Preventing Failure of High-strength Fasteners Used in Offshore and Subsea Applications
Figure 1: Failure of Cap Screw Made of Low-alloy Steel Due to Hydrogen Embrittlement
increase, the material properties, particularly strength, toughness and resistance to SCC, become the primary selection criteria.
To resist brittle fracture and improve fatigue strength and resistance to SCC and HE, material is specified with a certain level of ductility, toughness and controlled hardness. These parameters typically require a specific heat treatment to control strength or hardness. Typical factors used in the selection of fasteners are:
Note the intergranular morphology on the close-up of the fracture surface.
Table 1: Typical Materials for Subsea Fasteners
Alloy Group Material
Alloy steels
AISI 4140 AISI 4130 AISI 4340
Stainless steels Duplex SS 2507 A286
Custom 455 PH 13-8 Mo S17400 S17700 S15700
Ferralium 255
Ni-based alloys K-400 K-500
Alloy 718 Alloy 725 Alloy 925 Alloy 59
Alloy X-750 Alloy 625 Rene 41
Co–Ni alloys MP35N MP159
Be–Cu alloys C17200 C17300 C17000
• material strength (yield and ultimate strength); • material ductility expressed in elongation (generally a minimum of 7–18%);
Yield Strength Tensile Strength (ksi)
(ksi)
75–105 120 145
80–95
95–119 135–231 190–205 145–170 140–230 170–230 110 30 86
120–145 120 110
128–168 92
120 157 230 250
145–190 145–190 125–185
Ti base alloys Ti-6Al-4V, Ti-6Al-4V ELI 128 Ti 3Al-8V-6Cr-4Mo-4Zr 115 Ti-5Al-1Sn-1Zr1V-0.8Mo 111 Corona-5
Ti-4.5Al-5Mo-1.5Cr Ti-6Al-6V-2Sn Ti-10V-2Fe-3Al
Ti-6Al-2Sn-4Zr-6Mo Ti-7Al-4Mo
Ti-13V-11Cr-3Al Ti-8Mo-8V-2Fe-3Al Ti-11.5Mo-6Zr-4.5Sn Ti-15V-3Al-3Cr-3Sn Ti Beta 21S
120–153 145–160 150
150–160 150–160 160 170 175 190 205
100–125 135 160
116–145 150–190 160–260 220–235 200
130
75–100 105–150 150–180 165 168
149–170 162 165 206 280 260 – – –
200 120 126
– – – – – – – – – –
strength alloy steel bolts used for drilling riser applications with a specified hardness of 34 HRC in fact have hardness ranging from 34 to 38 HRC and are generally closer to 38 HRC than 34 HRC in one lot of 3.25-inch diameter bolts. Thus, adhering to specification and quality assurance should be a prime concern when procuring fasteners.
Design Criteria
Bolted joint designs are governed by American Society of Mechanical Engineers (ASME) Pressure Vessel Code or API Standards, ASME B16.58
and API 6A9
KI
for offshore equipment. The flange size and
pressure rating generally decide the number, size and strength of the fasteners. As the size, strength and demand for high reliability
76
= Cσ πa
(1)
where KI = stress intensity factor, C = shape factor (1.5 has been used for threads geometry),2
σ = nominal stress and a = depth of a surface breaking flaw.
EXPLORATION & PRODUCTION – VOLUME 8 ISSUE 1
• toughness, typically specified as Charpy impact values at the lowest expected temperature (generally average Charpy toughness values of 20–50ft.lb minimum);
• resistance to corrosion and SCC; • resistance to HE and other hydrogen-induced cracking (HIC); and • long fatigue life.
Material strength selection is determined by the design stress, which is determined by:
• design stress at rated working pressure (σsm); • the maximum allowable general primary membrane stress at
hydrostatic test pressure (σST); and
• the maximum combined primary and secondary stress (σSS). These stresses are limited to:
• σsm = the smaller of 2/3 σMSYS or 1/2 of σMSUTS; • σST = the smaller of 5/6 σMSYS or 2/3 of σMSUTS; and • σSS = the smaller of 2 σMSYS or σMSUTS,
where σMSUTS is the minimum specified ultimate tensile stress and σMSYS is the minimum specified yield strength. Increasing the pressure rating of the component or the combined stress intensity requires using higher-strength material.
To control corrosion and SCC, materials are selected for either their resistance to corrosion and SCC or their ability to be protected by CP. In subsea cases, it has been shown that CP with sacrificial anodes effectively controls corrosion of carbon steel fasteners in use with carbon steel or corrosion resistance alloy (CRA) connections.7
Resistance to SCC limits the strength of each material and, in many cases, some materials are not considered viable. This led designers to use high-strength grades of CRAs.
Defect-tolerant Design
To ensure the fastener materials have sufficient resistance to SCC, a defect tolerance approach should be considered. Stress intensity solutions for threaded components are given by the following equation:2
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