Contents lists available at ScienceDirect Ocean Engineering journal homepage: www.elsevier.com/locate/oceaneng Design of a supervisory fuzzy logic controller for monitoring the inflow and purging of gas through lift bags for a safe and viable salvaging operation A.K.D. Velayudhan Department of Mechanical Engineering, Muthoot Institute of Technology & Science, Varikoli, India ARTICLEINFO Keywords: Marine salvage Buoyant systems Breakout force Supervisory fuzzy logic controller ABSTRACT This paper presents a mathematical model and numerical time-domain approach to simulate the dynamics of a sunken ship/vessel being raised from seafloor by buoyancy (gas-inflating) systems in a form which is suitable for integrating control techniques to ensure hydrodynamic stability for a safe and viable salvaging operation. According to the two-degree-of-freedom equations of rigid-body vessel motion in diving plane, a conventional slidingmodecontrollerisdesignedastheprimarycontrollertoregulateflowrateoffillinggasinsidetheliftbags and a PID controller is designed as the secondary controller for regulating the purging of gas through the valves fittedonliftbags.Thenasupervisoryfuzzylogiccontrollerisdesignedtomonitororswitchbetweentheprimary and secondary controllers based on the buoyancy requirement. From the simulation studies, it is found that the supervisory fuzzy logic controller is capable to maintain hydrodynamic stability by suitably defining the lin- guistic fuzzy rules, which is created based on the author's experience in conducting numerical simulation using primary and secondary controllers. 1. Introduction Marine salvage is an operation of rescuing a ship/vessel, its cargo or other properties from impending peril. The salvage comprises of towing and refloating a sunken or stranded vessel with the main purposes to prevent the marine environment from the pollution and to clear a channel for the navigation. Ships sink or capsize because they lose their buoyancy or stability due to the collision, battle or weather damage, flooding and other means. The rescue of a damaged vessel is a very difficult task when compared to an intact ship in the same location. Salvaging of sunken ships requires both the recovery of sufficient buoyancy to bring the ship afloat and the suitable buoyancy distribu- tion to regain the satisfactory condition of stability, trim and strength (U.S.Navy, 2006). There are three methods commonly used in the marine salvage industry to extract the sunken objects from the sea bottom, i.e. by using the floating cranes, the Remotely Operated Ve- hicles (ROVs) and the buoyancy systems. Floating cranes can be used for water depths of 2000 m with a good controllability; however the weight of cables becomes more than that of the payload for deeper lifts and hence the process becomes awkward and costly. As the cranes are fitted onto a moving vessel, there will be the operational constraints due to the limiting sea state affected by weather conditions. Excessive cost of hiring and the limited availability of cranes are the major pro- blemsfacingthesalvageindustry.ROVs,ontheotherhand,canbeused in higher water depths and they are highly controllable. Nevertheless, they can be only used for lifting smaller objects as the lifting capacity is limited by the size and power of the thrusters used for the propulsion (Nicholls-Lee et al., 2009). Buoyancy systems have the advantage that they can be used for lifting any size of objects from any depth with comparatively less costs. The concept of using buoyancy systems (e.g. the gas inflated bags) for salvaging sunken vessels from the deep ocean has been around for centuries. This operation is based on the well-known ‘Archimedes’ principle for which the force on the object can be determined by sub- tracting the weight of the object in air from the weight of the fluid displaced by that object (Farrell, 2008; Rawson and Tupper, 2001). In general,thebottomsofinflatablebagsareattachedtothepayloadtobe lifted and inflated using pipes from the gas generating system. In sal- vage industry, there are mainly two types of lift bags available for re- covering sunken objects; one is parachute type and the other is cy- lindrical type. Parachute type bags are generally preferred for lifting purpose, whereas cylindrical type lift bags are used for providing sta- bility (JW Automarine, 2010; Subsalve, 2010). The main drawback of using the inflating bags for marine salvage operation is due to the difficulty in controlling the vertical speed and pitch motion as the ship ascends. Due to the suction break out force, a https://doi.org/10.1016/j.oceaneng.2018.10.049 Received 10 July 2017; Received in revised form 19 April 2018; Accepted 24 October 2018 E-mail address: arunkumardv@mgits.ac.in. Ocean Engineering 171 (2019) 193–201 0029-8018/ © 2018 Elsevier Ltd. All rights reserved. T