Complex Ocean Environment Research Articles

A Damage Mechanics Approach to Fatigue Assessment in Offshore Structures

This article is intended to describe the development of a fatigue damage model capable of assessing fatigue damage in offshore structures. This is achieved by for mulating a set of damage coupled constitutive and evolution equations which make the for mulation of a unified approach possible under both low and high cycle fatigue damage and consistent with the structural dynamic response of the changing/deteriorating material be haviors. The structural analysis for the whole designed period, say about 30 years, can be carried out with the aid of the proposed analytical procedure, in which the fundamental characteristics of sea wave statistics responsible for the structural dynamic response can be sufficiently considered. An offshore structure subject to complex ocean environment is described by a general stochastic system which embeds a group of stochastic subsystems, each characterizing a duty cycle. An effective analytical method is established by introduc ing the concept of duty strain range with a clear mathematical definition and its analytical solution which covers all possible spectral parameters. The history-dependent damage is also included in the damage model so that the overload effects can be analyzed. It should be pointed out that the whole procedure can be fully computerized such that the practical or engineering significance of varying design variables can be readily highlighted.

Implementation of focalization for processing field data

It is impossible to localize an acoustic source in the ocean with conventional matched-field processing techniques if there are significant uncertainties in the acoustic parameters of the environment. Focalization is an approach for overcoming this difficulty that involves searching for key environmental parameters in addition to searching for the source location [Collins and Kuperman, J. Acoust. Soc. Am. 90, 1410–1422 (1991)]. The feasibility of this approach was investigated using implementations that might be difficult to apply to data. An implementation of focalization involving the parabolic equation method for computing replica fields and eigenvector-based signal processing is applicable to problems involving data from complex ocean environments. The covariance matrix is formed for a frequency corresponding to a peak in the source spectrum. The input data to the processor is the eigenvector corresponding to the largest eigenvalue. A large signal-to-noise gain is achieved with this approach, and the eigenvector is often a good representation of the true source field. To determine the source location, the eigenvector is compared with replica fields using a cost function that is minimized over the environmental parameter space with simulated annealing.

Rapid computation of acoustic fields in three-dimensional ocean environments

Adiabatic and coupled-mode theory is amenable to precalculations that can subsequently be used in a nonredundant manner to perform rapid three-dimensional acoustic field computations for a complex ocean environment. Algorithms have been developed to take advantage of both horizontal and vertical precalculated quantities. Complex three-dimensional field computations are then reduced to ‘‘spreadsheet’’ type manipulations of partial solutions to the wave equation. The method is illustrated by applying it to a Gulf Stream environment near the continental shelf. Results from adiabatic, coupled-mode, and parabolic-equation computations are compared.

Wideband monopulse sonar performance: cylindrical target simulation using an acoustic scattering center model

A time-domain simulation that lends itself to the study of complex ocean environments, including complex seafloor shapes and suspended objects, has been developed. This simulation is used to study the response of a monopulse sonar to the presence of rigid cylinders near the seafloor. An acoustic-scattering-center model is developed to represent the cylinders, and several examples are presented and discussed.

A finite-element model for ocean acoustic propagation and scattering

The finite-element technique has the potential to provide a very accurate treatment of the physics of acoustic-wave propagation in inhomogeneous media. This article describes the development of a finite-element model for acoustic propagation in complex ocean environments and its validation. The computational model can handle range and depth dependence in both sound speed and density, as well as rapid variations in bottom topography.

Simulation of array processing in three-dimensional noise fields: Suggested experiments

The growing capability to model three-dimensional noise fields in complex ocean environments taken together with concurrent recent developments in matched-field processing (MFP) suggests noise experiments that will shed light onto the physical processes generating and propagating the noise fields. These experiments should especially yield information concerning the noise field as it impacts on signal and/or array processing. Some of the expected (modeled) characteristics of low-frequency ambient noise in a deep ocean basin with realistic complexity and its subsequent impact on MFP are reviewed. For example, the effects on MFP of down-slope conversion from discrete sources (surface ships) and distributed sources (storms), and the effectiveness of various rejection techniques are demonstrated. These results suggest experiments to investigate the most important processes.

Adaptive beamforming or matched field processing in media with uncertain propagation conditions

Adaptive beamforming or matched field processing provides high resolution with sidelobe control if accurate replica fields can be generated. The generation of these replica fields is a formidable problem requiring knowledge of the complex ocean propagation environment. On the other hand, the detection problem in the ocean may not require high resolution, whereas sidelobe control is still an important issue. Relaxing resolution requirements suggest that a certain tolerance incorporating uncertainty of the propagation conditions is permissible, or even desirable, because of the difficulties both in specifying the medium exactly and in identifying global peaks. This possibility of lowering the requirements on our knowledge of the environment is investigated with two methods: (1) by constructing multiple beam (constraint) algorithms and (2) by considering stochastic blurring by the medium. These two approaches are applied to plane‐wave beamforming and matched field processing.

Three-dimensional oceanographic acoustic modeling of complex environments

Remote sensing combined with oceanographic modeling opens up new possibilities for quantiatively describing very large area complex oceanographic-acoustic phenomena. A complex ocean environment consists of variable oceanography including mesoscale phenomena and variable topography such as continental slopes and seamounts. Predicting the acoustical effects from any to all of these phenomena is a computationally intense problem whose efficient and accurate solution will, no doubt, ultimately employ new and innovative numerical physics. As a stimulus to developing new approaches, three-dimensional modeling results for such complex environments are being looked at using a technique that can rapidly calculate propagation loss over a wide geographical area. This approximate technique, which is based on normal mode theory, has been applied to environments that contain both variable oceanography and topography. Of prime interest in these initial studies is a search for graphical patterns that emerge for complex environments where the acoustical results reflect or “image” the oceanography and topography. It is believed that these three-dimensional representations using color graphics to display and convey complex ocean-acoustic phenomena will provide the impetus to further understanding and computing the interaction of these phenomena.

Rapid computation of acoustic fields in three‐dimensional ocean environments

Adiabatic and coupled‐mode theory is amenable to precalculations that can subsequently be used in a nonredundant manner to perform rapid three‐dimensional acoustic field computations for a complex ocean environment. Algorithms have been developed to take advantage of both horizontal and vertical precalculated quantities. Complex three‐dimensional field computations are then reduced to ‘‘spreadsheet’’ type manipulations of partial solutions to the wave equation. The method is illustrated by applying it to a Gulf Stream environment near the continental shelf. Results from adiabatic, coupled‐mode, and parabolic‐equation computations are compared.