| Add RSS Feed

Lateral current may cause a large ‘leaning’ deflection, and/or vortex-induced oscillations of the buoyancy can and tower. Anti-VIV strakes will increase current drag and overall deflection from vertical. 2H and Subsea7 introduced the Grouped SLOR concept to overcome problems of widespread seabed connections, and buoyancy can and mooring line spacing with a plurality of SLOR risers. It may be possible to control lateral deflections of Grouped SLORs in current by installing guy lines to the upper riser guide frame.

A flowline to a single or multi-line tower base requires a connecting spoolpiece, which needs:

• Seabed space for installation
• Two mechanical connections with reliable seals
• Ability to cater for any flowline expansion.<.ul>

  Tower risers do not usually include umbilicals, which have been arranged separately in either lazy-S or lazy-wave configurations.

  Petrobras plans to install five single-line free-standing hybrid risers (FSHRs) to its disconnectable Cascade-Chinook FPSO slated for the US Gulf of Mexico in around 2,500 m (8,202 ft) water depth. These risers are similar to the SLOR-type described above.

  Lazy-S risers

  Coflexip installed the first single lazy-S flexpipe riser in the North Sea in 1984. The upper U-shaped catenary absorbs vessel and wave motions, and the tethered mid-water buoy maintains the lower J-catenary and touch-down point almost static. Multiple lazy-S riser systems were installed to North Sea semi-FPVs at Ivanhoe/Rob Roy (on stream 1989) and Buchan fields (operating via lazy-S since 1999).

  Many FPSOs operating worldwide have multi-line lazy-S risers arranged over tethered buoyant mid-water supports. A 2001 survey of 277 flexpipe risers operating in the North Sea and West of Shetlands offshore area showed that approximately 50% were arranged in the lazy-S configuration. In water depths to 300 m (984 ft), the buoys are often constructed as large horizontal tubes or cylinders, which can be de-ballasted by displacing a water-fill with air or nitrogen. Arches may be positioned over the tubes to support riser pipe overbends at larger radius. The associated tethers are made from synthetic fiber rope or steel chain.

  Deepwater riser support

  In March 1999, Foster Wheeler presented its deepwater riser support (DRS) concept at the Deepwater Pipeline Conference in New Orleans. The DRS for DLSRs comprises a 20-50 m (66-164 ft) long beam-type support for lower catenary hangers and overbend arches, supported by buoyancy tanks that are rigidly attached above each end. The assembly is held at setting depth by just two main tethers in a single plane.

  In a DLSR system, the J-catenaries can be flexpipes, umbilicals, integrated production bundles (IPBs), SCRs, or towed bundles. SCRs and steel bundles can only be used where the J-catenary is sufficiently long to allow the steel pipe to behave in a substantially flexible manner. Spare slots along the DRS will allow extra risers to be added when required.

  Previous Page
  | Next Page
  

  Page 2 of 5

  Volume 68  Issue 9   September 2008