The basic concept in the float-over is to transport the topsides on a vessel or vessels in one piece, position the vessel(s) over the substructure, and lower the topsides onto the substructure while maintaining the position of the vessel. While this basic principle of load-out and transportation remains the same, methods and systems used to execute the concept vary significantly, and pose numerous technical and operational challenges. Semar has a wide experience in float-over operations, for the installation of large topsides decks onto fixed or floating platforms. Semar personnel have been engaged in most of the topside inshore mating operations that have been carried out for platforms on the Norwegian sector of the North Sea. Semar performs all engineering associated with the float-over operation and supervises the marine operation. The float-over operation enables large integrated topsides to be installed, thereby minimising offshore hook-up and commissioning, without the use of large crane vessels.

›  Hydrostatic and hydrodynamic analyses

›  Global and local integrity verification

›  Structural design of:

›  Grillages

›  Vessel reinforcement

›  Seafastening

›  Removal aids

›  Outfitting

›  Bumpers and fenders

›  Marine operation

›  Fabrication follow-up

The Hebron field was developed with a fixed PDQ platform. The topside consists of an integrated deck that was installed on a single-shaft concrete GBS. The integrated deck structure consists of utility, well bay and process areas, and also supports the living quarters, the drilling facilities and a flare boom. The Hebron platform is located in harsh environment offshore Newfoundland and Labrador, 350 kilometres southeast of St. John’s. The field is approximately eight kilometres north of the Terra Nova project, 31 kilometres southeast of the Hibernia project, and 46 kilometres from the White Rose project. The water depth at Hebron ranges from 93 to 100 m.

The Hebron topsides weight is 50 900t, and is the worlds heaviest float-over mating ever conducted. The diameter of the GBS base is 122m with a concrete volume of 132 000 cubic metres.

The main activities of the Topsides Mating Operation included,

›  Preparatory activities

›  Concept development and detail design of structural items (grillage, seafastening, etc.), systems (ballast, sounding, survey, etc.) and equipment (winches, fairleads, etc.)
›  Developing of commissioning, testing and operational procedures
›  Procurement and prefabrication
›  Mobilisation of barges equipment and crew

› Marine operations

›  Lift-off of Topsides from the Pier supports
›  Tow to the deep-water site
›  Mating of Topsides and GBS

› Projects termination

›  Cleaning of mating barges
›  Demobilisation of barges, equipment and crew

The structural strength and of the barges, stability and ballasting of the barges were analysed by Semar through all the steps. Analyses were performed in AutoHydro to determine the ballasting configurations, PERMAS was used to verify the global structural strength, while Abaqus/Standard was used for the local grillage analyses. Semar modeled the actual float-over conditions, to establish geometrical clearances and potential clashes between the topsides, the barges and the grillages. Semar also designed the grillages, sea-fastening and the barge reinforcements. Semar personnel supervised the whole operation at Bulls Arm, New Foundland. The engineering support followed the operation doing continuous analyses verifying the Catamarans integrity throughout all the loadsteps.