SCOPE AND METHOD Risers for Floating Production Systems FPSs and Tension-Leg PlatForms TLPs 1.1.1 This document addresses structural analysis proce- dures, design guidelines, componen
General Functions of Risers
FPS risers serve as fluid conduits connecting subsea equipment to surface platforms, playing a crucial role in riser systems These systems must respond dynamically to environmental forces, acting as the interface between the static structures on the ocean floor and the dynamic FPS structures at the surface Ensuring the integrity of the riser system involves maintaining fluid and pressure containment, as well as structural and global stability.
Figure 3 highlights the complexities that can arise during the implementation of a specific riser design, as the straightforward conduit may be affected by various factors These include intermediate connections, alterations in material or cross-sectional shape, couplings, attachments such as buoyancy modules, and the presence of multiple flow paths.
Risers serve several essential functions in offshore operations, including the conveyance of fluids between wells and the Floating Production System (FPS), facilitating the import, export, or circulation of fluids to remote equipment or pipeline systems They also guide drilling or workover tools and tubulars into the wells, support auxiliary lines, and can be integrated into mooring elements Additionally, risers may perform specialized tasks, such as providing access to the well bore annulus for monitoring or fluid injection.
1.2.4 This document is intended to provide guidance for design of risers that may be categorized according to these functions.
Configurations of Risers
Risers exhibit a diverse array of configurations based on several key factors These include the complexity of their cross-section, distinguishing between single and multiple tubes, and their global geometry, which can be characterized by small or large deflections Additionally, risers can be classified by their structural integration, either as integral or non-integral types, and by their means of support, which may involve top tensioning with tensioners or buoyancy methods Structural rigidity is another important aspect, differentiating between metal and flexible risers, while continuity can be categorized as sectionally jointed or continuous tubes Finally, the materials used in riser construction also play a significant role in their configuration.
Designers can consult Section 2 for a catalog of riser and riser system configurations currently in service, along with proposed concepts for future use This document provides essential guidance for assessing the viability of specific systems and components illustrated in the figures.
What is Not (Fully) Covered
There are many topics, materials and concepts for riser applications that are of interest and evolving toward poten-
The API Recommended Practice (RP) 2RD focuses on established issues through practice or operator commitment, highlighting topics that are not fully addressed in this initial release While the document does not provide a comprehensive list of riser system topics, it emphasizes the importance of recognizing these gaps This acknowledgment serves to indicate the potential need for future guidance as new concepts emerge and gain acceptance in the industry.
This document covers new risers not reuse of existing risers
This document does not fully address the applications of risers within the FPS mooring system Specifically, when a riser or riser system is designed to act as a tendon for direct mooring restraint, it is essential to incorporate the guidelines provided for tendons in API RP 2T Additionally, the piping that is part of the anchor leg structure must also be taken into account.
Single Anchor Leg Mooring, is not addressed in this RP In such cases, the designer should find suitable guidance in A P I
RP 2% (for FPS mooring systems) and API RP 11 11 (for risers on fixed structures)
Control lines or umbilicals serve a functional role similar to risers by facilitating fluid transport between the Floating Production System (FPS) and subsea equipment While this Recommended Practice (RP) does not specifically cover their design, it acknowledges that umbilicals can be connected to risers, potentially impacting their design and analysis Designers are encouraged to consult API RP 17A for guidance on umbilicals.
1.4.3 Low Pressure Fluid Transfer Hoses
This RP 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 Fonun
Bonded flexible pipe is not specifically considered in this
W Where such is intended to be used, a level of safety com- parable to other riser systems should be documented
This document does not provide comprehensive coverage for the design of risers of composite (fiber-resin matrix) con- struction Introductory information is provided in Annex D.
Status of Technology
Riser systems technology is rapidly evolving, encompassing concepts, design methodologies, component manufacturing, testing, and maintenance Due to this ongoing evolution, the technology associated with any riser system or component may not have a long history of successful application Designers must implement robust quality control measures to prevent misplaced confidence in the outcomes of the complex design process, especially when analyzing the extensive numerical data generated by modern high-speed computational tools.
Quality Assurance
To enhance the integrity of a riser system, it is essential to implement quality systems throughout various stages, including design, procurement, construction, testing, operation, and maintenance.
1.6.2 When these systems are applied, refemnce shall be made to the relevant quality systems standard (IS0 9OOO series).
General
2.1.1 This section, containing supplementary elements, further deiìnes the essential features and functions of riser systems for FPS applications and describes riser components and their primary functions
Risers on Floating Production Systems (FPS) encompass a wide range of operations, including production, injection, drilling, completion, workover, and exporting Unlike Mobile Offshore Drilling Units (MODUs), which typically utilize risers for single-well scenarios, FPS systems must accommodate multiple risers of varying types in close proximity The unique design and operational requirements for risers, particularly during concurrent drilling and production activities, are further explored in Section 3, highlighting the specific components that address these challenges.
Essential System Features
The primary function of W S risers is to convey fluids to and from the vessel Depending on site-specific consider- ations, these risers are either metal or flexible pipe
Due to constraints on the maximum continuous length of metal pipes that can be effectively manufactured, transported, handled, installed, retrieved, and replaced offshore, it is often necessary to segment these conduits Onsite, these segments can be connected using mechanical connectors or welding techniques.
Risers can be transported to the site either by towing, dragging, or floating them out in one piece, or by using a reel method In the reel method, a length of pipe is assembled onshore and then coiled onto a reel or drum for transportation.
The coiled pipe is then transported to the site and unreeled
The coiling and uncoiling process of pipes leads to plastic deformation, which diminishes their impact resistance Unlike segmented pipes, the installation of coiled pipes is expedited since welding is only necessary for joining reels.
Flexible pipe is typically stored and transported on reels, baskets, or carousels, with reel size determining the maximum length of pipe that can be fabricated without connectors To achieve the necessary riser length, multiple sections may be required, each needing end fittings at both ends Segmentation of the flexible pipe allows for transitions from dynamic to more economical static sections, addressing reel capacity issues or enhancing cost-effectiveness.
When designing the riser body, it is crucial for the designer to address the interface requirements at both the top and bottom of the riser At the top, the vessel's equipment must be capable of handling the various riser loads, motions, and additional equipment necessary for maintaining riser integrity, such as tensioners, while facilitating essential operations like drilling, completion, and workover Similarly, at the bottom, the interface components need to be engineered to support riser loads and ensure the integrity of fluid conduits and pressure.
Fluid transport, control, and monitoring systems require conduits and structural support load paths to ensure continuous operation Essential components for connection, installation, maintenance, and disconnection should be available at both the top and bottom interfaces of these riser features.
2.3 FPS RISER SYSTEM DESCRIPTIONS 2.3.1 Productionhnjection Risers
Production risers are essential for transporting fluids from the reservoir, while injection risers facilitate the movement of fluids to the producing reservoir or for disposal and storage These systems can be designed for interchangeable operation and include apparatus for hanging the riser at the surface, connecting it to surface valves and piping The seabed section features equipment that links the riser to a wellhead or receptacle, along with methods to space out the riser and manage bending loads at both ends Additionally, risers may be equipped with bumpers, vortex-suppression devices, and buoyancy modules to enhance their functionality.
The cross sections of production injection risers are typically intricate, featuring multiple parallel or concentric tubing strings Additionally, they may include specialized equipment such as slip joints, packers, and control lines The necessary pressure rating is closely linked to the characteristics of the reservoir and the expected performance of the well.
Top-tensioned TLP risers are designed to offer surface access to wells similar to fixed platforms, thanks to their reduced heave and pitch motions These risers consist of metal pipe cross sections that can be viewed as an extension of the well.
6 API REC~MMENDED PRACTICE 2RD bore to the ocean's surface Similar risers, using buoyancy to create top tension have been used as SPAR production risers
Flexible-pipe production risers are linked to subsea trees, with various configurations available, as illustrated in Figure 5 These risers typically hang from the Floating Production System (FPS) due to their own weight, although external buoyancy modules can be added to attain the desired riser shape or curvature.
Export riser systems typically incorporate equipment similar to production injection risers, but with larger diameters to facilitate total production through the Floating Production System (FPS) The pressure ratings are influenced by the export pipeline and flow conditions, often necessitating a pig launcher Due to increased bending loads in larger diameter risers, designers must carefully consider stress joints or flex joints Unlike production risers, which may consist of multiple pipes and flow paths, the cross section of the export riser system generally features a single pipe for the flow path.
Figure 6 illustrates a steel export pipeline riser option, designed in a simple catenary configuration and linked to the surface platform at pontoon level To alleviate moments at the top end, a stress joint or elastomeric flex joint may be utilized.
2.3.2.3 TLPs have used toptensioned export risers that are analogous to the top-tensioned production risers described earlier, but with a single flow path compared to production risers' multiple paths
Drilling riser systems for mobile offshore drilling units (MODUs) play crucial roles in fluid transportation to and from the well, supporting auxiliary lines, guiding tools, and drilling strings Additionally, they function as a running and retrieving string for the blowout preventer (BOP) Low-pressure drilling riser systems utilized on floating production systems (FPSs) fulfill the same essential functions as those on drilling vessels.
Most TLP systems feature the drilling blowout preventer (BOP) positioned at the top of the riser Consequently, the drilling riser must be engineered to withstand the maximum formation pressure.
2.3.3.3 When used h m a MODU, the drilling riser is almost always deployed alone When used h m FPSs the drilling riser may be deployed amongst production risers
Consequently it must be treated as just one element of a sys- tem of risers, with appropriate safety and potential interfer- ence considerations for each riser
Completiod workover risers are essential for providing unrestricted access to a well, facilitating the completion or workover processes Typically custom-designed, these risers may also incorporate some standardization to help reduce costs.
2.3.4.2 A form of compliant riser that has been proposed for providing well bore access for workover is shown in Figure 9