# Performability Evaluation of Autonomous Underwater Vehicles Using Phased Fault Tree Analysis
###### Abstract  
This paper presents a phased fault tree analysis (phased-FTA)-based approach to evaluate the performability of Autonomous Underwater Vehicles (AUVs) in real time. AUVs carry out a wide range of missions, including surveying the marine environment, searching for specific targets, and topographic mapping. For evaluating the performability of an AUV, it is necessary to focus on the mission-dependent components and/or subsystems, because each mission exploits different combinations of devices and equipment. In this paper, we define a performability index that quantifies the ability of an AUV to perform the desired mission. The novelty of this work is that the performability of the AUV is evaluated based on the reliability and performance of the relevant resources for each mission. In this work, the component weight, expressing the degree of relevance to the mission, is determined using a ranking system. The proposed ranking system assesses the performance of the components required for each mission. The proposed method is demonstrated under various mission scenarios with different sets of faults and performance degradations. autonomous underwater vehicle; reliability; performance; performability; phased mission system; phased fault tree analysis +
Footnote †: journal: Marine Science and Engineering  
_Article_  
Three categories of system performance level were identified, and a phase-level FT was created for each. A phased-mission industrial network system with several performance levels was examined for performability, and a new decision-making technique, known as multiple-terminal binary decision diagrams, was proposed [7]. A Markov model was developed to ascertain the state of a multi-stage PMS, taking into account component failure and recovery [8]. The universal generating function (UGF) was utilized to examine the system's reliability based on each stage's system structure, using the Markov model as a basis. A technique was proposed for a PMS reliability analysis that takes incomplete failure reaction strategies into account and uses a UGF [9]. Structural optimization was studied to increase system reliability. A multidimensional UGF technique was proposed to assess the performability of a multi-state containerized IP multimedia subsystem while taking availability and performance into account [10]. In an earlier study by the authors [11], the FT-based index was utilized to assess the health of the system based on the AUV's reliability, performance, and weight. By taking into account the performance and dependability of every component, in the earlier work, it was possible to validate the AUV's overall operational capabilities, independent of the task the vehicle was currently carrying out. However, due to component failures or performance degradation, it is challenging to determine whether the current operation or specific task can be completed. The practical possibility of performing a task, that is, the operability of the system, may be assessed differently depending on how serious or significant component failures are with respect to the accomplishment of the missions. We present a phased FTA method that can be used to examine the system's performability in a PMS where mission-specific key functions change. Phased FTAs, a phased mission-based analytic technique, are utilized to evaluate the performability of AUVs on each mission. A phased FT is designed based on the AUV design produced in the project being carried out by the Korea Research Institute of Ships and Ocean Engineering. Faults and the performance of the components which are required for each mission are examined in the form of a phased FT in order to assess the performability of the whole vehicle. The ranking of performance is defined using Top-Down-Left-Right (TDLR), which ranks the performance based on the FT structure. The results are used in rank-summed weighting to define the weight of each performance. (Note that the criteria for selecting the weight were ambiguous in earlier work [11].)