A Physical-Based Damping Model of Gap and Moonpool Resonance in WAMIT

An engineering model to estimate and incorporate quadratic damping of the piston-mode moonpool responses in the proximity of the piston mode period is proposed. The model provides a physical-based equivalent linearized damping coefficient. The method is not limited to forced motion, but applicable to freely floating moonpool vessels. Further, it is not limited to moonopools, but can be generalized to gap resonance problems, such as side-by-side operations. The soundness of the proposed physical-based method is demonstrated using the panel code WAMIT with a linear damping term in the free-surface boundary condition inside the moonpool using two existing moonpool experiments as case studies; (1) a two-dimensional rectangular box with a moonpool subject to forced heave, and (2) a freely floating offshore vessel in incident waves. The WAMIT computations using the proposed method reconstructs the experimentally obtained piston-mode and vessel responses well. We suggest that the proposed method can be used with fair degree of confidence in an early design or operational analysis phase, in the (often) case that the quadratic damping is not known from either experiments or CFD. To our knowledge, this is the first general, physical-based piston-mode damping model that does not require any tuning from experiments.acceptedVersio

Sloshing-induced motions of a spar inside a cylindrical dock with baffles in waves

The motions of a free floating offshore wind turbine’s (OWT) spar-type platform inside a bottom-less moored cylindrical dock are investigated for incident wave frequencies near the first lateral sloshing resonance, focusing on surge on pitch motions. The radiation and diffraction problems of the two-body system are first solved through a domain decomposition (DD) approach under linear potential flow assumptions. This semi-analytical model is extended to include the effects of solid and perforated baffles in the annular domain between the dock and the spar, adapting the method developed in our previous paper for the dock alone. Results are compared with those obtained with model tests, performed at scale 1:100 for both regular waves with low steepnesses and irregular sea states. The resonant peak amplitudes of the spar’s surge and pitch motions are reduced by almost half when a solid baffle is installed, with a strong dependency on the incident wave height due to viscous dissipation caused by the flow separation at the sharp edge of the baffle.publishedVersio

An upright bottomless vertical cylinder with baffles floating in waves

Damping of the surge and pitch motions, as well as the first lateral sloshing mode in a rigid free-floating upright circular dock with bilge boxes and open bottom is investigated. Model tests are carried out on a 0.80 m diameter model in regular waves with wave periods near the highest natural sloshing period, and the internal free-surface elevation and model’s rigid body motions are measured. Perforated and solid annular baffles of relatively small widths are also installed inside the dock at various submergences. The experimental results are compared to a semi-analytical approach, where a three-dimensional domain decomposition method based on linear potential flow theory is adopted to calculate the hydrodynamic coefficients and exciting forces in heave, surge and pitch. A reduced natural sloshing frequency, as well as a damping ratio estimated from the energy dissipated due to flow separation from the baffles, are introduced in the free-surface boundary conditions to model the effects of the baffle. It shows good agreement with experimental data when the ratio between the draft of the baffle and the internal radius of the cylinder is dB/α = 0.27, and tends to under-predict the damping ratio for shallower drafts, most likely due to free-surface interactions. The solid baffle damps the sloshing response most efficiently, reducing the amplitude at the resonant peak by more than 56%.publishedVersio

Operability analysis of control system for ROV launch-and-recovery from autonomous surface vessel

The launch and recovery of equipment such as remotely operated vehicles (ROVs) is a critical task that defines the operability limits of many marine operations. This paper considers the analysis of control systems that are designed to maximize the operability limits for launch and recovery of a ROV from a small unmanned surface vessel (USV). We use numerical simulation for the analysis, where the method combines recent approaches for wave compensating dynamic positioning, active heave compensation, and positioning control of the ROV with multi-body dynamic simulation of the surface vessel and ROV, including hydrodynamic forces and dynamic interactions from wires that depend on the ROV depth and moonpool. The results show that the choice of control algorithms and their tuning parameters has a significant effect on the system’s operability, and should be carefully designed and tuned to optimize the operability limits for any given sea state, weather and operational setup. The results show that numerical analysis with a system’s simulation is an effective tool to verify operability and its sensitivity to various parameters for the given ROV recovery application.publishedVersio

On Common Research Needs for the Next Generation of Floating Support Structures

The world is facing several industrial and societal challenges, such as providing enough renewable energy and enough safe and healthy food as formulated in the United Nations sustainable development goals. Using floating stationary structures, the ocean can contribute to solving several of the challenges. New applications need new types of structures, with which we have limited experience. These support structures will be diverse, but also have essential research needs in common. Design of novel floating structures need reliable descriptions of the marine environment. This is particularly challenging for semi-sheltered coastal regions, with complex topography and bathymetry. Novel structures are likely to be compliant, modular and/or multi-body, requiring increased understanding and rational models for wave-structure interaction. Structures with sustainable, safe, and cost-efficient use of materials, including untraditional ones, must be developed. Smart, affordable, and reliable mooring systems and anchors for novel applications are necessary for station keeping. Digital solutions connecting the various stages of design and operation, as well as various design disciplines, researchers, and innovators, will be necessary. Sustainability will be an integral part of any new design. To unlock the potential of novel floating structures, we need to understand the requirements of the applications, as well as the associated technology gaps and knowledge and research needs. This paper highlights research needs for innovation within floating offshore wind, floating solar power plants, novel aquaculture structures, and coastal infrastructure.acceptedVersio

Potential-Flow Predictions of a Semi-Displacement Vessel Including Applications to Calm-Water Broaching

As a marine vehicle’s operational speed increases, hydrodynamic pressure plays an increasingly significant role in carrying the vessel’s weight. The shift of importance from hydrostatic to hydrodynamic pressure may cause a vessel, which is stable at rest or low speeds, to become dynamically unstable at high speeds. The nature of the dynamic instability depends mainly on the vessel’s type and speed. Among high speed vessels, semi-displacement mono-hulls are particularly susceptible to a nonoscillatory dynamic instability in sway, roll and yaw known as calm water broaching. This type of dynamic instability is the main reason why semi-displacement monohulls must not operate at Froude numbers higher than 1.2 (Lavis, 1980). The application of linear theory in predicting this type of instability motivated the present study. A linear dynamic model of the vessel, including its hydrodynamic coefficients, is needed for the dynamic stability analysis. Prediction of these coefficients is a challenging problem, requiring the solution of the flow around an advancing, oscillating vessel. A three-dimensional boundary element solver was constructed for this purpose. The free-surface and body boundary conditions were linearized using Neumann-Kelvin linearization. Since the focus here is on high speed vessels, this type of linearization is chosen instead of the double-body linearization. Boundary surfaces were discretized using numerical grid generation methods. Elements ranging from constant to cubic were used to represent the surfaces. Rankine sources and dipoles were distributed on the boundaries. The solver was programmed in a way to allow for the implementation of different boundary integral formulations using elements with different distribution orders in a convenient and compact form. The derivatives of the velocity potential on the body surface were calculated using shape functions. On the free surface, the direction of differentiation is known to be important, especially in the flow around high-speed vessels. Therefore, derivatives on the free surface were calculated using upstream finite difference operators to satisfy the radiation condition and avoid numerical instabilities. Semi-discrete Fourier analysis was used to investigate numerical dispersion and damping of a wave traveling on a discrete free surface with and without current. Different singularity distribution and differentiation methods were considered. Based on these studies, a set of practical guidelines were established to choose suitable differentiation methods and an appropriate number of elements for each problem, and assess the numerical accuracy of the results. Damping zones were introduced around the free surface boundaries in order to absorb the waves and ensure that the radiation condition was satisfied in the time-domain analysis. Three types of flow separation were accounted for indirectly in the present potential-flow solution: trailing edge flow separation by a vortex sheet method, transom stern flow separation by a hollow body model, and cross flow separation by a 2D+t drag model. A series of problems for non-separated potential flows with and without forward speed were solved both in the time-domain and steady-state. The results were validated against experimental and analytical data. The flow around an advancing, surface-piercing flat plate with steady drift was investigated using steady-state and time-domain solvers as an example of the tailseparated flows. A 2D+t cross-flow drag model was adopted in order to consider the cross-flow separation effects, which turned out to be important. Then, the problem was extended by adding oscillatory motions to the surface-piercing plate. The hydrodynamic coefficients in sway, roll and yaw were calculated for a series of Froude numbers and oscillation frequencies. The results were validated against existing experimental and numerical data. Next, the flow around monohull semi-displacement vessels was studied using linear theory. The dry transom stern effects were captured by introducing a hollow body model. The results were validated against experimental and numerical data in terms of free-surface elevation and steady vertical forces. The hollow body model was extended to solve the flow around an advancing semi-displacement vessel with constant drift angle. A simplified 2D+t cross-flow drag model explained the differences between numerical and experimental data. The hydrodynamic coefficients in heave were calculated using the extended hollow body model. This method captured the anticipated sharp drop in the values of added mass and damping close to the transom stern. Finally, dynamic stability in sway-yaw and sway-roll-yaw was investigated using linear stability analysis. A semi-displacement vessel with documented instability issues was chosen for validation. The hydrodynamic properties of the vessel were simplified to those of a flat plate. A sensitivity study was carried out to assess the importance of different parameters in the vessel’s dynamic stability. This simplified analysis predicted the presence of an instability around the reported unstable Froude number. The nature of the instability was, however, different than what has been reported in the literature. Further investigations are needed on this subject

Waves' numerical dispersion and damping due to discrete dispersion relation

In linear Rankine panel method, the discrete linear dispersion relation is solved on a discrete free-surface to capture the free-surface waves generated due to wave-body interactions. Discretization introduces numerical damping and dispersion, which depend on the discretization order and the chosen methods for differentiation in time and space. The numerical properties of a linear Rankine panel method, based on a direct boundary integral formulation, for capturing two and three dimensional free-surface waves were studied. Different discretization orders and differentiation methods were considered, focusing on the linear distribution and finite difference schemes. The possible sources for numerical instabilities were addressed. A series of cases with and without forward speed was selected, and numerical investigations are presented. For the waves in three dimensions, the influence of the panels’ aspect ratio and the waves’ angle were considered. It has been shown that using the cancellation effects of different differentiation schemes the accuracy of the numerical method could be improved.acceptedVersio

Investigating a Simplified Model for Moonpool Piston Mode Response in Irregular Waves

Estimating moonpool piston mode response at resonance is important for operation safety. This is a difficult task, in particular, due to nonlinear nature of the moonpool response connected to the damping imposed by the flow separation at the moonpool’s inlet. In the present work, the applicability of a simplified model, based on decomposing the problem into potential and viscous components, is investigated. The moonpool piston mode response is modeled as an additional degree of freedom. The coupling terms between this new degree of freedom and other vessel’s modes of motions are calculated based on potential flow calculations. Radiation and diffraction problems are considered separately. A finite volume solver with linearized boundary conditions is used to obtain the moonpool response under forced vertical motions. A quadratic damping model is fitted to the obtained responses and added to the free-surface condition of the potential flow formulation. The problem is solved both in frequency and time-domain. The validity of the obtained model is investigated by model test comparison for a dummy vessel with moonpool undergoing regular and irregular forced oscillations, as well as an offshore operation vessel with moonpool exposed to irregular waves. The benefits and shortcomings of the model are discussed. It is suggested that this method can be used as a practical tool to address moonpool piston mode response in irregular waves.acceptedVersio

Simulation of Low Frequency Motions in Severe Seastates Accounting for Wave-Current Interaction Effects

Today’s industry practice assumes wave drift forces on floating structures can be computed from zero current wave drift force coefficients for the stationary floater, while simplified correction models introduce current effects and slow drift velocity effects. The paper presents an alternative approach which overcomes some of the limitations of today’s procedures. The method, to be applied together with a time domain solution of the low frequency motions, is based on pre-calculation of mean wave drift force coefficients for a range of current velocities. During the low frequency motions simulation, the wave drift forces induced by the irregular waves are computed from the mean drift coefficients corresponding to instantaneous relative velocity resulting from the current and the low frequency velocities. A simple interpolation model, based on a quasi-steady assumption, is applied to obtain the drift forces in time-domain. Since calculation of the wave drift forces on Semi-submersibles in severe sea states with fully consistent methods is out of reach, a semi-empirical model is applied to correct the potential flow wave drift force coefficients. This model takes into account viscous effects, that are important in high seastates, and wave-current interaction effects. The paper compares the wave drift forces and the related low frequency motions computed by the proposed method, with results applying “standard” methods and with model test data. The test data was obtained in the scope of the EXWAVE JIP, with model tests designed to investigate wave drift forces in severe seastates and assess the wave-current interaction effects.acceptedVersio