Heave added mass and damping of a suction can in proximity to the sea floor

• Main Authors: Roe, T, MacFarlane, GJ, Drobyshevski, Y
• Format: Conference Object
• Language: English
• Published: American Society of Mechanical Engineers 2008
• Subjects: Engineering; Maritime Engineering; Marine Engineering; Arctic
• Online Access: http://www.asme.org/; http://ecite.utas.edu.au/53988

Suction cans are usually deployed by the crane of aconstruction vessel, which must have adequate capacity towithstand the dynamic hook loads generated by motions of thevessel and heave response of the suction can. Before thestructure is placed on the sea floor, it must be positioned abovethe target location; in this phase the suction can is manoeuvredinto position being suspended in proximity to the sea floor.Hydrodynamic properties of the structure in the positioningphase are different from those experienced during the decent,due to the effect of the bottom proximity. As a result, thedynamic hook loads experienced in this phase may be alsodifferent from the deep water condition.The objective of this study is to quantify these effects; inparticular the impact of the bottom proximity on the heaveadded mass and hydrodynamic damping. The added mass anddamping of a 6-metre diameter suction can, of dimensionstypical for Australian North West Shelf conditions, have beendetermined by testing a I: 10 model in the 4.2 m deep watertank of the Australian Maritime College. Free decay tests wereconducted at several heave frequencies, and the heave addedmass and damping determined. Four clearances of the modelfrom the sea floor were investigated ranging from 1.20 to 0.20of the can height. For each clearance, several sizes of openhatches were examined, by testing the model with 3 pairs ofhatches of various diameters, with up to 4.8% of the relativearea open.Model tests demonstrate that the heave added mass anddamping increase as the suction can approaches the sea floor.Increase in added mass is found to be within 20% of its deepwater value, and is made less pronounced by opening hatches of larger area. Linear (proportional to velocity) hydrodynamicdamping also increases moderately as the under-bottomclearance reduces. Quadratic (proportional to velocity squared)damping is strongly affected, especially at very smallclearances, with drag coefficient reaching unusually highvalues; this is attributed to substantial constraining effect of thebottom, which causes increasing flow velocities past the loweredge of the can. Results of the tests are presented, and theirapplication for the installation lift analysis is discussed