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Investigation Into the Sensitivity of the Dynamic Hook Load During Subsea Deployment of a Suction Can

[+] Author Affiliations
J. Ireland

Subsea 7, Tananger, Norway

G. Macfarlane

Australian Maritime College, Launceston, TAS, Australia

Y. Drobyshevski

INTEC Engineering Pty Ltd, Perth, WA, Australia

Paper No. OMAE2007-29244, pp. 497-507; 11 pages
doi:10.1115/OMAE2007-29244
From:
  • ASME 2007 26th International Conference on Offshore Mechanics and Arctic Engineering
  • Volume 4: Materials Technology; Ocean Engineering
  • San Diego, California, USA, June 10–15, 2007
  • Conference Sponsors: Ocean, Offshore and Arctic Engineering Division
  • ISBN: 0-7918-4270-3 | eISBN: 0-7918-3799-8
  • Copyright © 2007 by ASME

abstract

Suction cans are commonly used as foundations of fixed offshore structures, subsea equipment, and anchors of mooring lines. During the offshore installation phase, when a suction can is submerged, it attracts large heave added mass, which may be an order of magnitude higher than the mass of the can in air. Due to motions of an installation vessel the dynamic hook load may significantly exceed the submerged weight of the can. The dynamic hook load must be accurately predicted, as it governs selection of the vessel, lifting gear and rigging, and defines the allowable installation sea state. The objective of this paper is to examine the sensitivity of the dynamic hook load to hydrodynamic properties of the suction can, in particular its heave added mass and damping. This research is motivated by the lack of data on such properties, which are usually estimated by simplified methods with some engineering judgement and assumptions. A single degree of freedom system is considered and the frequency domain spectral analysis is used, which employs the stochastic linearization of the nonlinear damping component. The added mass and damping of a 6-meter diameter suction can of dimensions typical for Australian North West Shelf developments have been determined by testing a 1:10 model in the 4.1 m deep basin of the Australian Maritime College. Free decay tests were conducted at several frequencies and the added mass, linear and nonlinear damping components determined. The effect of open hatches on the hydrodynamic properties was examined by fitting the model with hatches of various diameters, with up to 4.8% of the relative area open. Results of the tests demonstrate that the added mass and damping are higher, when compared with estimates based on empirical data for non-oscillatory flow. Within the Keulegan-Carpenter number range of 0.1–1.0, open hatches impact significantly on the added mass and produce additional damping, which is found to be linear with the heave velocity. Results of the tests and their interpretation are discussed. Sensitivity analysis shows that if the model test results are used in the dynamic lift analysis for an installation vessel and sea states considered, the predicted hook load is generally less than its values obtained by using simplified estimates. In particular, the increase in linear damping due to open hatches is responsible for up to 20% reduction in the dynamic hook load, with 2.4% of the relative top area open.

Copyright © 2007 by ASME

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