Rolf Baarholm

Model Testing of Ultra-Deepwater Floater Systems: Truncation and Software Verification Methodology

Model tests for global design verification of floating production systems in depths beyond 1000m–1500m cannot be made directly at reasonable scales. Truncation of mooring line and riser models, software calibration, as well as extrapolation and transformation to full depth and full scale, are required. Here, the first two of the above three items are addressed. The paper emphasizes the important matters to be taken into account. The choice of proper procedures for the set-up and the interpretation, and consistent and well documented methods, are essential. A case study with a deep-water semisubmersible is presented. In general, good agreement between model test results and analytical results from time-domain coupled analysis of the floater system responses is found.Copyright

Vortex-Induced Vibration of Steel Catenary Riser Under Vessel Motion

A truncated steel catenary riser (SCR) model was experimentally tested in the ocean basin by oscillating the top end of the model to simulate the heave and surge vessel motion in order to investigate the vortex-induced vibration (VIV) features. Out-of-plane VIV responses were generally analyzed revealing that although the root mean square (RMS) strain distributed rather broadband, the displacement was quite consistent within a narrowband from 0.2D to 0.3D, and the touch-down point (TDP) area was found not to be the place suffering the maximum out-of-plane VIV response due to near wall effects. What’s more, strong wave propagations were firstly reported and summarized as a distinguished feature for VIV of a SCR under vessel motions, and further results reveal that wave propagation during the ‘lift up’ phase was quite different from that during ‘push down’ in terms of both wave speed and ‘power-in’ region location which is assumed to be caused by the tension variation along the model.Copyright

VIV of Flexible Cylinder in Oscillatory Flow

VIV in oscillatory flow is experimentally investigated in the ocean basin. The flexible test cylinder was forced to harmonically oscillate in various combinations of amplitude and period. VIV responses at cross flow direction are investigated using modal decomposition and wavelet transformation. The results show that VIV in oscillatory flow is quite different from that in steady flow; novel features such as ‘intermittent VIV’, amplitude modulation, mode transition are observed. Moreover, a VIV developing process including “Building-Up”, “Lock-In” and “Dying-Out” in oscillatory flow, is further proposed and analyzed.© 2013 ASME

Wave-in-Deck Impact: Comparing CFD, Simple Methods, and Model Tests

The slamming of waves on the lower deck of large volume offshore platforms has received increased attention over recent years. For many existing platforms, the problem of insufficient air-gap clearance has become more acute of late due to more extreme weather conditions than was used in their original design basis and/or due to issues such as subsidence of gravity based structures. To investigate this problem, MARINTEK’s Wave Impact Loads JIP has, in one of its sub-tasks, focused towards an idealised model test setup of a rectangular block in regular waves. The block is fixed at a distance h above the calm water line. Both 2D and 3D model test experiments of the block in regular waves have been carried out in Phase 1 of the JIP (2008). This paper considers results from the 2D model test setup, and compares the measured vertical loading on the deck against two simple potential theory based methods (Baarholm, OMAE 2009-79560) and against results from a CFD code (STAR-CCM+). The results demonstrate that a second impact event closely following a first impact event can have a much flatter free-surface profile (and stronger water entry force) as a result of its interaction with the (deck) diffracted wave from the first impact event. The importance of resolving this diffracted wave in the CFD analysis is demonstrated. The paper concludes that for isolated impact events the simple potential flow based models, which do not consider the influence of one impact event on another, are adequate to predict the vertical loading on the deck. However, from a design basis criteria, if there is the strong likelihood of steep wave groupings resulting in closely following wave-in-deck impact events, then the presented simple methods may be non-conservative, and a CFD (Computational Fluid Dynamics) analysis or model test may be advisable to predict the vertical wave-in-deck loading. However the horizontal loading was significantly under-predicted in the CFD analysis compared to the measurements, so more work still needs to be done in this respect.Copyright

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.

Suppressing Full-Scale Riser VIV With the VT Suppressor

The paper reports on recent full scale experimental tests with The Ventilated Trouser (VT), a novel device for VIV suppression of cylindrical structures exposed to external fluid flow. The VT suppressor is a loose fitting sleeve in the form of a light flexible net with integral bobbins in a special arrangement (Fig 1). It is omni-directional, rugged, and made from materials compatible with the offshore environment.The tests reported here, originated in an invitation from Statoil to test the VT on a slick riser section. They were undertaken at Marintek, Norway, with a 0.53m diameter riser in current velocities up to 2.3m/s, equivalent to post-Critical Reynolds Numbers of up to 1.2 × 106.The VT suppressed the maximum VIV amplitude of the slick joint by over 90%. This was consistent with the suppression performance of the VT from previous tests with model risers varying in size from 0.1m diameter to 0.3m diameter.The test results suggest the VT is a candidate suppressor fully capable of competing with conventional suppression devices.Copyright

Ultra-Deepwater Model Testing of a Semisubmersible and Hybrid Verification

Model test verification of floater systems in ultra-deep water meets limitations when it comes to available laboratory sizes. Systems in depths beyond 1000–1500 m cannot be tested at reasonable scales without the truncation of the mooring and riser system. The development of methods and procedures to overcome this problem has been addressed through extensive research programs at MARINTEK (VERIDEEP, VERIDEEP Extension, NDP, DEMO2000). This led to a hybrid verification procedure which combines reasonable truncation principles, model tests of the truncated system, and numerical simulations, to estimate the system’s response at full depth. There is, however, still a need to address the actual influence from the truncation procedure, and from the integration with simulations, on the final extrapolated full depth results. This paper presents a case study for the validation of the procedure, that compares full depth model test results of a semisubmersible in water depth 1250m against the extrapolated full depth results obtained from a truncated system of 500m. Results are presented for line tension and vessel responses in 3 seastates. In general the extrapolated full depth results were found to be in good agreement with the full depth model tests. However, the results confirmed expectation that the low frequency response has the greater uncertainties and presents the greatest challenge for the procedure.Copyright

Effect of Marine Growth on an Elastically Mounted Circular Cylinder

The Riser and Mooring project of the Norwegian Deepwater Programme (NDP) has undertaken a parametric study on the influence of marine growth on the effectiveness of helical strakes to suppress vortex-induced vibrations on circular cylinders. Helical strakes are one of the most commonly used devices to suppress vortex-induced vibrations (VIV) on marine risers. The aim is provide guidance on the effect marine growth has on the dynamic response of a riser fitted with such VIV suppression devices. The tests were conducted at MARINTEK in a controlled laboratory environment. Artificial marine growth was modelled, manufactured and tested. Both hard and soft marine growth of various heights and coverages were tested. The present paper discusses some results obtained in this test campaign.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