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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Component Analysis


This section addresses the detailed analysis of the riser components using the results obtained in the global analysis.

      1. Individual riser tubulars


The objective of analyzing individual tubulars is to evaluate them with regard to the appropriate design criteria and to generate loads and displacements required to evaluate their components. The analyses of the individual tubulars begins with the global analyses. Such analysis takes results obtained in the global analysis and generates the information required to satisfy this objective.

For riser systems which have one tubular that provides most of the riser stiffness (i.e. drilling risers, production risers with small inner tubes, etc.) the global displacements provide an accurate description of the predominant tubular's displacements. It may also be possible to determine the tensions, bending moments and stresses in the predominant tubular without additional analysis. However, in some cases additional analyses are required to determine the loads and stresses in the predominant tubular. In these riser systems, additional analyses may also be required to determine the loads and stresses in tubulars other than the predominant tubular. For riser systems which do not have one predominant tubular, additional analyses are usually required to accurately estimate the loads, stresses and displacements in individual tubulars.

The additional analyses are aimed at evaluating the interaction of the riser tubulars and determining the loads, stresses and displacements in each tubular. These may be obtained directly from the additional analyses or may be obtained using a procedure derived from the results of the global analysis and the analysis of the individual tubular. In most cases it is not practical to perform a specific analysis of each individual tubular for each load case to be evaluated. For these situations a procedure may be developed which relates the loads, stresses and displacements in the individual tubes to the loads, stresses and displacements generated in the global analyses.

The tubulars interact with each other through contact loads which occur at discrete points along the length of the riser. The frequency and size of the contact loads between the tubulars depends on the global riser displacements, the clearances between the tubulars (and tubular components) and the lateral stiffnesses of the tubulars. These discrete contact loads may occur anywhere along the length of the tubulars. The most likely points of contact are at the locations of centralizers, clamps, connectors, etc.

Finite element or finite difference computer programs are generally used to analyze the individual tubulars. Depending on the riser configuration and the particular objective of the analysis, it may be desirable to include all of the tubulars in a solution with each tubular modeled individually, or it may be desirable to model one tubular in a solution and perform one solution for each tubular. For some riser systems, a combination of both methods may be required. For both methods static solutions are used to perform the analyses. These analyses are used to obtain factors relating the responses of the individual tubulars to the global responses of the riser system. These factors are then used in conjunction with the global analyses to estimate the loads, stresses and displacements of the individual tubulars.

Because the lateral stiffness of an individual tubular is dependent on the tubular's effective tension, along with the cross-sectional properties of the tubular, it is important to accurately model the effective tension distribution. Some of the factors which should be considered when determining the tubular's effective tension distribution are listed below:



  • weight of the tubular;

  • densities of the fluids inside and outside the tubular;

  • internal and external tubular pressures;

  • temperature of the tubular;

  • boundary conditions at the ends of the tubular;

  • tubular installation procedure to determine the initial tubular tensions;

  • distance between the centerline of the tubular and the assumed centerline of the global riser model;

  • special joints (i.e. sliding or telescoping joints);

  • relative axial stiffnesses of the riser tubulars.

It is also very important to accurately model the free space or distance between the tubulars. The models should include free space reductions which may occur at the locations of centralizers, clamps, connectors or other components. Care must be taken when discretizing the model of the tubular to ensure that in critical areas, the response of the tubular between the components can be accurately determined. For top tensioned metal risers, these critical areas generally occur at the top and bottom of the riser and in regions where the effective tension of the tubular is low enough to allow buckling of the tubular.

Centralizers are used on tubulars inside larger tubulars to control the displacements of the inner tubulars. The more centralizers that are used, the more closely the riser behaves like the equivalent pipe riser model used for the global analysis. Centralizers are used to provide the inner tubulars with additional stability, control the locations where inter-tubular contact may occur and to protect other components. For such riser systems, optimization of the centralizer spacing is generally one of the tasks to be performed. This generally requires an iterative process in which a number of responses are evaluated to determine the optimum centralizer spacing.

The results which should be generated from the analyses will vary depending on the type of riser being analyzed and the objective of the analysis. Some of the results which may be generated include:


  1. estimation of the axial and transverse displacements of the tubulars. The axial displacements can be used to generate information on the relative sliding between tubulars and to generate stroke information for tubulars with sliding (stab-in) connections or telescoping joints. The lateral analyses are used to generate the load distributions and to determine the clearances between tubulars;

  2. estimation of the load distributions along the length of the tubulars. These load distributions are used for the analyses of the tubular connectors and to determine the stresses along the length of the tubulars. The loads obtained at the ends of the tubulars are used for the analyses of the components connected to the ends of the tubulars;

  3. estimation of the stresses along the length of the tubulars. These stresses are used to evaluate the tubulars with regard to the allowable stress and fatigue criteria;

  4. optimization of the centralizer and/or clamp spacing;

  5. estimation of the contact loads between tubulars. These contact loads can be used to design centralizers and evaluate wear.


      1. Connectors and stress joints


The objective of these analyses is to assure structural integrity and performance of these components as a part of the overall riser system. Analysis is intended to demonstrate that they are resistant to yielding and failure due to fatigue.

The following analysis procedures are recommended:



  1. Global analysis: Determine the net loads by global analysis. In global analysis the stress joint should be modeled appropriately with its varying stiffness. If flowlines and their guide tubes share bending loads with the stress joint, it may be assumed that there is a common radius of curvature for all these tubes. The bending stiffness of the guide tubes and the flowlines are then added to that of the stress joint. Connectors are usually represented by adding the weight and mass to the tubular distributed properties. Such global analyses include all possible sea-state and riser configurations, and the worst riser response is then determined. From the global analysis, determine loadings and displacements on the stress joint or connector, calculate stresses along various cross sections of the stress joints and compare with allowable stresses criteria;



  1. Finite element analysis: Detailed finite element analysis (FEA) is then usually performed. A two or three-dimensional model may be used. Symmetry of the component can be utilized in determining the stress state by using a two-dimensional axisymmetric FEA model. For such two-dimensional analyses the equivalent tension concept (API Spec 16 R) can be used to account for the bending load in element types which allow only axisymmetric loadings. Element formulations which allow axisymmetric modeling with asymmetric loading are also available and may provide improved results. Depending on the finite element model, displacements and rotations rather than forces and moments may be used to transfer the loadings from the global model to the local model;



  1. Boundary conditions: A fixed boundary condition may be used at the end of a stress joint attached to a foundation that is significantly stiffer than the stress joint. The flexibility of either the riser base or the upper riser package should be included using appropriate spring elements. Interface elements can be used to model the interactions between the stress joint and the lower connector package. This provides a method to determine the load at which separation of the faces of the connection between these two components initiates;



  1. Stresses: Once stresses are available from the FEA, they should be compared to the design allowables defined in Sections 4 and 5;



  1. Fatigue: Connectors and stress joints are critical members where fatigue loading should be analyzed. Either FEA based analysis or global analysis results may be used to calculate the fatigue life. In the case where the global results are used, stress amplification factors (SAFs) should be used for localized high stress regions.


      1. Flex joints


Local analyses as outlined for the stress joint can be conducted for the flex joint. The purpose of the local analysis is to determine high local stresses and life of the joint under fluctuating loads. Therefore, the model should include stiffness of the elastomers between spherical shaped steel rings and appropriate contact elements. Such models must include highly non-linear behavior of the rubber layers and call for specialized analyses capabilities.


      1. Effect of appendages on local stress


Examples of appendages that will have an effect on local stresses and may need to be analyzed are:

  • choke and kill lines;

  • anodes;

  • buoyancy.


      1. Tensioning system


Tensioner design requires information on maximum loads, stroke and angles. Loads comprise tensions in each component, for example: cylinder tensions and centralizing reactions. Stroke information determines the overall dimensions of the tensioner unit and will generally be quite different for up and down-stroke directions. Since such responses are likely to be non-gaussian in large seastates, care should be taken in developing extreme values from time series.


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