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### Research Papers

J. Offshore Mech. Arct. Eng. 2008;130(4):041001-041001-9. doi:10.1115/1.2904945.

The probability of failure of steel jacket platforms subjected to fatigue damage is computed by means of Monte Carlo simulations using limit state functions in which wave, wind, and deck loadings are expressed in terms of empirical functions of uncertain maximum wave height. Limit state functions associated with the base shear capacity of the jacket and the shear capacity of the deck legs were used. The sensitivity of the probability of failure to the coefficient of variation of resistance, of wave height, of resistance and loading biases, and to parameters in empirical loading functions, as well as the influence of the reserve strength ratio is analyzed using a simplified limit state function. Results from simulations are compared to those obtained with a formulation that relates the reserve strength ratio to the reliability index. An application to risk based inspection planning for extension of the service life of a platform is given.

Commentary by Dr. Valentin Fuster

### Ocean Engineering

J. Offshore Mech. Arct. Eng. 2008;130(4):041101-041101-15. doi:10.1115/1.2957913.

Any device aiming to harness the abundant clean and renewable energy from ocean and other water resources must have high energy density, be unobtrusive, have low maintenance, be robust, meet life cycle cost targets, and have a $10–20year$ life. The vortex induced vibration aquatic clean energy (VIVACE) converter—invented by Bernitsas and Raghavan, patent pending through the University of Michigan—satisfies those criteria. It converts ocean/river current hydrokinetic energy to a usable form of energy such as electricity using VIV successfully and efficiently for the first time. VIVACE is based on the idea of maximizing rather than spoiling vortex shedding and exploiting rather than suppressing VIV. It introduces optimal damping for energy conversion while maintaining VIV over a broad range of vortex shedding synchronization. VIV occurs over very broad ranges of Reynolds (Re) number. Only three transition regions suppress VIV. Thus, even from currents as slow as $0.25m∕s$, VIVACE can extract energy with high power conversion ratio making ocean/river current energy a more accessible and economically viable resource. In this paper, the underlying concepts of the VIVACE converter are discussed. The designs of the physical model and laboratory prototype are presented. A mathematical model is developed, and design particulars for a wide range of application scales are calculated. Experimental measurements on the laboratory prototype are reported in the sequel paper and used here for preliminary benchmarking.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2008;130(4):041102-041102-9. doi:10.1115/1.2904587.

The quasideterminism (QD) theory introduces a deterministic wave function (of both space and time) that shows what, most probably, will happen if an exceptionally large wave will occur at some point in a sea storm. This deterministic wave function holds for every configuration of the solid boundary, provided that the wave motion may be regarded as irrotational. This is the main feature of some conceptual novelties of the QD theory. The following invited lecture given at the 2006 OMAE Congress in Hamburg aims to emphasize these conceptual novelties through a fresh overall picture of the theory.

Topics: Waves , Seas
Commentary by Dr. Valentin Fuster

### Offshore and Structural Mechanics

J. Offshore Mech. Arct. Eng. 2008;130(4):041301-041301-4. doi:10.1115/1.2948956.

This paper presents a detailed finite-element analysis of unbonded flexible risers. The numerical results are compared to the analytical solutions for various load cases. In the finite-element model, all layers are modeled separately with contact interfaces between each layer. The finite-element model includes the main features of the riser geometry with very little simplifying assumptions made. The numerical model was solved using a fully explicit time-integration scheme implemented in a parallel environment on a 16-processor cluster. The very good agreement found from numerical and analytical comparisons validates the use of our numerical model to provide benchmark solutions against which further detailed investigation will be made.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2008;130(4):041302-041302-8. doi:10.1115/1.2904586.

The annulus between the pile and leg in jacket type offshore platforms may be filled with cement grout mainly to reduce horizontal deflections, inhibit corrosion, and increase the energy absorption capacity. This paper discusses an approach, which can be used to demonstrate an enhanced structural performance due to the both presence and lack of grouted piles. The compressive stress-strain response of the grout has been derived from the performed experiments. Having this response, the fiber beam column post-buckling element in the commercial code, DRAIN-3DX , was being used to investigate the behavior of grouted and ungrouted jackets and also the relative pile-leg interaction. It is therefore concluded that in the cases where the existing structure is ungrouted or incompletely grouted, adequate grouting can be considered as a relatively inexpensive method to improve the strength and performance of the structure. In fact, the cement filling of a tubular member increases its overall strength and also provides additional stability. The lateral force-deformation curves are equivalents for the cases where the axial force is less than 30% of the yielding force, $Pyielding$. However, as the axial force increases, the grouted portal element gradually gives a much better performance compared to the ungrouted element. By increasing the axial force, the lateral hysteretic behavior deteriorates in both grouted and ungrouted cases; however, this deterioration is more severe in the case of an ungrouted portal element.

Commentary by Dr. Valentin Fuster

### Materials

J. Offshore Mech. Arct. Eng. 2008;130(4):041401-041401-6. doi:10.1115/1.2426996.

In this paper, the deformability of line pipe with local metal loss was examined. A full-scale experiment and a finite element (FE) analysis were carried out for line pipe with local metal loss subjected to an axial compressive load. As a result, a good agreement was obtained between the analytical and experimental results. This indicated that the present analytical method was applicable to evaluate the deformability of line pipes with local metal loss subjected to a large ground movement. Parametric studies were then conducted to clarify the relationship between the geometry of the local metal loss and the deformability using the FE analytical method.

Commentary by Dr. Valentin Fuster

### Safety and Reliability

J. Offshore Mech. Arct. Eng. 2008;130(4):041601-041601-6. doi:10.1115/1.2948960.

The focus of the present paper is the extreme response statistics of drag dominated offshore structures subjected to harsh weather conditions. More specifically, severe sea states both with and without strong current are considered. The nature of the hydrodynamic forces acting on the structure becomes highly nonlinear. In addition to the drag forces, the so called inundation effect due to the wave elevation, corrected to include second order waves, is also taken into account. In the present paper, the Monte Carlo method along with a special extrapolation technique is applied. The proposed method opens up for the possibility to predict simply and efficiently long-term extreme response statistics, which is an important issue for the design of offshore structures.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2008;130(4):041602-041602-5. doi:10.1115/1.2904589.

This paper contains several investigations on the behavior of bulged piles. Bulge means the application of vertical steel plates somewhere near the ground surface to improve the lateral bearing capacity. The results of small scale tests in sand are illustrated, which demonstrate the effectiveness of such bulge. Some theoretical investigations are presented trying to apply standard methods like the $p‐y$-curve procedure to the design of bulged piles. An outline of possible calculation methods is given. Investigations on two different exemplary pile systems demonstrate the behavior of the bulged structure and give an idea of the advantages of this innovative system.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2008;130(4):041603-041603-10. doi:10.1115/1.2904949.

Three-dimensional, time-domain, wave-body interactions are studied in this paper for cases with and without forward speed. In the present approach, an exact body boundary condition and linearized free surface boundary conditions are used. By distributing desingularized sources above the calm water surface and using constant-strength flat panels on the exact wetted body surface, the boundary integral equations are numerically solved at each time step. Once the fluid velocities on the free surface are computed, the free surface elevation and potential are updated by integrating the free surface boundary conditions. After each time step, the body surface and free surface are regrided due to the instantaneous wetted body geometry. The desingularized method applied on the free surface produces nonsingular kernels in the integral equations by moving the fundamental singularities a small distance outside of the fluid domain. Constant-strength flat panels are used for bodies with any arbitrary shape. Extensive results are presented to validate the efficiency of the present method. These results include the added mass and damping computations for a hemisphere. The calm water wave resistance for a submerged spheroid and a Wigley hull are also presented. All the computations with forward speed are started from rest and proceeded until a steady state is reached. Finally, the time-domain forced motion results for a modified Wigley hull with forward speed are shown and compared to the experiments for both linear computations and body-exact computations.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2008;130(4):041604-041604-8. doi:10.1115/1.2960859.

The characteristics of hindcast data for extreme storms at a Northern North Sea location are shown to depend on storm direction, reflecting storm strength and fetch variability. Storm peak $HS$ over threshold is modeled using a generalized Pareto distribution, the parameters of which are allowed to vary smoothly with direction using a Fourier form. A directionally varying extreme value threshold is incorporated. The degree of smoothness of extreme value shape and scale with direction is regulated by roughness-penalized maximum likelihood, the optimal value of roughness selected by cross-validation. The characteristics of a $100-year$ storm peak $HS$, estimated using the directional model, differ from those estimated when ignoring the directionality of storms. In particular, the extreme right-hand tail of omnidirectional $HS100$ is longer using the directional model, indicating in this case that ignoring directionality causes underestimation of design criteria. Although storm peak data alone are used for extreme value modeling, the influence of a storm, in directional design sectors other than that containing its storm peak direction, is incorporated by estimating the storm’s directional dissipation directly from the data. An automated approach to selection of directional design sectors is described. Directional design criteria are developed using three different approaches, all consistent with an omnidirectional storm peak $HS$ nonexceedence probability of 0.5. We suggest a risk-cost criterion, which minimizes design cost for a given omnidirectional design specification, as an objective basis for optimal selection of directional criteria.

Topics: Design , Storms , North Sea
Commentary by Dr. Valentin Fuster

### Technical Briefs

J. Offshore Mech. Arct. Eng. 2008;130(4):044501-044501-5. doi:10.1115/1.2948954.

In the past few years, DNV has been involved in a variety of projects related to marine energy converters. All projects have been characterized for the handling of technical uncertainties due to the application of new technology or proven technology in different areas of application. A systematic approach based on the DNV RP-A203 Qualification of New Technology was applied combined with the Guidelines for Design and Operation of Wave Energy Converters (May 2005—work carried out by DNV under Commission of Carbon Trust as part of Marine Energy Challenge) to steer the third party activity, but, more importantly, to allow developers to systematically identify and deal with the risks in a rational manner with traceability of decisions throughout the development of the energy converter. From the very start of our engagement, it was clear that the handling of technical uncertainties was affected, not only by the technical barriers, but also by financial and time constraints. The establishment of the safety and functional targets to be achieved by the energy converter are to be based, not only on the safety and asset integrity aspects, but also on the financial∕business model. The experience of using the qualification process and the guidelines on these projects, achieving the right balance between the constraints, handling of uncertainties, financial targets, and safety and functional requirements, are briefly described in this paper as well as the future steps to be taken to improve the process and consolidate the experience so far. In this paper, the use of the DNV OSS-312 (Certification of Wave and Tidal Energy Converters) on the certification process of marine energy converters is also addressed.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2008;130(4):044502-044502-4. doi:10.1115/1.2948958.

Measurements have been made of two dimensional motion of an elastically supported circular cylinder in the wake of a fixed upstream cylinder. The experiments were made in a water flow channel with $6.35cm$$(2.5in.)$ diameter cylinders with a maximum Reynolds number of 77,000. The elastically supported downstream cylinder moves downward and inward toward the centerline of the upstream cylinder’s wake with increasing flow velocity, indicating the presence of a transverse lift force and reduced drag in the wake. These forces can cause the cylinders to clash. The measured motions correlate with the theory.

Commentary by Dr. Valentin Fuster

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