35 Sustainable Marine Structures | Volume 01 | Issue 01 | January 2019 Sustainable Marine Structures http://ojs.nassg.org/index.php/sms Distributed under creative commons license 4.0 DOI: *Corresponding Author: Surendran Sankunny, Department of Ocean Engineering, Indian Institute of Technology Madras, Chennai, India 600036 Email: sur@iitm.ac.in. AbstrAct Moonpools are openings right through the hull from continuous deck to bottom of the ship, allowing equipment or mini-submarines to be put into the water at a location on the vessel with minimum ship motion. Open moonpools in a drillship are causing additional resistance when the ship is in forward speed. It was shown that the water inside the moonpool started to oscillate at forward speed. the water mass in the moonpool is subjected to sloshing and pis- ton modes. the vertical motion is piston mode and the longitudinal one is called as sloshing mode. This water particle motion inside the moonpool is mainly depended on the geometry, moonpool depth, and encountered wave frequency. Out of this, moonpool geometry is one of the key factors for the performance of the moonpool. the varying cross-section geometry is one of the practically possible and economically feasible solutions to reduce the oscillation to a considerable level is attempted in this paper. Also the resistance caused by the moonpool and the free surface generated around the hull is investigated with the use of computer simulation. ArtIcLE INFO Article history: received: 26 December 2018 Accepted: 7 January 2019 Published: 18 January 2019 Keywords: Moonpool Free surface elevation Water motion Article Infuence of Moonpool on the Total Resistance of a Drillship by the effect of Water Motions inside the Moonpool Sivabalan Ponnappan Surendran Sankunny* Department of Ocean Engineering, Indian Institute of Technology Madras, Chennai, India. 1. introduction A moonpool is meant for access to underwater part from onboard ship. It is located mostly at the midship region of the drillship for under water operations either in square or rectangular in shape.In drillship, these openings are used for supply the raiser and other drilling equipment through it. this drillship moonpool is mainly subjected to two types of motions of water mass inside the column namely, piston mode and sloshing mode. the piston mode is a type of water particle motion where the water particles can be seen as a “plug” which oscillates in the vertical direction. In sloshing mode the movement of water plugs in the horizontal direction and makes a standing wave effect on the side wall of the moonpool. this piston and sloshing mode play a major role in vessel's surge and heave motion while the drill ship in operation/transit and its lead to a larger quantity of green water on the weather deck. In the case of rectan- gular moonpool the dominating mode is sloshing where as in square moonpool the piston mode of oscillation is the dominant one. because of this water motion inside the moonpool the drillship regular activities may be affected. Also this water motion is the reason for a large quantity of sea water can enter on to the continuous deck of the vessel. If this quantity of water added with the green wa- ter at the deck because of the waves then the net effect