1,721,013 research outputs found
A model for predicting pipeline sinkage induced by tunnel scour
No full textThe current design practice for subsea pipeline on-bottom stability (e.g. DNV-RP-F109) does not account the effect of sediment transport around a pipeline. Both field survey and small scale model test results show that seabed scour has a significant effect on pipeline embedment and therefore stability. Physical model tests carried out in an innovative large experimental facility, named the O-tube, at the University of Western Australia, have shown that tunnel scour and the subsequent pipe sinkage into the scour hole tend to stabilize a pipeline which might otherwise become unstable on an assumed stationary seabed, under ramping-up flow conditions. A simple calculation model that incorporates the three-dimensional scour and pipe sinkage due to the soil bearing capacity failure at the supporting span shoulders is proposed. The model parameters were calibrated using the O-tube experimental results. The model serves as a key element of a new pipeline stability analysis method that takes into account seabed mobility.</p
A new facility for studying ocean-structure-seabed interactions: The O-tube
No full textThis paper describes a unique new physical testing facility for studying ocean-structure-seabed interactions, and in particular pipeline on-bottom stability on erodible seabed under hydrodynamic loading. The facility, named the O-tube due to its shape, is a fully enclosed flume in which ambient and storm-induced near-seabed flows are generated by a computer-controlled flow pump. Combined steady and oscillatory flow can be generated by alternating pump flow directions in a controlled manner, and computer control also allows irregular flow to be generated. The design of the O-tube combines the capabilities of a conventional open channel flume (which provides steady current) with a U-tube (which provides oscillatory flow). The facility is designed to physically model severe storm conditions, as well as ambient or tidal flows. When studying pipeline stability on erodible seabed under severe hydrodynamic loading conditions, tests can be performed at a relatively large scale (typically 1/5) for large diameter pipelines (e.g. 40. in. gas trunklines) and at full scale for small diameter pipelines (<. 8. in.) to minimize potential scaling effects associated with movable bed model tests. The specifications of the O-tube, a model pipe and an actuator system that supports the model pipe are given in detail. Preliminary model testing results show that the facility has met its design expectations.</p
Calibration of UWA’s O-tube flume facility
No full textThe O-tube facility, designed and established at the University of Western Australia, is an innovative closed loop flume in which a random storm sequence can be reproduced via control of a large pump system. The O-tube facility is capable of simulating hydrodynamic conditions near the seabed and the interaction with seabed sediment and any infrastructure that is resting on it.The purpose of carrying out the O-tube calibration described in this paper is to obtain the relationship between the motor rotation movement and the flow velocity generated in the O-tube, such that any required storm history within the performance envelope of the O-tube can be reproduced. A range of flow velocities and the corresponding pump speeds were measured under steady current, oscillatory flow and combined flow conditions. It was found that the relationship between the pump speed and the flow velocity varies with the oscillatory flow period. Based on the pump characteristic curves and O-tube system curves, the correlation between the motor speed and the flow velocity was derived by applying hydraulic theory and the principle of energy conservation.The derived correlation is validated by reproducing a wide range of target storm series, including a (1:5.8) scaled 100-year return period storm from the North West Shelf of Western Australia in 40 m water depth
Effect of wave boundary layer on hydrodynamic forces on small diameter pipelines
No full textEffect of wave boundary layer on hydrodynamic forces on small diameter pipelines is speculated in this paper. It is found that use of the recommended calculation methods in DNV-RP-F109 leads to unrealistic predictions of high specific gravity (SG) requirements for small diameter pipelines subject to wave loading. Potential reasons for such high SG requirements under wave loading are speculated based on existing knowledge and experimental evidence, and a way forward is proposed. It is suspected that the unrealistic predictions are because the calculation methods ignore wave boundary layer effects on hydrodynamic forces.</p
A re-examination of the hydrodynamic forces acting on partially-buried submarine pipelines
No full textThe hydrodynamic forces exerted on a pipeline partially buried in a permeable seabed subjected to combined random oscillatory flow are investigated experimentally. The tests were carried out in a re-circulating flume that can generate steady currents, oscillatory flow and combined flow, known as the O-tube. The performance of the testing facility was validated against published experimental data under regular oscillatory flow conditions. Four tests with different embedment depths, under random oscillatory flow conditions were then carried out. The flow velocity, hydrodynamic pressure around the model pipe and pore pressure in the soil were monitored. The hydrodynamic forces were calculated through pressure integration around the model pipe. The hydrodynamic force reduction due to embedment was compared with the model given in the most widely-used pipeline stability design guideline, DNV-RPF109. A horizontal load reduction of 55% was found for the fully buried pipe, which was lower than the 70% reduction suggested by DNV-RP-F109. The variation of the vertical hydrodynamic load with embedment ratio (embedment depth to pipe diameter) was also found to be different from the model suggested by DNV-RP-F109. It was found the fully buried pipe experienced a large vertical force, even higher than that of the fully exposed pipe, under low KC number conditions.</p
Stability of subsea pipelines during large storms
No full textpipeline stability, scour, offshore hydrodynamics Author for correspondence
Modelling changes to submarine pipeline embedment and stability due to pipeline scour
No full textIn this paper a beam bending model is combined with existing predictive formulas for pipeline scour to study changes to pipeline stability during scour and lowering. The model is introduced and demonstrated for a range of simplified conditions, including scour-induced lowering of a pipeline resulting from multiple uniformly spaced scour initiation points. The model is then used with a synthetic seabed generated with a variety of length scales. In this simulation the pipeline is 'laid' onto the seabed, leading to the formation of 'natural' initiation points for scour. The distribution and spacing of the initiation points (which are a function of the pipeline bending stiffness, tension and seabed roughness) lead to different rates of pipeline lowering and stability. The resulting model may be used within a probabilistic framework to estimate changes to pipeline stability resulting from sediment mobility and scour.</p
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Filter units for cable stabilisation: the effect of fill material and volumetric filling ratio on performance
No full textFiter units (sometimes called rock bags) are a very popular solution to improve the stability of subsea cables and cable protection systems (CPS) in the offshore wind industry. They are easily filled quay-side from local rock quarries and installed from vessels that are already mobilized as part of a windfarm construction spread. However, very limited data and research has been presented on the hydrodynamic properties and stability limit states for these mesh bags. No prior known literature describes the influence of filter material properties on their lateral resistance and stability, especially in the typical case where they are placed over a cable and are subjected to external loading from the cable or CPS. This paper presents results from a parametric study involving scaled physical model tests at scale factor of around 1:10. The tests explore the performance of the bags when filled with different materials to different levels of filling. The results show that bag material and filling level have strong influence on the filter unit and cable stability due to their strong influence on the filter bag weight distribution between the cable and the adjacent seabed. Empirical models for use in engineering design are proposed based on the experimental data.</p
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