Preventing Failure of High-strength Fasteners Used in Offshore and Subsea Applications
a report by
Khlefa A Esaklul1 and Tawfik M Ahmed2
1. BP Exploration, Sunbury; 2. IONIK Consulting/JP Kenny Inc., Houston
Adequate strength and resistance to corrosion and environmentally assisted cracking are key elements in selecting fasteners for offshore and subsea systems.
Proper material selection is of paramount importance, along with proper material processing, to ensure suitability for the intended subsea service environment. This material selection becomes particularly significant as the oil and gas industry explores and develops reservoirs in deeper waters, where the operating and intervention cost is exceptionally high.
For assembly and construction ease, flanged connections continue to be an integral part of offshore developments, with fasteners being the primary assembly method. The combined presence of stresses and a corrosive environment requires, in addition to strength, good resistance to corrosion and environmentally assisted cracking to meet the high reliability and integrity requirements of these systems.
Developing reservoirs with deeper water depths, higher reservoir pressure and higher temperature inherently requires a class of materials with optimum combined properties that exceed typical subsea materials that have been in common use. Costly intervention and the demand for higher safety and environmental protection have increased the need for design reliability and parts with proven performance. As more complex and higher-pressure systems are being built, larger-diameter fasteners made with higher-strength materials are being used. Fasteners with diameters exceeding 100mm are becoming common.
Essentially, materials selection is based on the mechanical properties, corrosion resistance (general, galvanic and localised pitting and crevice) and resistance to environmentally assisted cracking (EAC), which includes stress corrosion cracking (SCC), hydrogen embrittlement (HE) and sustained load cracking (SLC). While alloy steels have shown adequate corrosion resistance in cathodically protected systems, their resistance to cracking has been a major concern, particularly for high-strength grades.
Although some materials may have the combined desired properties, alloy applicability is restricted by size limitation due to lack of uniform
Tawfik M Ahmed is a Lead Materials Engineer with IONIK consulting/JP Kenny Inc. He has more than 12 years of experience in failure analysis involving corrosion and hydrogen embrittlement, materials testing and specifications, damage tolerance, structural integrity, fracture mechanics analysis and fatigue life assessments (ECA/FFS) and the internal corrosion assessment of pipelines. He has a PhD from the University of British Columbia.
E:
Tawfik.Ahmed@ionik.net
thickness properties as a result of inadequate hardenability. In addition to these limitations, high cost is another limiting factor to using some of these alloys.
Causes of Failure
Fasteners can fail as a result of one or more of:
• overload; • corrosion; • fatigue; • corrosion fatigue; and • EAC.
Selection of fasteners and assembly processes must account for the above factors to ensure reliable long-term performance. Joint design and installation practices address the overload and fatigue issues. Corrosion, corrosion fatigue and EAC can be addressed through materials selection and applying corrosion protection measures.
High-strength steels are known to be prone to SCC and HE, and resistance decreases with increasing strength. The resistance of these materials is generally expressed in terms of a hardness limit above which the material is not recommended for use in a specific environment, such as seawater, or in terms of SCC or HE threshold
stress intensity factors – KISCC and KIHE, respectively. Steels with yield strength levels below 120ksi are generally resistant to SCC and HE.1,2
Above 120ksi yield strength, KIEAC decreases with the increase in yield strength with a typical threshold stress intensity value of 50–80ksi.√in for 145ksi yield strength material.1,2
Typical fracture
toughness value (KIC) for 145ksi yield strength quenched and tempered steels is 200ksi.√in.
In the absence of cathodic protection (CP), high-strength steels are typically more resistant to EAC in seawater. NASA has evaluated several aerospace-grade high-strength, low-alloy steels and showed AISI 4340 is resistant to SCC up to a tensile strength of 180ksi (40 HRC).3
and ASTM A193 B7M5
grades with a maximum hardness of 22 HRC, including the specified limit for sour service applications per MR0175.6
Studies have shown that limiting subsea fasteners to the sour service requirements is overly conservative and have shown that subsea fasteners exposed to CP can be used to a maximum hardness of 34 HRC.7
For cathodically protected components, alloy steel fasteners historically were limited to ASTM A320 L7M4
Figure 1 shows failure of a cap screw made of low-alloy steel due to HE under CP conditions when hardness exceeded 34 HRC. Resistance to HE due to CP must be treated in the same way as sulphide stress cracking, where the maximum hardness allowed must not be exceeded. Experience has shown that large-diameter, high-
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© TOUCH BRIEFINGS 2010
Design & Engineering
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