Crack growth rate (CGR) behavior of UNS N07718 was investigated as a function of K-rate in two different environments under cathodic potentials, a mild environment containing 3.5wt% NaCl and a more aggressive environment containing 0.5M H2SO4. The CGR in 3.5wt% NaCl at -1050mV SCE exhibited a plateau CGR that was a strong function of K-rate. CGR measurements in sulfuric acid exhibited K dependence. The CGR exhibited a weak dependence on K-rate in sulfuric acid. In the sulfuric acid environment stable cracking was sustained at constant displacement, which was not readily observed in the 3.5wt% NaCl at -1050mV SCE.
A test was performed to understand the fatigue crack growth rate behavior under gentle cycling at two different values of Kmax in 3.5wt% NaCl/pH = 8.2 at -1050 mV SCE. The measured fatigue crack growth rate increased as the frequency was decreased from 1 mHz to 0.01 mHz. The measured CGR at 1mHz with 86400s hold periods at a Kmax of 55MPa vm and 66MPavm was significantly lower than the CGR values measured in the rising displacement tests at the same K values. This suggested that crack tip strain rate may play a critical role in sustaining static crack growth.
A fracture mechanics based approach is currently being pursued for subsea equipment design by the Oil and Gas (O&G) industry in High Pressure High Temperature (HPHT) applications. A significant driver for this shift in the industry is due to the very high pressures (>15ksi) and temperatures (>350°F) that are expected in future Gulf of Mexico (GoM) fields.
The start-up period of a well is typically associated with an increase in pressure and temperature in the subsea components. The pressure increases from a low value to the full operating pressure of the well as it is brought on line. The increase in pressure in the subsea equipment usually occurs over a relatively short period of time on the order of tens of minutes to hours. The temperature of the various components increases from the seabed temperature of about 40°F to the operating temperature of about 350°F to 400°F. The rise in temperature usually lags the pressure increase and occurs over the course of a few days. This is typically followed by long periods at nominally constant pressure and temperature associated with normal operating conditions prior to the next shutdown and start-up sequence. API 17TR8 Annex D (Material Characterization Protocols) is currently being finalized to develop guidelines on the use of metallic materials (high strength corrosion resistant alloys (CRA's) and low alloy steels) for HPHT applications. The document suggests that there is currently a need to develop fatigue and fracture data for the materials of interest in not only production environments but also in seawater under cathodic protection environments. There has to date been very limited fatigue and fracture data generated under conditions relevant to subsea HPHT design.