This paper aims to study the importance of the wave inundation effect. A concentrated force acting at the mean water level of the platform leg is used to represent the inundation effect due to variable wave surface. Its effects on the total and maximum wave forces are investigated based on structural modal analysis and linear wave assumptions. After the inundation drag force is approximated as an asymmetric second-order term, the corresponding frequency-domain transfer functions of Volterra/Wiener representation are derived. The auto-spectra of the deck displacement of 2 typical jack-up platforms are numerically evaluated to show the significant contribution of the inundation effect to the structural dynamic responses.
Considerable works have already been carried out by many researchers in the study of nonlinear wave forces on jack-up platforms. It is commonly accepted that nonlinear dynamics has a significant effect on the response of many jack-up structures, and 2 main nonlinear effects are attributable to wave forces. One is the effect of the nonlinear drag force, which is usually the predominant wave-force component for the slender structural members of a jack-up platform and can be evaluated using the wellknown Morison formula. The other nonlinear effect can be related to the variable submerged height of structural members near the free water surface, or the inundation effect (Tickell and Bishop, 1985; Tung, 1996). The common practice by which to include nonlinear effects in dynamic analysis is to perform timehistory simulations. In order to have statistically meaningful results, many such time-consuming simulations are usually required. A more direct approach is to carry out stochastic analysis in frequency domain. However, nonlinear frequency-domain analysis is much more involved than the normal approach based on linear assumptions.
Green, K.J. (Industrial Control Services PLC) | Lewis, E. (Liverpool John Moores University) | Dutch, W.R. (Liverpool John Moores University) | Scully, P.J. (Liverpool John Moores University) | Deboux, B.J. (Liverpool John Moores University)
A portable data acquisition system for use with laboratory or field test projects is described in detail. Data in the form of low voltage D. C. is received, either directly or by FM radio, and stored on a home tape recorder. The data is processed off the tape as desired. The system is made up of readily available components that cost about $100, which prices it in the range of budget-minded researchers.
There is always a need for functional and dependable low cost test equipment, particularly in the area of research and development. From the Viewpoint of two individuals whose interest rather than livelihood is in oceanography, presently available commercial test equipment and instrumentation was found to be much too expensive. Therefore, it became necessary to design and develop the test equipment needed to pursue this interest. This effort resulted in an economical system for acquiring, transmitting, storing, and retrieving data for distances of up to one-fourth a mile over land or water.
The system was developed to aid in obtaining data from a wave direction measuring instrument, either in an experimental wave channel or in the near-shore ocean. It was designed to meet the authors' needs; however, because of its potential use by others faced with low budget research projects' it was decided to share this information through the presentation of this paper.
An ocean wave direction measuring instrument has been under development for several years and permanent records of test data were frequently required. The direction measuring instrument has a resistance which is proportional to the wave direction, and visual observation of a meter was not always satisfactory for accurate data evaluation. On a few occasions a Brush Mark II two-channel strip chart recorder was used. The Mark II was satisfactory; however, it was seldom available. A survey was made of strip chart recorders, both new and used, either to purchase or to lease, and it was determined that records were either too expensive or unsuitable. Consequently, development of the direction measuring instrument slowed because adequate data recording instrumentation was not available.
An article by Mr. J. Me Loe (Reference 1) reported on the use of a duty cycle modulation method for recording D. C. voltages on magnetic tape. The technique discussed was tried with limited success but the results were, nevertheless, encouraging, A Voltage Controlled Oscillator (Y. C. 0.) was used for modulating the input signal and means for recording a D.C. signal on tape was achieved. The duty cycle modulation technique for playback presented few problems. This completed the data storage and retrieval system.
Testing the direction measuring instrument in the ocean was difficult because the operator and the recording instrumentation were on shore and the test instrument was out in the water. Underwater electrical cables usually provide the data communication link but the time and expense required by this method were prohibitive. It was decided to incorporate a wireless data transmission link into the data acquisition system so that test data could be obtained from a floating buoy.