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Ocean Engineering

J. Offshore Mech. Arct. Eng. 2010;132(4):041101-041101-11. doi:10.1115/1.4001442.

This paper presents simulated results of a computational study conducted to analyze the impulse waves generated by the subaerial landslide at Lituya Bay, Alaska. The volume of fluid method is used to track the free surface and shoreline movements. The renormalization group turbulence model and detached eddy simulation multiscale model were used to simulate turbulence dissipation. The subaerial landslide is simulated using a sliding mass. Results from the two-dimensional simulations are compared with the results from a scaled-down experiment. The experiment is carried out at a 1:675 scale. In the experimental setup, the subaerial rockslide impact into the Gilbert Inlet, wave generation, propagation, and runup on the headland slope is considered in a geometrically undistorted Froude similarity model. The rockslide is simulated by a granular material driven by a pneumatic acceleration mechanism so that the impact characteristics can be controlled. Simulations are performed for different values of the landslide density to estimate the influence of slide deformation on the generated tsunami characteristics. Simulated results show the complex flow patterns in terms of the velocity field, shoreline evolution, and free surface profiles. The predicted wave runup height is in close agreement with both the observed wave runup height and that obtained from the scaled-down experimental model.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2010;132(4):041102-041102-16. doi:10.1115/1.4001440.

A great deal of works has been developed on the spar vortex-induced motion (VIM) issue. There are, however, very few published works concerning VIM of monocolumn platforms, partly due to the fact that the concept is fairly recent and the first unit was only installed last year. In this context, a meticulous study on VIM for this type of platform concept is presented here. Model test experiments were performed to check the influence of many factors on VIM, such as different headings, wave/current coexistence, different drafts, suppression elements, and the presence of risers. The results of the experiments presented here are motion amplitudes in both in-line and transverse directions, forces and added-mass coefficients, ratios of actual oscillation and natural periods, and motions in the XY plane. This is, therefore, a very extensive and important data set for comparisons and validations of theoretical and numerical models for VIM prediction.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2010;132(4):041103-041103-5. doi:10.1115/1.4001443.

An offshore wave energy converter (WEC) was successfully launched at the Swedish west coast in the middle of March 2006. The WEC is based on a permanent magnet linear generator located on the sea floor driven by a point absorber. A measuring station has been installed on a nearby island where all measurements and experiments on the WEC have been carried out. The output voltage from the generator fluctuates both in amplitude and frequency and must therefore be converted to enable grid connection. In order to study the voltage conversion, the measuring station was fitted with a six pulse diode rectifier and a capacitive filter during the autumn of 2006. The object of this paper is to present a detailed description of the Lysekil research site. Special attention will be given to the power absorption by the generator when it is connected to a nonlinear load.

Commentary by Dr. Valentin Fuster

Offshore Technology

J. Offshore Mech. Arct. Eng. 2010;132(4):041301-041301-12. doi:10.1115/1.4001429.

A new concept and a preliminary study for a monocolumn floating unit are introduced, aimed at exploring and producing oil in ultradeep waters. This platform, which combines two relevant features—great oil storage capacity and dry tree production capability—comprises two bodies with relatively independent heave motions between them. A parametric model is used to define the main design characteristics of the floating units. A set of design alternatives is generated using this procedure. These solutions are evaluated in terms of stability requirements and dynamic response. A mathematical model is developed to estimate the first order heave and pitch motions of the platform. Experimental tests are carried out in order to calibrate this model. The response of each body alone is estimated numerically using the WAMIT ® code. This paper also includes a preliminary study on the platform mooring system and appendages. The study of the heave plates presents the gain, in terms of decreasing the motions, achieved by the introduction of the appropriate appendages to the platform.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2010;132(4):041302-041302-12. doi:10.1115/1.4001436.

Level-crossing analysis of long-crested, Gaussian waves in space and time are studied in the context of wave loads on a fixed, horizontal deck-box above mean waterline. Vertical wave loads on decks due to insufficient airgap are a major concern for many in-service platforms. Reliable estimation of magnitude and duration of these loads is important in assessing structural and global response of an offshore platform. In the case of an irregular wave-impact on a flat deck of dimensions comparable to mean wavelength of the incident waves, both temporal and spatial variability of wave-kinematics need to be considered during the deck-wetting process. In the present study, we have used a multidimensional Gaussian formulation of incident wave-kinematics to derive a joint probability density function of deck-wetting (or exceedance) duration and its spatial extent. We have also derived a probability density function for initial slam force on deck. A numerical scheme for simulating wave-impact events on a two-dimensional deck is discussed, results from which are compared against corresponding analytical estimates. Vertical force on deck was estimated using the momentum method, which includes a von Kármán slamming model applied over the wetted-length determined from an undisturbed wave profile.

Commentary by Dr. Valentin Fuster

Materials Technology

J. Offshore Mech. Arct. Eng. 2010;132(4):041401-041401-11. doi:10.1115/1.4001437.

This paper studies the static stability of metal cones subjected to combined, simultaneous action of the external pressure and axial compression. Cones are relatively thick; hence, their buckling performance remains within the elastic-plastic range. The literature review shows that there are very few results within this range and none on combined stability. The current paper aims to fill this gap. Combined stability plot, sometimes called interactive stability plot, is obtained for mild steel models. Most attention is given to buckling caused by a single type of loading, i.e., by hydrostatic external pressure and by axial compression. Asymmetric bifurcation bucklings, collapse load in addition to the first yield pressure and first yield force, are computed using two independent proprietory codes in order to compare predictions given by them. Finally, selected cone configurations are used to verify numerical findings. To this end four cones were computer numerically controlled-machined from a solid steel billet of 252 mm in diameter. All cones had integral top and bottom flanges in order to mimic realistic boundary conditions. Computed predictions of buckling loads, caused by external hydrostatic pressure, were close to the experimental values. But similar comparisons for axially compressed cones are not so good. Possible reasons for this disparity are discussed in the paper.

Commentary by Dr. Valentin Fuster

Strutures, Safety and Reliability

J. Offshore Mech. Arct. Eng. 2010;132(4):041601-041601-11. doi:10.1115/1.4001415.

Hydroelasticity theory, considering the second-order fluid forces induced by the coupling of first-order wave potentials, is introduced briefly in this paper. Based on the numerical results of second-order principal coordinates induced by the difference-frequency and sum-frequency fluid forces in multidirectional irregular waves, the bending moments, as well as the vertical displacements of a floating plate used as a numerical example are obtained in an efficient manner. As the phase angle components of the multidirectional waves are random variables, the principal coordinates, the vertical displacements, and the bending moments are all random variables. Extreme values of bending moments are predicted on the basis of the theory of stationary stochastic processes. The predicted linear and nonlinear results of bending moments show that the influences of nonlinear fluid forces are different not only for the different wave phase angles, but also for the different incident wave angles. In the example very large floating structure (VLFS) considered in this paper, the influence of nonlinear fluid force on the predicted extreme bending moment may be as large as 22% of the linear wave exciting forces. For an elastic body with large rigidity, the influence of nonlinear fluid force on the responses may be larger than the first-order exciting forces and should be considered in the hydroelastic analysis.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2010;132(4):041602-041602-7. doi:10.1115/1.4001417.

Design S-N curves in design codes are based on fatigue test data, where the stress cycle is under external tension load. It is observed that during pile driving most of the stress cycle is compressive and the design procedure used for fatigue analysis of piles might therefore be conservative. In order to investigate this further, it was proposed to perform laboratory fatigue testing of specimens that are representative for butt welds in piles under relevant loading conditions. In the present project 30 test specimens made from welded plates were fatigue tested at different loading conditions to assess effect of compressive stress cycles as compared with tensile stress cycles. In 2006, the Edda tripod in block 2/7 was taken ashore. This platform has been in service since 1976 and the piles are considered to be representative for the piles installed in the North Sea jacket structures during the 1970s. Therefore it was suggested to investigate the pile weld at the sea bed in detail to assess the stress due to fabrication and 30 years of in-service life and the residual fatigue life of the pile. Six test specimens made from the Edda pile were fatigue tested. The results from the assessment and the fatigue testing are presented in this paper.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2010;132(4):041603-041603-8. doi:10.1115/1.4001418.

Calculated probabilities of fatigue failure depend on the analysis procedure used for design. Calculated probabilities of a fatigue failure also depend on long term stress ranges due to loading and uncertainties associated with this. In order to ensure the consistent safety level for assessment of fatigue failure, the design fatigue factors (DFFs) to be used for fatigue design should be dependent on the analysis procedure and premises used. In the present paper, an assessment of appropriate DFFs for piles subjected to dynamic actions from pile driving has been performed by probabilistic analysis based on: uncertainty with respect to dynamic cyclic stress during pile driving, and fatigue capacity of circumferential welds in piles. Accumulated probabilities of fatigue failures in pile butt welds are presented. An assessment of uncertainties involved in calculation of stress ranges during pile driving has been performed. It is shown that the uncertainty in loading when driving records are known is lower than that estimated on the basis of soil data. Thus, in order to obtain consistent safety levels, different DFFs should be used when calculated stress ranges are derived based on soil data only, as compared with the actual stress ranges and number of blows determined from driving records. The results from probabilistic analyses together with recommended design fatigue factors are presented in this paper.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2010;132(4):041604-041604-7. doi:10.1115/1.4001419.

The paper describes a novel method for predicting the appropriate extreme value distribution derived from an observed time series. The method is based on introducing a cascade of conditioning approximations to the exact extreme value distribution. This allows for a rational way of capturing dependence effects in the time series. The performance of the method is compared with that of the peaks-over-threshold method.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2010;132(4):041605-041605-4. doi:10.1115/1.4001432.

The effect of tether-riser dynamics on the response characteristics of deep water tension leg platforms in water depths 900 m and 1800 m under random waves is investigated using a developed nonlinear finite element analysis program in the time-domain. Updated Lagrangian coordinates and incremental iterative solution based on Newmark’s integration scheme are adopted. Linear wave theory is used. Relative velocity form of Morison’s equation is used for estimating the wave forces. Current forces are also included in the analysis. Results are reported in the form of statistical values of responses. The statistical values of responses are found to increase with water depth and significant increase is observed when risers are included.

Commentary by Dr. Valentin Fuster

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