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The IUP Journal of Mechanical Engineering
Seakeeping Analysis of an Indian Research Vessel
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A multidisciplinary research vessel Sagar Manjusha was considered for seakeeping analysis, which was exclusively used for deployment, maintenance and retrieval of data buoys, Tsunami buoys and so on. During rough sea conditions, the ship undergoes extreme rolling, which hinders deployment/retrieval operation. It also leads to seasickness for scientists and crew onboard. An experimental study was carried out to improve its seakeeping qualities. Ship Motion Reference Unit (MRU) data and the corresponding wave climate data were collected to study seakeeping capabilities. A model test was carried out in a towing tank facility and the design was optimized based on outcomes of test results. An Anti-Roll Tank (ART) was fabricated and installed on boat-deck. Inclination test was carried out in the presence of Indian Register of Shipping surveyor. Stability booklet was prepared as per class recommendation and the procedure was accepted by the class. It was observed during testing in rough sea condition that ship rolling had reduced by considerable amount, which in turn improved comfort and safety during extreme sea conditions. This experiment resulted in a transformation of an innovative idea into best outcome so that similar problems can be solved and the shipís comfort, safety, productivity and so on can be improved considerably. Seakeeping analysis and installation of ART resulted in improvement in the safety of vessel, onboard machineries, instrument used for scientific research and quality of life onboard.

 
 

Seakeeping evaluation of a vessel mainly depends upon environmental conditions. Parameters for seakeeping analysis include relative velocity, acceleration, propelleremergence, slamming and motion sickness incidence and so on. Seakeeping capability (Kadir and Ebru, 2005) decides a shipís performance. It is decided by the shipís motion in a particular sea environment that is exceeded by the vesselís capabilities. Seakeeping performance depends mainly on shipís wave response characteristics, sea environment conditions, and vessel speed and heading. Buoy tender-cum-research vessel Sagar Manjusha is a multidisciplinary research vessel of National Institute of Ocean Technology (NIOT), operated and maintained by vessel management cell. It has an overall length of 60 m, breadth of 11 m, draught of 3.2 m, gross weight of 1075 t and a cruising speed of 11.5 knots at 90% maximum continuous rating. It can accommodate 11 scientists, 8 officers and 10 crew and has an endurance of 20 days. Two numbers of articulated electrohydraulic cranes each having 5 t Safe Working Load (SWL) with an outreach of 8 m and an A-frame of SWL 12.5 t especially serve the need of buoy deployment and retrieval operations. This vessel has many sophisticated facilities, viz., a winch of 4500 m capacity for launching/retrieval of portable scientific equipments; a Conductivity Temperature and Depth (CTD) winch of 800 m capacity to collect water samples at different depth, microscope, centrifuge, deep freezer to collect, analyze and store core, sediment grab and other marine biological and chemical samples. The vessel is also facilitated with survey equipment like single beam and multibeam echosounders, used for bathymetry survey at Indian exclusive economic zone. Figure 1 shows Sagar Manjusha vessel and Figure 2 shows Anti-Roll Tank (ART) fitted at boat-deck.

Sagar Manjusha vessel is exclusively used for deployment, maintenance and retrieval of tsunami, meteorological buoys and fish aggregating device at Arabian Sea, Bay of Bengal and Indian Ocean. Since the vessel is having flat-bottomed hull and weight of the ship is considerably less as compared to cargo vessels, it will undergo heavy rolling at rough sea conditions. These roll motions make it difficult to perform onboard operations such as deployment/retrieval of buoys, deployment of dead weight, transfer and assembling of buoys and so on. The onboard scientists, crew and officers used to undergo seasickness due to vertical accelerations caused by rolling motion. An experiment is done in order to reduce these roll motions. Loads (Nils and Odd, 1970) acting at various points were considered for this study. As an initial attempt, a weight of 9.6 t was placed at the boat-deck and it was observed that the roll motions were reduced considerably. It was decided to go for further analysis to improve the stability.