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
Description of Riser Systems and Components
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- Essential system features
- Riser system descriptions
- Completion/workover risers
- Multi-bore hybrid risers
- Multi-bore top tensioned metal risers
Description of Riser Systems and ComponentsGeneralThis section, containing supplementary elements, further defines the essential features and functions of riser systems floating production system (FPS) applications and describes riser components and their primary functions. Risers on FPSs cover the full range of production, injection, drilling, completion, workover and exporting operations. Mobile offshore drilling units (MODU) normally use risers in single-well situations. Risers for FPS, on the other hand, have additional requirements associated with operating multiple risers of potentially different types in relatively close proximity. Section 6 iscusses the unique requirements of riser design and operations that can be imposed by concurrent drilling and production operations. Riser components associated with these requirements are the focus of this section. Essential system featuresRiser bodyThe primary function of FPS risers is to convey fluids to and from the vessel. Depending on site-specific considerations, these risers are either metal or flexible pipe. Metal pipeSegmented—Limitations on the maximum continuous length of metal pipe that can be reasonably manufactured, transported, handled, installed, retrieved and replaced offshore often require that metal pipe conduits be segmented. These segments can be joined onsite by mechanical connectors or welding. Continuous—Risers can be towed, dragged or floated out in one piece and upended on location, or they can be transported on a reel to the site. The reel method consists of assembling a length of pipe on shore. This pipe is then coiled onto a reel or a drum. The coiled pipe is then transported to the site and unreeled. The coiling and uncoiling of the pipe involves plastic deformation that reduces its impact resistance. Welding at the site is only required for joining reels, hence installation proceeds much more quickly than segmented pipe. Flexible pipeFlexible pipe is usually stored and transported on reels, baskets or carrousels. The reel size limits the maximum length of flexible pipe of a given pipe diameter that can be fabricated without connectors. Several sections may be required to achieve the riser length required. End fittings are required at both ends of each pipe segment. Flexible pipe can be segmented, for example, to accommodate a change over from a dynamic section to a less expensive static section. The pipe can be segmented when there is a reel capacity problem or when it is more economic to do so. System interfacesIn addition to the design of the riser body, the designer must consider interface requirements at the top and bottom of the riser. At the top, equipment on the vessel must be designed to accommodate the range of riser loads, motion, and ancillary equipment needed to maintain riser integrity (e.g. tensioners) and to enable necessary operations to be conducted (e.g. drilling, completion, workover, support equipment). At the bottom, interface components must also be designed to accommodate riser loads and maintain fluid conduit and pressure integrity. Conduits for fluid transport, control or monitoring system umbilicals and load paths for structural support must be provided and their continuous operation maintained. Components necessary for connection, installation, maintenance and disconnection may have to be provided at the top and bottom interfaces for each of these essential riser features. Riser system descriptionsProduction/injection risersProduction risers transport fluids produced from the reservoir. Injection risers transport fluids to the producing reservoir or a convenient disposal or storage formation. The systems can be designed to operate interchangeably. They consist of an apparatus to hang the riser at the surface and attach it to surface valves and piping. The sea bed portion contains an apparatus that connects the riser to a wellhead or receptacle. Included are also methods/equipment to space out the riser and to account for bending loads at the bottom and/or top of the system. The riser may also have bumpers, vortex-suppression devices and other attachments such as buoyancy modules. The cross sections of production/injection risers are often complex, as they can contain multiple parallel or concentric tubing strings. They also may contain special equipment like slip joints, packers and control lines. Required pressure rating is tied to the reservoir characteristics and anticipated well performance. Top-tensioned risers (Figure 4) of a float production system have been developed to provide surface access to wells in a manner analogous to fixed platforms. This can be accomplished because of the minimal heave and pitch/roll motions of these platform types. Such risers comprise metal pipe cross sections which may be thought of as a continuation of the well bore to the ocean’s surface. Flexible-pipe production risers are associated with subsea trees. A large number of flexible-riser configurations are possible, two of which are shown in Figure 5. In all cases, flexible risers hang from the offshore production system under their own weight, but external buoyancy modules may be attached to achieve the required riser shape or curvature. Export riser systems will usually include equipment similar to production/injection risers. The riser diameters may be larger so as to accommodate total production through the offshore production system. Pressure ratings are determined by export pipeline and flow conditions. Typically, a pig launcher will be required. Because bending loads in larger diameter risers are higher, the designer must pay particular attention to stress joints or flex joints. The system’s cross section is usually a single pipe forming the flow path compared to production risers which may have multiple pipes and multiple paths. Figure 6 shows one pipeline riser option, a steel export pipeline laid in a simple catenary and connected at pontoon level to the surface platform. Moment relief at the top end may be provided by e.g. a stress joint or an elastomeric flex joint. A top-tensioned riser could be used as export riser that are analogous to the top-tensioned production risers described earlier, but with a single flow path compared to production risers’ multiple paths.
Drilling risersThe major functions of drilling riser systems for mobile offshore drilling units (MODU) are to provide fluid transportation to and from the well; support auxiliary lines, guide tools, and drilling strings; serve as a running and retrieving string for the BOP. Low pressure drilling riser systems used on FPSs perform the same functions as riser systems on drilling vessels. Most TLP systems have been designed with the drilling BOP at the top of the riser. See Figure 7. In such cases, the drilling riser must be designed for the added requirement of containing maximum formation pressure. When used from a MODU, the drilling riser is almost always deployed alone. When used from FPSs the drilling riser may be deployed amongst production risers. Consequently it must be treated as just one element of a system of risers, with appropriate safety and potential interference considerations for each riser.
Completion/workover risersCompletion/workover risers (Figure 8) are used to provide full bore, unrestricted access to a well for the purposes of completing or working over the well. Typically, these are custom-designed, but some standardization may be implemented to reduce costs. A form of compliant riser that has been proposed for providing well bore access for workover is show in Figure 9. Hybrid risers such as the one shown in Figure 10 provide multiple flowpaths from the seabed to an FPS by a combination of vertical metal piping over most of its water column and flexible piping between its top and the FPS. There are many possible variations. The key components of the hybrid riser in Figure 10 are as follows: Structural member – steel tubular providing structural backbone to the vertical metal piping and buoyancy modules, support for the top assembly and providing reaction path to the riser base. The structural member may also provide buoyancy and contain fluid conduits for import or export; Buoyancy modules – syntactic foam buoyancy formed in two or more segments around the riser circumference providing guide tubes for the peripheral lines and attached to the structural member by strapping; Peripheral lines – vertical steel pipe flowlines running full length of metal riser section; Flexible jumper hoses - flexible pipes connected from the goosenecks at the top assembly of the metal riser to the support points on the FPS providing compliancy needed to allow relative motion between vessel and riser top assembly; Top assembly or upper riser connector package (URCP) – upper termination point for metal peripheral lines having isolation valves and emergency disconnect package (EDP). A tether may be attached from this point to the FPS to maintain compatibility of lateral displacements in extreme conditions. Goosenecks and their support structure are provided to attach flexible jumpers; Air tanks – near surface buoyancy tanks attached to the top of the riser providing fixed and variable components of tension to the metal riser section; Riser base – foundation to which the riser is attached by a stress joint and hydraulic connector. The base provides resistance to riser tensioning forces, overturning moments and lateral loading. It may consist of an independent piled steel structure. Flowline connection porches located on the periphery are hard piped to a central location for connection to peripheral lines. Alternatively, the riser may be connected directly to a well template. Multi-bore top tensioned metal risersA multi-bore top-tensioned metal riser (Figure 11) pierces the water surface and extends its rigid geometry all the way up to a deck of the vessel. At the top, standard tensioning equipment provides support for the riser. Multi-bore top-tensioned risers have been used for applications such as import and/or export of fluids coming from subsea manifolds or templates. A top-tensioned metal riser can also support metal freestanding flowlines. The flowlines vary in functionality, number, size, dry weight, submerged weight, fluid content and pressure characteristics, as dictated by different field requirements such as: (1) satellite trees, (2) subsea manifolds and (3) flowlines coming from other platforms. The metal riser joints, stress joint, lower riser connector package (LRCP) and riser base follow the same pattern as described for the hybrid riser.
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