Lazy-S risers offer advantages in the ultra deep
Installation uses proven TLP tendon technology
Keith Shotbolt, Consultant
In ultra deepwater -- beyond 1,500 m (5,000 ft) deep -- riser plus umbilical weight off a floating production vessel (FPV) can total many thousands of tons. Weathervaning FPSOs have turret swivel bearings with limited load capacity. Riser system weight applied to a disconnectable FPSO should be kept low because after disconnect, the turret buoy has limited self-buoyancy.
Riser configurations with integral buoyancy include:
- Lazy-wave, with a series of small buoyancy collars around the overbend
- Free-standing multi- or single-line tower, with a large buoy at the top of the steel column
- Lazy-S, with a tethered buoy between the lower J-catenaries and the upper U-catenaries.
Lazy-wave risers
Lazy-wave flexpipe risers are in place with many FPSOs around the world. The lazy-wave overbend with multiple small buoys usually is set fairly deep to minimize riser movement from current and wave action. As depth increases beyond 1,000 m (3,280 ft), setting lazy-wave riser buoyancy at >50% of total depth leaves significant weight applied to the turret.
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A weathervaning FPSO with lazy-S risers (Courtesy Foster Wheeler).
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The disconnectable Stybarrow FPSO is operating in 825 m (2,707 ft) water depth off Australia using lazy-wave flexpipe risers. A major FPSO turret designer has estimated that the maximum practical depth for lazy-wave flexible risers to a disconnectable turret is around 1,500 m, depending on the numbers of lines and lateral current velocity.
Shell plans to install the first S-shaped steel catenary risers (SSCR) or steel lazy-wave risers to a weathervaning FPSO on its BC-10 field in 1,780 m (5,840 ft) water depth off Brazil. The SCR and the SSCR are unlikely to be used with a conventional disconnectable FPSO because they apply too much weight to the turret buoy, and they probably need a flexjoint at the turret connection, which is difficult to access. SBM-Atlantia has recently proposed the MoorSpar concept, which would have sufficient buoyancy to support SCRs, and a swiveling arm for mooring an FPSO.
Tower risers
The ‘vertical steel tower + flexpipe catenary jumper’ type riser, described in US patent 4,182,584 (granted in 1980), has a large buoyancy tank at the top of the tower. The Girassol FPSO off Angola came on stream in 2001 through circular bundle tower risers, and the Plutonio FPSO also has a 1,260 m (4,134 ft) long bundle tower riser containing multiple lines. All bundled tower riser lines must usually be incorporated on ‘day one’ because it is difficult to add extra lines. It may be necessary to incorporate expansion relief for individual lines in this type of construction if they operate at different temperatures.
2H Offshore introduced the single line offset riser (SLOR) at the Kizomba field, which started producing in 2005. The concentric offset riser (COR) version has an outer pipe around the riser pipe, and the annular space between them can be used for transporting lift gas.
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Volume 68 Issue 9
September 2008
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