Issue 30

M. N. James, Frattura ed Integrità Strutturale, 30 (2014) 293-303; DOI: 10.3221/IGF-ESIS.30.36 300 amongst materials, welding fabrication and fatigue and fracture. Such difficulties may also include misinterpretation of the requirements and constraints imposed by fatigue design codes. Failure case studies The main example of such problems in the present paper centres on two large horizontal rotating cylindrical shells in which author was recently involved as an expert witness. Each cylindrical shell rotates at 1 rpm, and is around 65 m long and 4.5 m in diameter. There is a significant corrosive distributed load inside the shell at any time as well as a substantial self-weight. The capital cost of the installation was around $24M and the design life was specified to be 20 years against fatigue and corrosion. A large through-shell crack was discovered in one cylinder within 5 months from the date of practical completion whilst the second cylinder experienced similar cracking within a further 2 months. The initial cause of the failure was identified as fatigue cracking from internal detail attachment welds associated with two process-related features: firstly knife features designed to assist the internal process taking place in the cylinders. Forty of these features were arranged in an array inside the shell from low stress entry positions to high stress positions further along the cylinder. Secondly, wear bars in the form of channel sections were stitch welded to the inside of the shell as the internal process was known to be abrasive. The problem was ascribed to inadequate fatigue design, as the stress concentrating effect of these features, in particular the knives whose geometry created large stiffness transitions at their attachment points, had not been considered in the original stress calculations done for the shells. Further design issues were identified as:  Transition tapers between shell segments of different thickness (25 mm to 40 mm to 70 mm) that did not meet the requirements for the weld class used in the fatigue life calculations of the circumferential butt welds between cylindrical strakes. There was high concern at this point as further fatigue life calculations indicated that in a free corrosion environment, cracks of the size found (in a very limited survey undertaken of the large number of internal welds) would very quickly lead to further through-shell cracks.  There was a 35 m unsupported span of 25 mm shell in the central region of each cylinder, which a strain gauging and FE analysis showed became oval during rotation and that therefore led to two biaxial stress cycles occurring during each revolution of a shell. Short-term remedial measures were accordingly implemented which included removing all the knives except those in low stress areas of the shell along with associated repair work, and a long-term re-design process was commenced along with a court case by the operators. It was interesting that the removal of the bulk of the knives had no discernible effect on the efficacy of the internal process inside the cylinders. This raises the interesting question as to why they had been specified as part of the original design. It was a point of note that the original design specification from the process owners made no mention of a highly corrosive internal environment, although it was clear from operational measurements that such an environment was present in at least some parts of the cylinder. Further investigation demonstrated that additional problems had also contributed to the very short observed fatigue life. These included alloy substitution for the knife backing plates, which were fillet welded to the shell, from Grade 250 steel (yield strength around 250 MPa) which had been specified for all knives except those in low stress positions, to abrasion resistant steel with typical yield strength of 1,200 MPa. The backing plates of knives in low stress positions had been specified by the designers to be fabricated from abrasion resistant steel with a 0.2% proof strength of around 1,070 MPa. This alloy substitution meant that tightly controlled welding procedures should have been followed by the fabricator in terms of consumables, pre-heat and interpass temperatures. In the event, the procedures followed were not fully documented, while the hardness values and their variation across the weld zone implied that hydrogen cracking was a distinct possibility (this was supported by the observed positions of some cracks at the attachment welds). In addition, solidification cracks were present at some of the detail attachment welds. The final issue of relevance to the ongoing legal argument regarding responsibility, replacement or repair, emerged from semi-annual non-destructive monitoring of sections of the shells, using automated ultrasonic inspection. It became apparent that significant shell abrasion was occurring between the wear bars, although the process design was supposed to lead to an adherent abrasion-resistant coating being deposited on the inner surface of the cylindrical shell between the wear bars. It is interesting to note that annual maintenance of these shells was supposed to occur in a very short shut- down period of about a week. These difficulties could have been avoided through better communication amongst process owner, designers, fabricators and operator and by better familiarity with the requirements of weld design and fabrication for fatigue. The legal argument and associated preparation of reports from a variety of experts eventually pushed the settlement costs of the case to perhaps $40M and extended the time taken for achieving a long-term solution to more than 5 years, by which point the only option was total replacement.