Experimental evaluation of CTD package hydrodynamic behavior and recommendations for improved lowering techniques

Abstract

On the Cover and the Cover SheetContract Number should be N00014-82-C-0019 4.5.6. 7.8. 9.10. 11.12.13.14. This included quality control and acceptance testing procedures for new cables and instructions for installing these cables aboard ship. Proper maintenance procedures for both cables and winches were also reviewed.Operational limits for both depth of cast and payout speed were calculated for different sea states. Alternate CTD lowering systems using special steel armored cables, synthetic fiber and fiber optics cables were evaluated.The results of this investigation were condensed in a first·report analyses considering modifications to the standard package in symmetry, weight and drag. From thi-s,.an optimal configuration was selected.Criteria for the fonnulation of a half sca.le model were developed to satisfy geometric, kinematic and dynamic similarity. The report concluded with recommendations to improve the hydrodynamic behavior of the package through an increase in its tenninal velocity by changes in symmetry, drag reduction and the addition of weight. 1A paper reviewing these studies and presenting the results of tank model tests was published in Deep Sea Research (Berteaux and Walden, 1983, Reference 3). This paper also presents the results obtained at sea which confirmed our suspicions that conditions of zero tension in the wire did indeed occur when the combination of ship motions and payout speed were too great. Severe wire damage could occur from the subsequent snap loading.The report which follows concludes our CTD study. It presents in detail the instruments and procedures used to observe and document th~ hydrodynamic behavior of standard and improved CTD packages. This report also presents a number of recommendations to improve the efficiency and the reliability of instrument lowerings.The first section describes how the terminal velocity of free falling CTD packages was actually measured. The effects that drag reduction and increased weight have on package terminal velocity are discussed and numerical results presented.A half scale model of the CTD package was built and its 'flight pattern observed as it was dropped in the large water tank of the Naval Surface Weapons Center. The second section describes the criteria of similarity used for the fabrication of the model; it outlines the test procedures, and reviews the results obtained.The next section covers the tests performed at sea during actual CTD In the final section the report formulates a number of recommendations to improve the efficiency and the reliability of instrument package lowering operations. Prudence and common sense will dictate that limits be set on payout speeds and lengths' payed out as a function of sea state.Theoretical considerations and measurements made during the study permit quantifying these limits. Ways of specifically improving CTD packages are discussed. A brief review of ship motion compensation and of its benefits for cable lowering applications concludes this report. The operating technique for using this ...