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Effect of Low Frequency on the Fatigue Response of Wellhead Extension Joint Welds Exposed to Simulated Seawater with Cathodic Protection

Picture for Effect of Low Frequency on the Fatigue Response of Wellhead Extension Joint Welds Exposed to Simulated Seawater with Cathodic Protection
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This work considers wellhead extension joint welds for high-pressure deepwater Gulf of Mexico prospects with a design shut-in tubing pressure (SITP) above 15000 psi. The structural welds joining 75K 8630M low pressure wellhead housing (LPWH) forgings to API 5L Grade X80M PSL 2 conductor casing are expected to be fatigue-sensitive due to water depths approaching 6000 ft. and substantial estimated weight of a 20K BOP stack. Cyclic loading is expected to be from wave‑induced motions (WIM) and vortex‑induced vibration (VIV). The wellhead extension joints extend 10 to 15 ft. above the mudline and will be exposed to seawater as well as cathodic protection when the wellhead is connected to either the BOP stack or the tubing head spool and subsea tree.The present work considered both S-N and fracture mechanics based designs. Fatigue curves such as those in BS (British Standard) 7608 are not yet proven for structural steels with yield strengths in excess of 400 N/mm2 (58 ksi) in the presence of cathodic protection and fatigue crack growth laws such as those in BS 7910 for steels in a marine environment are only applicable to materials with yield strengths less than 600 N/mm2 (87 ksi). Materials in the subject assemblies may have yield strengths in excess of 690 N/mm2 (100 ksi) so both S-N and fatigue crack growth rate (FCGR) testing in a simulated seawater with cathodic protection environment were determined to be appropriate. This paper describes the environmental fatigue testing of welds and the relationship between the test data established fatigue curves and published literature. Tests were performed in a 3.5% NaCl solution at 40 °F with pH adjusted to 8.2 and an applied potential of -1100 mV Ag/AgCl. Frequency scans were conducted in several weld microstructures to assess growth rate sensitivity of each microstructure to cyclic frequency and the fatigue response at the low end of the frequency range was explored in greater detail.

Product Number: 51319-13071-SG
Author: Adam Rowe
Publication Date: 2019
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$20.00
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Picture for Case Studies of Thermal Fatigue Damage in Duplex and Stabilized Stainless Steel
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Case Studies of Thermal Fatigue Damage in Duplex and Stabilized Stainless Steel

Product Number: 51319-13271-SG
Author: Naif Alabri
Publication Date: 2019
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Thermal fatigue is a specific type of fatigue failure mechanism that is induced by cyclic stresses generated due to repetitive fluctuations in the temperature. These temperature change in the material induces thermal expansion (or contraction) and if surrounding material or external constraints hinder this expansion thermal stresses arise. The degree of damage is affected by the magnitude and frequency of the temperature swings and typically appears in the form of one or more surface cracks on the component.This paper presents two case histories of thermal fatigue failure along with detailed metallurgical analysis of stainless steel pressure components in a downstream Aromatics plant.The first case study is on failure of a UNS31803 duplex stainless steel heat exchanger tubing that occurred after four and half years in service. The damage was in the form of multiple parallel circumferential surface cracks with few cracks propagating through the wall thickness of tube. The equipment was subjected to a temperature differential of about 100 deg C every 4 hours of operation during alternate heating & cooling cycle.The second failure discussed is on 6” diameter UNS32100 process piping after around ten years of service. This is primarily a thermal mix point wherein demineralized water is injected in a hot gas stream resulting in a temperature gradient of more than 100 Deg C. The failure mode was also characterized by many circumferential cracks at location of injection as well as about 2-3 diameters downstream injection point.Keywords: Thermal Fatigue Stainless Steel Cyclic stresses Temperature differential
Picture for Evaluation Of The KIEC Of UNS N07718 With And Without Delta Phase In Simulated Seawater With Cathodic Protection Using Double Cantilever Beam Tests
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Evaluation Of The KIEC Of UNS N07718 With And Without Delta Phase In Simulated Seawater With Cathodic Protection Using Double Cantilever Beam Tests

Product Number: 51321-16652-SG
Author: Adam C. Rowe; Suresh Divi
Publication Date: 2021
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00214 CORROSION FATIGUE CRACK GROWTH BEHAVIOR OF DUPLEX STAINLESS STEELS WITH DIFFERENT NITROGEN CONTENTS

Product Number: 51300-00214-SG
ISBN: 00214 2000 CP
Author: Chuan-Ming Tseng, Horng-Yih Lieu, and Wen-Ta Tsai
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