Acoustic emission monitoring of composite containment systems

This paper considers two different types of composite containment system, and two different types of acoustic emission (AE) monitoring approach. The first system is a composite reinforced pressure vessel (CRPV) which is monitored both during construction and in-service using a broadband modal acoustic emission (MAE) technique. The second system is a membrane cargo containment system which is monitored using both a global as well as a local AE technique. For the CRPV, the damage assessment is concerned mainly with the integrity of the composite outer layer at the construction stage, and possible fatigue cracking of the inner steel liner at the inservice stage. For the membrane tank, the damage assessment is concerned with locating and quantifying any abnormal porosities that might develop in-service. By comparing and contrasting the different types of structural system and different monitoring approaches inferences are drawn as to what role AE monitoring could take in the damage assessment of other types of composite containment system. (Detailed technical data have not been included, due to client confidentiality constraints.) Introduction Lloyd’s Register EMEA (LR) carry out a wide range of independent safety assessments, as part of our wider role as Classification and Certification Authority. This involves us with independent analyses, inspection, testing and measurement, throughout the lifecycle of design, construction and in-service operation. This paper considers two scenarios, involving composite containment systems, which are of growing interest across a range of industries. Scenario 1 Composite Reinforced Pressure Vessels (CRPVs) in ships LR are involved with marine projects to transport compressed natural gas (CNG) in cylindrical pressure vessels, of diameter circa 40 inch and lengths up to 80 foot. The steel cylinders are overwrapped with a fibre reinforced plastic (FRP) composite material. A process known as “autofrettage” is used during construction such that the composite overwrap is used to induce residual compressive stresses within the steel cylinder, to improve in-service performance. Some general arrangement images are shown in Figures 1 and 2. Figure 1. Cutaway concept view of CRPVs in CNG ship. 9th International Conference on Damage Assessment of Structures (DAMAS 2011) IOP Publishing Journal of Physics: Conference Series 305 (2011) 012044 doi:10.1088/1742-6596/305/1/012044 Published under licence by IOP Publishing Ltd 1 Figure 2. Pair of CRPVs end frame and other details. There is an LR safety requirement that an in-service inspection and testing plan be developed and approved for the CRPVs. It has been established that it is not considered appropriate to consider the in-service inspection and testing in isolation, as it is necessary to additionally link in to elements from: the inspection and testing carried out during manufacturing; the construction quality assurance (CQA) and quality control (QC) procedures, and also: the underlying design approach. In other words the approach to the complete lifecycle of design, construction (manufacture at works and installation in ship) and operation (in-service) needed to be considered together. Philosophy for design, construction and operation of CRPVs In essence, once the CPRVs are installed within the CNG carriers they become extremely difficult to inspect and test in service. As a consequence the overall design philosophy has been to: design with conservative margins of safety, and a “leak-before-break” philosophy ; construct under a strict QA and QC regime, including use of LR approved materials; inspect and test thoroughly at the construction stage (100% coverage); inspect and test as far as reasonably practical, in-service. The in-service inspection and testing proposals include: regular visual inspection, as far as reasonably practical; regular monitoring of vessel pressure and temperature by the crew; having in place a continuous monitoring system to detect and signal leaks, such that any leaking CRPV can be safely taken out of service before compromising safety; periodic pressure testing, using air (as hydrotesting is impractical in service); periodic acoustic emission testing (AT), focussing on the three areas identified as being most at risk from fatigue cracking, namely the circumferential welds at the centres of the CRPVs, the longitudinal seam welds, and in the vicinity of the nozzles at the extremities of each CRPV. In respect of acoustic emission testing (AT), this is an established technique for testing at the construction stage, in relation to the composite outer layer of the CRPV. However it is less well established as a technique for testing the steel cylindrical liner at the in-service stage, in relation to fatigue cracking in the CRPVs, and hence in this respect it was only to be regarded as an additional safeguard, which might give early warning of abnormal problems, but which was not relied upon in terms of the “safety case” that the operator was putting forward to the Classification Society (LR). 9th International Conference on Damage Assessment of Structures (DAMAS 2011) IOP Publishing Journal of Physics: Conference Series 305 (2011) 012044 doi:10.1088/1742-6596/305/1/012044