Float over deck installation is becoming increasingly popular and reliable in recent years. The float over technology uses a variety of float over systems and lets large platform topsides be installed as a single integrated package without the use of a heavy lift crane vessel. This allows not only the elimination of expensive day-rate derrick barges, minimization of offshore hookup, and maximization of onshore testing and commissioning, but also freedom of equipment layout within the deck compared to modular lifting designs.
A complete dynamic mooring analysis tool has been developed which will perform a standardized dynamic mooring analysis as well as code check. The tool will determine the first and second order dynamic response of the vessel using 3-D diffraction on a 3D representation of the vessel hull. The calculated vessel responses include the stiffness effects of the mooring lines. To improve the efficiency of the analysis it has been chosen to perform the analysis in the frequency domain.
The model’s output is a ready for issue report describing the used input, the models employed and the results and conclusions of the analysis. The model runs on the state-of-the-art marine simulation software MOSES from Bentley.
Broadly, the float over analysis is divided into several stages.
An example of stages during passive float over could be as below:
- Standby: The barge with topside is moored in a safe distance from the substructure to start the preparations, such as the vessel’s ballast system, waiting for the installation weather window. For standby stage, operation and survival conditions can be checked.
- Entry (docking): The barge position is controlled by winching in/out the mooring system to enter the jacket slot. An additional mating mooring system that connects the barge to the jacket is normally used to assist the positioning.
- Pre- and mating: The environmental loads are balanced by the mooring and mating systems that keep the barge in position.
In pre-mating phase, the barge is ballasted down to match the LMUs with receptors on top of the substructure legs. The remaining tie-downs are then removed. The stabbing cone is just in contact with the docking cone and none of topside weight is transferred yet.
In mating phase of passive float over operation, the vessel is ballasted rapidly to lower the topside onto the substructure until the entire topside weight is transferred.
- Undocking and exit: The barge is continuously ballasted to create a gap between the underside of deck and the vessel’s Deck Support Frame (DSF) and then the vessel is withdrawn from the jacket slot.
Mooring Model Specification:
- Multibody analysis capability
- 3D Hydrodynamic analysis of mooring spread 1st and 2nd order motions
- Check for two environmental conditions and two tidal current conditions
- Database of steel wire, chains, and synthetic lines
- Check of pipeline crossings
- Offshore spread mooring and/or berth moorings
- Damaged line check including time domain transient mooring analysis
- Results presented as capacity rosettes as function of environmental heading
- Compliant with API, DNV Posmoor, GL-Noble Denton, IMO guideline
- Approved and used for various offshore projects
