Tc 67/sc 4 n date: 2005-03-9 iso/wd XXXXXX ISO tc 67/sc 4/wg 6 Secretariat: Design of dynamic risers for offshore production systems Élément introductif — Élément central — Élément complémentaire Warning
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- Loading types and flow conditions
- Oscillatory flow due to waves and vessel motions
CurrentFor a specified sea state, the associated current velocity profile should be used in the riser analysis. In some areas, the current may include contributions from areawise phenomena such as loop/eddy current and solitons, tide and circulation of ocean currents.8,9 In general, wind generated current is found in the top layer of the ocean. It can be represented by a shear or slab profile; whereas the loop/eddy current and solitons penetrate deeply in the water column. The corresponding velocity profile does not change rapidly with time. Because of this, the velocity profile may be treated as time invariant for each seastate in riser analysis. Some current profiles are shown in Figure 33. In time-domain analysis, current profiles should be stretched to the free surface using one of the approaches given in API RP2A. Loading types and flow conditionsHydrodynamic forces on risers can be classified by three basic categories: a) inertia forces due to the acceleration of the fluid and/or the riser body, b) forces induced by flow separation and vortex formation; c) skin friction on the surface of the riser (Forces in this category are of a higher order. In general, they are combined with the drag force in category b); Force vectors in category b can be decomposed into two components, a drag force in line with the incident flow vector and a lift force orthogonal to the incident flow. The drag or lift force may be further expressed by two terms. One represents the component which is a function of the square of incident flow velocity, and the other represents the resultant of pressure fluctuation due to vortex shedding. These two terms represent the loadings in two different frequency ranges. A brief summary of the flow conditions is given in the following: Oscillatory flow due to waves and vessel motionsCondition 1. Stationary vertical riser in wavesThis flow condition is characterized by unsteady flow in the wave field. The fluid particles follow cyclic orbital movements that decay exponentially with depth. The flow surrounding the riser is three dimensional. Since the wake reverses its position relative to the riser in each motion cycle, continuous vortex shedding at a nominal frequency is unlikely but possible. Accordingly, the fluctuating lift and drag forces induced by vortex shedding are often ignored in common practice, unless there is potential for lock-on. Referring to the definition in the nomenclature located at the end of Section 6, the fluid force per unit length is defined by the following two components: Inertial Force  ...(15)
Drag Force  ...(16)
Since the flow incident to the riser may be contaminated by the reversal of the wake from the previous motion cycles, the fluid forces may demonstrate a hysteretic phenomenon. Scattered data is anticipated in model test experiments and field measurements if the inertia and drag coefficients are computed on a wave-by-wave basis. In this regard, it is appropriate to determine the inertia and drag coefficients based on the least-squared fit of the whole force measurement record. 10,11 The coefficients Cd and CM are governed by the geometry of the riser cross section, the Keulegan-Carpenter number, KC, the Reynolds number, Re.12, and the surface roughness. Typically, the value of CM is in the range of 1.5 to 2.0 for a smooth circular cylinder. The value of Cd in the post-critical steady-state is about 0.6 to 0.7. Directory: standards standards -> Request for proposal contract form for the transit industry standards -> Indiana Academic Standards Resource Guide Grade 8 United States History – Growth and Development (to 1877) Updated April 2016 standards -> Us history Dictionary Based on the sc state Standards standards -> Integrating Standards Education into the Business School Curriculum standards -> Use of Sprint U301 3G/4g mobile Broadband usb device Installation standards -> International organisation for standardisation organisation internationale de normalisation standards -> Standards of Excellence standards -> Common Core State Standards for Mathematics (ccssm) Download 2.34 Mb. Share with your friends:
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