G.L. Kuiper

Experimental Investigation of the Dynamic Behaviour of a Water Intake Riser

This paper considers the dynamic stability of free-hanging water intake risers. Suspended from a barge, these risers convey a great volume of cooling water, which is needed for offshore liquefaction process of natural gas. There is a contradiction between theoretical predictions and experiments for cantilever pipes pumping up water. Reported small-scale experiments did not show any instability, while theory predicts instability beyond a critical fluid velocity. To investigate whether the previous experimental setups did not allow to observe the instability or the pipe aspirating water is unconditionally stable, a new test setup was built which could attain a higher internal fluid velocity than the predicted critical velocities. A cantilever pipe of about 5 m length was partly submerged in water. The experiments clearly showed that the cantilever pipe aspirating water becomes unstable by self-excited oscillatory motion (flutter) beyond a critical velocity of water convection through the pipe. Below this velocity the pipe is stable, whereas above it, the pipe shows a complex motion that consists of two alternating types of motion. The first type is a nearly periodic orbital motion with the amplitude of a few pipe diameters and the second one is a quasi-chaotic motion with very small amplitude. Translating these results to offshore water intake risers, shows that for realistic internal flow velocities the riser might become unstable.© 2007 ASME

Instability of a Simplified Model of a Free Hanging Riser Conveying Fluid

Studying stability of a vertically suspended, fully submerged pipe conveying water (free-hanging water intake riser), researchers have found that if a critical flow velocity is reached, these pipes may flutter. However, there are different predictions for the value of this critical velocity. Researchers have mentioned values changing from infinitely small fluid velocities up to velocities which are unachievable in practice. The nonlinear hydrodynamic damping caused by the surrounding water seems to be crucial for correct description of the stability of the submerged riser aspirating fluid. In this paper, using the Morison’s equation, the nonlinear drag is taken into account as a function of the relative velocity between the current and the velocity of the riser itself. The nonlinear system is studied employing the Galerkin method. Ten-mode discretization turns out to be necessary to obtain an accurate result. It is shown that the current has a strong stabilizing effect. If the internal fluid flow exceeds a critical velocity the riser performs self-sustained oscillations with the amplitude smaller than one diameter.Copyright

Ice Model Testing of Structures With a Downward Breaking Cone at the Waterline JIP: Presentation, Set-Up and Objectives

Two large ice model test campaigns were performed in the period 2007–2010 as a part of a Joint Industry Project. The objectives of the project were to investigate different floater geometries and ice model test set-ups (model fixed to a carriage and pushed through the ice vs. ice pushed towards a floating model moored to the basin bottom) and their influence on the ice failure mode and structure responses in the various tested ice conditions. This paper presents the objectives and motivations for the project, the models tested, the target test set-up for the various tested configurations and the test matrix. Initial results from a fixed model tested in three first-year ice ridges with similar target ice properties are also presented and compared. Fixed models of both deep and shallow water platforms were tested in various ice conditions. All models except one had a downward breaking cone at the waterline. The influences of the waterline diameter, the angle of the downward breaking cone and the vertical cone height on the ice failure mode and the resulting ice load were investigated. Tests were conducted in level ice with a thickness ranging from 2 to 3 m and variable ice drift speeds ranging from 0.1 to 1.0 m/s in full scale values. The models were subjected to tests in managed level ice with varying speeds, ice concentrations and ice floe sizes. Fixed structures were also subjected to testing in typical first-year design ice ridge conditions with ridges of different depths and widths, as well as one multi-year ice ridge. One fixed model was also utilised for testing of the repeatability of scaled ice model testing. Moored models with the same waterline geometry as the fixed models were also tested. The moored models were tested in ice conditions similar to those of the fixed models with the objective of comparing their influences on the ice load due to structural responses.Copyright

On the Dynamic Interaction Between Drifting Level Ice and Moored Downward Conical Structures: A Critical Assessment of the Applicability of a Beam Model for the Ice

Downward conical structures are believed to be an interesting concept of a floating host for oil and gas developments in deeper Arctic waters. The conical structure forces the ice to fail in bending, thereby limiting the ice loads on the structure. During the last two years, several conical structures were investigated at the Hamburg Ship Model Basin (HSVA) as part of a Joint Industry Project. This paper presents a numerical model for drifting level ice interacting with a moored downward conical structure. The goal of this development was to get insight in the key processes that are important for the interaction process between moving ice and a floating structure. The level ice is modelled as a moving Euler-Bernoulli beam, whereas the moored offshore structure is modelled as a damped mass-spring system. The ice-structure interaction process is divided into two phases. During the first phase, the ice sheet bends down due to interaction with the structure until a critical bending moment is reached at a cross-section of the beam. At this moment, the beam is assumed to fail at the critical cross-section in a perfectly brittle manner. During the second phase, a broken off block of ice is pushed further down the slope of the structure. These phases were built into one, piece-wise in time continuous model. A key result found by means of the numerical analysis of the model is that the motions of the moored floating structure do not significantly influence the bending failure process of level ice. Also the influence of the in-plane deformation and the heterogeneity of ice on the bending failure process is negligible. As a consequence, the dynamic response of the structure is for the biggest part determined by the ice failure process. Although the response of the structure can be dynamically amplified due to resonance for some particular ice velocities, no frequency locking of the ice failure onto one of the natural frequencies of the structure was observed. The model output showed qualitative agreement with the HSVA test results. It was however concluded that one-dimensional beamlike models of level ice sheets cannot accurately predict loading frequencies on downward conical moored floating structures because the ice blocks that break off are too long. Modelling level ice in two dimensions using plate theory is expected to give better results, since it takes into account the curvature of a structure and both radial and circumferential ice failure.Copyright