Hydraulic and earth engineering – Marine structure or fabrication thereof – With anchoring of structure to marine floor
Reexamination Certificate
2002-03-05
2004-01-06
Will, Thomas B. (Department: 3671)
Hydraulic and earth engineering
Marine structure or fabrication thereof
With anchoring of structure to marine floor
C114S264000, C405S224300, C166S355000
Reexamination Certificate
active
06672804
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates in general to the field of bottom-to-surface type connections of the type including a vertical underwater pipe referred to as an up column or riser, connecting the sea bed to a floating support installed on the surface.
BACKGROUND OF THE INVENTION
Once the depth of water becomes large, production fields and in particular oil fields are generally operated from floating supports. In general, the floating support has anchor means to keep it in position in spite of the effects of currents, winds, and swell. It generally also includes oil storage and processing means together with means for off-loading to off-loading tankers. Such tankers arrive at regular intervals to off-load production. Such floating supports, which are generally of the barge type, are commonly referred to by the initials “FPSO” for floating production storage off-loading. Numerous variants have been developed such as SPARs (which are long vertical floating cigar-shaped objects held in position by catenary anchoring) or indeed TLPs (tension leg platforms) which are platforms whose “legs” are anchor lines under tension, which lines generally extend vertically.
Wellheads are often distributed over an entire field and production pipes, together with water injection lines and inspection and control cables, are all placed on the sea bed to extend towards a fixed location vertically below the position of the floating support on the surface.
Some wells are thus situated vertically beneath the floating support and the insides of such wells can then be accessed directly from the surface. Under such circumstances, a wellhead fitted with its “Christmas tree” can be installed on the surface on board the floating support. It is then possible to use a derrick installed on said floating support to perform all of the drilling, production, and maintenance operations required on the well throughout its lifetime. This is referred to as a “dry” wellhead.
With SPARs and TLPs, dry wellheads are brought together in a limited zone of the platform over which the derrick is displaceable to take up a position vertically over each of the wells so as to perform drilling operations or maintenance operations on a well throughout the lifetime of the field.
Since a drilling derrick is a tall structure, possibly 60 meters (m) tall and possessing a hoisting capacity that can exceed 500 tonnes (t), the means required for moving it from one well to another and for keeping it in position during operations on any particular well give rise to structures that are complex and expensive.
In order to maintain the riser fitted with its 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 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 indeed by a combination of those two techniques.
French patent FR 2 754 011 describes a barge and a guidance system for a riser, in which the riser is fitted with floats.
SPARs and TLPs are likewise fitted with a multiplicity of risers supported by floats that are held in position by guidance systems.
In FPSOS, risers reach the surface in a central cavity of the floating support that is referred to as a “wellbay”. The cavity passes right through the hull vertically over a height of about 30 meters, drawing water to a depth of about 20 meters. It is generally installed on the axis of the floating support, at equal distances from its ends since this is the zone in which the amplitudes of movements and of accelerations are the smallest when the vessel is subjected to the phenomena of roll, pitch, and yaw.
The depth of water over some oil fields exceeds 1500 m, and can be as much as 2000 m to 3000 m, so the weight of risers for such depths requires vertical forces to be deployed to keep them in position that can be as great as or more than several hundreds of tonnes. Buoyancy elements of the “can” type are used which are installed at various levels along risers connecting the surface to ultragreat depths (1000 m-3000 m).
The floats concerned are of large dimensions and in particular they have a diameter in excess of 5 m, and a length of 10 m to 20 m, with each float delivering buoyancy that can be as much as 100 tonnes.
The float and the riser are subjected to the effects of swell, and of current, but since they are connected to the FPSO on the surface, they are also indirectly subjected to the effects of wind. This gives rise to lateral and vertical movements of the assembly comprising the riser, the floats, and the barge, which movements can be large, as much as several meters, particularly in a zone that is subject to swell.
To ensure that risers do not interfere with one another and do not interfere with the hull of the floating support, said risers are spaced apart from one another by several meters and also by several meters from the walls of the wellbay, which means that such wellbays can often be as much as 80 m long and 20 m wide on FPSO barges that are themselves as much as 350 m and 80 m wide, and rising by as much as 35 m above the water line. Such barges have a deadweight that can be as much as or greater than 500,000 tonnes deadweight.
These riser movements give rise to large differential forces between a riser and the guidance systems secured to the floating support.
The amplitude of the displacements and the very high level of the forces in the risers make it necessary to design guidance systems capable of withstanding not only extreme conditions, but also phenomena of fatigue and wear of the kind that can accumulate over the lifetime of such an installation, which can exceed 25 years.
The present invention relates to guiding risers within the generally central cavity when in a production position or when in a position in which drilling operations and heavy maintenance operations can be performed on the well, i.e. using a derrick which is fixed relative to the floating support, or indeed on the means for transferring the riser between these various positions.
The present invention also relates to transferring risers within the generally central cavity between their production positions and a position in which drilling operations and heavy maintenance operations can be performed on the well, i.e. using a derrick which is fixed relative to the floating support.
Well drilling is thus performed on the main axis of the derrick through a “drilling” riser whose function is to guide the drill strings and to contain the mud returned from a well that is being drilled. Such a drilling riser is made up from unit lengths that can be as much as 50 m long, with the entire assembly being lowered step by step as the said riser is assembled. At the end of drilling, the portion of the riser corresponding to the depth of water is disconnected from the well at the sea bed, and is then taken to a parking position after its length has been shortened by removing one or two of the unit lengths. By proceeding in this way, the drilling riser remains suspended with its bottom end situated at 50 m to 100 m from the sea bed.
The production riser can then be assembled step by step in the same manner until it reaches the entrance to the well. Floats are installed on the top portion as it is put into place, and finally the bottom of the production riser is connected to the well. The well is then fitted with various items of production tubing and the “Christmas tree” of the dry wellhead is put into place.
The assembly is then transferred to its “slot”, i.e. its production position in which it will remain throughout the lifetime of the field, except when certain maintenance operations are performed that require said riser to be returned to the main axis of the derrick in order to perform heavy maintenance operations.
Means are known for guiding a riser relative to a floating support. However, those guide systems usually imply cable-tensioning systems which are difficult to implement (GB
Baylot Michel
Hallot Raymond
Rocher Xavier
Bouygues Offshore
Cohen & Pontani, Lieberman & Pavane
Mayo Tara L.
Will Thomas B.
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