Material selection for downhole applications has become more difficult as the number of alloys continues to increase. On one hand, stainless steels like 316 offer a relatively low initial cost, but are not suited for many severe applications or environments. Other alloys, like MP35N, offer considerable strength and corrosion resistance, but with a much higher cost. Thus, an attempt has been made to create an alloy that spans the considerable gap between the 300 series of stainless steels and the nickel or cobalt base alloys. The resulting 6% Mo stainless steels offered increased strength and corrosion resistance without a drastic cost increase. The first generation of superaustenitics still falls considerably short of the strength and corrosion resistance provided by established nickel or cobalt-base alloys. In an attempt to further bridge the remaining gap, a new superaustenitic stainless steel has been developed that maintains the attractive cost of the 6% Mo alloys, but enhances both corrosion resistance and strength to open up environments that were too severe for 6% Mo alloys. The development of this alloy along with localized corrosion resistance, qualification testing, and mechanical testing are discussed.
One of the frequent and major problems encountered in the oil and gas production is the
internal corrosion of carbon steel pipelines. Corrosion can be categorized into uniform (or
general) corrosion, localized corrosion and erosion-corrosion. Uniform corrosion causes
overall metal loss and general thinning of metal. Localized corrosion has the appearance
of pits or grooves.
Upstream oil and gas companies operate oil gathering systems comprising a flowline network and process facilities that transport the flow of produced fluids from the wells to a main processing plant. The frequency of corrosion related leaks has increased recently despite a corrosion inhibitor is injected at the wellhead into all flowlines. A root-cause analysis conducted by several companies revealed that severe internal corrosion was caused by a low fluid flow velocity an increasing water cut and the presence of sulfate-reducing bacteria (SRB) in the production streams. Nevertheless it was not clear why some of the flowlines may leak while others do not leak despite the composition of produced fluids principal design parameters (diameter and length) dosage of corrosion inhibitor and environmental conditions of the flowlines are similar. A diagnostic analysis of different oil flowlines of was carried out to gain an understanding of why a first group of oil flowlines is developing leaks and why a second group of flowlines has not experienced leaks. The methodology used for the diagnostic analysis comprises 1) Ultra-High Definition simulation of 3-phase or 4-phase flow of gas oil water and solids; 2) 3D imaging of phase distributions inside critical sections of the oil flowlines as per NACE ICDA; 3) mapping adverse operational conditions; and 4) the determination of probability of failure in the critical sections based on criteria depending on the severity of operating conditions inside and outside the flowlines. It was found that multiple sections were exposed to stagnant water and/or had a fraction of internal surface area covered by a stationary bed of solids (formation solids produced from the well). The identified causes of potential leaks comprise the following failure mechanisms: a) metal loss caused by colonies of SRB b) composed load acting on the pipe wall and c) cyclic" thermal expansion/contraction of the flowlines due to seasonal ambient temperature variations. One of the surprising findings of this study was that a shorter flowline with a lower water cut may have multiple leaks while a longer flowline with a higher water may not leak at all approximately for the same period after commissioning. This result was explained with help of maps of adverse operational conditions constructed for the two groups of flowlines. Immediate corrective mitigation actions and preventive actions were implemented to reduce leak frequency including the installation of a novel automatic flushing system.
The power plant is a natural gas-fired, combined cycle plant with three combustion turbines and a single steam turbine. A large stainless steel surface condenser is used to condense steam off of the turbine, and provide high purity steam condensate return to the boiler system. The steam condenser was put into service approximately 15 years ago. This plant takes makeup water for its open recirculating cooling tower water system from a river location that is inland from an ocean coastal area.
The Hanford site contains approximately 55 million gallons (2.08 x 108 liters) of radioactive and chemically hazardous wastes arising from weapons production, beginning with World War II and continuing through he Cold War era. The wastes are stored in 177 carbon steel underground storage tanks, of which 149 are single-shell tanks (SSTs) and the remaining are double-shell tanks (DSTs). Historically, tank failures have been associated with the SSTs
Pre-commissioning hydrostatic testing of pipelines and the resulting corrosion (MIC) issues are often linked to test water quality, as well as post-test cleaning operations. In a 1998 study, it was reported that localized corrosion (pitting/crevice corrosion) accounted for 20% of failures in the chemical process industry with an estimated one half of those being MIC failures. Identification of MIC failures is not straightforward. Common characteristic features such as pit clustering, “tunneling” of pits, tuberculation, high microbiological counts, presence of sulfides (in the case of sulfate reducing bacteria (SRB)) and preferential weld attack have been used to anecdotally pinpoint field failures towards MIC.
Stainless steels and Ni-base alloys are often considered as construction materials in applications where highly corrosive conditions are expected. High levels of halides, low pH and high temperatures are factors that often contribute to the selection of such materials.
Ten different alloys have been included in this work, representing a range of highly alloyed stainless steels and Ni-base alloys. The purpose has been to evaluate the corrosion resistance of stainless steels with alloying content in the 6Mo range or higher, and competing Ni-base materials. The austenitic grade N08904 and two super-duplex grades have also been included for reference.