A New Spectral Method for Modeling Dynamic Ice Actions

This paper presents a method of evaluating the response of a vertical offshore structure that is subjected to dynamic ice actions. The model concerns a loading scenario where a uniform ice sheet is drifting and crushing against the structure. Full scale data obtained at the lighthouse Norstro¨msgrund is used in the derivation of a method that applies both to narrow and wide structures. A large amount of events with directly measured local forces was used to derive formulas for spectral density functions of the ice force. A non-dimensional formula that was derived for the autospectrum applies for all ice thicknesses. Coherence functions are used to define the cross-spectra of the local ice forces. The two kind of spectral density functions for local forces can be used to evaluate the spectral density of the total ice force. The method takes account of both the spatial and time correlation between the local forces. Accordingly, the model provides a tool to consider the non-simultaneous characteristics of the local ice pressures while assessing the total ice force. The model can be used in conjunction with general purpose FE programs to evaluate the dynamic response of an offshore structure.

Crack Growth During Full Scale Reeling Simulation of Pipes With Girth Welds

Installation of pipelines by reeling has proved to be an effective method. However, the pipe bending results in very high stress and strain and cannot be handled by conventional design rules, as stated in design codes, e.g. [2]: High strain crack growth must be assessed according to specific case-by-case selected criterions. In the present work the performance of 10” and 12 3/4” pipes with typical weld defects is studied — from initiation of cracks at notches to final fracture. Information was obtained from several sources: full scale cyclic bending of pipes, FE simulations, and small-scale tests. The plasticity during reeling operations results in substantial non-linear behavior due to varying cross section properties, cyclic creep, and different material response at tensile and compression side of the pipe. Hence, a full scale reeling simulation must be carefully planned and include sufficient tolerances. Critical cracks in pipe girth welds initiate mainly from the surface (undercuts, lack of penetration, or lack of fusion), but potentially also internally (lack of fusion or large pores). Various configurations of these parameters were investigated in full scale pipe tests. It was possible to verify both crack propagation during the reeling cycles, and the point of final fracture (for ECA verifications). In pipe design on must assure safe conditions for both reeling operations and for later in-service loading. Proper design tools must be available. Several methods for high strain crack growth analysis were considered and also compared to small-scale specimen data. Conventional strain-life methodology failed to predict the crack propagation accurately. A new approach including a tensile strain range parameter offered promising results.

Conditional Short-Crested Second Order Waves in Shallow Water and With Superimposed Current

For bottom-supported offshore structures like oil drilling rigs and oil production platforms, a deterministic design wave approach is often applied using a regular non-linear Stokes’ wave. Thereby, the procedure accounts for non-linear effects in the wave loading but the randomness of the ocean waves is poorly represented, as the shape of the wave spectrum does not enter the wave kinematics. To overcome this problem and still keep the simplicity of a deterministic approach, Tromans, Anaturk and Hagemeijer (1991) suggested the use of a deterministic wave, defined as the expected linear Airy wave, given the value of the wave crest at a specific point in time or space. In the present paper a derivation of the expected second order short-crested wave riding on a uniform current is given. The analysis is based on the second order Sharma and Dean shallow water wave theory and the direction of the main wind direction can make any direction with the current. Numerical results showing the importance of the water depth, the directional spreading and the current on the conditional mean wave profile and the associated wave kinematics are presented. A discussion of the use of the conditional wave approach as design waves is given.

The Effect of Directional Spreading on Interaction of Focused Wave Groups With a Cylinder

The problem of diffraction of a directionally spread focused wave group by a bottom-seated circular cylinder is considered from the view point of second-order perturbation theory. After applying the time Fourier transform and separation of vertical variable the resulting two-dimensional non-homogeneous Helmholtz equations are solved numerically using finite differences. Numerical solutions of the problem are obtained for JONSWAP amplitude spectra for the incoming wave group with various types of directional spreading. The results are compared with the corresponding results for a unidirectional wave group of the same amplitude spectrum. Finally we discuss the applicability of the averaged spreading angle concept for practical applications.

Nonlinear Water Waves in the Presence of Submerged Elliptic Cylinder

The fully nonlinear problem on unsteady two-dimensional water waves generated by elliptic cylinder, that is horizontally submerged beneath a free surface, is considered. An analytical boundary integral equations method using a version of Milne-Thomson transformation is developed. Boundary equations (the BEq system) determine immediately exact wave elevation and fluid velocity at free surface. Small-time solution expansion is obtained in the case of accelerated cylinder starting from rest.

Nonlinear Hydroelastic Response of a Two-Dimensional Mat-Type VLFS by the Green-Naghdi Theory

The two-dimensional, nonlinear hydroelasticity of a mat-type VLFS is studied within the scope of linear beam theory for the structure and the nonlinear, Level I Green-Naghdi (GN) theory for the fluid. The beam equation and the GN equations are coupled through the kinematic and dynamic boundary conditions to obtain a new set of modified GN equations. These equations model long-wave motion beneath an elastic plate. A set of jump conditions that are necessary for the continuity (or the matching) of the solutions in the open water region and that under the structure is newly derived through the use of the postulated conservation laws of mass, momentum and mechanical energy. The resulting governing equations, subjected to the boundary and jump conditions, are solved by the finite-difference method in the time domain. The present model is applicable, for example, to the study of the hydroelastic response of a mat-type VLFS under the action of a solitary wave, or a frontal tsunami wave. Good agreement is observed between the present results and other published theoretical and numerical predictions, as well as experimental data. The nonlinear results show that consideration of nonlinearity is important for accurate predictions of the bending moment of the floating elastic plate. It is also found that the rigidity of the structure also greatly affects the bending moment and displacement of the structure in this nonlinear theory.

A Knowledge Based Approach Towards Design of Mechanical Systems and Components for Deep Sea Mining

Development of systems, machinery and components for deep sea mining from 2000–6000 meters depth in the ocean is a major technological challenge. The existing engineering system concepts have been demonstrated for short duration and have to be well evaluated for long term mining operations. The need for conceptualization of new systems has often been emphasized. A Knowledge Based Design Support System has been developed which can be used as a tool to aid a system designer in evaluating existing systems as well as conceive new systems and components. The architecture for evaluating available products and development of evolvable products, hybrid products and genetically modified products are presented. System synthesis using far and near vision fused with conventional methods are described. System variants can be evaluated using a fusion of Analytical Hierarchy Processes (AHP) and Altschullerian approaches.

Effect of Wave Induced Loading and Line Loading in Fully Stochastic Fatigue Assessment for West Africa FPSO

The fatigue assessment for a caisson of the interface between a caisson and a FPSO hull has been performed based on the FMS (Fatigue Methodology Specification) [1]. Investigation is focused on the various loading effects including inertia loading due to ship motions from a site specific swell, sea and line loading from the submerged caisson connected to the side shell of FPSO. The fully stochastic method was used for estimating damage levels from wave loadings. The component-based method was used for line loading effects. The two kinds of results from each case were combined the variance and mean period combination. For the inertia loading, it is shown that a hull deforming due to vertical bending moment is the principal effect for a fatigue assessment. For the line loading, it is found that the loading effect due to the submerged piping system connected to the side shell of FPSO is not significant for the fatigue of the side shell supporting structure. In conclusion, the fatigue damage from the site-specific swell is dominant effect among overall fatigue damage components of FPSO in the specific site condition of West Africa.

Hydroelastic Response of a Mat-Like Floating Circular Plate Advancing in Waves: Analytical Solution

The boundary-value problem is formulated to predict the hydroelastic response of a mat-like floating circular plate advancing slowly in waves. The potential flow theory is employed with low forward-speed assumption. The plate is modeled as an elastic plate with zero draught. This assumption allows the steady disturbance potential due to forward speed be neglected, simplifying considerably the problem. By applying the eigenfunction-expansion domain-matching method analytical solutions are derived for the scattering and radiation potentials up to the leading-order terms of the speed-dependent parts. Numerical results are presented for the typical plate geometry. It is shown that the forward-speed effect on the hydroelastic response, especially on the bending strain response, of the plate is significant.

Interaction of a Second-Order Solitary Wave With a Vertical Permeable Plane Breakwater

The purpose of this paper is to develop mathematical models to investigate the interaction between long non-linear water waves and dissipative/absorbing coastal structures. The diffraction of a plane second-order solitary wave at a vertical permeable plane barrier standing in front of an impermeable wall, with calculation of the second-order wave loading is investigated. An incident plane second-order solitary wave is the Laitone solution of Boussinesq equations. The analytical solution is obtained by means of a small parameter development and Fourier transformation techniques. Computational results were performed using the software MATHEMATICA version 4.0.1.0.