AN EFFICIENT APPROACH TO MODEL RESPONSE OF SUBMERGED STRUCTURES TO BLAST LOADING (Paper n. OMAE96-493)
G. M. Manfredini and F. Lagasco, D'Appolonia S.p.A.. Genoa, Italy
James L. Drake, Applied Research Associates, Inc., Vicksburg, Mississippi, USA
ABSTRACT
Blast loading on submerged offshore structures due to fishing activities and hostile attacks is a concern to Oil Operators.
The dynamic response of structures interacting with the fluid has been extensively detailed in the past (e.g. Jackson and Cost, 1980; Jackson and Cost, 1981; Jackson and Jamnia, 1984; Dawson and Sullivan, 1995).
An efficient state-of-the-art methodology is presented to evaluate the structural behavior of underwater nearby blasts to steel and concrete elastoplastic structures.
Contact charges require analyses by means of hydrocodes and are not covered in this contribution.
The water shock model and the structure-medium interaction model are treated in detail, while a general description is given for the two dimensional lumped mass incremental implicit beam element code. Due to its nature the code is particularly suited to analyze the response of one dimensional extended structures, but appropriate results have been found also in cases where three-dimensional effects are predominant.
Results of the blast code include time hystories of structural ductility, which can be interpreted as a damage indicator in the consequence analyses.
The performance of the blast methodology is evaluated by comparing numerical results with an experimental test of underwater explosion against a stiffened plate.
TUNED VIBRATION ABSORBERS FOR OFFSHORE STRUCTURES
Mikhail F. Dimentberg, Zhikun Hou, Mohammad N. Noon and Shiyu Chen,
Department of Mechanical Engineering, Worcester Polytechnic Institute, Worcester, MA 01609
ABSTRACT
A preliminary feasibility study is made of efficiency of tuned vibration absorbers for reducing response of offshore platforms, to random ocean waves.
The absorbers are submerged spring-loaded flaps, with stiffnesses of the springs being adjusted for tuning to any specific (rigid-body) mode of the platform (heaving, pitching, etc.), whereas actual design of the flaps should provide the desired amount of drag-induced damping. Mean square response analysis of the system is made for the case of a narrow-band random excitation due to ocean waves by using method of moments in conjunction with statistical linearization (with Gaussian closure) for quadratic damping term. Extensive parametric study is made of the efficiency of vibration control through the use of absorbers, as represented by the RMS response reduction factor. In particular, the efficiency increases with the absorber/platform mass ratio, and it may be of some practical importance even at values of this ratio as small as one per cent. The importance of the excitation bandwidth is demonstrated. It is shown also, that whereas optimal damping of the absorber depends on the level of wave excitation, the RMS response reduction is fairly constant within a rather broad range of (nonlinear) damping, so that adjusting absorber damping for storm level may not be necessary.
