A current challenge in the offshore industry is the designof subsea equipment for pressures exceeding 15,000 psi and temperaturesexceeding 250°F. This combination of pressure and temperature has been fairlyaccepted as the start of the high-pressure, high-temperature (HPHT) region. API17 Technical Report 8 (API 17TR8) and API 17G provides the High Pressure HighTemperature (HPHT) design guidance.Schlumberger 20 ksi landing string uses 7-1/16″ API flangewith BX 169 in the following locations at the Swivel top connection, Swivelbottom connection, Top of Flow head block and Bottom of Flow Head Master block.The document is to describe the finite element analyzes of 7-1/16″ API flange(HPHT) that were performed to determine the structural and functional capacityof the flange at the temperature 350°F and Fatigue performance at roomtemperature of 70°F according to the methodology described in API 17G. Inaddition, ratcheting behavior was verified as per the ASME section VIII div.
3and the assessment was considered with room temperature initially for FactorAcceptance Test (FAT) loads and proceeded with 350°F temperature for the cyclicloadings.Plastic collapse load criterion method (global failure) perAPI 17G: The static capacity of the 7-1/16″ 20 ksi flange wasevaluated for (bending, internal pressure, and external tension) using PlasticCollapse method of API 17G. For the global plastic collapse load methodyielding criterion, incremental load is applied on the component till the modelstops converging because of large plastic deformation (API 17G).
For this method, the structural capacity isthe global collapse load divided by 1.2. Structural Capacity based on the global failure Local failure assessment as per KD-232, ASME Sec. VIII Div.3:The strain damage at load capacity values arrived from theglobal assessment. As per this criterion KD-232, ASME Sec.
VIII Div. 3, failureload is determined as the load wherein the total strain damage anywhere in themodel exceeds 1.0. Structural Capacity based on the local failure Functional Capability Study:The gasket sealing considered to be adequate if the contactpressure on the gasket sealing faces is 3x the internal pressure over a sealinglength of at least 0.060 in. (Cameron & One-Subsea internal standard). Forall the loading conditions, Functional capacity was determined based on thegasket sealing capacity criterion for normal operating and accidentalconditions according to the Cameron and One-Subsea internal standards.
Contact pressure (psi) plot on the gasket for the appliedloading conditionFatigue Life calculation – M-N Curve—Bending:The M-N curve is generated based on the API 17G 3rd edition2016 draft and has been redacted in the 2017 draft. The M-N curve representsthe Moment range versus Number of cycles based on the fatigue hot spots of theflange assembly. The moment ranges (M) are from the given histogram input dataand the Number of cycles (N) are calculated based on the DNV RP-C203 equation2.4.1.
The number of cycles are presented for Single Bending Fatigue at HotSpot Locations of the flange assembly. (?M-N Curve) Moment range versus number of cyclesFatigue Life calculation – T-N Curve—Tension:The T-N curve is generated based on the API 17G 3rd edition2016 draft and has been redacted in the 2017 draft. The T-N curve representsthe tension range versus number of cycles based on the fatigue hot spots of theflange assembly. The tension range (M) from the given histogram and Number ofcycles (N) are calculated based on the DNV RP-C203 equation 2.4.
1. The numberof cycles are presented for Single tension fatigue at Hot Spot Locations of theflange assembly. (?T-N Curve) Tension range versus number of cyclesElastic-Plastic Ratcheting Assessment:Ratcheting assessment is an elastic-perfectly plasticanalysis were the flange assembly is verified with the five load combinations.In ratcheting criteria, the growth of plastic strain is zero, results there isa development of elastic core in primary components which satisfies the requirementas per the ratcheting criteria KD-234 of ASME section VIII Div.3.
Equivalent Plastic Strain (in/in)The project provided an opportunity to learn the fatigueanalysis method based on the API 17G 3rd edition 2016. This project wassupported and Mentored by the Mircea Ionescu, Houston.