RP 16Q: Design, Selection, Operation, and Maintenance of Marine Drilling Riser Systems ing. In cases of extended drilling on a harsh environment location, such as on a North Sea template, a fatigue analysis of the riser may be advisable. The mean and maximum flex/ball joint angle limits given for the normal drilling mode are intended to prevent wear and keyseating damage to the riser and flex/ball joint. Prudent operational procedure should strive to maintain these angles as small as possible, and consider 2.0 degrees (mean) and 4.0 degrees (maximum) as upper bounds. The maximum flex/ball joint angle limits for the connected nondrilling mode and disconnected mode are intended to prevent damage to the riser, flex/ball joint, and BOP stack. The upper flex/ball joint angle rarely has a significant effect on riser design; however, this angle should be considered when evaluating clearances in the moonpool area.
The purpose of the maximum stress analysis is to ensure that the riser is strong enough to support the maximum design loads. This is accomplished by requiring the riser to support the maximum design loads while keeping the maximum stresses below the allowable stress. This limit is intended to prevent structural deformation that could lead to failure and includes a margin of safety. All stresses in Table 3.1 refer to the von Mises stress criterion (see Higdon, et al (1976)). Local peak stresses (see Glossary, Appendix A) are not considered for the maximum load analysis; however, these peak stresses could be of concern for evaluating the fatigue fife of the riser. Fatigue analysis is discussed in Appendix C, Section C.2.
A minimum tension setting is required to ensure the stability of the riser. The tension setting should be sufficiently high so that the effective tension, as addressed in Section 3.4.3, is always positive in all parts of the riser even if a tensioner should fail. In most cases the minimum effective tension is encountered at the bottom of the riser.
The minimum top tension, T^, is determined by:
min SRmin f where,
TSRmin = Minimum Slip Ring Tension = W f . - B f, + A.[d H - d H ]
awt nbt i mm ww and
WB = Submerged Riser Weight above the point of consideration fwt = Submerged Weight Tolerance Factor (minimum value = 1.05 unless accurately weighed)
Bn = Net Lift of Buoyancy Material above the point of consideration fbt = Buoyancy Loss and Tolerance Factor resulting from elastic compression, long term water absorption, and manufacturing tolerance. (Maximum value = 0.96 unless accurately known by submerged weighing under compression at rated depth)
Aj = Internal Cross Sectional Area of Riser including choke, kill, and auxiliary fluid lines d = Drilling Fluid Weight Density
Hm = Drilling Fluid Column to point of consideration dw = Sea Water Weight Density
Hw = Sea Water Column to point of consideration including storm surge and tide
N = Number of Tensioners Supporting the Riser n = Number of Tensioners Subject to Sudden Failure
Rf = Reduction Factor Relating Vertical Tension at the Slip Ring to Tensioner Setting to account for fleet angle and mechanical efficiency (usually 0.9 - 0.95)
Note that in the above equation for TSRmjii, the exterior pressure, dwHw, is multiplied by the internal cross sectional area of the riser, A., rather than the exterior cross sectional area, A. This is because the buoyancy of the riser pipe walls, dwHw (Ao - A_), has been included in the submerged riser weight, Ws.
See Sample Calculation D.2 in Appendix D for a determination of minimum tension setting.
The significant dynamic stress range limit should also be used in conjunction with the maximum load analysis. This limit is intended to provide some control on the fatigue damage accumulated by the riser. Incorporation of this limit in the maximum load analysis eliminates large dynamic stresses which can lead to accelerated fatigue.
Additional operating modes which might influence design should be considered. Specifically, the disconnected mode, handling tool interfaces, hang-off on either spider or riser hang-off structure, special handling situations, and emergency conditions should be reviewed for their impact on riser system design.
The load and resistance factor design (LRFD) procedure, based on reliability rather than working stress, is rapidly gaining acceptance. This approach is described in Appendix C, Section C.3.
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