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Statistical Characterization of Slacking and Snap Loading During Offshore Lifting and Lowering in a Wave Environment

[+] Author Affiliations
Kevin W. Thurston, Frans Kopp

Shell International Exploration and Production Inc., Houston, TX

Richard C. Swanson

RCS Engineering, Houston, TX

Paper No. OMAE2011-49273, pp. 269-277; 9 pages
doi:10.1115/OMAE2011-49273
From:
  • ASME 2011 30th International Conference on Ocean, Offshore and Arctic Engineering
  • Volume 1: Offshore Technology; Polar and Arctic Sciences and Technology
  • Rotterdam, The Netherlands, June 19–24, 2011
  • ISBN: 978-0-7918-4433-5
  • Copyright © 2011 by ASME

abstract

During offshore lifting operations, appropriate characterization of the dynamic forces is critical to ensuring that the proper lifting equipment has been selected and designed. When a load is supported by a wire rope, or any member that will not support compression, there is a possibility of the rope becoming slack during the lift. The resulting shock load when the rope becomes taut again can produce significant impact loads in the lifting equipment, which is a situation preferably avoided. Thus, of particular importance is the reasonably accurate estimate of the probability of occurrence of sudden loading events. During offshore lifting operations, this situation can occur under the following circumstances: 1) During a transfer of a weighted load between two vessels in an offshore wave environment. 2) Lowering a load through the water surface, particularly in a wave environment. 3) Lowering a load through the water column with significant motions of the support at the surface. This paper deals with the first two situations. First, a model to predict the probability of slacking the rope while lowering a load through the water surface in a wave environment will be presented. Secondly, some commentary will be made about the possible magnitude of the shock loads that can occur during vessel to vessel transfers and lowering through the water surface. Finally, a brief comparison will be made between these models and others that have been proposed within the industry, some of which have been incorporated in design standards and guidelines.

Copyright © 2011 by ASME

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