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 design considerations General
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General design considerationsGeneralDesigning a riser system for a FPS is a multi-disciplinary task. Since the riser is a part of a larger system, its design is influenced by the environment and performance requirements as well as by interaction with both the FPS and the sea floor equipment. This section further develops material in Section 1 by describing general design parameters that should be considered by the riser designer. This section begins with safety, risk and reliability and continues with functional, operational and structural considerations and materials standards. It also identifies some applicable design codes and standards. The intent is to provide an introduction to the considerations that govern an FPS riser design. This section also gives designers of other major FPS systems an appreciation of the interfaces their systems may have with the riser system. Safety, risk, and reliabilityThis paragraph is from 1st paragraph of Safety, Risk, And Reliability, Section 3.2, API-2RD-1998 First Ed.: Designers of riser systems need to consider personnel safety and protection of the environment and equipment within a framework of efficient, cost-effective design. Designers carefully assess the risks associated with operating a riser system and strive to minimize the probability and consequences of an uncontrolled release of riser system contents. Such a release could be caused by either internal or external events. In addition, riser systems are often non-redundant structures conveying pressurized hydrocarbons between seabed terminations and surface vessel. Therefore, risk assessment plays an essential role in riser design. Riser systems should be designed recognizing that they are an integral element of the overall offshore production system. The following 2 sub-sections are from Safety, Section 3.14, API-2RD-1998 First Ed.:
SafetyPersonnel and platform safetyMarine riser safety issues should be integrated with the platform and/or vessel safety manual and procedures. Of particular concern are the handling of the large riser components and operating conditions that could lead to emergency disconnects or component failures. All personnel should be made aware (by training and guidance provided in the safety manual) of contingency measures and the consequences of riser system failures. The riser system designer should help ensure that these issues are included in the operational safety documentation. A two-independent barrier design should be considered for any riser when it is operating with environmentally damaging fluids inside. This could be two tubulars, one inside the other, such that the inner tubular carries the working fluid, and the annulus catches any leaks. This could also be a BOP or shear valve type mechanism installed on the seabed to stop the flow of working fluid should a riser leak develop. Other features such as SCSSV or completion kill fluid in the riser may also be considered as a barrier. Contingency plans would have to be made to purge, retrieve and repair the riser. The following is from General Design Considerations of Section 3.2, API-2RD-1998 First Ed.: Risk and reliabilityAlthough risk assessment is essential in the design process, the designer will have some latitude in selecting among the various qualitative and quantitative risk assessment techniques, depending on a project's specific regulatory environment. Whether qualitative or quantitative, risk assessment fits well within the framework of developing an efficient, cost-effective design through the early identification of hazards, assessment of failure consequences and frequencies of occurrence and identification of mitigation and prevention measures. Risk assessment is also particularly useful in comparing relative risks among riser design options. The consequences of failure are often assessed in terms of the possible hazards to personnel, effects on the environment and potential financial loss. Although the integrity of the riser system is the primary concern, a riser risk assessment should encompass a larger scope, including well system operations, marine operations (including escape and evacuation procedures) and system interfaces at the top and bottom of the riser. System deployment and retrieval procedures are also considered when assessing risk. The types of events to be considered in a risk assessment should include leaks (especially hydrocarbon releases), riser structural failure, component functional failure and major surface events: riser system leaks can occur in any number of components, from the riser (joints/couplings), downhole (casing/tubing, packers, seals, safety valves, etc.) or external (leak in neighboring riser, process area, etc.). Leaks can affect riser operability, riser structural integrity and well control; structural failure can result from excessive load, accidental impact, corrosion or fatigue; failure and/or inadvertent actuation of mechanical components can lead to undesirable events such as dropping the BOP stack and/or riser or disconnect of the subsea wellhead connector; major surface events include accidents such as fires, explosions, blowouts or vessel collisions that could be a serious detriment to riser system integrity. All FPS subsystems and interfaces should be considered in assessing the risks of these events to the riser system. The term “reliability” may have different connotations with reference to offshore structures. In its simplest form, reliability is synonymous with dependability (e.g., component reliability), and there are several databases that catalog the reliability (e.g., number of expected failures per year) of common components on offshore facilities. In formal risk assessment, reliability analysis focuses on estimating the probability of an event (e.g., release of hydrocarbons, component failure, etc.). In structural engineering, structural reliability analysis assesses a structure's ability to accommodate loads in excess of its design loads and estimates the likelihood of structural failure given anticipated loading conditions (e.g., determining a platform's reserve strength). Reliability is also the key word in the reliability-based structural design method, which differs from the working stress design approach followed in this RP by explicitly accounting for uncertainties in loading and in a structure's resistance to loads. One application of reliability-based design methods is the load and resistance factor design (LRFD) method that has been implemented for fixed platforms in API RP2A-LRFD. Reliability, as used here, relates to estimating the likelihood that the riser system will fail because of component failure, operator error, an external event or structural loading. Therefore, reliability assessment is an integral part of evaluating risk, which by definition combines both the frequency of occurrence and the consequences of an undesirable event. The validity of riser reliability assessments is a function of accumulated field experience. As more field data is compiled, the accuracy of these assessments will improve. 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