The offshore installation method that uses controlled ballasting to land a topside directly onto its jacket
Definition
Float-over installation is an offshore construction method in which a topside module — the upper deck of an oil or gas platform — is transported on a barge, floated over the substructure jacket, and landed onto the jacket leg supports by controlled ballasting of the barge. The topside is supported during transport on Deck Support Units (DSUs) mounted to the barge. The barge descends as water is pumped in, the topside transfers load progressively onto the jacket supports, and the barge is deballasted and withdrawn once load transfer is complete.
The method allows large integrated topsides to be installed as a single integrated operation, eliminating the need for offshore module integration and the extended offshore hook-up campaign that modular installation requires. It is particularly suited to topsides that are too heavy for conventional crane vessels to lift, or where crane vessel availability or cost is prohibitive.
Float-over topsides have ranged from a few thousand tonnes to approaching 50,000 tonnes. The heaviest documented to date is the 50,900-tonne Hebron topside (2017), reported by marine contractor Semar as the largest float-over mating yet conducted†. The method has been applied in the North Sea, the Gulf of Mexico, offshore West Africa and in shallow-water Asian developments.
The Float-Over Sequence
The topside is fabricated onshore and skidded or lifted onto the transport barge. The barge is towed to the installation site and manoeuvred between the jacket legs — typically using mooring winches, tugboats and thruster-assisted positioning. Clearances between the barge hull and the jacket legs are typically very small, requiring precise control in wave and current conditions.
With the barge positioned and the topside directly over the jacket supports — fitted with Leg Mating Units (LMUs) and guidance cones — ballasting begins. As the barge descends, the topside load transfers from the barge onto the jacket through the LMUs, which absorb vertical and horizontal mating loads. Wave-induced motions during this phase must be managed carefully; dynamic loads at the mating interface are the governing structural consideration.
Once load transfer is complete, the structural connections between topside and jacket are made, and the barge is deballasted and removed. Offshore hook-up and commissioning then proceed.
Commercial Risk Factors
Float-over operations are weather-sensitive. The installation window is typically defined by maximum wave height and current conditions at the mating interface. A vessel mobilised and on-station is incurring day-rate regardless of whether conditions permit the operation to proceed. Weather downtime can extend campaigns significantly, with consequent day-rate exposure.
The jacket must be installed and verified before the float-over can proceed. Any delay to jacket installation — from fabrication, transportation or seabed preparation — directly delays the topside float-over and extends the offshore campaign.
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Float-over installation — 6-step sequence, leg mating unit detail and commercial risk factors
Original diagram — EE&HL Network 2026 · In preparation
Diagram: Original — EE&HL Network 2026
† Reported topside weight for Hebron varies by source (47,000–50,900 t) depending on scope basis. 50,900 t is the figure used by the marine contractor for the mating operation itself.
Frequently Asked Questions
Float-over installation is an offshore construction method in which a topside module is transported on a barge, floated over the substructure jacket, and landed onto the jacket leg supports by controlled ballasting of the barge.
The barge is manoeuvred between the jacket legs and positioned with the topside directly over the jacket supports. Ballasting descends the barge, transferring load progressively onto the jacket. Once load transfer is complete, structural connections are made and the barge is withdrawn.
Weather. The installation window is defined by maximum wave height and current conditions. A vessel on-station accumulates day-rate regardless of whether conditions permit the operation to proceed.
A Leg Mating Unit (LMU) is a shock-absorbing structural interface, typically incorporating engineered elastomeric pads, mounted on the jacket leg or topside leg. LMUs provide guidance during docking and absorb the vertical and horizontal loads generated as the topside is lowered onto the jacket during ballasting.
