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Research Papers: Ocean Engineering

J. Offshore Mech. Arct. Eng. 2018;140(6):061101-061101-9. doi:10.1115/1.4040244.

Towed marine seismic streamers are extensively utilized for petroleum exploration. With the increasing demand for efficiency, leading to longer and more densely spaced streamers, as well as four-dimensional (4D) surveys and more complicated survey configurations, the demand for optimal streamer steering has increased significantly. Accurate streamer state prediction is one important aspect of efficient streamer steering. In the present study, the ensemble Kalman filter (EnKF) has been used with two different models for data assimilation including parameter estimation followed by position prediction. The data used are processed position data for a seismic streamer at the very start of a survey line with particularly large cable movements due to currents. The first model is a partial differential equation (PDE) model reduced to two-dimensional (2D), solved using a finite difference method (FDM). The second model is based on a path-in-the-water (PIW) model and includes a drift angle. Prediction results using various settings are presented for both models. A variant of the PIW method gives the overall best results for the present data.

Commentary by Dr. Valentin Fuster

Research Papers: CFD and VIV

J. Offshore Mech. Arct. Eng. 2018;140(6):061801-061801-14. doi:10.1115/1.4038939.

This study illustrates a comparison of two numerical methods under a unified computational platform for solving fluid–structure interaction (FSI) problems. The first is an arbitrary Lagrangian–Eulerian (ALE)-based fluid model coupled to a structural finite element (FE) method (ALE-FE/FE), and the second is a smoothed particle hydrodynamics (SPH) method coupled to the same structural FE code (SPH/FE). The predictive capabilities and computational efficiency of both the numerical methods are evaluated and validated against a canonical problem of a rapidly varying flow past an elastic gate for which experimental data are available. In both numerical solutions, the fluid flow is governed by the Navier–Stokes equation, and the elastic gate is modeled as a flexible structure. Numerical simulation results show that the ALE-FE/FE continuum approach not only captures the dynamic behavior properly but also predicts the water-free surface profiles and the elastic gate deformations accurately. On the other hand, the coupled purely Lagrangian approach of the SPH/FE under an identical computational platform is found to be less accurate and efficient in predicting the dynamics of the elastic gate motion and the water-free surface profiles.

Commentary by Dr. Valentin Fuster

Research Papers: Ocean Renewable Energy

J. Offshore Mech. Arct. Eng. 2018;140(6):061901-061901-10. doi:10.1115/1.4040234.

This paper focuses on load mitigation by implementing controllable trailing-edge slotted flaps on the blades of an offshore wind turbine (OWT). The benchmark NREL 5 MW horizontal axis OWT is subjected to coupled stochastic aerodynamic-hydrodynamic analysis for obtaining the responses. The OWT is supported on three different fixed-bottom structures situated in various water depths. Blade element momentum (BEM) theory and Morison's equation are used to compute the aerodynamic and hydrodynamic loads, respectively. Presently, the load reduction obtained by means of the slotted flaps is regulated using an external dynamic link library considering the proportional-integral-derivative (PID) controller. BEM theory is presently modified to account for unsteady effects of flaps along the blade span. The present analysis results show reduction up to 20% in blade and tower loads for the turbine with different support structures on implementing controllable trailing edge flaps (TEFs). This study can form the basis for evaluating the performance of large-scale fixed OWT rotors.

Commentary by Dr. Valentin Fuster

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