Reliability Metrics From Two Decades of Indian Ocean Moored Buoy Observation Network

surface parameters are required for cyclone tracking, and large-scale spatiotemporal oceanographic parameters are required for effective modeling of the ocean dynamics and precise monsoon predictions (Ravichandran, 2015). Precision monitoring and reporting of water level changes in the deep ocean during tsunamigenic earthquakes are required for tsunami early warning systems (Nayak & Kumar, 2008). The Indian Ocean observation network comprising multiple MSB are deployed in the challenging deep ocean environments and coastal locations using position moorings. The MSB house power systems, meteorological, sea surface and subsurface oceanographic instruments, and data acquisition systems communicate with the shore stations through satellite and terrestrial telemetry networks (Venkatesan et al., 2013a). The Indian tsunami early warning system (ITEWS) comprises seabedlocated bottom pressure measurement systems communicating with moored tsunami buoys using acoustic telemetry and, in turn, to the shore centers via satellite (Kumar et al., 2012a). The rapid technological advancements in electronics, telecommunication, mooring materials, energy storage, computational infrastructure, May and numerical modeling tools pave the way for realizing robust moored buoy observation networks. Reliability, availability, andmaintainability are the key requirements for the MSB networks as the data are used for societal protection, climate change studies, and moreover, offshore system outages result in loss of time-critical data, costly interventions, and reinstallations. The need for assessing the reliability of the Indianmoored buoy networks used for monitoring cyclones and tsunamis is evident from the postevent analysis conducted by the Japan Meteorological Agency after the tsunamigenic earthquakes that affected the Pacific /June 2018 Volume 52 Number 3 1 coast of Japan in 2011. Based on the return of experiences over the past two decades, this paper summarizes the technical maturity, achieved reliability metrics, and implemented integrity management strategies for the National Institute of Ocean Technology (NIOT)-operated Indian Ocean MSB network from position mooring till data reception. The paper for the first time presents the reliability determination methodology adopting applicable industrial and offshore standards, and the achieved reliability metrics shall serve as a baseline for further technical improvements and integrity management of similar ocean observation networks. Indian Moored Buoy Network and Its Importance The Indian Ocean is land-locked by the Asian landmass and hence cannot transport the heat gained in the tropics to the higher northern latitudes. At the same time, it gains additional heat/mass from the tropical Pacific through the Indonesian throughflow. This unique geography among the global oceans has important implications on ocean circulation, chemistry, biology, and consequently on the climate and biogeochemistry of the ocean. As a result, the Bay of Bengal (BoB) and the Arabian Sea basins are the locations where a large proportion of the South Asian coastal cyclones and impacts occur, and the frequency of intense cyclones is in an uptrend (South Asian Disaster Knowledge Network, 2009). In order to understand the variability and its influence in the Indian subcontinent and on the global climate, a series of shipbased experiments were carried out since the 1970s. Important among 2 Marine Technology Society Journal them are the summer monsoon experiment series done during the 1970s; the BoB Monsoon Experiment, the Monsoon Trough Boundary Layer Experiment, and the Joint Air-Sea Monsoon Interaction Experiments in the 1990s; the Arabian Sea Monsoon Experiment in 2004; the Ocean Mixing and Monsoons and Air Sea Interactions in the Northern Indian Ocean in 2013; and the BoB BoundaryLayer Experiment in 2016 (Bhat et al., 2001; Bhat & Narasimha, 2007; Wijesekera et al., 2016). The need for systematic and sustained longterm ocean observations with increased spatiotemporal measurements are essential for advancing our understanding of the role of the Indian Ocean in the climate system, monsoon variability, interseaosnal variability, Indian Ocean dipole, ocean circulation, heat budget, and biogeochemical cycles. Subsequent to theOceanObs99meeting, IndOOS, a multiplatform long-term observing system, which consists of a surface moored array, Argo floats, surface drifting buoys, tide gauges, voluntary observing ship-based XBT/XCTD systems, and satellite measurements, was taken up (Ravichandran, 2015). For enabling sustained long-term ocean observations, the Government of India (GoI) established the Ocean Observation Systems (OOS), formerly called the National Data Buoy Program at the NIOT in 1996, for developing, operating, and maintaining Indian moored buoy observational systems and related telecommunication networks in the Indian waters (Venkatesan et al., 2016a); the Research Moored Array for African AsianAustralian Monsoon Analysis and Prediction (RAMA) during 2009 to understand the variability in the IndianOcean and its influence in the monsoons (McPhaden et al., 2009); and the Ocean Moored Buoy Network in the Indian Ocean (OMNI) developed and operated byNIOT-OOS to understand the variability in the upper ocean thermohaline and current structures on several timescales, which has important bearing on the evolution of the seasonal monsoons and cyclones in 2011 (Venkatesan et al., 2013b). In the Indian Ocean, there are two tsunamigenic zones, the AndamanSumatra trench and the Makran coast. As a result, tsunamis are an ever present threat in India’s 7,500-km-long coastline where more than 30% of the national population resides. In response