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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- Wave frequency motion response
- Low frequency motion response
- Combination of motion components
- Tensioner modeling
- Flex joints
- Stress joints
- Solution methods
Riser boundary conditionsTop endVessel motionsVessel motions are required to perform uncoupled dynamic riser analysis. This type of analysis is valid for both column stabilized and monohull vessels. Vessel motions important for riser design are: wave-frequency motions; low-frequency motions at the surge/sway natural periods; static offsets. The analysis required can be summarized as follows: first order motion responses in the six-degrees-of-freedom; wind and current force coefficients and slowly varying dynamic wind responses; mean wave drift forces and slowly varying wave drift responses; maximum platform offset including all motion response contributions. This section is intended to summarize the main features of this type of analysis. Details may be found in API RP 2 FPI and API RP 2 T. The calculation of first order vessel motions is usually performed in the frequency domain for uncoupled riser analysis. The motion responses and static offsets due to the first-order wave forces, second-order, wave-drift forces and wind and current forces are calculated separately. The maximum vessel offset and motion are estimated based on combination of the individual contributions. The dynamic wind response of the vessel is a combination of a steady component and a time varying component. The steady component is calculated by summing the wind drag forces and moments on each member above the water line. In addition to the steady wind forces, the vessel will exhibit a low frequency motion responses induced by wind dynamics. The total wind forces are treated as a constant or a combination of a steady component and a time varying component. The time varying component is also known as low frequency wind force. The low frequency wind force will induce low frequency resonant surge, sway and yaw motion. The frequency distribution of wind speed fluctuation can be described by a spectrum. A wind spectrum for use in this analysis is given in the 19th edition of the API RP 2A-WSD. Wave frequency motion responseThe vessel’s first order wave responses in six degrees of freedom are generally calculated using 3D diffraction techniques. These are characterized by response amplitude operators (RAOs) and phase angles. TLPs require computation of a wave frequency setdown resulting from surge/sway. Heave -- and roll/pitch -- are usually negligible for TLP riser analysis. Low frequency motion responseA dynamic riser analysis is generally performed with the vessel offset to its mean position due to the steady environmental loads (wind, wave and current) plus the maximum low frequency response of the vessel. The low frequency wave responses of the vessel are excited by the second order wave drift forces and moments and by wind gust forces. Although the magnitude of these forces is small compared to the first order wave forces, the frequency of the oscillating drift force may correspond very closely to the natural frequency of the moored vessel. In this case the horizontal motion of the vessel may be significant. The low frequency responses are also highly dependent on the mooring stiffness and the total damping included in the system. Both the slow-drift oscillations and the wave-frequency motion are calculated about the mean offset position described above. A simple one-degree-of freedom (surge) simulation of the vessel's behavior can be used to calculate the low frequency surge and sway motions of the vessel.72 In this method, by knowing the vessel mass, the mooring stiffness and surge damping, the low frequency surge motion can be estimated. Combination of motion componentsMaximum vessel offsets are calculated as the superposition of the mean offset and the maximum dynamic excursion. For riser analysis, mean and low frequency motions are generally added to get a quasi-static offset to determine a quasi-static mean riser configuration. Wave frequency motions are then calculated about this point for by the riser analysis program. Tensioner modelingTop tensioned risers are attached to the FPS by tensioners so that modeling of their response characteristics is important to accurately simulate the global riser response. In general, the load-displacement curve of a riser tensioner may resemble one of the following characteristics: flat curve close to constant tension with respect to tensioner stroke. In the analysis, riser top tension is maintained constant; linear relation between riser top tension and tensioner stroke. The tensioner can be modeled by a linear spring between the riser and the platform; nonlinear relation between riser top tension and tensioner stroke. Top tension response and tensioner stiffness can be modeled by a non-linear beam or truss elements. If the riser top tension increases significantly with respect to the tensioner stroke, the tension variation may change the stiffness in flexural bending of the riser. In this case, the coupling between the axial and bending response may be important. Bottom endFlex jointsFlex joints are often used at the lower end of metal risers to allow the riser to articulate with minimal bending resistance. A flex joint can be modeled as a beam element or preferably as a linear spring having the appropriate rotational stiffness properties. Stiffness is a function of deflection magnitude, and this flex joint property can be important for fatigue. It should be ensured during the motion analysis that the design angular limits of the flex joints are not exceeded. Stress jointsStress joints are special joints at the bottom or top of the riser designed to control the curvature and hence the stress in the riser where it attaches to a stiff support structure. They are usually tapered from a thick-walled section to a thinner-walled section with constant ID. A stress joint can be modeled for gross stiffness purposes as a number of straight-walled beam sections with decreasing wall thickness. A numerical solution to the equilibrium equations is typically obtained by assembling equations for each region comprising the riser into a system of equations describing the force-displacement relationships for all degrees of freedom (dof). By combining all equations for elements connected to a particular node, in a manner consistent with requirements for equilibrium at the node and compatibility between elements, equations relating forces at all global dof to displacement at each dof at the node are obtained. Assembling all such equations for N global degrees of freedom leads to a system of N coupled algebraic equations. These equations can be expressed in matrix form as  ...(40)
where [M] = N x N system mass matrix [C] = N x N system damping matrix [K] = N x N system stiffness matrix {R} = N x 1 system load vector {  0} = N x 1 acceleration vector
{  0} = N x 1 velocity vector
{U} = N x 1 displacement vector where each row represents the equilibrium equation for a global degree of freedom, obtained by adding contributions from each connected element. Matrix columns contain coefficients specifying mass, damping and stiffness coupling between the various dof. The coupling terms arise from elements which are connected between nodes at which the coupled dof are defined. The following sections address various issues pertinent to the numerical solution of the global equilibrium equation. 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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