Hydraulic and earth engineering – Marine structure or fabrication thereof – With anchoring of structure to marine floor
Reexamination Certificate
2000-10-17
2002-10-08
Shackelford, Heather (Department: 3673)
Hydraulic and earth engineering
Marine structure or fabrication thereof
With anchoring of structure to marine floor
C166S350000
Reexamination Certificate
active
06461083
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a bottom-to-surface method and system for an underwater pipe installed at great depth.
The technical sector of the invention is the field of manufacturing and installing rising production columns for underwater extraction of oil, gas, or other soluble or fusible materials or a suspension of minerals from an underwater well head for the purpose of developing production fields installed at sea off-shore. The main application of the invention lies in the field of oil production.
2. Description of the Related Art
The present invention relates to the known field of links of the type comprising a vertical tower anchored to the sea bed and having a float situated at the top of the tower, which float is connected to a floating support installed on the surface by means of a pipe whose own weight causes it to take up the shape of a catenary.
In the present description the production fields are considered as being oil fields. Once the underwater depth of such fields becomes large, they are generally worked from floating supports. The well heads are often distributed over the entire field and production pipes and also water injection lines and control command cables are placed on the sea bed going towards a fixed location having a floating support positioned vertically above it on the surface.
In general, the floating support has anchor means so as to enable it to remain in position in spite of the effects of current, wind, and swell. It also generally includes means for storing and processing oil and means for off-loading it to off-loading tankers, which arrive at regular intervals to take away the production. These floating supports are known as “floating production storage and off-loading” (FPSO) supports and the initials “FPSO” are used throughout the description below to designate such a support.
Such FPSOs are either anchored by a series of anchor lines running from each of the corners of the floating support, in which case the FPSO maintains a substantially constant heading regardless of surrounding conditions, or else an FPSO has a turret secured to its structure and anchored by a series of anchor lines. Under such circumstances, the FPSO is free to revolve relative to the turret, and it is the turret that maintains a constant heading; under such circumstances the FPSO takes up a heading that corresponds to the resultant of the forces due to wind, current, and swell on the hull of the vessel. In the following description, the bottom-to-surface links are described for the most part as being connected to the side of an FPSO that is anchored and that therefore has a substantially constant heading (as shown in FIG.
2
), whereas if the FPSO has a turret, then they should be connected to the turret itself (as shown in FIG.
6
).
The bottom-to-surface link pipe is known as a “riser”, which term is used in the present description, and it can be implemented in the form of a pipe rising continuously from pipes placed on the sea bed and going directly to the FPSO, thereby giving rise to a catenary configuration whose angle relative to the vertical at the FPSO is generally in the range 3° to 15° (a catenary riser).
When the water depth is less than several hundred meters such links must necessarily be made using pipes that are flexible, however once the depth reaches or exceeds 800 m to 1000 m, flexible pipes can be replaced by pipes that are strong and rigid, being constituted by tubular elements that are welded or screwed together and made of rigid material, such as composite material or thick steel. Such rigid risers of thick strong material and taking up a catenary configuration are commonly referred to as “steel catenary risers” (SCRs) and the initials SCR are used in the present description regardless of whether the riser in question is made of steel or of some other material such as a composite.
A flexible pipe and an SCR type rigid riser when subjected to the forces of gravity only, and when they are of the same height, present the same angle relative to the vertical where they connect to the FPSO, and have the same curvature over their entire suspended length. Mathematically, this curve is accurately defined and is known as a “catenary”. However, SCRs are much simpler than flexible pipes technically speaking and they are much less expensive. Flexible pipes are structures which are complex and expensive and which are made from multiple spiral-wound sheaths and composite materials.
The depth of certain oil fields is greater than 1500 m and can be as great as 2000 m to 3000 m. The tension induced at the FPSO by each SCR can be as great as 250 metric tonnes to 300 tonnes and the large number of risers needed to develop certain fields leads to reinforcing the structure of said FPSOs considerably, and can give rise to unbalance if starboard and port loading is not the same. In addition, during circular movements of the FPSO about its mean position, the catenary formed by an SCR changes and the point of contact on the sea bed moves forwards and backwards and also from left to right at the same rate as the FPSO moves, putting down or picking up a portion of the pipe. These movements are repeated over long periods of time and they dig a furrow in poorly consolidated beds of the kind commonly encountered at great depth, thereby modifying the curvature of the catenary and leading, if the phenomenon amplifies, to risks of the pipes being damaged, i.e. the underwater pipes can be damaged and/or the SCRs can be damaged.
Because of the multiplicity of lines that exist in installations of this type, it is preferred to use a solution of the tower type in which the pipes and cables converge on the foot of a tower and rise up the tower, either all the way to the surface, or else to a depth that is close to the surface, with flexible pipes then extending from that depth to provide links between the top of the tower and the FPSO. The tower is then provided with buoyancy means so as to keep it in a vertical position and the risers are connected at the foot of the tower to the underwater pipes via flexible coupling sleeves which accommodate the angular movements of the tower. The resulting assembly is commonly referred to as a “hybrid riser tower” since it makes use of. two technologies: firstly a vertical portion, the tower, in which the riser is constituted by rigid vertical pipes; and secondly a top portion of the riser which is constituted by flexible pipes in a catenary configuration connecting the top of the tower to the FPSO.
French patent FR 2 507 672, which corresponds to U.S. Pat. No. 4,462,717, discloses such a hybrid tower comprising a surface float connected to the FPSO via flexible pipes and carrying suspended guides through which there pass solely the top portions of the vertical fluid transfer pipes. The hybrid tower is anchored to the sea bed by a cable under tension that gives the assembly a certain amount of flexibility in vertical movement, the bottom portions of the pipes being free and forming bends at the sea bed, against which they bear.
The advantage of such a hybrid tower lies in the freedom allowed to the FPSO to move away from its normal position while giving rise to a minimum amount of stress in the tower and in those portions of the pipes that are in the form of suspended catenaries, whether at the sea bed or at the surface. The FPSO is generally anchored by means of a multitude of lines connected to a system of anchors resting on the sea bed. Such an anchor system gives rise to return forces that maintain the FPSO in a neutral position. The bottom-to-surface links give rise to additional vertical and horizontal forces which have the effect of offsetting the axis of the FPSO relative to said neutral position. In the absence of current, wind, or swell, and when the tide is at its mean level, the position of the FPSO corresponds to a “reference position” P
0
. Under the combined effects of environmental conditions, both on the hull of the FPSO and also on the various elements constituting th
Pionetti Regis
Rocher Xavier
Bouygues Offshore
Cohen & Pontani, Lieberman & Pavane
Kreck John
Shackelford Heather
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