Float-Over High Air Gap: New Solution

Abstract

Abstract Technip has previously developed, and has widely used, the jacking assisted float-over deck installation concept, called "UNIDECK®", for decks requiring a relatively low air gap. This has demonstrated the advantages of the float-over method which minimizes the overall cost of transport, installation and hook-up by having a deck that can be installed on a jacket as a fully completed and tested single unit without using a conventional crane barge. That concept has been successfully used for relatively benign areas of the world where a low air gap is acceptable but installation is problematic due to the impact of long period swells on the dynamic behavior of the installation vessel. To allow the float-over method to be extended to harsher environmental areas of the world, a methodology is required to allow deck transportation at low level, but then enable the deck to be installed with a high air gap. This paper describes a new float-over solution, which is applicable to platforms requiring air gaps of 20m (66 ft) or more, and deck weights up to 20,000 t (22,000 short tons) and extendable to higher weights. It is called "float-over high air gap" - FOHAG. Introduction With the discovery of oil & gas fields located in remote harsh environment areas of the world, a cost effective alternative is required to relying on crane vessels for installation, the availability of such vessels becoming limited as demand for their services increases around the world. The FOHAG concept greatly extends the geographical range for deck installation using the float-over method as an alternative to using a crane vessel. If only using ballasting for float-over deck installation, the stability criteria during transportation restricts the height of the deck above the transportation vessel and hence limits the maximum finally achieved air gap. To achieve a higher final air gap, the use of jacks allows the deck to be raised up once at site and under suitable weather window conditions. Technip has developed a jacking assisted float-over installation system, called "UNIDECK®", which has been successfully used for decks up to 18,000 t (Ref 1).. This technology uses hydraulic jacking to provide a safe and quick transfer of the deck weight onto the pre-installed jacket. However, the height to which jacks can lift the deck is constrained by the sensitivity of the jacks to lateral forces (at high elevation) and their limited stroke. The newly developed FOHAG solution consists of replacing the limited stroke jacks by a rack and pinion system that has no stroke limitations within all conceivable air gap requirements. The main characteristics of the system are that, during transportation, the deck is at a minimum elevation from the vessel in order to maximize the floating stability and reduce horizontal acceleration applied on the deck. Once arrived at the installation site, the deck is elevated to a sufficient height using a rack and pinion system for the vessel to enter above the preinstalled substructure (see Fig 1).