High-Performance Bolting Technology for Offshore Oil and Natural Gas Operations (2018) / Chapter Skim
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Appendix I: Drilling Riser Design
Appendix I: Drilling Riser Design
Pages 195-216
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The rig itself, including rig performance in various ocean environment conditions 195
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Currents As stated in Appendix D, surface and subsea currents can be significant in the Gulf of Mexico. These currents can be created by the loop current, eddy currents that are spun off the loop current, and topographical Rosby currents.
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Appendix I 197 FIGURE I.1 Typical drilling riser system and its sub-systems and components. SOURCE: FESAus, "Glossary: Riser," http://fesaus.org/glossary/doku.php?
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198 FIGURE I.2 Example of burst pressure on riser.
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The lateral displacement of the rig and riser caused by both waves and current will place a lateral load on the top of the BOP resulting is some seemingly minor displacement of the top of the BOP. This load will transfer as a bending moment to the wellhead connector and the wellbore casing through bottom BOP flange.
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200 H i g h - P e r f o r m a n c e B o lt i n g Te c h n o l o g y FIGURE I.3 Currents causing lateral offset of rig.
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FIGURE I.5 Actual versus predicted bending moments.
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3. Wave height, direction, and wave period impact the amount and direction of lateral movement of the drilling rig, and resultant forces on the riser, the BOPs, and the wellhead connector.
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On very heavy risers buoyancy enhancement devices may be installed.5 The three pieces of equipment that manage riser tension during rig operations in response to movement induced by wind and waves are • The rig tensioner system attached to the riser tensioner ring, • The riser slip joint, and • The riser flex joint (or ball joint)
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Figure I.8 is a photograph of riser tensioners coming down through the moon pool to hold the top of the riser, with an extended slip joint. Consider this example of how rig heave and lateral displacement combine to potentially cause failure of the riser system.
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Appendix I 205 FIGURE I.9 Effect of rig offset on riser length (not to scale)
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The plug at the bottom of the test tool rest against the bottom of the pipe rams or the annular as the pressure below it acts like a piston trying to push up on the test tool. The test pressure between BOP components creates a tensile load on the flanges between the components.
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Appendix I 207 FIGURE I.10 Lateral Load on blowout preventer (BOP) /Lower Marine Riser Package (LMRP)
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. FIGURE I.12 Typical blowout preventer test tool.
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It is necessary that designers, contractors, and operators realize that the individual components are recommended and selected in a manner suited to the overall performance of that system." It is for just this reason that this appendix will discuss overall riser system analysis processed before delving deeper into the design processes for flanges and flange bolts. Because of the complexity of the design and operation of a drilling riser system, API RP 16Q presents several types of design methodologies that can be used at the discretion of the lease operator, the drilling contractor, a drill rig shipyard, or the OEM.6 The most common are • Operability analysis, • Failure analysis, and • Fatigue analysis.
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Operability Analysis Operability analysis is performed that a riser of a specific design can be safely deployed and operated on a specific rig, in a specific location, during a set of an ticipated drilling/wellbore conditions, and under a set of assumed environmental conditions. This analysis is done to ensure the following: • Riser can be safely run and retrieved, • Riser can stay connected to allow safe drilling, • Riser (LMRP)
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In some cases, the minimum effective tension may occur at another location." • Maximum lateral offset -- the maximum rig lateral offset that can be tolerated without causing mechanical failure of the riser or any of its components by excessive tension or bending moment. Since lateral offset is the primary cause of so many failure modes for a riser/BOP system it is critical that it be accurate, conservative, and adhered to in the field.
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As a rig moves towards the POD limit: • Ball joint is maxed out, • Tension increases due to riser length at angle, • The riser slip joint strokes out, • The riser tensioning system reaches its lower limit, • Tension is added to riser top, • Bending moment on wellhead and BOP results, or • Current could add or subtract a small amount.
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From page 213... ...
There are critical human factors considerations here in determining the time for individual drilling crews to make these preparations. The following observations about the watch circle should be noted: • The condition that results in the most conservative calculation of the POD circle is the condition where the riser telescopic joint strokes out and outer barrel/tension ring is at its lowest elevation in the moonpool.
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From page 214... ...
For example, while POD circle calculations are most conservative when the riser slip joint strokes out and outer barrel/tension ring is at its lowest elevation, for weak point considerations the operating situation that results in the least amount of extension of the telescopic joint and the barrel/tension ring is at its highest elevation in the moonpool will typically be conservative. This is "because the eventual bottoming out occurs at a larger offset thereby producing larger bending moment and stresses." 12 OAA, Ocean Explorer, "Acoustic Doppler Current Profiler," http://oceanexplorer.noaa.gov/ N technology/tools/acoust_doppler/acoust_doppler.html, accessed October 22, 2017.
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In addition, in deepwater Gulf of Mexico locations near the Sigsbee Escarpment, Rossby Waves can produce significant currents near the seafloor. Interestingly these is no requirement to analyze fatigue for deepwater riser systems except for HPHT wells, although fatigue can be a bigger issue in shallower water The definition for "HPHT" can be found in API TR8.14 Fatigue analysis looks primarily at the conductor pipe, the wellhead and wellhead connector, and the riser string.
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Yamazaki, "A Method for Esti S mating Quasi-Static Riser Deformation and Applied Forces from Sparse Riser Inclination Measure ments," OMAE2015-41286, 34th International Conference on Ocean, Offshore and Arctic Engineer ing, May 31-June 5, 2015, http://proceedings.asmedigitalcollection.asme.org.
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