H. R. Riggs

The hydrostatic stiffness of flexible floating structures for linear hydroelasticity

The formulation of the hydrostatic stiffness for linear rigid body hydrodynamics is well known. An explicit formulation for an analogous hydrostatic stiffness in linear hydroelasticity, which is applicable to both rigid body and flexible displacement, is not as well-known. Three such formulations have been proposed previously in the literature, none of which is quite correct; all produce an unsymmetric stiffness matrix. An explicit formulation for the complete hydrostatic stiffness for flexible floating structures at rest in calm water is derived based on a consistent linearization of the external hydrostatic pressure and the internal structural stresses. The symmetry of the present formulation for a floating structure is proven analytically, and the unsymmetry of the hydrostatic stiffness for individual finite elements is discussed. The formulation will be of most interest to those who wish to extend linear potential theory hydrodynamic codes for rigid body analysis to deformable bodies. Several issues that are significant for practical implementation are discussed and several examples are presented.

APPROXIMATE METHODS FOR DYNAMIC RESPONSE OF MULTI-MODULE FLOATING STRUCTURES

Abstract Two-dimensional and three-dimensional approximate hydroelasticity theories are used to predict the dynamic response of 5- and 16-module very large floating structures in regular waves. For the two-dimensional approach, strip theory is combined with a beam model of the structure to determine the dynamic response. However, the fluid forces from strip theory are augmented with ‘surge’ forces calculated from Morisons equation. For the three-dimensional approach, a rigid module, flexible connector structural model is used with fluid loading obtained with the three-dimensional double composite source distribution technique. Fluid-structure interaction effects are studied and connector moments are obtained. These approximate methods of hydroelastic analysis offer alternatives to the more computationally demanding, fully three-dimensional flexible module, flexible connector model in which linear potential theory is used.

Procedure for Site Assessment of the Potential for Tsunami Debris Impact

AbstractTsunami debris can place large demands on the structures it impacts. The types of potential debris and impact forces they generate are not well understood, and relatively little consideration is taken for the risk of tsunami debris strikes during structural design. A procedure is outlined to assess the site-specific potential for debris impact and its significance to structures. The procedure involves a categorization of potential debris based on fundamental characteristics. It includes an assessment of the ability and likelihood of debris transport, as constrained by the topography and the constructed environment. Data from aerial surveys and on-ground surveys after the March 2011 Tohoku tsunami are used to demonstrate and validate the considerations proposed for an assessment of debris and its transport. Instances of structural damage found during the site survey and that potentially resulted from debris strikes are reported. These cases are evaluated to correlate the proposed debris categories ...

Hydraulic Experiments on Impact Forces from Tsunami-Driven Debris

AbstractImpact on a column by an idealized 1:5-scale shipping container propelled by tsunami flow was modeled in a large-scale wave flume. Results from hydraulic experiments were compared with corresponding data from in-air impact experiments using the same experimental configuration to assess the hydrodynamic effects on impact force and duration. Experiments were conducted by varying flow conditions, velocity, and nonstructural mass. An aluminum specimen was tested empty and with the addition of nonstructural mass to simulate partially loaded shipping containers. The measured peak impact forces from the longitudinal test in water were observed to have an increase no greater than 17% of the corresponding measured peak impact forces from the longitudinal test in air. The impact duration measured from the in-air test provided a lower bound for the impact duration measured for the in-water tests. Hydraulic effects were shown to increase the impact duration by an average of 20%. The additional nonstructural m...

Full-Scale Experimental Study of Impact Demands Resulting from High Mass, Low Velocity Debris

AbstractTsunamis can generate a considerable amount of flow velocity on land. The associated hydrodynamic effects coupled with the plethora of unrestrained objects and frangible structures produce significant debris that can travel similar velocities as the flow. Design of structures to resist the tsunami-driven debris requires a conservative estimation of the forces generated at impact. To quantify the forces generated, an experimental study was conducted on a full-scale wood utility pole, steel tube, and ISO shipping container subjected to in-air axial impacts. The impact force is found to vary linearly with the impact velocity and the impact duration remains constant for elastic response of the debris. For inelastic axial impact of the debris, the duration of the impact event increases and the impact force demands reach a limit. The results are compared with a simplified method, which is found to provide an accurate estimate of the impact demands. The model presented in this paper is developed for use ...

A comparative study of RMFC and FEA models for the wave-induced response of a MOB

Abstract A comparative study of the linear, wave-induced response of a 5-module mobile offshore base (MOB) based on two structural models is reported herein. The lumped parameter, rigid module, flexible connector (RMFC) model and a more detailed finite element shell model were used. The 1500 m ×152 m MOB corresponds to McDermotts design concept, where the modules are hinge-connected at the deck. The wave-induced response was determined based on linear hydroelasticity, which involves linear potential theory for the fluid forces. The response in both regular and irregular seas is compared. The results show that the simplified RMFC model can predict the response very well if the natural frequencies and associated normal modes correspond well to the FEA models frequencies and modes. Under these conditions, the simpler RMFC model, which is especially useful for preliminary and conceptual design during which parametric studies are carried out, can therefore be used with a good deal of confidence.

Computationally efficient techniques in the hydroelasticity analysis of very large floating structures

Abstract Two techniques are introduced in the three-dimensional hydroelasticity theory to increase the computational efficiency for the determination of the dynamic response of very large floating structures (VLFS). One technique is related to the convergence of the Green function and its derivatives, namely the introduction of a criterion used to truncate the influence of the Green function and its derivatives. The other involves using an iterative sparse solver for the linear system of equations. The principle motivation behind the application of these two techniques stems from the fact that a source makes a very small contribution to the potential at a point “far away” from the source point. By employing these two techniques in the hydroelastic analysis of a VLFS, the CPU time and required storage are considerably reduced and therefore it is now possible to analyze the dynamics of a VLFS as large as a floating airport by using the three-dimensional panel method.

TSUNAMI BORE FORCES ON WALLS

A series of experiments have been carried out at Oregon State University to quantify tsunami bore forces on structures. Phase 1 of the tests was carried out in the Tsunami Wave Basin (TWB), while Phase II of the tests were carried out in the Large Wave Flume (LWF) at approximately twice the scale of the Phase I tests. These latter tests included ‘offshore’ solitary waves, with heights up to 1.3 m, that traveled over a flat bottom, up a sloping beach and breaking onto a flat ‘fringing reef’. Standing water depths on the reef varied from 0.05 m to 0.3 m. Resulting bores on the reef measured up to approximately 0.8 m. After propagating along the reef, the bores struck a vertical wall. The resulting forces and pressures on the wall were measured. The test setup for the Phase II tests in the LWF is described and the experimental results are reported. The results include forces and pressure distributions. Results show that the bores propagated with a Froude number of approximately 2, and that the forces follow Froude scaling. Finally, a design formula for the maximum impact force is given. The formula is shown to be an improvement over existing formulas found in the literature. The lateral forces are shown to be quite significant compared to traditional lateral loads on vertical wall elements.Copyright

Comparison of Formulations for the Hydrostatic Stiffness of Flexible Structures

The formulation for the hydrostatic stiffness (restoring force) for linear rigid body hydrodynamics is well known, whereas there are several formulations in literature for the corresponding stiffness of flexible structures. Which of these formulations to use is not immediately obvious. This paper clarifies the relationship and the differences between the formulations and the selection of the appropriate one. In addition, it will be shown that a general formulation of the hydrostatic stiffness for flexible structures involves the internal stress distribution under gravity loads, just as it does the corresponding hydrostatic pressure distribution.

Water-Driven Debris Impact Forces on Structures: Experimental and Theoretical Program

Water-driven debris generated during tsunamis and hurricanes can impose substantial impact forces on structures that are often not designed for such loads. This paper presents the design and results of an experimental and theoretical program to quantify these potential impact forces. Two types of prototypical debris are considered: a wood log and a shipping container.Full-scale impact tests at Lehigh University were carried out with a wooden utility pole and a shipping container. The tests were carried out in-air, and were designed to provide baseline, full-scale results. A 1:5 scale shipping container model was used for in-water tests in the Oregon State University large wave flume. These tests were used to quantify the effect of the fluid on the impact forces.Results from both experimental programs are presented and compared with theoretical predictions. The analytical predictions are found to be in sufficient agreement such that they can be used for design. A fundamental takeaway is that the impact forces are dominated by the structural impact, with a secondary affect provided by the fluid. Both forces are quantified in the paper.Copyright