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

摘要

This work presents laboratory testing performed to assess the fatigue response of wellhead extension joint welds for high-pressure,deepwater prospects subjected to low-frequency cyclic loading.The wellhead extension joints are exposed to seawater and are cathodically protected during those periods when the wellhead is connected to either the BOP stack or the tubing head spool and subsea tree.Materials in the subject assemblies may have actual yield strengths in excess of 690 N/mm~2(100 ksi),strength levels at which published S-N and fatigue crack growth rate(FCGR)design curves are not proven.This paper describes the environmental fatigue testing of welds and the relationship between the test data,established fatigue curves,and published literature.Both S-N and FCGR tests were conducted in simulated seawater with cathodic protection environments.The tests were performed in a 3.5% NaCI solution at 4 °C(40 °F)with pH adjusted to 8.2 and an applied potential of -1100 mV Ag/AgCI(0.1 M KCI).Frequency scans were conducted to assess growth rate sensitivity of various weld microstructures to cyclic frequency,and the fatigue response at the low end of the frequency range was explored in greater detail.