Hydraulic and earth engineering – Marine structure or fabrication thereof – Structure protection
Reexamination Certificate
2000-08-24
2004-02-03
Lee, Jong-Suk (James) (Department: 3673)
Hydraulic and earth engineering
Marine structure or fabrication thereof
Structure protection
C405S224200, C114S264000, C114S243000
Reexamination Certificate
active
06685394
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to methods and apparatus for suppressing vortex-induced vibrations (“VIV”) In another aspect, the present invention relates to apparatus and methods for suppressing VIV in flowing-fluid environments by the use of shrouds, and to flowing-fluid elements incorporating such shrouds. In even another aspect, the present invention relates to apparatus and methods for suppressing VIV in aquatic environments by the use of shrouds, and to aquatic elements incorporating such shrouds. In still another aspect, the present invention relates to apparatus and methods for suppressing VIV utilizing a partial shroud with perforations easily attached to and/or encircling a flowing-fluid element.
2. Description of the Related Art
Whenever a bluff body, such as a cylinder, experiences a current in a fluid, it is possible for the body to experience vortex-induced vibrations (VIV) These vibrations are caused by oscillating hydrodynamic forces on the surface which can cause substantial vibrations of the structure, especially if the forcing frequency is at or near a structural natural frequency. The vibrations are largest in the transverse (to flow) direction; however, in-line vibrations can also cause stresses which are sometimes larger than those in the transverse direction.
Drilling for and/or producing hydrocarbons or the like from subterranean deposits which exist under a body of water exposes underwater drilling and production equipment to water currents and the possibility of VIV. Equipment exposed to VIV includes structures ranging from the smaller tubes of a riser system, anchoring tendons, or lateral pipelines to the larger underwater cylinders of the hull of a minispar or spar floating production system (hereinafter “spar”).
Risers are discussed here as a non-exclusive example of an aquatic element subject to VIV. A riser system is used for establishing fluid communication between the surface and the bottom of a water body. 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 riser system normally consists of one or more fluid-conducting conduits which extend from the surface to a structure (e.g., wellhead) on the bottom of a water body. For example, in the drilling of 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.
Also, the newly developed spar production facilities are used in aquatic environments of great depths. Strong water currents often occur at these greater depths in ocean environments. The hulls of spar production facilities, therefore, can be exposed to excessive vortex-induced vibrations.
This drilling for and/or producing of hydrocarbons from aquatic, and especially 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 or anchor tendons 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, tendons or spars is generally a function of and increases with the velocity of the water current passing these structures and the length of the structure.
It is noted that even moderate velocity water currents acting on linear structures can cause stresses. Such moderate or higher currents are readily encountered when drilling for offshore oil and gas at greater depths in the ocean or in an ocean inlet or near a river mouth.
Drilling in ever deeper water depths requires longer riser pipe strings which because of their increased length and subsequent greater surface area 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 a riser or other structure caused by strong and shifting currents in these deeper waters increase and set up stresses in the structure which can lead to severe fatigue and/or failure of the structure 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 underwater structure (“vortex-induced vibrations”) in a direction perpendicular to the direction of the current. When water flows past the structure, vortices are alternately shed from each side of the structure. This produces a fluctuating force on the structure transverse to the current. If the frequency of this harmonic load is near the resonant frequency of the structure, large vibrations transverse to the current can occur. These vibrations can, depending on the stiffness and the strength of the structure and any welds, lead to unacceptably short fatigue lives. In fact, stresses caused by high current conditions have been known to cause structures such as risers to break apart and fall to the ocean floor.
The second type of stress is caused by drag forces which push the structure in the direction of the current due to the structure's resistance to fluid flow. The drag forces are amplified by vortex induced vibrations of the structure. For instance, 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 sub sea structures.
Some of these methods to reduce vibrations caused by vortex shedding from subsea structures operate by stabilization of the wake. These methods include streamlined fairings, wake splitters and flags.
Streamlined or teardrop shaped, fairings that swivel around a structure have been developed that almost eliminate the sheading or vortexes. 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 structure is challenged by long-term operation in the undersea environment. Over time in the harsh marine environment, fairing rotation may either be hindered or stopped altogether. A non-rotating fairing subjected to a cross-current may result in vortex shedding that induces greater vibration than the bare structure would incur.
Wake splitters are flat plates that extend from the back of a cylindrical structure parallel to the current flow direction. These wake splitters have been found to be effective in creating 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. Unfortunately, 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 structure
Allen Donald W.
Henning Dean L.
Gilbreth J. Mark
Gilbreth Mary A.
Gilbreth & Associates P.C.
Lee Jong-Suk (James)
Shell Oil Company
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