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Research Papers: Ocean Engineering

J. Offshore Mech. Arct. Eng. 2019;141(6):061101-061101-10. doi:10.1115/1.4042939.

In this paper, detailed experimental results are reported to study the effect of the surface wave of different frequencies on unidirectional current over the bed-mounted train of rib roughness. The model roughness used in this study is transverse square ribs that lengthened across the entire width of the recirculating wave channel. The center-to-center rib pitch (P) was constant during the experiments, thus generating a broad range of near-bed flow patterns for each of the three different surface wave frequencies studied here. The relative submergence associated with the roughness height (k) was 8, which fall in the category of large roughness. Velocity measurements were conducted using acoustic Doppler velocimeter (ADV), and a surface wave of different frequencies was generated using the plunger-type wavemaker. The measured velocity data were analyzed to determine the relative importance of mean flow over the train of rib roughness. Mean velocity profiles illustrate the well-known downward shift from the flat surface data of the semi-logarithmic portion of the law of the wall. The width of the turbulent boundary layer increases with the superposition of surface wave compared to that of the current-only flow. The results also show that the mean reattachment length decreases due to the superposition of surface wave on unidirectional current.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2019;141(6):061102-061102-10. doi:10.1115/1.4042942.

A method for the generation of steep nonlinear broad-banded wave trains having an arbitrary prescribed shape is developed. It is shown that the second-order contributions to the velocity field are negligible in deep water, while the second-order bound components of the surface elevation are significant. This fact allows improvement of an iterative method of the wavemaker driving signal adjustment that increases the accuracy of excitation of wave train with the prescribed free waves’ spectrum. The decomposition of the complex amplitude spectrum of the surface elevation into free and bound components is based on the approach adopted in the derivation of the Zakharov model. The iterative adjustment of the driving signal is carried out using the numerical wave tank based on the boundary element method. It is demonstrated that accurate wave train excitation is attained for different values of the wave steepness. The method allows decreasing the number of iterations needed for the driving signal adjustment. The surface elevation values measured in the laboratory wave tank agree closely with those obtained in the numerical simulations. The measured and the simulated frequency spectra are in agreement as well.

Topics: Waves , Signals
Commentary by Dr. Valentin Fuster

Research Papers: Structures and Safety Reliability

J. Offshore Mech. Arct. Eng. 2019;141(6):061601-061601-11. doi:10.1115/1.4042941.

In order to study the compressive behavior of flexible pipes, a nonlinear finite element model was developed. This fully tridimensional model recreates a five-layer flexible pipe with two tensile armor layers, an external polymeric sheath, an orthotropic high strength tape, and a rigid inner nucleus. The friction coefficient is known as a key parameter in determining the instability response of flexible pipes’ tensile armor. Since the featured model includes all nonlinear frictional contacts between the layers, it has been used to conduct several experiments in order to investigate its influence on the response. This article includes a description of the finite element model itself and a case study where the friction between the layers of the pipe is changed. The procedure of this analysis is described here, along with the results.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2019;141(6):061602-061602-11. doi:10.1115/1.4042946.

Jacket structures have been widely used in oil and gas industry and are increasingly becoming competitive as a support structure of wind turbines at different water depths. These types of structures usually fix in transition or shallow waters where numerous field observations and experiments have shown that water particles tend to exhibit non-Gaussian characteristics. However, current engineering practice ignores the wave nonlinearity for the analysis and design of these structures. The application of linear irregular models might result in considerable uncertainties in the obtained wave loads and consequently the dynamic response and thus it is highly questionable. Therefore, it is crucial to calculate the dynamic response of jacket structures under both linear and nonlinear wave models to investigate the validity of linear wave models in different sea states. In this paper, the finite element (FE) model of a jacket structure located in Persian Gulf (SP17 jacket) is setup and applied to perform a comparative study of the dynamic response to both linear and weakly nonlinear random waves. The fatigue life of the jacket structure is then calculated under both wave models. This paper will substantially improve the understanding of the dynamic response of jacket structures under fatigue damage.

Commentary by Dr. Valentin Fuster

Research Papers: Ocean Renewable Energy

J. Offshore Mech. Arct. Eng. 2019;141(6):061901-061901-9. doi:10.1115/1.4042938.

This paper summarizes the control design work that was performed to optimize the controller of a wind turbine on the WindFloat structure. The WindFloat is a semisubmersible floating platform designed to be a support structure for a multimegawatt power-generating wind turbine. A controller developed for a bottom-fixed wind turbine configuration was modified for use when the turbine is mounted on the WindFloat platform. This results in an efficient platform heel resonance mitigation scheme. In addition, several control modules, designed with a coupled linear model, were added to the fixed-bottom baseline controller. The approach was tested in a fully coupled nonlinear aero-hydro-elastic simulation tool in which wind and wave disturbances were modeled. This testing yielded significant improvements in platform global performance and tower-base bending loading.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2019;141(6):061902-061902-8. doi:10.1115/1.4042943.

Several types of oscillating water column (OWC) type wave energy converters (WECs) are researched and developed in the world. They are floating types and fixed types. In case of a fixed type, wave-dissipating caissons could be replaced by WECs of an OWC type. In OWC types, installation of the projecting walls (PWs) is useful in order to improve power take-off (PTO) performance. In this study, a double-dissipating caisson was used as an OWC type WEC with PWs. A front caisson of the double caisson seems to be the area surrounded by PWs and a back caisson can be seen as an OWC. The paper studied the basic property of the primary conversion from wave power to pneumatic power from model tests in a wave tank. It was found that the wave height strongly affects the behaviors of OWC motion and air pressure. Finally, the primary conversion was affected by wave height. Besides, the concept of use of a double caisson was useful for the primary conversion over 80% evaluated using test data.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Offshore Mech. Arct. Eng. 2019;141(6):064501-064501-6. doi:10.1115/1.4042940.

Both experimental and numerical studies were conducted to investigate the effectiveness of composite patch repair on underwater structures, especially aluminum alloy structures. Physical samples were prepared using 5XXX aluminum plates with a premachined hole and E-glass woven fabric layers. The epoxy resin was selected such that it could be cured underwater. Test samples were prepared under different curing conditions such as dry curing and wet curing with different durations of in-water exposure. Strain gages were attached to all samples. The samples were tested for both tensile and four-point bending loads. Furthermore, numerical modeling and simulations were conducted, and the numerical models were validated against the experimental measurements. Then, the interface normal and shear stresses were determined from the numerical models so as to understand the delamination failure at the interface between the aluminum and composite patches. Underwater composite patching showed good interface strength and potential for successful usage in repairs.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2019;141(6):064502-064502-6. doi:10.1115/1.4042945.

In-pipe robots are a powerful tool for hydrate plug removal inside ultradeepwater pipes. Most of these robots operate with the energy supplied by umbilical cables. The present work focuses on the development of a general strategy for computing the required forces for pulling such cables confined in ducts of generic length and geometry. Based on classical mathematical models applied in cable friction evaluation, a new equation set was developed and implemented in a computational algorithm designed to evaluate the static friction force related to the cumulative effects along the arbitrary set of curves present in a generic pipe. Therefore, the proposed computational routine can calculate the static friction forces associated with a cable inside a given pipe, whose coordinates are fed by the user. To evaluate the simulation performance, the achieved results were compared with the data obtained through experimental tests performed using a cable with polymeric coating positioned inside ducts. Different geometries, loads, and lubricating conditions were tested, and the analytical model could suitably estimate the required force to move an umbilical cable inside pipes.

Commentary by Dr. Valentin Fuster

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