API Spec 16R:1997(R2010) pdf download.Specification for Marine Drilling Riser Couplings
4.4 Stress Distribution Verification Test
After completion of the design studies, a prototype (or multiple prototypes) of the riser coupling shall be tested to verify the stress analysis. The testing has two primary objec- tives: to verify any assumptions which were made about pre- loading, separation behavior, and friction coefficients and to substantiate the analytical stress predictions. Strain gauge data shall be used to measure preload stresses as they relate to make-up load or displacement. Friction coef- ficients shall be varied (including at least two values) to estab- lish sensitivity. The coupling design load shall be applied to verify any assumption made in the analysis regarding separation. Strain gauges shall be placed as near as physically possible to at least five of the most highly stressed regions as predicted by the finite element analyses performed in accordance with 4.3 and in five locations away from stress concentrations. Rosettes shall be used. All strain gauge readings and the asso- ciated loading conditions shall be recorded in a manner that they may be retained as part of the coupling design documentation. Normal design qualification tests may be performed simul- taneously with this stress distribution verification testing. These are defined in 8.2. Note: It is often difficult to acquire sufficient strain data to totally correlate with the analytical results. High stress areas may be inaccessible and are sometimes so small that a strain gauge gives an average rather than the peak value. The testing should serve to verify the pattern of strain in regions sur- rounding the critical points.
4.7 Stress Amplification Factor
Field experience suggests that the most likely cause of a riser coupling failure is propagation of a fatigue crack which has initiated at a point of stress concentration. It is, therefore, incumbent upon the designer to endeavor to minimize the conditions leading to the initiation and propagation of fatigue cracks. The Stress Amplification Factor (SAF) is intended to provide the coupling user with information needed to esti- mate fatigue damage for a particular application without extensive fatigue testing of the coupling. The SAF is a func- tion of the double amplitude range of alternating stress. It is important to note that the SAF value depends largely on the exhaustiveness of the finite element analysis and the validity of assumptions in the analysis. Assumptions such as load distribution, the correctness of preloading in field ser- vice, and finite element size at critically stressed points neces- sitate individual evaluation for each design case. Calculation of SAF is not intended to substitute for a comprehensive fatigue life analysis.
API Spec 16R:1997(R2010) pdf download
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