Comprehensive Riser VIV Model Tests in Uniform and Sheared Flow

Despite of considerable research activity during the last decades considerable uncertainties still remain in prediction of Vortex Induced Vibrations (VIV) of risers. Model tests of risers subjected to current have been shown to be a useful method for investigation of the VIV behavior of risers with and without suppression devices. In order to get further insight on VIV of risers, an extensive hydrodynamic test program of riser models subjected to vortex-induced vibrations was undertaken during the winter 2010 by Shell Oil Company. The VIV-model test campaign was performed in the MARINTEK Offshore Basin Laboratory. A new test rig was constructed and showed to give good test conditions. Three different 38m long riser models were towed horizontally at different speeds, simulating uniform and linearly varying sheared current. Measurements were made In-Line (IL) and Cross-Flow (CF) of micro bending strains and accelerations along the risers. The test program compromised about 400 tests, which give a rich test material for further studies. In the present paper the test set-up and program are presented and selected results are reported

How typhoons trigger turbidity currents in submarine canyons

Intense turbidity currents occur in the Malaylay Submarine Canyon off the northern coast of Mindoro Island in the Philippines. They start in very shallow waters at the shelf break and reach deeper waters where a gas pipeline is located. The pipeline was displaced by a turbidity current in 2006 and its rock berm damaged by another 10 years later. Here we propose that they are triggered near the mouth of the Malaylay and Baco rivers by direct sediment resuspension in the shallow shelf and transport to the canyon heads by typhoon-induced waves and currents. We show these rivers are unlikely to generate hyperpycnal flows and trigger turbidity currents by themselves. Characteristic signatures of turbidity currents, in the form of bed shear stress obtained by numerical simulations, match observed erosion/deposition and rock berm damage patterns recorded by repeat bathymetric surveys before and after typhoon Nock-ten in December 2016. Our analysis predicts a larger turbidity current triggered by typhoon Durian in 2006; and reveals the reason for the lack of any significant turbidity current associated with typhoon Melor in December 2015. Key factors to assess turbidity current initiation are typhoon proximity, strength, and synchronicity of typhoon induced waves and currents. Using data from a 66-year hindcast we estimate a ~8-year return period of typhoons with capacity to trigger large turbidity currents

Sedimentation Management in Combined Sewer Overflow Storage Reservoirs Using Water Jets (HES 76)

Metropolitan Water Reclamation District of Greater Chicago and U.S. Army Corps of Engineers (Chicago District)unpublishednot peer reviewe

Turbidity Current Test 57 BL

<p>Bedload transport due to turbidity current.</p> <p>Sequeiros, O.E. ; Spinewine, B., Beaubouef, R.T., Sun, T., Garcia, M.H. and Parker G. 2010. Bedload transport and bed resistance associated with density and turbidity currents<em>.</em> Sedimentology, Vol. 57, no. 6, p. 1463-1490.</p> <p>Sequeiros OE, Spinewine B, Beaubouef RT, Sun T, García MH, Parker G. 2010. Characteristics of velocity and excess density profiles of saline underflows and turbidity currents flowing over a mobile bed. Journal of Hydraulic Engineering 136(7): 412-433.</p

Stratigrafia

The code in the workbook “Stratigrafia” computes * longitudinal profiles; * water surface elevation; * sediment transport rates; * time for the flow and the sediment transport to reach equilibrium in a water – feed laboratory flume.unpublishednot peer reviewedOpe

Modelling the air-sea-land interactions responsible for the direct trigger of turbidity currents by tropical cyclones

Tropical cyclones directly trigger turbidity currents in submarine canyons as a consequence of storm surges, high waves, onshore blowing winds and extreme currents. The resultant supply of sediment at the heads of the canyons plays a crucial role in the genesis of turbidity currents and thus is key in understanding frequency and duration of their flows. Here we present a single numerical framework capable of modelling turbidity currents driven by cyclone-induced winds and waves through resolved, quasi-3D hydrostatic equations. Our simulations predict the occurrence of a canyon-confined turbid underflow induced by a tropical cyclone that caused a seafloor pipeline to shift. Turbidity current occurrence is shown to be related not just to tropical cyclone intensities but also to their tracks with respect to the canyon head. The modelled underflow is well approximated by similarity profiles from laboratory and field observations which demonstrates the reliability of the model in capturing the structure of turbidity currents. The proposed triggering of turbidity currents off the centre of coastal embayments is likely to occur when the abrupt rotation of incoming winds induced by a passing cyclone remains always coastal-bound all across the cyclone's waxing and waning stages. Our results show that these conditions can give rise to simultaneous, opposite alongshore currents and eventually result in offshore-bound rip currents. Conversely, it is unlikely that turbidity currents will be triggered by cyclone -induced rip currents when the cyclonic rotation results in peak offshore winds (coming from the land), as no fetch is available for generating large breaking waves to induce simultaneous, opposite alongshore currents. Nevertheless, the sole presence of strong alongshore currents deflected at the headlands of a coastal embayment (or delta) is likely to trigger sediment-gravity flows and eventually result in turbidity currents offshore the edge of the embayment without the aid of rip currents

Typhoon-induced megarips as triggers of turbidity currents offshore tropical river deltas

Intense rip currents caused by tropical cyclones can drive sediment-laden turbidity flows down submarine canyons, according to numerical simulations. Shoreline shape, bathymetry and incoming wave direction are key factors controlling this phenomenon

In-Line VIV Based on Forced-Vibration Tests

Excitation and added mass functions determined from forced vibration tests of a rigid cylinder undergoing harmonic motion in the flow are used in the semi-empirical software VIVANA to predict the VIV response of pipelines. An advantage of this approach, as opposed to the more-commonly-used response function approach, is that it can account for changing conditions along the length of the pipe, like changing current velocity, seabed proximity, and/or pipe diameter. This makes it useful for pipelines as well as for risers when such changes occur. Further, for pipelines, travelling wave effects play less of a role than for risers, so the VIVANA approach can be simplified by assuming the phase angle of the harmonic response is constant along the span. The interactions between cross-flow and in-line response that complicate the prediction of cross-flow VIV by the excitation function approach, do not arise for pure inline VIV. For the latter case, using the pure in-line forced vibration test data of Aronsen (2007), it is found that both VIVANA approach and simplified ‘SIVANA’ approach thereof predict VIV amplitudes consistent with experiments on flexible pipe (Ormen Lange umbilical VIV tests), and the DNVGL-RP-F105 response function for a range of structural and soil damping values. In a companion paper, this approach is applied partially strake-covered pipeline spans, to show that a relatively small fraction of well-placed strake coverage is enough to suppress in-line VIV.acceptedVersio

Forced Vibration Tests for In-Line VIV to Assess Partially Strake-Covered Pipeline Spans

A series of experiments is performed in which a strake-covered rigid cylinder undergoes harmonic purely in-line motion while subject to a uniform “flow” created by towing the test rig along SINTEF Ocean’s towing tank. These tests are performed for a range of frequencies and amplitudes of the harmonic motion, to generate added-mass and excitation functions are derived from the in-phase and 90° out-of-phase components of the hydrodynamic force on the pipe, respectively. Using these excitation- and added-mass functions in VIVANA together with those from experiments on bare pipe by Aronsen (2007), the in-line VIV response of partially strake-covered pipeline spans is calculated. It is found that as little as 10% strake coverage at the optimal location effectively suppresses pure in-line VIV. Further advantages of strakes rather than intermediate supports to suppress in-line VIV include: strakes are not affected by the scour which can lower an intermediate support (in addition to creating the span in the first place). Further they do not prevent self-lowering of the pipeline or act as a point of concentration of VIV damage as the spans to each side of the intermediate support grow again.acceptedVersio

Experiments on wedge-shaped deep sea sedimentary deposits in minibasins and/or on channel levees emplaced by turbidity currents. Part I. Documentation of the flow

Decelerating turbidity currents commonly emplace sedimentary wedges. Here "sedimentary wedge" is used as a generic term for a sediment deposit, the thickness of which gradually decreases in the downdip direction. Examples of sedimentary wedges relevant to the research reported here include a) deposits in submarine minibasins, b) deposits on zones of lower slopes of stepped profiles, and c) deposits on the levees of submarine channels. In the present work, a generic configuration is used to study the flows that emplace sedimentary wedges. These flows consisted of a succession of sustained saline density underflows, which were used as surrogates for turbidity currents driven by fine-grained material (mud) that does not easily settle out. Although the flow naturally decelerated in the downstream direction, deceleration was ensured by the presence of a barrier to the flow at the downstream end of the study reach. The density underflows carried a load of lightweight plastic particles, from which the depositional wedge was constructed. The experiments were not designed to model any specific field configuration. This notwithstanding, the experimental configuration provides an analog for a) decelerating flows into confined minibasins, as well as b) levee-constructing overflows from submarine channels. This paper documents the nature of the flows that emplaced the wedge. The sedimentary wedge itself is documented in a companion paper