Buoys – rafts – and aquatic devices – Buoyancy providing attachment for pipe – log – or line
Reexamination Certificate
2002-02-26
2003-02-25
Basinger, Sherman (Department: 3617)
Buoys, rafts, and aquatic devices
Buoyancy providing attachment for pipe, log, or line
C166S367000, C405S224200
Reexamination Certificate
active
06524152
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the known field of bottom-to-surface links of the type comprising a vertical submarine pipe known as a “riser” connecting the sea bed to a floating support located on the surface.
2. Description of the Prior Art
Once the depth of water becomes large, production fields, and in particular oil fields, are generally operated from floating supports. In general, such a floating support has anchor means to keep it in position in spite of the effects of currents, winds, and swell. Such a support generally also has means for storing and processing oil, and means for off-loading oil to off-loading tankers. Such tankers arrive at regular intervals to off-load production. Such floating supports are referred to by the initials FPSO (for floating, production, storage, off-loading). Numerous variants have been developed such as SPARS which are long floating cigar shapes held in position by catenary anchor systems, or indeed TLPs, i.e. tension leg platforms, in which the tension legs are generally vertical.
Well heads are often distributed over an entire field, and production pipes together with pipes for injecting water and cables for monitoring and control are placed on the sea bed to converge on a fixed installation with the floating support being positioned on the surface vertically above said installation.
Some wells are situated vertically beneath the floating support and the insides of such wells are. therefore directly accessible from the surface. Under such circumstances, a well head fitted with its “Christmas tree” can be installed at 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 a well throughout the lifetime of said well. This is referred to as a “dry” well head.
To keep the riser fitted with its dry well head in a substantially vertical position, it is appropriate to exert upwardly directed traction near its top end, which traction can be applied either by a cable-based tensioning system using winches or hydraulic actuators installed on the floating support, or else by means of floats distributed along the riser and installed at various depths, or else by a combination of those two techniques.
U.S. Pat. No. 2,754,011 (IFP) discloses a barge and a guide system for a riser, the riser being fitted with floats.
SPARS and TLPs are also fitted with risers tensioned by floats.
When floats are used for tensioning purposes, it is necessary:
either to make syntactic foam elements that are installed as a half-shell around the riser;
or else to make floats out of metal or composite material, which floats are filled with gas, preferably an inert gas such as nitrogen.
Syntactic foam is a foam containing microbeads of glass impregnated in an epoxy or polyurethane type resin. This type of foam has exceptional ability to withstand pressure and it is commonly used at great depths. However this type of foam suffers from the drawback of being very expensive and difficult to make.
Since the depth of water over some oil fields exceeds 1500 meters (m), and can be as great as 2000 m to 3000 m, the weight of a riser over such a depth requires forces that can be as much as or greater than several hundreds of (metric) tonnes in order to enable them to be kept in position. For such extreme depths (1000 m-3000 m), buoyancy elements of the “can” type are used which are installed on the risers at various depths. The floats concerned are then of large dimensions, and in particular they can be of a diameter lying in the range 1.5 m to 5 m, or even greater than 5 m, and they can extend over a length of 10 m to 20 m in order to achieve buoyancy of as much as 100 tonnes.
The float and the pipe are subject to the effects of swell and of current, and because they are connected to an FPSO on the surface, they are also subjected indirectly to the effects of wind. This gives rise to considerable lateral and vertical movements (several meters) in the system comprising the riser, the floats, and the barge, particularly in the zone thereof that is subject to swell. These movements give rise to large differential forces between the riser and the float. In addition, the curvature taken up by the riser can give rise to very large bending moments in the change of second movement of area that arises where there is a connection between the riser and a float.
In order to minimize the forces generated by current and swell acting on the riser-and-float system, floats are generally circular and they are installed coaxially around the riser.
In addition, floats are generally fixed to a riser in such a manner as to ensure that the connection between a riser and a float is leakproof and capable of confining the filler gas within the float. The commonly-employed solution consists in mutual interfitting engagement between the float and the riser at the top and bottom ends of the float, backed up by welding. Numerous reinforcements are added to ensure that the assembly has sufficient strength.
At such a junction between a riser and a float, the second moment of area of the assembly (i.e. its resistance to bending) varies considerably on going from the section of the riser to the section of the float.
Such large variations in second moment of area give rise to poor stress distribution, thus giving rise to highly localized zones where stress can become unacceptable and can lead either to sudden breakage or else to fatigue, leading in turn to the appearance of cracks and then to ruin. In order to reinforce the sensitive zone, these localized stresses require transition pieces to be used, generally large conical pieces referred to as “tapered joints”. In some cases, these pieces can be as much as 10 m long, and in the best of cases they require very high performance steels to be used. However, it is often necessary to use titanium which is about five to ten times more expensive than the best steels. Furthermore, these pieces are generally complex in shape and they must be made to extremely high standards in order to guarantee the service expected of them over the lifetime of such equipment which commonly exceeds 25 years.
U.S. Pat. Nos. 3,952,526 and 3,981,357 disclose junction systems between floating tanks and risers, where parts are used that are made of elastomer material.
Those buoyancy systems enable the tensioning system on board the floating support to be reduced and, in general, they are distributed over a large fraction of the depth of water, and in addition they present small buoyancy of up to a few hundreds of kg, or perhaps as much as 1 tonne or 2 tonnes.
The junctions are situated in the top portions of the floats, while the bottom portions of the floats are generally left open. Such devices can transfer loads corresponding to reducing the weight of a limited length of pipe, but they are not suitable for floats that are intended (on their own and without help from additional tensioning systems secured to the floating support) to support very long lengths of riser, e.g. 500 m to 1000 m, or even more, of the kind to be found in off-shore oil fields at greatdepth, i.e., in particular, at depths of more than 1000 m. The buoyancy required for providing tensioning by means of floats alone requires considerable forces to be transferred vertically and transversely, said vertical forces applied to the head of the riser being capable of reaching several hundreds of tonnes, and in particular lying in the range 300 tonnes to 500 tonnes.
SUMMARY OF THE PRESENT INVENTION
The object of the present invention is to provide a novel type of junction between a riser and a can so as to enable large loads to be supported and transferred while mitigating the drawbacks of the above-described floats assembled around said pipe by mutual engagement.
An object of the present invention is thus to provide a novel riser-and-float junction means that is simple, flexible, and reliable, mechanically speaking, and in particular
Dauphin Raphaël
Gassert Michaël
Quiniou Valérie
Rocher Xavier
Basinger Sherman
Cohen & Pontani, Lieberman & Pavane
Saipem s.a.
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