Hydraulic and earth engineering – Marine structure or fabrication thereof – Structure protection
Reexamination Certificate
2001-01-31
2003-11-11
Lee, Jong-Suk (Department: 3673)
Hydraulic and earth engineering
Marine structure or fabrication thereof
Structure protection
C405S195100, C405S224200, C166S367000, C166S359000, C114S243000, C114S264000, C114S06700A, C114S06700A
Reexamination Certificate
active
06644894
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to methods and apparatus of reducing and/or controlling vortex-induced vibrations (“VIV”), current drag, low frequency drift oscillations due to random waves, and low frequency wind induced resonant oscillations. In another aspect, the present invention relates to methods and use of passive apparatus for reducing and/or controlling VIV, current drag, low frequency drift oscillations due to random waves, and low frequency wind induced resonant oscillations. In even another aspect, the present invention relates to methods and use of flow thru members with marine members for reducing and/or controlling VIV, current drag, low frequency drift oscillations due to random waves, and low frequency wind induced resonant oscillations.
2. Description of the Related Art
When drilling for and/or producing hydrocarbons or the like from subterranean deposits which lie under a body of water, it is necessary to provide a marine riser system for establishing fluid communication between the water surface and the marine bottom. The principal purpose of the riser is to provide a fluid flow path between a drilling vessel and a well bore and to guide a drill string to the well bore.
A typical marine riser system normally consists of one or more fluid-conducting conduits which extend from the water surface to a structure (e.g., wellhead) on the marine bottom. For example, in drilling a submerged well, a drilling riser usually consists of a main conduit through which the drill string is lowered and through which the drilling mud is circulated from the lower end of the drill string back to the surface. In addition to the main conduit, it is conventional to provide auxiliary conduits, e.g., choke and kill lines, etc., which extend parallel to and are carried by the main conduit.
This drilling for and/or producing of hydrocarbons from offshore fields has created many unique engineering challenges. For example, in order to limit the angular deflections of the upper and lower ends of the riser pipe and to provide required resistance to lateral forces, it is common practice to use apparatus for adding axial tension to the riser pipe string. Further complexities are added when the drilling structure is a floating vessel, as the tensioning apparatus must accommodate considerable heave due to wave action. Still further, the lateral forces due to current drag require some means for resisting them whether the drilling structure is a floating vessel or a platform fixed to the subsurface level.
The magnitude of the stresses on the riser pipe are generally a function of and increase with the velocity of the water current passing the riser pipe, and the length of the riser pipe.
It is noted that even moderate velocity water currents acting on a riser can cause stresses. Such moderate or higher currents are readily encountered when drilling for offshore oil and gas in a marine inlet or near a river mouth.
Drilling in ever deeper water depths requires longer riser pipe strings which are subject to greater drag forces which must be resisted by more tension. This is believed to occur as the resistance to lateral forces due to the bending stresses in the riser decreases as the depth of the body of water increases. Accordingly, the adverse effects of drag forces against the riser caused by strong and shifting currents in these deeper waters increase and set up stresses in the riser which can lead to severe fatigue and/or failure of the riser if left unchecked.
There are generally two kinds of water current induced stresses.
The first kind of stress is caused by vortex-induced alternating forces that vibrate the riser (“vortex-induced vibrations” or “VIV”) in a direction perpendicular to the direction of the current. When water flows past the riser, vortices are alternately shed from each side of the riser. This produces a fluctuating force on the riser transverse to the current. If the frequency of this harmonic load is near the resonant frequency of the riser, large vibrations transverse to the current can occur. These vibrations can, depending on the stiffness and the strength of the riser and the welds between the riser joint, lead to unacceptably short fatigue lives. In fact, stresses caused by high current conditions have been known to cause risers to break apart and fall to the ocean floor.
The second type of stress is caused by drag forces which push the riser in the direction of the current due to the riser's resistance to fluid flow. The drag forces are amplified by vortex induced vibrations of the riser. A riser pipe that is vibrating due to vortex shedding will disrupt the flow of water around it more than a stationary riser. This results in more energy transfer from the current to the riser, and hence more drag.
Many methods have been developed to reduce vibrations of subsea risers.
Some of these methods to reduce vibrations caused by vortex shedding from subsea risers operate by stabilization of the wake. These methods include streamlined fairings, wake splitters and flags.
Streamlined, or teardrop-shaped fairings that swivel around a riser have been developed that almost eliminate the sheading or vortices. The major drawbacks to teardrop shaped fairings is the cost of the fairing and the time required to install such fairings. Additionally, the critically-required rotation of the fairing around the riser is challenged by long-term operation in the undersea environment. A non-rotating fairing subjected to a cross-current may result in vortex shedding that induces greater vibration than the bare riser would incur.
Wake splitters are flat plates that extend drom the back of a riser parallel to the current flow direction. These wake splitters have been found to be effective to create a symmetric vortex pattern so that each vortex “sees” an image created by the rigid splitter plate giving symmetry with respect to the axis in the direction of flow. Splitter plates also stabilize the separation points, decrease the wake width and reduce drag. Splitter plates suffer from most of the same detrimental effects as teardrop shaped fairings for off-axis currents. They must therefore either be rotatable or be used only where the directions of a significant current does not vary.
Flags are similar to wake splitters, but are flexible. They are not generally as effective as wake splitters, but have the advantage that they can wrap around a riser and remain somewhat effective with varying current directions without being rotatable. Flags are not commonly used in subsea applications due to the high probability of the flag wrapping itself around the riser and becoming ineffective, and because of the difficulty and expense of attaching the flag to the riser along the length of the riser.
Other of these methods to reduce vibrations caused by vortex shedding from subsea risers operate by modifying the boundary layer of the flow around the riser to prevent the correlation of vortex shedding along the length of the riser. Examples of such methods include the inclusion of helical strakes around the riser, axial rod shrouds and perforated shrouds.
Where possible, it is generally preferred to utilize strakes over fairings, wake splitters and flags.
There exists a need in the art for methods and apparatus of reducing and/or controlling VIV, current drag, low frequency drift oscillations due to random waves, and low frequency wind induced resonant oscillations.
There is another need in the art for methods and apparatus of reducing and/or controlling VIV, current drag low frequency drift oscillations due to random waves, and low frequency wind induced resonant oscillations that do not suffer from the disadvantages of the prior art.
There is even another need in the art for methods and apparatus that act passively to reduce and/or control VIV, current drag, low frequency drift oscillations due to random waves, and low frequency wind induced resonant oscillations.
There is still another need in the art for methods and apparatus that use fluid passages pa
Allen Donald Wayne
Shu Hongbo
Gilbrath & Associates, P.C.
Gilbreth J. M. (Mark)
Gilbreth Mary A.
Lee Jong-Suk
Shell Oil Company
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