Trygve Kristiansen

Studies on Resonant Water Motion Between a Ship and a Fixed Terminal in Shallow Water

This work focuses on the hydrodynamical problem of a Liquid Natural Gas (LNG) carrier near a Gravity Based Structure (GBS) -type offshore terminal subject to incoming waves in medium deep to shallow water conditions. The work is restricted to 2D, and the ship is restrained from moving. The resonant behavior of the fluid in the gap between the ship and the terminal is investigated. The problem is investigated by means of a numerical model and model tests. Potential theory is assumed, and a linear as well as a nonlinear time-domain numerical wavetank based on a boundary element method with a mixed Eulerian-Lagrangian approach is implemented for this purpose. Model tests (near 2D) of a midship section near a vertical wall are carried out in a 26.5 m long and 0.595 m wide wave flume in model scale 1:70. In full scale the ship beam is 45 m and the ship draft is 12 m. The ship model is constructed in such a way as to avoid flow separation, i.e., no sharp corners. Several parameters are varied: water depth, wave period, and wave steepness. Wave elevation is measured at 12 locations.

Wave-induced response of a floating two-dimensional body with a moonpool

Regular wave-induced behaviour of a floating stationary two-dimensional body with a moonpool is studied. The focus is on resonant piston-mode motion in the moonpool and rigid-body motions. Dedicated two-dimensional experiments have been performed. Two numerical hybrid methods, which have previously been applied to related problems, are further developed. Both numerical methods couple potential and viscous flow. The semi-nonlinear hybrid method uses linear free-surface and body-boundary conditions. The other one uses fully nonlinear free-surface and body-boundary conditions. The harmonic polynomial cell method solves the Laplace equation in the potential flow domain, while the finite volume method solves the Navier–Stokes equations in the viscous flow domain near the body. Results from the two codes are compared with the experimental data. The nonlinear hybrid method compares well with the data, while certain discrepancies are observed for the semi-nonlinear method. In particular, the roll motion is over-predicted by the semi-nonlinear hybrid method. Error sources in the semi-nonlinear hybrid method are discussed. The moonpool strongly affects heave motions in a frequency range around the piston-mode resonance frequency of the moonpool. No resonant water motions occur in the moonpool at the piston-mode resonance frequency. Instead large moonpool motions occur at a heave natural frequency associated with small damping near the piston-mode resonance frequency.

Kinematics in a Diffracted Wave Field: Particle Image Velocimetry (PIV) and Numerical Models

As the use of CFD in industrial applications increases, so does the need for verification and validation of the theoretical/numerical results. This paper focuses on tools for validation and in particular, on the use of Particle Imaging Velocimetry (PIV) as such a tool. Diffraction of regular waves due to a single, fixed vertical cylinder is investigated. Theoretical results of wave run-up and wave kinematics are compared to measurements from model tests. Theoretical results are obtained by second order potential theory and by fully non-linear CFD computations. The second order potential theory frequency-domain results are computed by the industry standard code WAMIT, while the fully nonlinear time-domain simulations are performed by the commercial CFD code Flow-3D. Measurements are obtained by means of wave probes, PIV and snapshots taken with a highspeed camera. The experiments are made with the model in place as well as without the model, for validation of the incident flow field. For the identification of non-linear effects, the steepness of the waves is varied. The surface elevation is measured by means of the wave probes, while the PIV equipment measures the kinematics. High quality photos taken by the high-speed camera give a detailed overview of the surface elevation for inspection. In addition to focusing on validation tools, the paper also addresses some critical aspects associated with the CFD computations, such as the modeling of boundary conditions.

Validation of a Hybrid Code Combining Potential and Viscous Flow With Application to 3D Moonpool

When operating with a moonpool, a main concern is the large-amplitude piston-mode motion at resonance. This limits the time-window for operations inside the moonpool. Longer time-windows are desired. Further, the moonpool size is expected to increase for dedicated vessels. There has therefore recently been an increased attention to moonpool design. Potential theory highly over-predicts the water motion at moonpool resonance, and may not be used for analyzing moonpool. Viscous damping has been shown to be important, and hence vital for the moonpool functionality. We present new numerical results with a hybrid method that combines potential and viscous flow. The simulations are done with a newly implemented code called PVC3D (Potential Viscous Code). The free-surface motion is governed by potential theory, while a Navier-Stokes solver provides the solution in the main bulk of the water. With the presently considered set-up with simple geometries, the computational time remains similar to that of pure potential flow time-domain solvers, while the important flow separation that provides viscous damping is captured. The application is to a 3D moonpool set-up. The inlet of the moonpool has sharp corners, and viscous damping is significant. Good agreement with experiments is demonstrated.Copyright

Interaction and Clashing Between Bare or Straked Risers: Analyses of Experimental Data

Hydrodynamic interaction and clashing of two long flexible cylinders in uniform steady current have been studied. Model tests have been performed in the towing tank at Marintek in Trondheim. The riser models were free to move in vortex induced vibrations (VIV) and wake induced vibrations (WIO). Model tests were performed both for straked and bare risers, and for risers with and without bumper elements attached. The model tests where presented in an earlier paper. This paper will briefly present the tests while the primary objective of the present paper is to describe advanced analysis and results. The emphasis in the analysis of the recorded data has been to assess the spatial distribution of clashing, the relative impact velocities, WIO and VIV.Copyright

Experimental Investigation of Dual Riser Interaction

Hydrodynamic interaction and clashing of two long flexible cylinders in uniform steady current have been studied. Model tests have been performed in the towing tank at Marintek in Trondheim. The riser models were free to move in vortex induced vibrations (VIV) and wake induced vibrations (WIO). The main objectives of the experimental campaign were to acquire high quality data that can be used to enhance the understanding of the mechanisms that drives riser clashing and to provide benchmark data for riser clashing code validation. Model tests were performed both for straked and naked risers, and for risers with and without bumper elements attached. Riser spacing, inflow angle, riser pretension and current velocity were systematically varied. The analysis of the recorded data has emphasized on assessing the spatial distribution of clashing, the relative impact velocities, WIO and VIV.Copyright

Experimental and Numerical Investigation of Ship Roll Damping With and Without Bilge Keels

This paper describes an experimental and numerical study on the effect of bilge keels on a specific offshore vessel. Forced roll and free decay experiments were carried out at MARINTEK in a joint project between VARD, DNV GL and MARINTEK during Nov. 2012. The 1:25 ship model was tested both with and without bilge keels. 19 forced roll periods were tested at 4 roll angles (2, 4, 6 and 10deg). Free decay tests were performed for initial roll angles 2, 4, 6, 10 and 15deg. Damping was extracted and compared with numerical simulations from a potential flow strip theory program (VERES) and a commercial CFD tool (StarCCM+).Both qualitative and quantitative agreement between the experimentally and numerically obtained damping parameters was documented. Discrepancies were identified and discussed. The three different methods of investigation gave together a basis for discussing the effect of bilge keels. From the present study, it is recommended that both experimental and numerical work in combination, for both forced roll and free decay tests, should be performed to assess the damping efficiency of bilge keels.Copyright

Resonant Water Motion Between a Ship and a Terminal in Shallow Water

This work focus on the hydrodynamical problem of an LNG carrier near a GBS-type offshore terminal subject to incoming waves in medium deep to shallow water conditions. The work is restricted to 2D and the ship is restrained from moving. The resonant behaviour of the fluid in the gap between the ship and the terminal is investigated. The problem is investigated by means of a numerical model and model tests. Potential theory is assumed and a linear as well as a nonlinear time-domain numerical wavetank based on a boundary element method with a Mixed Eularian-Lagrangian approach is implemented for this purpose. Model tests (near 2D) of a mid-ship section near a vertical wall is carried out in a 26.5m long and 0.595m wide wave flume in model scale 1:70. In full scale the ship beam is 45m and the ship draft 12m. The ship model is constructed in such a way as to avoid flow separation, i.e. no sharp corners. Several parameters are varied: Water depth, wave period and wave steepness. Wave elevation is measured at twelve locations.Copyright

Bilge Keel Induced Roll Damping of an FPSO With Sponsons

The bilge keel induced roll damping of an FPSO with sponsons is investigated numerically and experimentally. The influence of the bilge keel size, on the roll damping is studied. Free decay tests of a three-dimensional ship model, for three different bilge keel sizes are used to determine roll damping coefficients. The dependency of the quadratic roll damping coefficient to the bilge keel height and the vertical location of the rotation center is studied using CFD. A Navier-Stokes solver based on the Finite Volume Method is adopted for solving the laminar flow of incompressible water around a section of the FPSO undergoing forced roll oscillations in two-dimensions. The free-surface condition is linearized by neglecting the nonlinear free-surface terms and the influence of viscous stresses in the free surface zone, while the body-boundary condition is exact. An averaged center of rotation is estimated by comparing the results of the numerical calculations and the free decay tests. The obtained twodimensional damping coefficients are extrapolated to 3D by use of strip theory argumentations and compared with the experimental results. It is shown that this simplified approach can be used for evaluating the bilge keel induced roll damping with efficiency, considering unconventional ship shapes and free-surface proximity effects.

A Numerical Parameter Study on Current Forces on Circular Aquaculture Net Cages

The focus in this paper is on current forces on circular aquaculture nets, and how they vary with different parameters. The main parameters are current velocity and profile, solidity ratio of the net, sinker weights and net depth. The drag force on the net is calculated by a newly developed screen type force model. A subset of the calculations are validated by recent experimental data. The presented results are meant to represent a basis for discussing how current loads on a net cage vary with different environmental and strucural parameters. Scaling effects are discussed.Copyright