibility study to be carried out to assess if the available caissons meet the stability requirements. In some cases, project specific additional caissons may need to be added however build, installation and removal costs should be considered here when comparing similar vessels.

Multibody interaction
The interaction between the cargo and barge can lead to multibody effects commonly known as sueing loads. For many float-off operations, it is possible to match the trim of the cargo and barge so that the cargo floats off with negligible sueing loads. When there is a significant difference in trim angle, which would cause one end of the cargo to lift off the supports first, a large sueing load occurs. This is often less than the static load on the supports in air, however as the trim difference increases, the load on a single support point can be overloaded. This type of sueing load occurs during the stern trim operations where the cargo floats off at level trim. In this instance, multibody analysis should be carried out to assess the sueing loads throughout the operation to ensure the supports or cargo integrity are not compromised.

Mooring arrangements
Float-offs are typically carried out either in port, against a quayside or in open water. Float-off mooring arrangements usually consist of mooring winches which can be adjusted throughout the operation to maintain the position of the barge. For quayside operations these are usually positioned on the quay and for open water these can be positioned on the barge. For free floating open water float-off operations, where the vessel excursions are less critical, catenary mooring arrangements may be utilised. Unlike most mooring arrangements, where the vessel draught remains constant as the tide rises and falls, float-off mooring arrangements should also consider the mooring performance in the fully submerged condition where the line angles will be vastly different from the surfaced condition.

Tow off
Once the cargo is afloat it needs to be held in place until the required tow off clearance has been achieved. This is typically done using guideposts or tugger lines. Once the clearance has been confirmed the cargo can be towed off using winches or tugs. Consideration should be made when using
tugs whether the selected tug can pass over the barge deck.