3.1 General Considerations
The marine riser coupling is in the primary load path of the riser and must be designed to the same loads as the riser pipe. Procedures for determination of those loads are described in API RP 2Q. Riser couplings should also be designed to facilitate use, maintenance and inspection. Guidelines for care and operation are given in API RP 2K.
Because of its large slenderness ratio (L/r), a drilling riser's ability to resist environmental loading depends primarily on tension. The environment consists of the hydrodynamic forces of current and waves, the motions induced by the floating vessel's dynamic response and the loads imposed by the contained drilling fluids.
The determination of a riser's response to the environmental loading, and the mechanical loads developed in the riser, requires specialized computer programs and analyses. The general procedure used to determine riser system response and design loads is described in Section 3.0 of RP 2Q. The recommended limit for riser loading is based on the maximum riser pipe stress resulting from combined tension and bending.
Additional sources of applied load may significantly affect the coupling design and should be included in the design calculations. Of noteworthy importance are:
1. Loads Induced by Choke & Kill and Auxiliary Lines
Riser couplings typically provide support for Choke & Kill and auxiliary lines. This support constrains the lines to deflect compatibly with the riser pipe. Loads can be induced on the coupling both from pressure in the lines and from imposed deflections on the lines.
2. Loads Induced by Buoyancy
Riser couplings may provide support for buoyancy, which induces loads on the couplings.
3. Effects of Internal/External Pressure and Temperature
In some extreme cases the pressure or temperature differential across the wall of the coupling may be significant and should be considered. A pressure seal should be provided to sustain anticipated bore pressure.
These are temporary loads induced by suspension from the handling tool and/or spider. Also, sufficiently strong lugs must be provided at the pickup points on each riser joint.
The recommended design stress levels and verification testing procedures contained herein deal with structural failure. Structural failure is defined as fracture or excessive material yielding. Design stress levels and testing procedures are independent of coupling design or method of makeup.
Structural failure by fracture can result either from overload or from fatigue cracking. Structural failure by yielding results when excessive permanent strain is developed. Loss of preload and dimensional interference are degrading effects of yielding. Significant wear at an interface is a failure closely related to yielding, also resulting in loss or reduction of preload.
Preload is used in riser couplings to provide a rigid joint and to increase fatigue life. Preloading a riser coupling establishes a compressive bearing load across
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