Ships – Towing or pushing – Cable fairing
Reexamination Certificate
1999-10-27
2001-06-12
Avila, Stephen (Department: 3617)
Ships
Towing or pushing
Cable fairing
Reexamination Certificate
active
06244204
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an improved fairing and fairing assemblies for underwater cables, particularly towed cables, to reduce drag resistance compared with a unfaired cable, when the cable is moving relative to the water. The invention also relates to a faired cable provided with the improved fairing.
BACKGROUND OF THE INVENTION
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 <<ballooning>> resulting from the fairing <<puffing>> 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.
SUMMARY OF THE INVENTION
A first aspect of the invention relates to a airing for a towed cable comprising a pliable material folded over the cable and stitched together to give a substantially drop shaped cross section, wherein the flexible material is segmented into individual sections in the longitudinal direction of the cable, that each section at its upstream end is fastened to a free-rotating, but along the cable non-sliding fastening devise and that each section independent of each other may rotate around the cable together with its is fastening devise.
Preferably the fastening devise is prevented from sliding along the cable by an anti stacking ring fixed to the cable at the downstream end of the fastening means.
It is also preferred that the fastening devise is prevented from sliding along the cable by an anti stacking ring fixed to the cable and wherein the anti stacking ring is placed in a cut out slot it the nose section of the fastening devise.
Preferrably the fastening devise has a substantially drop shaped cross section.
It is also preferred that a bridge is stitched across the cross section of the pliable part of the fairing to prevent the fairing from ballooning during towing.
A second aspect of the invention relates to a faired tow cable provided with a fairing comprising a pliable material folded over the cable and stitched together to give a substantially drop shaped cross section, wherein the fairing is segmented into individual sections where each section at its upstream end is fastened to a free-rotating, but non-sliding fastening devise attached to the cable and that each section independent of each other may rotate around the cable together with its fastening devise.
A third aspect of the invention relates to an anti stacking ring for a towed cable, wherein the ring is made of a glassfibre tape coated with water activated polyurethane resins, where the ring is moulded in situ onto the cable.
A fouth aspect of the invention relates to a method for the production of a anti stacking ring for a towed cable, said method comprises the following steps:
a) cleaning and rubbing of a segment of the cable;
b) attaching two clamps in a distance to each other equal to the width of the antistacking ring;
c) soaking of a glassfibre tape coated with polyurethane resins that are activated by water;
d) wrapping the tape around the cable between the clamps;
e) smoothening of the ring surface;
f) removing of the clamps.
The fairing according to the invention is primarily for use on seismic lead-in tow cables.
It also has the potential to be used on ROV and other navy and oceanographic tow cables where it is important that the faired cable can be repeatately wrapped on a winch drum without damaging the fairing.
REFERENCES:
patent: 1104254 (1914-07-01), Eddelbuttel-Reimers
patent: 3241513 (1966-03-01), Rather
patent: 3454051 (1969-07-01), Goepfert et al.
patent: 3712261 (1973-01-01), McLelland et al.
patent: 3899991 (1975-08-01), Chatten
patent: 4075967 (1978-02-01), Silvey
patent: 4365567 (1982-12-01), Kuhar
patent: 4567841 (1986-02-01), Hale
patent: 4700651 (1987-10-01), Hale
patent: 4756269 (1988-07-01), Holcombe et al.
patent: 5410979 (1995-05-01), Allen
Weyman H. Nicolas
Weyman Heather
Avila Stephen
Nixon & Vanderhye PC
Odim Holding ASA
Weyman Heather
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