Digital Twin of a Maneuvering Ship: Real-Time Estimation of Derivatives and Resistance Coefficient Based on Motion Sensor

Retrieving certain hydrodynamics coefficients from a marine craft during a maneuver can be useful for various reasons, such as the validation of project specifications or the rapid verification of structural changes that could impact the vessel movement. Intended to estimate some of these parameters, the present work proposes a method purely based on traditional Extended Kalman Filter (EKF) focused for limited drift angles. Albeit not posing as a replacement to conventional estimations, such as from Computer Fluid Dynamics (CFD) — which solve equations in order of millions — and experimental tests — with its time-consuming preparation setups and post-analyses — the method can possibly present itself as a convenient and quicky technique to estimate the hydrodynamics coefficients in real time, as each iteration resorts only into a few dozen of equations. Preliminary results in the simulated environment called pyDyna — a python version of the Numerical Offshore Tank ship maneuvering simulator — indicate this procedure is faster along with an acceptable margin of accuracy, possibly pointing as a feature for future digital twin applications.

THE HYDRODYNAMIC COEFFICIENTS FOR OSCILLATING 2D RECTANGULAR BOX USING WEAKLY COMPRESSIBLE SMOOTHED PARTICLE HYDRODYNAMICS (WCSPH) METHOD

ABSTRACT: An implementation of the weakly compressible smoothed particle hydrodynamics (WCSPH) method is demonstrated to determine the hydrodynamics coefficients through radiation problem of an oscillating 2D rectangular box. Three possible modes of motion namely swaying, heaving, and rolling are carried out to establish the influence of oscillating motions in predicting the added mass and damping. Both solid boundary and fluid flow are modelled by WCSPH and validated by the potential flow and experimental results. Discrepancies observed at lower frequencies are further investigated using different particle resolutions, different time steps, and extending the domain with longer runtime to demonstrate the performance of WCSPH. Finally, flow separation and vortices are discussed and compared with experimental results. ABSTRAK: Bagi fenomena yang melibatkan radiasi dalam air, segiempat kotak 2D diosilasikan dengan menggunakan simulasi WCSPH untuk memperoleh pekali hidrodinamik. Mod osilasi terbahagi kepada 3 iaitu sway, heave dan roll. Osilasi dengan mengguna pakai kotak akan mempengaruhi pergerakan air dalam menentukan nilai penambahan jisim dan rendaman. Keseluruhan domain air dan sempadan telah dimodelkan dengan menggunakan WCSPH. Semua model tersebut kemudiannya akan dibandingkan melalui keputusan eksperimen dan teori. Jika keputusan melalui kaedah WCSPH ini berbeza, terutama pada frekuensi rendah, penyelidikan lanjut akan dilakukan dengan menggunakan zarah resolusi yang berbeza, langkah masa yang berbeza dan menambah masa domain ujikaji bagi menilai keputusan WCSPH. Akhirnya, kriteria aliran dan kadar pusaran yang terhasil di sekeliling kotak akan dibincang dan dibandingkan bersama keputusan eksperimen.

Using the Floating Body Symmetries to Speed Up the Numerical Computation of Hydrodynamics Coefficients With Nemoh

The open source potential flow BEM solver Nemoh (developed at École Centrale de Nantes) is widely used today, notably for the development of wave energy converters. The use of a linear potential flow theory allows quick estimations of the hydro-dynamic properties of the device, such as its added mass or its radiation damping. Nonetheless, their computation with Nemoh can still be time consuming and could be optimized to facilitate the R&D process. Many wave energy converter concepts present symmetric shapes: for instance rotational symmetry for point absorbers or translation invariance for cylindrical shapes. These symmetries can be exploited in the BEM solver to significantly speed up the computations. In this paper, the mathematical effect of symmetries on the BEM resolution will be discussed and some results of its implementation in Nemoh will be presented.

Simplified Hydrodynamic Design Procedure of a Submerged Floating Tube Bridge Across the Digernessund of Norway

The paper will look into the hydrodynamic loads and responses on the proposed Submerged Floating Tube Bridge (SFTB) through the Digernessund by the Norwegian Public Roads Administration (Statens vegvesen, NPRA). The aim is to show how different hydrodynamics aspects during the prelimiary design can be simply addressed under the given environmental conditions. Different SFTB systems are introduced as the first step. A simplified method based on modal analysis is introduced and implemented for evaluation of the motions and stress, bending moments along the bridge. Firstly, a 2D Boundary Element Method (BEM) solver is developed and verified, which is further used for solving the hydrodynamics coefficients of different bridge cross sections. The 3D hydrodynamic coefficients of pontoons are solved by the commercial software AQWA. The analysis procedure of the simplified method for the global SFTB responses is presented. The Eigen periods of the Bjørnefjord SFTB is re-calculated by the present model as a first validation of the implementation. The loads and responses of the bridge under given wave conditions are then estimated. The evaluation of the possibility of vortex induced vibrations of the current SFTB design is given.

On the concept of sloped motion for free-floating wave energy converters

A free-floating wave energy converter (WEC) concept whose power take-off (PTO) system reacts against water inertia is investigated herein. The main focus is the impact of inclining the PTO direction on the system performance. The study is based on a numerical model whose formulation is first derived in detail. Hydrodynamics coefficients are obtained using the linear boundary element method package WAMIT. Verification of the model is provided prior to its use for a PTO parametric study and a multi-objective optimization based on a multi-linear regression method. It is found that inclining the direction of the PTO at around 50° to the vertical is highly beneficial for the WEC performance in that it provides a high capture width ratio over a broad region of the wave period range.

Numerical Simulation of a Submersible Buoy Using a Wake Oscillator Model Calibrated for VIM

New discoveries of petroleum reservoirs in ultra deep-water depths, like Pre-salt fields in Santos Basin, are demanding new riser systems concepts. In this scenario, the Free-Standing Hybrid Riser (FSHR) system is a viable choice. A submersible buoy connected by rigid and flexible risers constitutes this riser system. The sea current can cause the Vortex-Induced Motion (VIM) of the buoy, which can increase significantly the riser fatigue damage. Although the VIM phenomenon is similar to Vortex-Induced Vibration (VIV), it generally occurs in rigid bodies with low aspect ratio, where end effects causes tridimensional flow behavior. Therefore, the vortex wake characteristics and the hydrodynamics coefficients found for VIV is no longer valid for VIM. In this context, wake oscillator models used for VIV prediction in actual form is not adequate for the VIM prediction of the buoys. In this paper, a VIV wake oscillator model is calibrated for VIM, through hydrodynamic coefficients found in the technical literature. In order to verify accuracy, the VIM calibrated wake oscillator model is used to reproduce some FSHR reduced model tests. The results of amplitude and frequency of oscillation against the reduced velocity obtained from the numerical simulation are compared with the experimental results. The numerical results presented the same trend with some differences in amplitude. The amplitude deviation could be related to the hydrodynamics coefficients used in the calibration of the wake oscillator model.

Numerical Simulation of VIM Response of a Submersible Buoy Using a Semi-Empirical Approach

Nowadays, discoveries of petroleum reservoirs are located in ultra deep-water depths. In this scenario, risers systems generally demand submersible buoys to support the riser, in order to reduce weight in floating platform and riser tensioners. Usually, these buoys are installed below 200 meters depth to avoid the wave forces. However, in this condition the sea current cause the Vortex-Induced Motion (VIM) of the buoys, which can increase significantly the riser fatigue damage. Although the VIM phenomenon is similar to Vortex-Induced Vibration (VIV), it generally occurs in rigid bodies with low aspect ratio, where end effects causes tridimensional flow behavior. Therefore, the vortex wake characteristics and the hydrodynamics coefficients found for VIV is no longer valid for VIM. It makes complex the prediction of VIM in buoys. In this paper, a semi-empirical model using hydrodynamic coefficients found for low aspect ratio cylinders are presented. In order to verify accuracy of numerical simulations, results are compared with experimental data presented in the literature and a good agreement is found.

Hydrodynamic Interaction of Floating Structure in Regular Waves

Floating structures play an important role for exploring the oil and gas from the sea. In loading and offloading, motion responses of offshore floating structures are affected through hydrodynamic interaction. Large motions between floating bodies would cause the damage of moorings, offloading system and may colloid to each other. This research studies on hydrodynamic interaction between Tension Leg Platform (TLP) and Semi-Submersible (Tender Assisted Drilling (TAD)) in regular and irregular waves with scenario as follows: fixed TLP and 6-DOF floating semi-submersible and 6-DOF both TLP and semi-submersible. Under these conditions, hydrodynamics coefficients, mooring and connectors forces, motions and relative motions of TLP and Semi-Submersible will be simulated numerically by using 3D source distribution method. As the scope is big, this paper only presents model experiment of floating TLP and semi-submersible in the regular wave. The experiment is carried out in the UTM Towing Tank.

Numerical Simulation of Hydrodynamics of a Circular Disk Oscillating Near a Seabed

This paper studies how the hydrodynamics coefficients of added mass and damping varies when an oscillating disk approaches a seabed. Analysis was performed by OpenFOAM code using the ‘PIMPLE’ algorithm. The simulations considered the flow as laminar and hence no turbulence model was used. Simulations were conducted for a solid disk of 200 mm diameter, 2 mm thick, oscillating at amplitudes varying from 1–48 mm and elevation ‘h’ of the disk from the seabed varying from 0.2–2 times the disk radius. The geometry and parameters used here were the same as that of Wadhwa et al. (2010) [1] and Vu et al. (2008) [2]. The forces on the disk were calculated using a Tool for post-processing force/lift/drag data with function tool available in OpenFOAM. The motions of the disk were restricted to axial (heave) direction. The calculated forces and displacement were analyzed using a Fourier projection to separate the added mass and damping effects. Numerical results were compared with the experiments conducted by Wadhwa et al. (2010) [1] with a sandy bottom. Results show that the added mass and damping increase monotonically with the Keulegan-Carpenter number (KC) up to a critical value, beyond which the behavior becomes random. The critical KC increases linearly with increasing distance from the seabed. The hydrodynamic problem has important applications in structures such as foundation templates and subsea structures oscillating in proximity to the seabed. The computations show vortex lines of the flow, and the influence of the seabed on the flow around the structure.

Coupled Transient CFD and Diffraction Modeling for Installation of Subsea Equipment/Structures in Splash Zone

In development of deep water oil and gas fields, successfully and economically installing subsea equipment and structure is critically important. This paper presents a state-of-the-art methodology for predicting the motions and loads of subsea equipment/structure during such operations basing on time domain simulations of the combined installation vessel and subsea equipment/structure. The time domain diffraction simulation of the moving lifting vessel is coupled with multiphase CFD simulation of subsea equipment/structure in splash zone. Transient CFD model with rigid body motion for the equipment/structure calculates added masses, forces and moments on the equipment/structure for diffraction analysis, while diffraction analysis calculates linear and angular velocities for CFD simulation. This paper has many potential applications, such as, installation of pile, manifold, subsea tree, PLET/PLEM, or other subsea equipment/structure. This coupled approach has been successfully implemented on a cylindrical structure. The results show that total load level, and dynamics of the subsea equipment/structure due to waves in splash zone are predicted. Current practice of installation analysis in accordance with the recommendations from DNV-RP-H103 [1] cannot determine in detail the wave loads either during the passage through splash zone, or added mass and damping when the equipment/structure is submerged. In order to determine wave loads in detail, model tests are needed. In the absence of tests, simplified equations or empirical formulations have to be used to calculate/estimate these hydrodynamics coefficients as recommended in DNV-RP-H103. Steady-state CFD simulations on a stationary equipment/structure are usually used to predict drag and added masses on submerged structures. However the steady-state assumption in CFD ignores the resonating motion of equipment/structure in calculating hydrodynamics coefficients, which can severely affect the accuracy of these predictions. The above methods often give overly conservative results for allowable sea state which results in uneconomical vessel time or inaccurate results for installation. The methodology of this paper gives more accurate results, and provides potentially economical vessel time during installation. The intent of this paper is to demonstrate the solution and methodology.