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


General General functions of risers



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General

  1. General functions of risers


Dynamic risers of offshore production systems are fluid conduits between subsea equipment and the surface platform. Figures 1 and 2 introduce the essential functional elements (or features) of risers and riser systems. An offshore production system responds dynamically to environmental forces. The riser system is the interface between a static structure on the ocean floor and the dynamic offshore production system structure at the ocean's surface. Riser system integrity includes not only fluid and pressure containment, but structural and global stability as well.

Figure 3 introduces some aspects of the complexity that may evolve when implementing a specific riser design. The simple conduit may be complicated by intermediate connections, changes in material or form of cross-section construction, couplings, attachments (e.g., buoyancy modules), and multiple flowpaths.

Risers may perform the following specific functions:


  • convey fluids between the wells and the offshore production system (i.e., production, injection, or circulated fluids);

  • import, export or circulate fluids between the offshore production system and remote equipment or pipeline systems;

  • guide drilling or workover tools and tubulars to and into the wells;

  • support auxiliary lines;

  • serve as, or be incorporated in a mooring element;

  • other specialized functions such as well bore annulus access for monitoring or fluids injection.

This document is intended to provide guidance for design of risers that may be categorized according to these functions.

    1. Configurations of risers


Risers, regardless of function, have a wide range of possible configurations. It is possible to differentiate between various riser configurations on the basis of:

  • cross-section complexity (a single vs. multiple tubes),

  • global geometry or behaviour (small vs. large deflection),

  • structural integration (integral vs. non-integral risers),

  • means of support (top tensioned with tensioners or hard mountings vs. concentrated or distributed buoyancy),

  • structural rigidity (metal vs. flexible risers),

  • continuity (sectionally jointed vs. continuous tube),

  • materials.

The designer may refer to Section 5 for a catalog of riser and riser system configurations that are (or have been) in service, as well as some concepts (proposed for imminent use) that serve as examples of the range of possible configurations. The designer should find guidance within this document for establishing the viability of specific systems and components indicated by those figures.

    1. What is not (fully) covered


There are many topics, materials and concepts for riser applications that are of interest and evolving toward potentially advantageous applications. This document is intended to cover only such issues that are considered established through practice or operator commitment. This section tries to identify topics that are not intended to be fully covered in this release of the document. The following headings, of course, cannot be a completely comprehensive listing of the possible riser system topics that are not covered by this document. One reason for noting these issues or options here is to highlight that there may be need for future efforts to provide necessary guidance for future releases of the document as new concepts gain acceptance.

This document covers new risers, as well as reuse of existing risers.


      1. Risers as mooring elements


This document does not provide comprehensive coverage for applications of risers for service as (or part of) the mooring system of the offshore production systems. For example, when a riser or riser system is intended to function as a tendon to provide direct mooring restraint of an offshore production system, structural design must also consider the recommendations outlined for tendons in API RP 2T. Further, piping that is integral to anchor leg structure of, for example a Single Anchor Leg Mooring, is not addressed in this document. In such cases, the designer should find suitable guidance in API RP 2SK (for mooring systems) and API RP 1111 (for risers on fixed structures).

      1. Control lines or umbilicals


Control lines or umbilicals fit some of the functional definition of a riser in that they may provide a conduit for fluids between the offshore production system and subsea equipment. This document does not address their design specifically, although they may be attached to risers and thereby influence the riser's design and analysis. The designer may refer to API RP 17A for guidance on umbilicals.

      1. Low pressure fluid transfer hoses


This document is not intended to provide guidance for the design of low pressure hoses for such service as cargo transfer. Appropriate guidance is available through documentation prepared by the Oil Companies International Marine Forum (OCIMF).

      1. Bonded flexible pipe


Bonded flexible pipe is not specifically considered in this document. Where such is intended to be used, a level of safety comparable to other riser systems should be documented.

      1. Composite (fiber-reinforced) materials


This document does not provide comprehensive coverage for the design of risers of composite (fiber-resin matrix) construction. (See Annex D ??)

    1. Status of Technology


The reader should be aware that riser systems technology (i.e., concepts, design and analysis methodologies and criteria, components manufacturing and testing, operational roles and demands, maintenance and inspection, etc.) is in a state of rapid and continuing evolution. This evolutionary status means that the technology relating to any given riser system or component is not likely to be well-proven by years of practical, successful application. Therefore, designers are advised to take appropriate measures to ensure that their practice incorporates suitable quality control to avoid errors of unquestioning, unfounded confidence in the results of any phase of the complex design process. This advice is particularly applicable when evaluating the vast quantities of numerical results that can be facilitated by modern high speed computational methods and tools.


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