| 摘要 |
Buoyancy modules are integrally shaped externally to reduce VIV formation. The shaped modules are configured and dimensioned to pass intact through a rotary table, so that no time-consuming assembly or disassembly on board a drilling rig is required. Integral molding of the VIV interfering shapes does not add appreciable cost to the system. The integrally molded shaped externally modules of the present invention are inherently less volumetrically efficient, suffering a reduction of 20 % to 30 % less volume than conventional cylindrical modules of the same external diameter. This loss in volume is reflected in an equivalent reduction in buoyancy. The present invention obviates this problem by taking advantage of the fact that it is not necessary to cover the entire riser string with integrally molded shaped modules in order to effect the desired reduction in VIV. Ocean currents are most often restricted to relatively shallow depths, typically the uppermost 300 to 500 feet. In the case of the deep water wells, this amounts to only about 10 % of the total riser length being at risk of damage from currents. In many cases, the lower density of the uppermost modules can be used to advantage to offset some if not all of the buoyancy deficit resulting from their non-cylindrical contours. Thus, in even the most extreme cases, overall buoyancy loss with a system according to the present invention will be only 2.0 to 3.0 % less than that which might otherwise be achieved with conventional systems.
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