API TR 17TR12-2015 pdf download.Consideration of External Pressure in the Design and Pressure Rating of Subsea Equipment.
3 Terms, Definitions, Acronyms, and Symbols 3.1 Terms and Definitions For the purposes of this document, the following definitions apply. 3.1.1 design validation Process of proving a design by testing to demonstrate conformity of the product to design requirements. 3.1.2 design verification (assessment) Process of examining the result of a given design or development activity to determine conformity with specified requirements. 3.1.3 depth adjusted working pressure The maximum internal pressure a piece of equipment can contain and/or control with consideration of the equivalent external pressure at a specified water depth (measured in “psia”, absolute pressure). 3.1.4 differential working pressure The difference between the upstream and downstream pressures on a pressure-controlling element that defines the working pressure for the pressure-controlling equipment (measured in “psid”, differential pressure). 3.1.5 manufacturer (equipment) Individual or organization that is normally responsible for the design and manufacture of the equipment. 3.1.6 operator Individual or organization that normally uses the equipment (also referred to as “end-user”). 3.1.7 pressure The ratio of force to the area over which that force is distributed (i.e. pound-force applied to an area (in. 2 ), measured in “psi”, etc.): 1) absolute pressure: the internal pressure that the equipment is designed to contain and/or control or zero- referenced against a perfect vacuum, measured in “psia”; 2) differential pressure: the difference in pressure between any two points (p1 and p2), measured in “psid”; 3) gauge pressure: measured relative to the ambient pressure (e.g. atmospheric for surface application, hydrostatic for subsea application), measured in “psig”. 3.1.8 pressure-containing equipment Part whose failure to function as intended results in a release of wellbore fluid to the environment. EXAMPLE Subsea tree valve body, bonnet, stem.
3.1.9 pressure-controlling element The closure mechanism contained within pressure-controlling equipment. EXAMPLE Valve gate, choke, tubing hanger. 3.1.10 pressure-controlling equipment Part intended to control or regulate the movement of pressurized fluids. EXAMPLE Valve-bore sealing mechanism, choke trim, and hanger/packoff. 3.1.11 rated working pressure The maximum internal pressure a piece of equipment is designed to contain and/or control. NOTE For the purposes of this technical report, rated working pressure is defined as the absolute internal pressure minus 14.7 psia (see API 6A or API 17D). 3.1.12 specified water depth In situ or installation water depth for the subsea equipment as related to depth adjusted working pressure and differential working pressure.
4 Rating and Design Considerations 4.1 General In the context of this technical report, there are several considerations that need to be discussed in order to provide background information as related to the consideration of external pressure in the design of subsea equipment. 4.2 Definition of “Rated Working Pressure” There can be differing definitions for the term “rated working pressure” (RWP) when evaluating the effects of internal and external pressures in the design of subsea equipment. This technical report provides clarification of rated working pressure for subsea equipment which is subjected to external hydrostatic pressure from the in situ environment. Relative to API 6A, API 16A and API 17D oilfield equipment, the term “rated working pressure” is currently defined as “the maximum internal pressure that the equipment is designed to contain and/or control” and usually interpreted as absolute internal pressure minus 14.7 psia. This has been commonly simplified to the “absolute internal pressure” of the fluid contained within the equipment, measured in “psia” (psi absolute). However, wording between API 6A and API 17D are inconsistent. Confusion can arise when the terms “absolute” and “gage” or “gauge” pressure are used in subsea applications. Gauge pressure is the pressure relative to the local atmospheric or ambient hydrostatic pressure. At sea level, the absolute pressure in air is 14.7 psia, and the gage pressure is 0 psig. These terms could also be adopted for subsea, but the equipment designer needs to then be careful with the nomenclature and application.