It is well known that faired cables gives less resistance to motion, or cable drag, of a cable that is moving through the water.
The marine seismic exploration companies are attempting to put more and more cables into the water to improve efficiency, performance and the area explorated per unit of time. Drag reduction and elimination of cable strumming becomes therefor all-important.
Prior art fairings ranges from ribbon or hairy (close-knit weave around cable, with tufts or strings) fairings, to "flag" style triangular-shaped material (typically sewn canvas or similar), to hydrofoil-shaped rigid segments along the cable. The ribbons and hairy fairings have the advantage of being easy to handle and withstand wrapping in multiple layers of cable on a winch drum, and then retain its original form when deployed. The primary objective from using these types of fairings is to reduce cable vibration, or strumming; however, their hydrodynamic performance as it relates to actual drag reduction is limited.
Improved hydrodynamic performance is possible with a drop shaped fairing. For example U.S. Pat. No. 5,410,979 describes non-rotary drop shaped fairing made of metal to reduce vibration on marine tubular pipes (e.g. at an offshore rig). This fairing is acceptable on permanent pipes where the direction of flow does not change but it is not satisfactory on a towed cables.
Hydrofoil-shaped rotary fairings are normally made of relatively hard plastic or rubber-like materials. Some of the hard fairings must be stripped away form the cable before the cable is wrapped on a drum. One solution for rapid and relatively easy wrapping and stripping off a cable is described in U.S. Pat. No. 4,365,567. However this method is not fully acceptable for towed cables that is wrapped on a drum when not in use.
To give some flexibility to hydrofoil-shaped fairing to make is possible to wrap the faired cable on a winch drum, U.S. Pat. No. 4,567,841 and U.S. Pat. No. 4,700,651 describes fairing segmented into short interconnected sections. A faired cable according to those solutions may be wrapped on a one layer drum and is therefor acceptable for short cables. For long cables (for example, in excess of 250 meters) as used for seismic systems, however, this solution is not fully acceptable as the hard fairing are too vulnerable for damage if wrapped up on an multiple layer drum.
Prior art fairings does also include fairings made of pliable material folded around the cable and stitched together to make a drop shaped fairing. A fairing of this kind may be wrapped on a drum without breaking the fairing. However, the fairing does often fail to resume the drop shape as it is twisted around the cable. On long cables the pliable material is additionally subject to an enormous drag that can tear the material. An additional problem is that the fairings made of pliable material is what is called &lt;&lt;ballooning&gt;&gt; resulting from the fairing &lt;&lt;puffing&gt;&gt; out loosing its hydrodynamic shape.
There is, therefor, still a need for a fairing that is pliable enough to withstand being wrapped in multiple layers on a winch drum and then come back to its original hydrodynamic profile when deployed.