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Some additional information applicable to fatigue sensitive components:
Flange Connection and Girth Weld Fatigue Testing
The pipe-connector girth weld on the drilling riser stress joint (and pipe-pipe and pipe-connector welds on standard drilling riser joints) should be qualified for fatigue by full-scale resonance fatigue testing. The girth welds should demonstrate a fatigue performance exceeding the target cycles calculated for the DNV CN No. 30.2/BS 7608 C class S-N curve using the test variables such as alternating stress range, number of specimens (n), and the level of significance (probability). The flanged connector should demonstrate a fatigue performance that exceeds that of the girth welds.
Fatigue test methodology and protocol, i.e., target cycles, specimen marking, and post-test inspection, should be according to industry recognized practices. The level of significance for test cycles should be 2.5% (97.5% probability). All welds, both failed and run-outs, should be sectioned and examined after testing for minimum wall thickness, High/Low, and ground area surface finish. For failed welds, the sectioned sample should be frozen, broken open to expose the crack face, and then photographed. The SCF at the crack should also be calculated. For run-outs, the sample should undergo magnetic particle inspection at the weld to identify any surface-breaking indications.
Actual riser joint materials fabrication procedures and practices that are to be used for the production welds should be used to fabricate the fatigue test samples. This includes weld bevel preparation, component fit-up and alignment methods, in-process verification of internal surface mismatch or High/Low, weld procedures, non-destructive examination methods, defect acceptance criteria, and surface conditioning/grinding.
Since there are many articles explaining what Top Tension Risers (TTR) are, I will be discussing the Material Selection aspects when working/selecting the fit-for-purpose TTR.
All primary forgings, pipe, and structural materials should meet the requirements of API STD 2RD, Section 7, Materials.
Exposure to a cathodic protection system should be considered in the selection of all materials.
Forgings should meet the requirements of API STD 2RD. Forgings and Extrusions should meet ASTM A182 F22 grade material.
Structural equipment materials should meet the requirements of API STD 2RD.
All seals must be suitable for the fluid service intended. Typical practice is to design or specify corrosion resistant metal seals for use in flanged joint connections. An elastomer seal should be used to keep sea water away from primary flange seals.
Bolting for drilling riser flange connections should meet the standards of API STD 2RD, Section 7.10.3, with the exception that maximum bolt hardness should be limited to 32 HRC.
Pad-eyes and Lifting Gear
All material for pad-eyes, spreader bars, and lifting eyes should be 100% ultrasonically tested for laminations in accordance with ASTM A578 (acceptance standard Level II). Pad-eye holes should be bored or reamed and should have a uniform milled finish. All lifting pad-eyes, spreader bars, tri-plates and other lifting tools should be certified by a recognized classification society. All pad-eyes spreader bars and lifting eyes should be load tested to 1.5 times the safe working load with non-destructive testing performed prior to and after load testing.
Welding, Fabrication and Nondestructive Testing
Welding, fabrication and nondestructive testing (NDT) should meet the requirements of API STD 2RD.
All weld procedures should be qualified in accordance with applicable standards and reviewed/approved prior to use.
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