Today hard riser, dry tree systems not carried on a bottom founded structure are supported by either a TLP or Spar. Both of these systems directly or indirectly use buoyancy to achieve the riser tensioning. The use of buoyancy requires that the relative motions of the buoyancy and the risers be kept small. The TLP does this by tendon restraining the buoyancy, while the Spar does this by creating a somewhat quiescent water shaft in which buoys can effect a relatively uniform riser tension. The physics of an FPSO require that the tensioning system cope with draft changes and larger relative motions between riser and floater. A weight-based riser tensioning system has been developed and tested which is able to cope with both the large stroke and relative motions required to make hard risers practical for FPSO systems.
Deep-water hydrocarbon production in depths above 500 m uses two generic modes of development. This involves either an FPSO with wet subsea trees and flexible risers, or motion restrained floaters like TLP or Spar with dry trees and hard risers. Dry tree systems are the preferred means for producing these reservoirs as it gives better well control, intervention and workover leading to optimum reservoir depletion. Unfortunately developments of fields in remote areas often require storage and thus FPSOs with wet trees make better economic sense. To overcome this dilemma, a riser tensioning system has been developed which enables dry tree production from a spread moored FPSO.
To realize a proper FPSO tensioning system, the system must be designed to cope with draft changes and with larger and faster motions between the FPSO and riser. To deal with this a tensioning system based on weight rather than buoyancy was developed. The weight lateral motion is controlled by placing it in the water, well below the FPSO from which they are suspended by tension members running over sheaves. This riser tensioning system named Tension Leg Deck (TLD) will be detailed. The basic physics of this tensioning system will be described along with corroborating model test results. The present day existence of hardware required to construct this tensioning system will also be discussed.
Systems that can support hard risers for dry production trees and surface BOP offer many benefits over those using wet trees and subsea BOP. The main benefits are those of surface control, access and the ability to perform low cost interventions and workover over the life of the field. The floating platforms required to support these hard risers in deep water are however more costly than floating platforms for wet subsea production and drilling. Examples of these lower cost platforms are weathervaning FPSOs which are being used to receive, process, store and offload crude from subsea or fixed surface facilities. More recently spread moored FPSOs are being used and considered for milder deep water areas like West Africa and Brazil.