Prognostics Framework for Remaining Life Prediction of Cutty Sark Iron Structures Yasmine Rosunally1, Stoyan Stoyanov1, Chris Bailey1, Peter Mason2, Sheelagh 3 4 Campbell and George Monger 1 School of Computing and Mathematical Sciences, University of Greenwich, Greenwich, London, SE10 9LS, UK [email protected] [email protected] [email protected] 2 The Cutty Sark Trust, 2 Greenwich Church Street, Greenwich, London, SE10 9BG, UK [email protected] 3 Applied Electrochemistry Group, School of Pharmacy and Biomedical Science, University of Portsmouth, St Michael’s Building, White Swan Road, Portsmouth, PO1 2DT, UK [email protected] 4 Conservation & Museum Services, Unit 6a Glebe Farm Business Units, Woodland Close, One House, Suffolk, IP14 3HL, UK [email protected] ABSTRACT corrosion data was generated using temperature, humidity and time as the factors causing corrosion.* The Cutty Sark is undergoing major conservation to slow down the deterioration of the original Victorian fabric of 1. INTRODUCTION the ship. The conservation work being carried out is “state The Cutty Sark is a composite-built vessel with a of the art” but there is no evidence at present for wrought iron frame and teak and rock elm strakes predictions of the effectiveness of the conservation work fastened to it. Conservation work is currently being 50 plus years ahead. A Prognostics Framework is being carried out due to extensive deterioration of the developed to monitor the “health” of the ship’s iron wrought iron frame and timber planking (Campbell, structures to help ensure a 50 year life once restoration is 2005). On completion of the conservation work, a completed. This paper presents the prognostics framework decision support system will be put in place to being developed using three prognostics approaches: monitor and predict the “health” of the ship’s iron Physics-of-Failure (PoF) models, Data-driven methods and structures in the future. The decision support system a fusion approach integrating both former approaches. will be built upon the prognostics framework to “Canary” and “Parrot” devices have been designed to achieve these aims. Corrosion is the main cause of mimic the actual mechanisms that would lead to failure of deterioration of the wrought iron framework. the iron structures. A PoF model based on decrease of Various forms of corrosion are prevalent on Cutty corrosion rate over time is used to predict the remaining Sark and monitoring of the rates of these different life of an iron structure. Mahalanobis Distance (MD) is corrosion types is a challenge. Common intrusive used as a precursor monitoring technique to obtain a single measurement techniques have high risk of damage comparison metric from multiple sensor data to represent to the Victorian fabric of Cutty Sark structures. anomalies detected in the system which could lead to Additionally, information on the corrosion of failures. Bayesian Network models integrating remaining wrought iron structures in the literature is scarce. life predictions from PoF models with information of There is are few models that attempt to predict possible anomalies from MD analysis, are used to obtain corrosion rate due to different influencing factors more accurate predictions of remaining life. As a such as relative humidity, temperature, time of demonstration, PoF models and MD analysis are applied to a pair of “canary” and “parrot” devices for which * This is an open-access article distributed under the terms of the Creative Commons Attribution 3.0 United States License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Annual Conference of the Prognostics and Health Management Society, 2009 wetness, chloride concentration and other contaminants in 2.2 Prognostic Framework for Cutty Sark Iron the environment. Structures The aim of this paper is to present the prognostics framework being developed for the iron structures of the The Prognostic Framework for Cutty Sark Iron Cutty Sark. This paper is organized into the two main Structures is based on the model shown in Figure 1. sections. The first section describes the prognostics Descriptions and details of how the three main framework along with the different methodologies prognostics approaches, as mentioned in section 2.1, integrated within. The second section provides a will be used to predict the “Health” of Cutty Sark demonstration of the prognostics framework developed. iron structures are discussed in Sections 3-6. 2. PHM APPROACH FOR CUTTY SARK IRON STRUCTURES The prognostics and diagnostics terminologies are used to describe the broad range of processes which aim to determine material condition at present time and also at a later predetermined time. Diagnostics is the process of determining the current state of a system (Hess et al., 2005) while prognostics is the process of predicting its future state. 2.1 Overview of Prognostic Methodologies The environmental conditions and lifecycle loads under which a system is subjected to are not usually taken into account by commonly used reliability prediction methods. Figure 1: Prognostics Framework for Cutty Sark Prognostics and health management approach accounts for Iron Structures environmental conditions and lifecycle loads subjected to a system to assess the current and future “health” of a system (Pecht et al., 2007). The prognostic techniques 3. CANARY AND PARROT DEVICES employed for the Cutty Sark can be classified into the following (Kumar and Pecht, 2007): 3.1 Origin of Canary Devices The word “canary” is derived from one of the coal 1. Canary Devices: These are prognostic devices integrated mining’s earliest systems for warning of the into a specific system incorporating the same failure presence of hazardous gas using the canary bird. mechanisms as the embedded system, but failing at a Because the canary is more sensitive to hazardous faster rate than the actual system. gases than humans, the death or sickening of the 2. Physics-of-Failure(PoF): Failures in a system are canary was an indication to the miners to get out of usually due to the processes occurring within and around the shaft. The canary thus provided an effective the system. The PoF methodology aims to carry out early warning of catastrophic failure that was easy prognostics by first calculating the cumulative damage to interpret (Vichare and Pecht, 2006). In PHM, the accumulation due to various failure mechanisms within same idea is used such that canary devices are used a particular environment of a system and then analyzing in the actual systems providing advance warning of this information to give predictions of remaining life of failures. Canary devices are accelerated devices the system. which will fail according to similar failure mechanisms to that of the actual system being 3. Precursor Monitoring and Data Trend Analysis: Two monitored and will fail faster than the actual system main steps are involved: (i) FMEA to identify the under the same environmental and operational precursor variables for monitoring, (ii) Development of loading conditions, thus providing an early warning a reasoning algorithm to correlate the change in the of failure. precursor variable with the impending failure. 4. Data-Driven Methods: These methods are typically 3.2 Design of Canary and Parrot Devices for derived from machine learning techniques such as (i) Cutty Sark and HMS Warrior models that establishes a set of interconnection relationships between input and output where the A similar methodology will be implemented for the parameters of the relationship are adjusted with more prognostics framework of Cutty Sark iron information, (ii) detection algorithms that learn a model structures, the main purpose of which is to provide of the nominal behaviour of a system and the indicates an awareness of the onset of degradation an anomaly when new data fails to match that model mechanisms before any major failure of the iron (Vichare and Pecht, 2006). structures occurs. Along with Canary devices, Parrot devices have been introduced to be used in situations where the quantitative measurements on the actual system are hard to obtain. The parrot 2 Annual Conference of the Prognostics and Health Management Society, 2009 devices are built with similar material and represent the certain degree for diagnostics purposes, same configurations of the actual iron structures. implementation of these approaches for prognostics Trials of the Canary and Parrot Devices are currently purposes is still very much at an exploratory stage. being commissioned on HMS Warrior for a one year period. These Canary and Parrot devices will be placed in 5.2 Precursor Monitoring using Mahalanobis pairs in locations experiencing different environmental Distance Analysis conditions within HMS Warrior. The degradation of the canary devices will be A precursor indication is usually a change in a assessed using accelerated testing and the degradation measurable variable that can be associated with levels will be calibrated and correlated to the degradation subsequent failure. (Vichare and Pecht, 2006) levels of the parrot devices which in turn will be calibrated Failures can then be predicted by using a causal and correlated to the actual failure levels of the iron relationship between a measured variable that can structures. be correlated with subsequent failure. Prediction of remaining life using PoF models very often 4. PHYSICS OF FAILURE MODEL