Environmentally Assisted Cracking Assessment of HPHT Products Under Cathodic Protection and Seawater Environment

Subsea High Pressure and High Temperature (HPHT) equipment in oil and gas applications are often required to be produced from high strength low alloy steels. In order to manage the harsh subsea corrosive environment these products generally feature cathodic protection systems to allow long term installation. Such HPHT equipment is governed by the industry standard API 17TR8 which applies to equipment operating at pressures greater than 15,000 psi working pressure and/or operating temperatures greater than 350°F. Equipment that is designed to operate above these parameters requires verification per the requirements of API 17TR8. Part of the verification required is to assess the products for their robustness against the potential for environmentally assisted cracking (EAC) in the subsea environment. Environmentally assisted cracking is defined by API 17TR8 as “cracking of a material wherein an interaction with its environment is a causative factor in conjunction with tensile stress, often resulting in brittle fracture of an otherwise ductile material”. HPHT equipment in a subsea environment that is connected to a cathodic protection system can result in the formation of atomic hydrogen at the products surface. This hydrogen generated can be absorbed and diffused into the metal, leading to hydrogen embrittlement. With the high loading and pressures seen in HPHT equipment, in parallel to a drive for cheaper and lighter products, the average stress seen in complex geometries and components can lead to greater stress concentrations and areas for potential crack initiation due to higher hydrogen accumulation in those areas. This in conjunction with the metal embrittlement will lead to a reduction in material properties which can in turn have an impact on the integrity and performance of the subsea product. Industry standards such as API 17TR8 therefore drive the requirement to consider such potential failure modes in the design. While general guidance is given on materials testing and assessment requirements; no detailed methodology is presented. This paper will outline a methodology for assessing for EAC in HPHT and other subsea products.