API Offshore Standards - Underlying Risk Values and Correlation WIth ISO

Abstract

This paper provides an overview of the current efforts of API to provide an overarching standard within the API Series 2 standards for the design and construction of offshore structures which would be similar to ISO 19900. The paper provides a historical perspective of the current suite of API standards, discussion of the current practices, current issues in the proposed standard (referred to as API RP 2GEN), and future plans.Offshore structures are designed according to ISO / API standards. The ISO 19901 series lists specific requirements for design, construction and operation of offshore facilities with provisions that are applicable to different facility types (fixed, floating, or arctic structures), different materials and the possible range of operating environments. The structure-dependent requirements for various types of offshore facilities are listed in ISO 19902-19906 and corresponding API series.The ISO / API standards for offshore structures generally follow reliability-based design, i.e. the design procedures have been developed or calibrated to ensure minimum target reliability levels depending on the life safety, environmental and economic consequence of failure. It is important to recognize that ISO / API procedures enforce minimum standards on a wide range of assumptions and activities: description of actions, modeling and simulation of action scenarios, structural or global analysis, design rules, model testing, workmanship, quality control etc. Hence the modification of one or more aspects of design standards can disturb the delicate balance in the design and lead to unintended consequences in the target reliabilities.The main objective of API 2GEN standard is to introduce consistency in terms / definitions / assumptions and high level requirements, with special emphasis on those that are used in setting design reliability targets in the lower level ISO / API documents. The API committee that is developing API 2GEN is recommending several new definitions related to exposure levels and ULS / ALS requirements. The emphasis of the committee recommendations is on conditions applicable to the Gulf of Mexico (GOM), e.g. shut-in of the wells and evacuation of personnel ahead of forecasted hurricanes Background - Structure of StandardsThe history of the API series of offshore structures standards, Recommended Practices (RPs), Specifications (Specs) and Bulletins began in the 1960's and has been well documented (1) (2). The first standard was API RP 2A released in 1967 focusing on fixed steel jackets and decks. The RP was quite expansive in scope covering steel member design, hydrodynamic loading (wind/wave/current), soils and foundations, fabrication, etc. Starting as a 16 page document, the RP rapidly expanded as the industry matured to well over 200 pages by the early 1990s and included additional topics such as inspections, installation, welding, etc. With the relative complete coverage of areas such as metocean, foundations and seismic, other API documents, such as RP 2T (Tension Leg Platforms), 2...

Site-specific variability of load extremes of offshore wind turbines exposed to hurricane risk and breaking waves

“…An example of a prescriptive standard that is designed to result in a uniform reliability for conditions including hurricanes regardless of site and structure is the so‐called RR by API RP 2A . This method states that, rather than using fixed load factors to amplify loads at a given MRP to achieve a target failure MRP, the structure should be designed directly for the environmental conditions at the targeted failure MRP (i.e., amplifying loads by the ratio of the load at the target failure MRP to that at the design MRP).…”

“…This method states that, rather than using fixed load factors to amplify loads at a given MRP to achieve a target failure MRP, the structure should be designed directly for the environmental conditions at the targeted failure MRP (i.e., amplifying loads by the ratio of the load at the target failure MRP to that at the design MRP). This method enables explicit and site‐specific consideration of the variability in loads relative to changes in the MRP of the load . The effect of the RR method is evaluated in this paper by comparing the fixed load factors recommended by the IEC method with load ratios calculated based on the RR method for normal and abnormal conditions and for 16 site‐specific monopiles designed according to the IEC method.…”