A 4,000-Foot Riser
Authors: Heuze L.R. , Chaussumier D. , Guesnon J. , Simondon D. 
Published: December 2007

This paper presents conclusions derived from theoretical studies of the relative influence of factors that must be considered in designing a 4,000-ft drilling riser. Also given are qualitative results of tests on different equipment proposed for solving one of the most important problems inherent in deep-water drilling and production-buoyancy. problems inherent in deep-water drilling and production-buoyancy. Introduction

Many wells have been drilled in water between 500 and 1,000 ft deep, with the present record held by Shell Oil Co. at 2,290 ft. About 20 drillships or semisubmersities have been ordered and should be equipped for depths between 2,000 and 4,000 ft, with some oil companies already studying risers of 6,000 and even 10,000 ft.

This paper focuses on the risers used in drilling. The problems posed in designing risers are among the most important in deep-sea drilling, and any solution to them must guarantee security in all circumstances. This is why TOTAL, ELF-Aquitaine, and I.F.P. jointly developed computer programs involving the use of static and dynamic calculations, tested flotation equipment, and developed a new technology where extrapolation of existing techniques was not possible.

Description of the Program

This program calculates the forces and strains in the steel under dynamic or static conditions in two dimensions, as well as the riser deformation whether or not it is connected to the wellhead.

Appendix A describes the external forces that are considered by the program, the simulation of the connections at the top and at the bottom of the riser, and the hypotheses and the principle of the calculation.

The riser is defined as an assembly of several clements. For these elements, it is necessary to specify (1) the length of each one; (2) the outside diameter and thickness of the resistant section (that is, the section considered for calculating the stresses); (3) the mass per unit length; (4) the hydrodynamic diameter, d, that is considered for calculating the drag forces (the drag and inertia coefficients are calculated automatically by the program from the Reynolds number and the Keulegan-Carpenter program from the Reynolds number and the Keulegan-Carpenter number shown in Figs. 1A and 1B); (5) the diameter, d', considered for calculating the buoyancy; and (6) the modulus of elasticity.

For a static analysis, it is also necessary to state (see Fig. 2) (1) the rotational and vertical stiffness of the connection at the top of the riser, S RT and S VT (see Appendix A); (2) the position and rotational stiffness of possible intermediate ball joints; (3) the rotational possible intermediate ball joints; (3) the rotational stiffness of the connection at the bottom of the riser, S RB; (4) the tension at the top of the riser, F T; (5) the mud density inside the riser, p m; (6) the offset of the floating support from the vertical axis of the wellhead (the last five parameters should be defined only when the riser is connected); and (7) the current velocities at as many depths as necessary to allow a reasonable simulation of reality by linear interpolation between successive depths.

For a dynamic analysis, we need to include (see Fig. 3) the Airy wave conditions and the sinusoidal motions of the floating support (heave, sway or surge, and roll or pitch).

It is also possible to impose a constant-speed drift on the sinusoidal motions of the floating support, with this drift beginning and ending anytime during the calculation.

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