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TECHNICAL PAPERS

J. Offshore Mech. Arct. Eng. 2001;123(3):93-102. doi:10.1115/1.1382593.

A method for the identification of the damping, restoring, and coupling parameters in the equations describing the coupled heave and pitch motions for an underwater robotic vehicle (URV) sailing near sea surface in random waves using only its measured responses at sea is presented. The random decrement equations are derived for the URV performing coupled heave and pitch motions in random waves. The hydrodynamic parameters in these equations are identified using a new identification technique called RDLRNNT, which uses a combination of a multiple linear regression algorithm and a neural networks technique. The combination of the classical parametric identification techniques and the neural networks technique provides robust results and does not require a large amount of computer time. The developed identification technique would be particularly useful in identifying the parameters for both moderately and lightly damped motions under the action of unknown excitations effected by a realistic sea. Numerically generated data for the coupled heave and pitch motion of a URV are used initially to test the accuracy of the technique. Experimental data are also used to validate the identification technique. It is shown that the developed technique is reliable in the identification of the parameters in the equations describing the coupled heave and pitch motions for an URV.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2001;123(3):103-111. doi:10.1115/1.1382594.

The method of large admissible perturbations (LEAP) is a general methodology, which solves redesign problems of complex structures without trial and error or repetitive finite element analyses. When forced vibration constraints are incorporated into the redesign problem, damping and added mass due to the presence of fluid must be included into the model. The corresponding terms introduce theoretical and numerical difficulties, which are treated in this paper. The LEAP method has been implemented into a Fortran computer code RESTRUCT, developed at the University of Michigan. The redesign process is mathematically formulated as an optimization problem with nonlinear constraints, called general perturbation equations. First, a finite element analysis of the initial structure is executed. Then, the results are postprocessed by code RESTRUCT using an incremental scheme to find the optimum solution for the problem defined by the designer. Accurate determination of nonstructural terms, such as fluid added mass, is generally detrimental as far as forced response analysis is concerned. In redesign problems, however, simple but realistic models can be used. A simple transformation of the structural mass matrix is used to compute the added mass matrix and its dependency on the redesign variables. The presence of non-structural terms in the general perturbation equations requires the development of a new LEAP algorithm for solution of the optimization problem. A simple cantilever beam with 100 degrees of freedom is used to validate the fluid added mass model. The developed method and algorithm are then applied to a partially submerged 4,248 degree of freedom complex structure modeled with beam elements.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2001;123(3):112-117. doi:10.1115/1.1372195.

Two separate studies are presented here that deal with analytical predictions of the air gap for floating structures. 1) To obtain an understanding of the importance of first and second-order incident and diffracted wave effects as well as to determine the influence of the structure’s motions on the instantaneous air gap, statistics of the air gap response are studied under various modeling assumptions. For these detailed studies, a single field point is studied here—one at the geometric center (in plan) of the Troll semi-submersible. 2) A comparison of the air gap at different locations is studied by examining response statistics at different field points for the semi-submersible. These include locations close to columns of the four-columned semi-submersible. Analytical predictions, including first and second-order diffracted wave effects, are compared with wave tank measurements at several locations. In particular, the gross root-mean-square response and the 3-h extreme response are compared.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2001;123(3):118-123. doi:10.1115/1.1377867.

The air gap response of a specific semi-submersible platform subjected to irregular waves is considered. Detailed model tests for this structure are studied in depth. Using time-histories of both motions and air gap, statistical analyses both for the absolute near-structure wave elevation (with respect to a fixed observer), and the relative wave elevation (with respect to the moving structure) are performed. Statistics of wave crest amplification, due to diffraction, are established. Corresponding amplification factors are derived from linear diffraction theory, and the results of theory and observations are critically compared.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2001;123(3):124-133. doi:10.1115/1.1377598.

A finite-difference scheme and a modified marker-and-cell (MAC) method are used for numerical wave tank (NWT) simulations to investigate the characteristics of nonlinear multidirectional waves. The Navier-Stokes (NS) equations are solved for two fluid layers and the boundary values updated at each time step by a finite-difference time-marching scheme in the frame of rectangular coordinate system. The fully nonlinear kinematic free-surface condition is satisfied by the density-function technique developed for two fluid layers. The directional incident waves are generated from the inflow boundary by prescribing a snakelike motion along the wavemaker direction. The outgoing waves are numerically dissipated inside an artificial damping zone located at the end of the tank. Using the NS-MAC NWT with both solid and transparent side-wall conditions, the effects of side-wall reflections are studied. Bull’s-eye waves are also numerically generated by the phase control of neighboring wavemaker segments or the reverse process of cylindrical wavemakers. The simulation results are compared with the computations by an independently developed potential-based NWT and the experiments conducted in the Offshore Technology Research Center’s 3-D wave basin.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2000;123(3):134-140. doi:10.1115/1.1376715.

The failure of a cracked submarine pipeline subjected to the underwater explosion shock is probabilistically analyzed in this paper. A simple procedure is presented, in which the Monte Carlo method is used to estimate the bending stress distribution, and an analytical method is used to estimate the fracture failure probability. The results obtained from the present method are compared with those obtained from direct Monte Carlo simulations. Both are in good agreement. From the studies in this paper, it is concluded that the most dangerous case is that the crack is perpendicular to the tension stress. The influence of crack model uncertainty is significant. FOSM method is also used, which greatly underestimates the failure probabilities for the problem studied in this paper.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2001;123(3):141-146. doi:10.1115/1.1371233.

Optimization of weight, cost, and performance of deepwater offshore structures demands the increased utilization of high strength, light weight, and corrosion resistant materials such as titanium alloys. Titanium alloy Ti-6Al-4V has been considered for several critical components such as risers and taper joints. Because of the novelty of use of titanium alloys in the offshore industry, there is currently no standard governing design of titanium components for offshore structures. Since these structural components are subjected to a complex spectrum of environmental loading, assessment of defect tolerance using fatigue crack growth analysis is generally considered an important design parameter. In this paper, more than 60 crack growth data sets from 20 independent laboratories were collected and analyzed to develop crack growth rate equations for use in defect assessment. These data include the results of fatigue testing of both base material and welded joints in air and seawater with and without cathodic protection and at different R-ratios and test frequencies. The results suggest that for crack growth rates above 10−7 in./cycle, crack growth of Ti-6Al-4V appears to be independent of testing condition and materials processing. At the low crack growth rate (below 10−7 in./cycle), the review revealed that data are very limited. These limited data, however, suggest that the crack growth threshold is dependent on the R-ratio and slightly dependent on material processing. Comparison between crack growth rates of steel and titanium alloy (Ti-6Al-4V) showed that the two materials have very similar behavior.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2001;123(3):147-151. doi:10.1115/1.1376716.

This paper provides an overview of mechanisms for high-pH and near-neutral pH stress corrosion cracking of underground pipelines. Characteristics and historical information on both forms of cracking are discussed. This information is then used to support proposed mechanisms for crack initiation and growth.

Commentary by Dr. Valentin Fuster

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