Sensors and Actuators A 229 (2015) 59–67 Contents lists available at ScienceDirect Sensors and Actuators A: Physical j ourna l h o mepage: www.elsevier.com/locate/sna Distributed damage detection of offshore steel structures using plastic optical ﬁbre sensors K.S.C. Kuang ∗ Department of Civil and Environment Engineering, Block E1A, #07-03, 1 Engineering Drive 2, Faculty of Engineering, National University of Singapore, Singapore 117576, Singapore a r t i c l e i n f o Article history: Received 18 August 2014 Received in revised form 20 March 2015 Accepted 20 March 2015 Available online 28 March 2015 Keywords: POF sensors Structural health monitoring Distributed monitoring Crack detection Steel structures a b s t r a c t In this paper, results demonstrating the potential application of plastic optical ﬁbre (POF) sensors for dam- age detection of offshore steel structures are summarized. Graded-index perﬂuorinated plastic optical ﬁbre (GIPOF) was used for crack detection in tubular steel specimens. A high-resolution photon-counting optical time-domain reﬂectometer (-OTDR) was used for interrogating the optical signal. The study will show that the technique adopted in this study is able to determine the position of the crack in the host structure with high accuracy and repeatability. The technique was also found to be capable of monitoring crack growth in the steel specimens used based on a serpentine sensor conﬁguration. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Offshore platforms, in general, consist of a superstructure and a substructure. The substructure, typically built using steel tubular jackets, are fabricated by joining tubular steel members together using welding and then pin-jointed to the seabed with steel piles. In a complex structure such as that of an offshore platform, there are many potential failure hot spots, which may result from fatigue due to cyclic stresses generated by sea waves. Cracking in the steel tubular members may occur at locations of high stress-intensity due to the physical discontinuities at the intersections of brace and chord members. At welded joints where abrupt changes in geome- tries are expected, cracks may initiate and propagate under cyclic load. Stress concentration could be as high as twenty times that of continuous non-welded sections at certain critical sections [1–3]. Therefore, detecting and monitoring such cyclical stress-induced damage e.g. crack growth, would give an objective measure of the rate at which the crack is propagating and therefore the level of urgency for repair work to be carried out to arrest the crack. In addition, there has been increasing demand for documentation of safety in the offshore industry by classiﬁcation societies to carry out inspection and structural checks. The need for an effective struc- tural integrity damage detection system in offshore structures has ∗ Tel.: +65 65164683; fax: +65 67791635. E-mail address: kevinkuang@nus.edu.sg indeed been highlighted in many reports available in the public domain [4,5]. Over the past few decades, the concept of structural health mon- itoring (SHM) has emerged as an attractive but challenging area of research [6,7]. At the most elementary level, an SHM system requires a sensing system capable of obtaining speciﬁc informa- tion from which the health of the structure can be inferred. While there are many sensors available, optical ﬁbres offer unique advan- tages due to their insusceptibility to corrosion as well as their non-ﬂammable and non-conductive nature. Their insensitivity to electromagnetic interference and long term stability are some of the differentiating features compared to other electrical-based sen- sors. Another signiﬁcant advantage of optical ﬁbre sensors is their potential for distributed monitoring along a single ﬁbre while offering damage detection capability with high spatial-resolution, allowing large structures to be reliably monitored. This is a signiﬁ- cant advantage over placing a large number of individual sensors on the host structure, requiring immense amount of electrical cables to be linked up. Besides being capable of true distributed monitor- ing capability, optical ﬁbres sensors have, over recent years been demonstrated to be capable of monitoring a variety of parameters including measurement of strain, load, displacement, the extent of bio-fouling, detection of moisture and the presence of cracks [8–10]. Polymer-based optical ﬁbre has, in recent years, been attract- ing signiﬁcant amount of attention for sensing applications [11,12]. Plastic optical ﬁbre (POF) offers ease of handling, termination and http://dx.doi.org/10.1016/j.sna.2015.03.028 0924-4247/© 2015 Elsevier B.V. All rights reserved.