Hydraulic and earth engineering – Marine structure or fabrication thereof – With anchoring of structure to marine floor
Reexamination Certificate
2002-07-15
2004-03-30
Shackelford, Heather (Department: 3673)
Hydraulic and earth engineering
Marine structure or fabrication thereof
With anchoring of structure to marine floor
C405S223100, C166S350000, C166S367000
Reexamination Certificate
active
06712559
ABSTRACT:
PRIORITY CLAIM
This is a U.S. national stage of application No. PCT/FR01/00203, filed on Jan. 22, 2001. Priority is claimed on that application and on the following application: Country: France, Application No.: 00/00865, Filed: Jan. 24, 2000.
BACKGROUND OF THE INVENTION
The present invention relates to the known field of bottom-to-surface links of the type comprising a vertical underwater pipe referred to as a “riser” connecting the bottom of the sea to a floating support installed on the surface.
Once the depth of water becomes large, production fields and in particular oil fields are generally worked from a floating support. In general a floating support has anchor means to enable it to remain in position in spite of the effects of currents, winds, and swell. It generally also has means for storing and processing oil and means for unloading to off-loading tankers. These supports are known as floating production storage off-loading (FPSO) supports and numerous variants have been developed such as submersible pipe alignment rigs (SPARs), which are long floating cigar-shaped objects held in position by catenary anchoring, or tension leg platforms (TLPs) where said legs are generally vertical.
Wellheads are often distributed over an entire field and production pipes together with water injection lines and control cables are disposed on the sea bed heading towards a fixed location with the floating support being positioned on the surface vertically thereabove.
Some wells are situated vertically below the floating support and the inside of the well is then directly accessible from the surface. Under such circumstances, the wellhead fitted with its “Christmas tree” can then be installed on the surface on board the floating support. It is then possible using a derrick installed on said floating support to perform all of the drilling, production, and maintenance operations required by the well throughout its lifetime. This is said to be a “dry” wellhead.
To maintain the riser fitted with the dry wellhead in a substantially vertical position, it is appropriate to exert upward traction which can be applied either by a cable tensioning system using hydraulic actuators or winches installed on the floating support, or else by using floats that are distributed at various depths along the riser, or indeed to use a combination of both techniques.
U.S. Pat. No. 2,754,011 in the name of IFP describes a barge and a guide system for a riser, which riser is fitted with floats.
In some oil wells, the depth of water exceeds 1500 meters (m) and can be as much as 3000 m, so the weight of a riser over such a depth requires forces that can reach or even exceed several hundreds of (metric) tonnes in order to keep them in position. Use is made of buoyancy elements of the “can” type which are added to underwater structures, mainly to risers connecting the surface to very great depths (1000 m to 3000 m). The underwater pipe then consists in a rising column comprising an underwater pipe assembled with at least one float comprising a can surrounding said pipe coaxially and having said pipe passing through it.
The means joining said can to said pipe preferably include a leakproof hinge assembled around said pipe at at least one of the top and bottom orifices of said can.
The floats in question are of large dimensions, and in particular of a diameter in excess of 5 m, and a length in the range 10 m to 20 m, and their buoyancy can be as much as 100 tonnes.
They are generally placed in a string one beneath another.
In general, the floats extend over a length corresponding to no more than about 10% of the length of the bottom-to-surface link, and in particular over a length of 100 m to 200 m.
The riser is put under tension by the floats and is guided, preferably at the floating support, by roller guides situated in a plane to hold and guide the riser relative to the floating support. Cable tensioning means acting as guides can be used.
FR 2 754 021 discloses a device for guiding a riser which is fitted at its top with floats, the device comprise wheels enabling the riser to slide vertically, and also enabling it to pivot about a horizontal axis, while guiding its horizontal displacements, such that the movements of the riser in horizontal translation follow substantially those of the floating support. Thus, FR 99/10417 discloses an improved guide device having wheels and friction pads disposed radially around the pipe. Finally, various guide systems are known that require tensioning to be performed by cable.
The entire riser then behaves like the string of an instrument under tension between the bottom of the sea and the point situated on the axis of the guidance system level with the floating support.
The riser is subjected to the effects of swell, of current, and in addition to the horizontal movements of said floating support which is itself likewise subjected to the same effects. Water movements in the depth of water create drag effects on the riser structure and on its floats, thereby giving rise to large forces of variable direction.
In certain configurations of water particle movement, interaction effects occur between the fluid and the riser which give rise to vortices being shed alternately from opposite sides of the riser as shown in FIG.
13
.
When the period of these alternating vortex separations is close to the natural excitation periods of the riser, a phenomenon known as “capture” occurs which causes the riser to vibrate.
The intensity of the vibrations generated by shedding vortices during “capture” increases with increasing length of riser from which vortices are being shed simultaneously on the same side. This length is referred to by the person skilled in the art as the “correlation length”.
If the riser is considered as being an instrument string that is attached at both ends, its overall behavior is subject to transverse displacements of several meters presenting a natural excitation frequency and harmonics corresponding to a mode referred to as “guitar mode”, i.e. the riser vibrates between its two ends like a guitar string.
For risers without floats whose section characteristics (in particular diameter, second moment of area, stiffness) are substantially uniform or continuous, the only vibration observed is vibration of this “guitar” type as shown in
FIGS. 3 and 4
.
Similarly, for a high-buoyancy riser as in WO 99/05389 in which buoyancy is distributed over the entire length of the riser to obtain buoyancy of about 95% to 98%, the only vibrations observed are in “guitar” mode.
In WO 95/27101 and WO 99/05389, stabilizers are described for risers that are subjected to “guitar” type vibrations associated with the disturbing effects of vortices or turbulence around the riser.
In WO 99/05389, the floats are distributed along the entire length of the riser in the form of a cylindrical shell of syntactic foam. The stabilizer consists in modifying the shape of the floats in the top portion corresponding to the depth of water which is subject to swell, so as to obtain a non-cylindrical surface of hexagonal section. The solution proposed in WO 99/05389 reduces the volume of the floats in the top portion and thus reduces their buoyancy compared with the remainder of the riser for equivalent overall size. That type of plane geometry modification has the effect of increasing drag and decreasing the excitation induced by shedding vortices, and stabilizes the riser solely by absorbing energy.
In WO 95/27101, a tubular pipe that is likewise subjected to “guitar” mode vibration only, is stabilized by being fitted with a plurality of perforated envelopes at different levels around the standard portion of the pipe. These perforated envelopes are slidable around the pipe so as to be placed specifically at the locations of zones that are subjected to vibrations which correspond to vibration antinodes.
In U.S. Pat. No. 4,768,984, a stabilizer is described for a riser that is provided with a free buoy at its head, said buoy supporting an installation that includes a working platform. The surface installation
Couprie Stephane
Gassert Michel
Lenormand Olivier
Saipem SA
Saldano Lisa M.
Shackelford Heather
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