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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Special Purpose AnalysisClearanceClearance between risers, between a riser and a moving obstruction (e.g., a FPS pontoon), or between a riser and a stationary obstruction (e.g., subsea wellhead) is typically assessed by performing a global riser analysis and calculating mean clearance and relative motions. Statistics on minimum clearance and impact velocity can then be developed. Environmental conditions that are usually checked in clearance analyses are storms and high currents. WavesClearance can be estimated for wave-induced motions in the frequency domain by calculating mean clearance and estimating relative displacements due to slow-drift and wave-frequency responses. Statistics for minimum clearance, contact rate and potential collision velocities can then be estimated based on reasonable simplifying assumptions and standard formulas for extreme values. Mean clearance can be estimated as the average clear distance between risers with the vessel at its mean slow-drift offset. If riser natural frequencies are well separated from vessel drift frequencies, then the standard deviation of relative displacement due to slow drift of the vessel can be estimated as the change in clearance that occurs as the vessel moves from its mean slow drift offset to the mean plus slow drift standard deviation position. This will often be an adequate representation for connected risers. However, some risers may exhibit appreciable dynamic response to vessel drift during installation or retrieval operations (e.g., running a subsea BOP without guidelines). For these cases, a dynamic analysis is appropriate to determine a relative displacement spectrum due to slow-drift, using the vessel motion slow-drift spectrum as a dynamic boundary condition. Finally, the standard deviation of relative displacements at wave frequencies is estimated by differencing the wave-frequency displacement transfer functions of the two risers. These transfer functions must be linearized about a particular vessel position, due to the effect of vessel motion on riser tension. A complete description of clearance in the frequency domain includes the mean clearance plus a relative displacement response spectrum, which typically comprises response due to vessel slow-drift and wave-frequency motions and loadings. There are no generally accepted procedures for calculating extreme response statistics for a combined slow-drift and wave-frequency relative displacement process. However, the general approach is to develop an envelope function for the combined process and then estimate extreme values of the envelope using classical level-crossing techniques.87,88,89 There are no specific examples for riser clearance analysis in the literature at this time, but the extreme response of a vessel is an analogous problem that has been studied extensively.90 Similar quantities may be calculated via time-domain analysis, in which case the calculation of relative displacement is straightforward if both slow-drift and wave-frequency vessel motions are included in the simulation. Relative velocities are typically computed via numerical differentiation of the relative displacement. CurrentsDue to possibly large static deflections in high currents the risk of corresponding collisions in riser systems should be evaluated. Considering top-tensioned risers, typical of TLPs and spars, the static deflection of such a riser due to current drag is proportional to the square of its length, provided other parameters such as diameter, pretension and current speed are kept equal. Large static deflection of a riser does not necessarily represent any problem as long as it is approximately equal for all risers in the array. However, for risers, even relatively small difference in deflection can exceed the top and bottom end spacings and lead to mechanical contact or collision between adjacent risers. Thus it becomes necessary to calculate the deflection of each riser in the array to determine if any two will collide or if there will be collision with mooring lines, platform pontoons, etc. In this calculation it is recommended to account for the shielding effect of risers situated upstream in the current field. For calculating the shielding effect it is recommended: to apply the theoretical formulation for fully developed turbulent wakes; to sum up wake contributions from all upstream risers by rms summation. The wake field behind a cylinder in homogeneous flow is: 94, 95  ...(52)
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