Vibration and drag reduction system for fluid-submersed hulls

Details Vibration and drag reduction system for fluid-submersed hulls Vibration and drag reduction system for fluid-submersed hulls

2000-10-25

2002-02-26

Swinehart, Ed (Department: 3617)

Ships

Floating platform

C114S06700A

Reexamination Certificate

active

06349664

ABSTRACT:

TECHNICAL FIELD
This invention relates to a system for reducing vibrations and drag in fluid-submersed hulls. More particularly, the invention relates to a boundary layer control system that reduces the hydrodynamic drag and shed-vortex induced vibrations on bluff hulls at least partially submersed in water, such as SPARs, marine risers that connect a floating drilling vessel to the ocean floor, and semi-submersible ocean drilling vessels.
BACKGROUND
FIG. 1A
is a side-view of a SPAR
100
. The SPAR
100
is a large un-propelled vessel of generally circular-cylindrical form that is oriented in the sea
102
with its long axis
104
vertical. When ballasted, the SPAR
100
exhibits a very deep draft in comparison with its diameter
106
and/or its freeboard
108
, thus providing a very stable platform of large volume for offshore petroleum production and storage. SPAR vessels can be positioned and restrained by an elaborate system of moorings, such as moorings
110
, which may be anchored to the ocean floor
111
.
When beset by ocean currents
112
, a SPAR
100
will exhibit substantial drag forces and large scale, long period, shed-vortex induced vibrations (“VIV”). As shown in
FIG. 1B
, which is a top-view of SPAR
100
, VIV is induced when currents
112
travel around the hull of the SPAR
100
, forming a down-current pressure gradient
114
and assymetrical circular eddies
116
. The assymetrical nature of these eddies
116
causes the SPAR
100
to oscillate orthogonally relative to the currents
112
. These oscillations are known as VIV.
A system that has been used to attenuate such VIV is the addition of large scale helical “strakes”
118
to the exterior of the hull of the SPAR
100
. While the addition of the strakes
118
may reduce VIV, the strakes
118
actually increase drag and cause the mooring systems
110
to be overwhelmed in the face of currents. This forces the development and use of means other than the strakes
118
to reduce drag and relieve the stress on mooring systems.
FIGS. 2A and 2B
show a marine riser
200
. Marine risers
200
are used to connect a floating drilling vessel
202
to the ocean floor
204
and to provide a conduit for a drill string and drilling fluids. Like SPAR
100
, when beset by ocean currents
206
, marine riser
200
will exhibit substantial hydrodynamic drag forces and VIV. Such forces and motions induce mechanical stresses in, and deflections of, the marine riser
200
and its connection
210
to the drilling vessel
202
and connection
212
to the ocean floor
204
, which ultimately may result in failure or interference with drilling operations.
Drag and VIV have been reduced by the application of fairings
214
to the marine riser
200
. The fairings
214
are enabled passively to rotate about the riser
200
in order to align with the direction of the current
206
to minimize drag. While some drag and VIV reduction is thereby obtained, the procedure for applying and removing fairing segments from riser joints while they are being run and retrieved is lengthy. Slowed riser deployment and retrieval reduces availability and safety of the drilling rig, with important economic consequences. Fairings
214
suffer another disadvantage, in that fairing sections are bulky, expensive, and subject to damage when being deployed through the ocean surface wave zone.
FIGS. 3A and 3B
show a semi-submersible drilling vessel
300
. Such vessels
300
are often configured as a platform
302
supported well above the ocean surface
304
on submerged longitudinal cylindrical buoyancy pontoons
308
. When beset by ocean currents
310
, the pontoons
308
, being generally bluff, cylindrical objects, exhibit substantial hydrodynamic drag due to flow separation. In order to maintain position relative to the ocean floor
312
, such vessels
300
are fitted with a system of moorings
314
and/or powered thrusters
316
to counter the drag forces.
Both moorings
314
and thrusters
316
, however, are expensive, and moorings
314
become impractical in very deep water. The presence of mooring winches
318
adds substantially to the topside weight carried by semi-submersible drilling vessels
300
. This increase in weight reduces the payload capacity of the vessel
300
and impairs its hydrostatic stability.
Hydrodynamic drag of a semi-submersible drilling vessel
300
can be reduced to a degree by rotating the vessel
300
so that the submerged pontoons
308
are aligned with the direction of current
310
and those located down-current are relatively “shadowed” by those located up-current, as shown in
FIG. 3B
, which is an end-view of FIG.
3
A. Newer designs of semi-submersible drilling vessels are intended to be more azimuthally uniform in their hydrodynamic drag characteristics. This uniformity obviates directional drag reduction.
Boundary-layer-control (“BLC”) has been investigated to attain high-lift on aircraft wings and to promote laminar, low friction, flow on wings and elongated bodies. But no such system has been proposed or applied to fluid-submersed bluff bodies to reduce pressure drag and VIV. Unlike aircraft wings, fluid-submersed hulls, such as SPARs, marine columns and risers, and semi-submersible drilling vessels, are generally large, bluff, unstreamlined vertical circular-cylindrical forms or non-circular and/or horizontal cylindrical forms.
The presence of high drag levels and VIV on SPARs, marine risers, and semi-submersible drilling vessels. Drag and VIV may prevent operation of such ocean-deployed vessels, at a high cost to drilling operations. Thus, the inability to substantially reduce drag and VIV may have high economic costs.
Accordingly, while various systems and methods exist for reducing VIV and hydrodynamic drag in fluid-submersible objects, no such system or method reduces both VIV and hydrodynamic drag to a substantial degree. Moreover, while BLC has been applied to streamlined aircraft wings to attain high-lift, BLC has not been designed or applied to fluid-submersed hulls, such as SPARs, marine risers and columns, and semi-submersible drilling vessels. Accordingly, a need exists for a system and method for reducing VIV and hydrodynamic drag in fluid-submersed hulls and for thereby preventing suspension of drilling operations and other functions in the presence of currents in the fluid.
SUMMARY
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.
The present invention is a system for reducing the hydrodynamic drag and VIV on a bluff hull submersed at least partially in a fluid, such as water. The term “bluff hull,” as used herein, means vertical and horizontal circular-cylindrical forms, vertical and horizontal elliptical-cylindrical and elliptical-semi-cylindrical forms, spherical forms, semi-rectangular forms with rounded corners, and other bodies that present a broad underwater profile against currents flowing in the water. The term “bluff hull” thus includes SPARs, marine risers, and pontoons and floatation bodies for semi-submersible drilling structures, all of which are generally deployed in the ocean, in which swift currents often travel that induce VIV and drag on a submersed bluff hull. The term “bluff hull,” as used herein, also includes other types of bluff bodies that are at least partially submersed in fluid, including bridge and pier stanchions and footings, ship and boat hulls, submarines, underwater communication cables, and underwater tunnel exteriors, to cite some examples. This definition of the term “bluff hull” also makes clear that the invention can be applied not only to floating bodies, such as ships and SPARs, but also to bodies that anchored to the bottom of the fluid, such as bridge stanchions and marine risers. Also, the invention is useful in any fluid that experiences or conveys currents and other disturbances, but is especially useful in oceans, seas, lakes, and rivers, all of whic

Affiliated with

Also associated with

Rating

Vibration and drag reduction system for fluid-submersed hulls does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Vibration and drag reduction system for fluid-submersed hulls, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Vibration and drag reduction system for fluid-submersed hulls will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2937567