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
Download 2.34 Mb.
|
- Navigate this page:
- Load Combinations for Design Cases
- Design Criteria
- Interference
- Fatigue and Service Life
- Degradation of syntactic foam buoyancy
Structural ConsiderationsThe following is from Structural Considerations of Section 3.4, API-2RD-1998 First Ed.: Once functional and operational considerations are established, the structural components are designed to perform their function and satisfy allowable stress limits and design life requirements. In this RP, structural design is based on an allowable stress approach that defines acceptability on the basis that calculated stresses in the riser are below allowables established herein for all applicable loading conditions. With such an approach, the designer should be aware of the range of external and internal loads to which a riser will be subjected (Section 4). These loads are combined to establish extreme conditions used to verify that the riser design satisfies specific design acceptance criteria. The designer should use the types of riser analyses that provide sufficient riser response information to compare predicted riser stresses and deflections with allowables. This subsection introduces the primary issues that the structural designer and analyst should consider in designing a riser system. A more thorough and detailed description of structural design loads and criteria for FPS risers are contained in Sections 4 and 5 of this RP. Section 6 gives additional guidance on how to analyze risers.
Load Combinations for Design CasesA design case is a combination of loads calculated for a specific operational phase and particular system and environmental conditions. The design cases to be evaluated are outlined in Section 4. For each design case, appropriate load combinations of the applicable external and internal forces should be developed. An example of the general components of a typical load combination is shown in Figure 21. For extreme event analysis, the riser should be designed for the loading condition combinations (of reasonable probability of occurrence) that produce the most severe effects on the riser. A sufficient number of load combinations should be developed to represent all installation conditions, in situ conditions and unusual event conditions according to the guidance given in Section 4. Design CriteriaAllowablesThe following design limits should be considered where applicable: allowable stress (include burst, tensile, and combined), allowable deflection, collapse, fatigue/service life, inspection/replacement interval, temperature limits, minimum bending radius, permeability of flexible pipe, abrasion and wear, interference. The determination of appropriate allowable stresses is addressed in Sections 4 and 5. InterferenceThe riser system design should include evaluation or analysis of potential riser interference (including hydrodynamic interaction) with other risers, mooring legs, tendons, hull, the seabed and with any other obstruction. Interference should be considered during all phases of the riser design life including installation, in‑place, disconnected and unusual events. If contact is to be permitted, resulting collision loads should be determined to demonstrate that structural integrity is maintained. The estimated accuracy and suitability of the selected analytical technique should be assessed when determining the probability and severity of contact.
Fatigue and Service LifeFatigue analysis of risers involves calculation of the fatigue damage caused by waves, vortex induced vibration, vessel motions, and thermal and pressure cycles. The goal is to ensure that a component's calculated fatigue life exceeds its service life multiplied by a safety factor (see 5.6). Service life analysis requires the calculation of the long term effects of chemical, biological or ultraviolet exposure on non-metallic riser components. The following items should be included in a riser fatigue analysis: all causes of cyclic stress variations should be identified including wave actions, VIV and vessel motions (low and wave frequency), and thermal and pressure cycling; the applicable set of hydrodynamic coefficients should be developed for each flow condition; the fatigue analysis procedure may use a fracture mechanics or cumulative damage analysis (S-N) approach, as appropriate. Normally, the S-N approach is used for design, while fracture mechanics is used to establish inspection criteria during both fabrication and in-service operation of components in riser systems; if the cumulative damage approach is followed, an appropriate S-N curve should be selected or generated for each component subject to cyclic stress variations. The component corrosive environment, fabrication method and surface finish should be considered; the designer should include a sufficient number of seastates and approach directions to accurately predict the long term distribution of stress ranges. For spectral techniques, an appropriate seastate spectrum type should be identified; an appropriate Stress Concentration Factor (SCF) for each component in the load path should be applied to the calculated cyclic stress variations; the fatigue design load cases for each seastate should account for the appropriate functional loads based on their statistical probability of occurrence during the given seastate; an appropriate method for calculating fatigue damage should be established; if wear is a factor on any component in the load path, its effects should be included in the analysis; determination of riser fatigue life should take into account any effects of internal and external corrosion, biofouling, chemical deterioration and ultraviolet rays; safety factors should reflect component maintenance, inspection and replacement program. Degradation of syntactic foam buoyancyThe designer should select the type and quantity of materials to provide the required buoyant lift over the intended service life while accounting for the predicted degradation of buoyancy. Factors which may affect syntactic foam buoyancy performance relative to specific applications include: hydrostatic pressure, duration of service, cycling of hydrostatic pressure, mechanical loads and load cycles (buoyancy modules are usually designed and installed for service in a manner that avoids or limits the imposition of bending or tensile loads), temperature, chemical or UV exposure. Syntactic foam exhibits a progressive buoyancy loss resulting from water absorption over time. The rate of buoyancy loss (due to water absorption) is inversely proportional to the strength (and density) of the syntactic foam. Typically, heavier or stronger syntactic foam materials will be required for service at greater depths and/or over longer periods of time in service. Syntactic foam manufacturers maintain extensive data on the performance of specific materials in various densities at various depths, as well as extrapolation methods which permit the prediction of degradation of lift over extended time in service. Selection of syntactic foam should be based on test data. 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:
|