1、Subsea Equipment Pressure RatingsAPI TECHNICAL REPORT 17TR4SECOND EDITION, XXXXX 2016Special NotesAPI publications necessarily address problems of a general nature. With respect to particular circumstances, local,state, and federal laws and regulations should be reviewed.Neither API nor any of APIs
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16、rs. A one-timeextension of up to two years may be added to this review cycle. Status of the publication can be ascertained from theAPI Standards Department, telephone (202) 682-8000. A catalog of API publications and materials is publishedannually by API, 1220 L Street, NW, Washington, DC 20005.Sugg
17、ested revisions are invited and should be submitted to the Standards Department, API, 1220 L Street, NW,Washington, DC 20005, standardsapi.org.iiiContents 1 Scope . 1 2 Normative References . 1 3 Definitions and Nomenclature . 1 3.1 Definitions . 1 3.2 Nomenclature . 1 4 Design Issues . 2 5 Example
18、Application . 3 6 Discussion 5 7 Conclusion 5 Bibliography 7 Figures 1 Example Vessel Under Pressure and Longitudinal Loading. 3 2 Loading on Example Vessel Broken into Two Components . 4 Subsea Equipment Pressure Ratings 1 Scope The impact of operation in deep water on the pressure rating of equipm
19、ent is a special concern. The objective of this document is to foster a better understanding of the effects of simultaneous internal and external pressures on the rated working pressure of equipment covered by the scope of API Specification 17D, Subsea Wellhead and Tree Equipment. Additionally, it i
20、s intended to provide a high-level overview of issues that should be considered if a user elects to consider differential pressure in their design, especially in components with irregular geometry and with high stress concentrations. It is not intended to serve as a design specification. This docume
21、nt was prepared in response to a request from the API Subcommittee 17 (SC17). 2 Normative References The following referenced documents are essential when considering the examples outlined in this document. For dated references, only the edition cited applies. For undated references, the latest edit
22、ion of the referenced document (including any amendments) applies. API 17TR12, Consideration of External Pressure in the Design and Pressure Rating of Subsea Equipment ASME Boiler and Pressure Vessel Code, Section VIII, Division 2 1, “Rules for Construction of Pressure Vessels, Alternative Rules, 20
23、10 Edition ASME Boiler and Pressure Vessel Code, Section VIII, Division 3, Rules for Construction of Pressure Vessels, Alternative Rules for Construction of High Pressure Vessels, 2010 Edition 3 Definitions and Nomenclature 3.1 Definitions For the purposes of this document, the following definition
24、applies. 3.1.1 rated working pressure The maximum internal pressure a piece of equipment is designed to contain and/or control. Source: API Spec 17D, API Spec 6A 3.2 Nomenclature A cross-sectional area of vessel wall Diinside diameter of vessel Dooutside diameter of vessel d diameter variable, such
25、that Did Do F external applied force 1ASME International, 3 Park Avenue, New York, New York 10016-5990, www.asme.org. 2 API TECHNICAL REPORT 17TR4 Piinternal pressure Poexternal pressure Saaxial stress Shhoop stress Srradial stress SVMEvon Mises equivalent stress 4 Design Issues During the design of
26、 any piece of equipment, all loads and conditions that may realistically occur must be considered, including accidental loads. A complete functional understanding of the system is needed to appropriately define design loads and operating conditions. API 17D defines rated working pressure (RWP) as th
27、e maximum internal pressure that the equipment is designed to contain and/or control (see API 17D 2011, 3.1.42). This is an absolute pressure. The API specifications state that the effects of external load, such as external pressure, should be taken into account in the design, but the use of externa
28、l pressure to increase the equipment RWP is not recommended. The equipment design must be evaluated for fitness for service and shown to provide sufficient margins for all relevant failure modes in a consistent manner. As stated in API RP 17G, failure is an event causing an undesirable condition, e.
29、g. loss of component or system function, or deterioration of functional capability to an extent that the safety of the unit, personnel or environment is significantly reduced (see API 17G 2006, Section 3.1.50). Failure of a pressure vessel is commonly described using the following forms: ductile fai
30、lure (called plastic collapse in ASME BPVC); brittle failure resulting from use of brittle materials or from environmental cracking; fatigue failure resulting from cyclic loading; failure from service criteria (as defined by the manufacturer, such as elastic deflection resulting in binding of compon
31、ents). Systems must be evaluated for additional potential failure modes such as: failure of sealing mechanisms failure of non-metallic sealing materials failure of closure bolting Therefore, limits must be defined for materials and loads to provide protection against the appropriate types of failure
32、. ASME BPVC Section VIII, Div. 2 and ASME BPVC Section VIII, Div. 3 require that the design of the vessel parts shall be limited to values that ensure an adequate safety margin against relevant failure modes under the stated conditions. Examples of loads and conditions or combination of conditions t
33、hat must be considered during the equipment design phase include: internal pressure, external pressure, SUBSEA EQUIPMENT PRESSURE RATINGS 3 axial loads (tension and compression), bending loads, collapse and buckling loads, cyclic loads, temperature effects, corrosion/erosion/wear/galling, fluid comp
34、atibility. It is essential that the designer be able to justify that the design has adequate margins to protect against failure due to any reasonable combination of possible events during the life cycle of the product. Safety margins are addressed in industry-specific codes, standards, and recommend
35、ed practices. However, these documents are usually not intended to be used as a handbook and must be applied in conjunction with education, experience, and careful engineering judgment. API 1111 states the following: “Nothing in this RP should be considered as a fixed rule for application without re
36、gard to sound engineering judgment” (see API 1111 1999, Section 1.5). API 17D further states that: “Users of this standard should be aware that additional or different requirements might better suit the demands of a particular service environment, the regulations of a jurisdictional authority or oth
37、er scenarios not specifically addressed” (see API 17D 2011, Introduction). 5 Example Application The following example of a closed-end cylinder is shown only to illustrate the effect of external pressure on the stresses of the cylinder. The example does not represent a complete design verification a
38、nalysis of the vessel, but only one aspect of the design verification analysis. Figure 1 represents a simple closed-end cylinder being acted on by both simultaneous internal pressure (Pi) and external pressure (Po). There is an external longitudinal force (F) in addition to the pressure loads. Figur
39、e 1Example Vessel Under Pressure and Longitudinal Loading 4 API TECHNICAL REPORT 17TR4 Figure 2 presents a pair of free-body diagrams of forces that, with superposition, are equivalent to Figure 1. In Figure 2 are (1) a closed-end vessel with only internal pressure and an externally applied force pl
40、us (2) the effects of hydrostatic pressure acting both internally and externally on the vessel. Figure 2Loading on Example Vessel Broken into Two Components Radial, hoop, and longitudinal stresses on the sides of the vessel can be determined using the Lame equations for an elastic cylinder without s
41、tress concentration areas. Radial Stress: o22o2i2o2ioirP)dD1(DDD)PP(S = foroiDdD Hoop Stress: o22o2i2o2ioihP)dD1(DDD)PP(S += foroiDdD Longitudinal Stress: oioioiaPDDDPPAFS +=222)( where the cross-sectional area, /422(A = D oD i) The von Mises equivalent (VME) stress is defined as: VME Stress: ( ) (
42、) ( ) 22221haarrhVMESSSSSSS += which, after substituting in the stress definitions, reduces to SUBSEA EQUIPMENT PRESSURE RATINGS 5 VME Stress: ( )2222223 +=AFdDDDDPPSoioioiVMEforoiDdD 6 Discussion Note that the VME stress calculation for the cylinder is a function of differential pressure, (Pi Po),
43、and external force, F. Therefore, the failure criterion based on the VME stress is affected only by differential pressure and F. If the relative difference between Piand Podoes not change, as is the case with an internally pressurized vessel evaluated at both atmospheric conditions and in deepwater,
44、 the VME stress does not change. For a constant differential pressure, the VME stress is constant regardless of the level of the external pressure, Po. However, the principal stresses are affected by Po. The radial, hoop, and longitudinal stress all have Poterms in addition to differential pressure
45、terms. The principal stresses are important in analyzing failure from cyclic loading. Also note that the externally applied force is usually different for subsea equipment as compared to surface equipment because of different system requirements and loads. This is why the designer must fully underst
46、and the operating conditions and apply all appropriate loads in the design verification and validation processes. To emphasize the point, while VME stress can be easily accounted for in a differential pressure design analysis for subsea equipment, full, meticulous consideration of the principal stre
47、sses and their effects must also be completed and resolved. A single load cycle analysis of equipment is typically based upon the VME stress level and an adequate safety margin for the design. As shown, the VME stress is a function to the differential pressure across the pressure vessel. However, th
48、e life cycle analysis of the equipment, fatigue is based upon the principal stresses. In deepwater applications, the principal stresses are affected by the high hydrostatic external pressure and the differential pressure. Hence, the fatigue life of subsea equipment will be different from surface equ
49、ipment even though the differential pressure remains constant. In addition to the general stress conditions, localized stress concentration areas also need to be well understood in engineering design for deep water. These localized stress concentration areas may likely be loaded into the plastic stress region. Examples of localized stress concentration areas are fillets, welds, the root diameter of threads, areas of discontinuity, and the intersection of cross-bores. These regions, ma
