Hydraulic and earth engineering – Marine structure or fabrication thereof
Patent
1993-04-12
1995-09-05
Corbin, David H.
Hydraulic and earth engineering
Marine structure or fabrication thereof
405224, 405202, 166367, E02B 1700, F16L 912
Patent
active
054473900
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a process for manufacturing a line of locally variable stiffness and a variable stiffness element allowing the line to have the variable stiffness.
This invention may notably be applied for the manufacturing of a line connected to a fixed point, with the other end of the line being likely to move, and the fixed point may be located at the bottom of a water body.
The present invention is notably applicable to taut lines, for example, anchoring lines, petroleum production transfer lines, such as risers.
BACKGROUND OF THE INVENTION
Current production and drilling strings are generally connected to the sea bottom through a joint composed of a ball joint or a flex joint allowing an angular movement of the order of 10.degree. in any direction. The exception to this rule is provided by the tension-leg platforms installed on the Hutton field in the North Sea and the Jolliet field in the Gulf of Mexico, where the production strings are directly embedded in the wellheads which are combined on the platform vertical.
Such an embedding is advantageous for several reasons. Firstly, it avoids imposing a significant bending on the tubings which are located inside the string. It further reduces the angular clearances thereof. Finally, it is more compact, less costly, and requires less maintenance than a ball joint.
In the case where a drilling riser is used, such a link considerably reduces the wear of the pipes.
The drawback of embedding is that the moments induced by the lateral offset of the platform, a well as by the effect of the sea current, may be extremely high. In order to reduce the bending stresses, which would otherwise exceed the limit allowable in the string, it becomes necessary to give the string or line a variable stiffness over part of the length thereof, more particularly close to the embedding. This is achieved by means of a variable stiffness element.
The element may be designed so that the curvature caused is substantially constant over the total length thereof. This requires that the stiffness in bending (EI) evolves in a precise way along the element.
The data of the problem are the following: joint, stiffness element, and the element joint and by putting ##EQU1## R.sub.e =the minimum allowable bending radius of the element (=(EI).sub.O /MO) and
To understand these designations better, see FIG.7 of the present application.
It can be demonstrated that the following relations are approximately correct:
The length (L) necessary for the element ##EQU2##
The angle (A.sub.e) by which the element must bend is ##EQU3##
The required evolution of the stiffness in bending (EI) along the element ##EQU4##
The maximum moment at the lower end of the element ##EQU5##
It can be deduced from Equation (2) that the lower the mimimum stiffness in bending (EI).sub.O of the element is, the shorter the element may be.
Equation (5) shows that the maximum moment transmitted to the foundation is in direct relationship with the length (L) of the element and with the allowed bending radius (R.sub.e). It is therefore desirable to make this element as supple as possible.
In the case where the string is made of a supple material such as, for example, a composite material (carbon fibers/glass fibers/resin) and if the element consists of a stiff material such as steel for example, it is possible that the allowable moment (M.sub.O) at the joint between them is much lower in the string than in the element.
There are two possible solutions.
The element may be made longer than it needs to be for itself. The alternative consists in introducing a transition joint several meters long and of constant section between the string and the variable stiffness element.
The optimum solution in order to avoid having to introduce a transition joint between the string and the variable stiffness element is to provide the element with an upper end at least as supple in bending as the string itself.
It is the same way in the case of a string made of a stiff material such as steel for example, an
REFERENCES:
patent: 4362521 (1982-12-01), Puck et al.
patent: 4589801 (1986-05-01), Salama
patent: 4605385 (1986-08-01), Puck et al.
patent: 4854781 (1989-08-01), Sparks et al.
Auberon Marcel
Metivaud Guy
Odru Pierre
Sparks Charles
Aerospatiale
Corbin David H.
Institut Francais du Pe'trole
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