Pipe Stress Analysis Scope and Specifications for Clients, Engineers and Contractors

Pipe Stress Analysis Scope and Specifications for Clients, Engineers and Contractors. by Meena Rezkallah, P.Eng. (piping stress engineer)
Pipe Stress Analysis Scope and Specifications for Clients, Engineers and Contractors. by Meena Rezkallah, P.Eng. (piping stress engineer)

This Article sets the rules for the piping stress analysis of the process and utilities piping systems of onshore and offshore petroleum installations.

Its scope is essentially the same as that of ASME Code B31.3. The following are specifically excluded: onshore and offshore pipelines, furnaces and boilers piping, building heating and ventilation piping, etc.

This document does not include the structural attachment between piping supports and the main structure.


The reference documents listed below form an integral part of this General Specification. Unless otherwise stipulated, the applicable version of these documents, including relevant appendices and supplements, is the latest revision published at the EFFECTIVE DATE of the CONTRACT.

Process and utilities piping shall comply with the requirements of ASME Code B31.3.

This document clarifies or complements some requirements of ASME Code B31.3 when deemed necessary considering COMPANY experience.

Piping systems covered by this specification shall be designed and analyzed in accordance with this Code and with the documents listed hereafter.

Any divergence between any of the Reference Documents, or between this specification and any Reference Document, shall be reported to the COMPANY for decision. In such a case, and unless otherwise agreed or decided by the COMPANY, it is understood that the more stringent requirement shall apply.

Exceptions to, or deviations from this specification are not permitted unless previously accepted in writing by the COMPANY. For this purpose, requests for substitutions or changes of any kind shall be completed with all pertinent information required for the COMPANY assessment.


ASME B16.5 Pipe Flanges and Flanged Fittings (NPS ½ through NPS 24)

ASME B16.47 Large Diameter Steel Flanges NPS 26 through NPS 60


WRC 107 Welding Research Council Bulletin 107 - Local Stresses in Spherical and Cylindrical Shells due to External Loadings


WRC 297 Welding Research Council Bulletin 297 - Local Stresses in Cylindrical Shells due to External Loadings on Nozzles - Supplement to WRC Bulletin 107


API 520 Part 2 Sizing Selection and installation of Pressure relieving devices in refineries

API 610 Centrifugal Pumps for General Refinery Service

API 617 Centrifugal Compressors for General Refinery Service

API 661 Air-Cooled Heat Exchangers for General Refinery Service

API 662 Plate Heat Exchangers for General Refinery Service

API RP 2A-WSD Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms - Working Stress Design

ASME B31.1 Power Piping

ASME B31.3 Process Piping

ASME III Div. 1 Subsection NB Rules for Construction of Nuclear Facility Components Class 1 Components

ASME VIII Div. 1 Rules for Construction of Pressure Vessels

BS 7159 Design and Construction of Glass-Reinforced Plastics (GRP) Piping Systems for Individual Plants or Sites

UKOOA Specifications and Recommended practice for the Use of Glass Fiber Reinforced Plastic Piping Offshore

PD 5500 Specification for Unfired Fusion Welded Pressure Vessels

1. Design basis 1.1 General Piping systems shall have sufficient flexibility to prevent thermal expansion or contraction from causing: · Failure of piping or supports from overstress or fatigue · Leakage at joints · Detrimental stresses or distortion in piping and valves or in connected equipment. Piping systems shall be designed in accordance with the requirements of the ASME B31.3 Code and the present specification. They shall take into account all the different conditions expected during their lifetime: operating conditions, environmental conditions, occasional and accidental conditions. All piping shall be adequately supported (anchors, guides, limit stop, etc.) to prevent excessive stresses from the different expected conditions. Pipe stress analysis shall be carried out at pressure and temperature conditions as stated in the Critical Line List. The purpose of this General Specification is to: · Categorize piping systems into Critical stress levels · Define criteria and coefficients so that loading conditions may be applied · State allowable stress levels to ensure safe plant operations.

1.2 Design Data The main design data of each line shall be taken from the following documents: · Line list: Operating and design pressures, operating and design temperatures, type and thickness of insulation (if any), type of hot tracing (if any), test conditions (pressure, fluid, etc.) · Piping material classes: Material, diameter, wall thickness, type of fittings, flanges and valves · Project Specification: Site data e.g. the minimum/maximum ambient temperature, the maximum solar temperature, the wind data such as wind speed for each grade level, the earthquake data, etc. 1.3 Method of Analysis The piping layout shall be analyzed by the individual consideration of each line affected by restraints (temperature, imposed displacements, etc.) to determine the reactions produced at anchors and terminal points and the stresses in the piping. The methods of analysis are classified into three Stress Analysis Levels. The levels of requirements and the corresponding methods of analysis are defined as follows: 1.3.1 Level 1: Visual inspection (or by analogy) It is an approximate, visual inspection method. It shall be restricted to lines that are similar to other calculated lines, or lines having a clear and adequate flexibility. If not, these lines shall be classified as level 2 or 3. No actual values of forces and moments acting on supports are requested for a level 1 analysis. 1.3.2 Level 2: Simplified analysis This method includes the use of charts, nomographs and simplified formulae which may only be accepted if they are used in the range of configuration for which their accuracy is acceptable. In case of doubt, the line shall be classified as level 3. The result of a level 2 analysis may be shown only on the calculation isometric, or in a simplified report including isometric and a computer calculations output restrain summary. 1.3.3 Level 3: Comprehensive analysis It is a comprehensive method by computer calculations exclusively that shall meet the requirements of the piping code. A complete calculation report shall be issued (refer to the § 12). The accuracy level selection shall be indicated in a document called "Critical Line List". This document shall include all the lines to be reviewed by the stress analyst and shall be submitted to the COMPANY for approval.

1.3.4 Exemptions A stress calculation needs not to be performed on the piping systems that meet one of the following requirements according to ASME B31.3, paragraph 319.4.1: · Piping systems which are duplicated, or are revised without significant changes of successfully operating operations · Piping systems which can be readily judged adequate by comparison with previously analyzed systems. 2. Critical Line List All lines of accuracy levels 2 and 3 shall be reported in the Critical Line List. These lines must be classified in accordance with the criteria shown on Appendix 1 (General Chart and Specific Cases). This requirement applies also to lines included in packages. To facilitate tie-ins and assign responsibilities, vendors will be responsible for the pipework design within their package and an anchor support must be provided at the limit of all packages. 2.1 Critical Line List for Basic Engineering At the Basic Engineering stage, a special care shall be taken to design the piping routing in a proper way to allow a good flexibility and to avoid any major changes during the detailed engineering when the piping stress analysis is finalized. Lines of level 3 to be calculated at Basic Engineering shall be identified in Critical Line List. This Critical Line List shall be updated during the Detailed Engineering in accordance with the possible modifications or Project up-dating. Unless otherwise required by the project, diameter 18” and above lines which are submitted to differential displacements (platforms or pipe racks differential displacements due to earthquake or to special operating cases) shall be checked as per level 3 lines. Piping stress analysis level 3 shall also include: · Lines submitted to wellhead displacement · Lines with oblique connections · Lines with expansion bellows · Carbon Steel and Low Temperature Carbon Steel lines used at a temperature lower than the minimum shown on the Piping Class · GRP lines larger than a diameter to be defined for each project. 2.2 Critical Line List for Detailed Engineering At the Detailed Engineering stage, the flexibility of all the piping systems shall be checked in the different conditions expected during their lifetime. The piping systems shall be able to withstand applied loads without permanent deformations at any time of the fabrication (i.e. load-out, transportation, lifting, partial hydrostatic test, etc.).

Lines of levels 2 and 3 to be calculated at Detailed Engineering shall be identified in the Critical Line List. This Critical Line List shall be regularly updated in accordance with the possible modifications. In addition to the Appendix 1 (General Chart and Specific Cases), the following criteria shall be considered in order to finalize the checking level: · Lines having substantial concentrated loads such as valves, fittings, unsupported vertical risers and branches · 3” and larger closed pressure relief system piping, where design temperature exceeds 93°C (200°F) or is less than -73°C (-100°F) · Lines having local reduction in strength due to installation of special fittings (i.e. oblique connections) · Piping 3” and larger connected to: - Rotating equipment such as pumps, blowers and compressors - Air cooler - Plate and frame heat exchanger - Gas turbine. · Blowdown and flare header or vent piping, but not atmospheric vent piping · Piping with pressure surge, slug and two phases flow · Normally dry firewater piping and normally water-filled firewater ring main piping · Piping subject to short term variations such as steam-out or purge piping · Pig receiver and launcher lines up to and including off-shore riser support · Relief valve or rupture disc piping reaction forces · Expansion bellows · Lines affected by deck deflection, platform settlement, wellhead movement or any other significant displacement · Lines subject to vacuum conditions · Cryogenic lines: - It is considered as cryogenic service all applications below -46°C · Lines which may create large forces or moments on structure or skid base · Unbalanced piping configuration, such as a long run of pipe with a short branch connected to anchor · Piping subjected to severe cyclic temperature conditions · GRP lines larger than a diameter to be defined for each project · Lines with special design requirements · In addition, the effect on piping of other conditions such as temperature gradients that could cause thermal bowing or where piping is connected to equipment with significant thermal growth · Lines design to B31.3 chapter IX · All piping with t > OD/6 or P/SE > 0.385 · 4” and larger with t > 10% OD · Lines with OD/t > 120 Where: · OD is the piping Outside Diameter · t is the piping thickness · P is the Design Pressure · SE is the Allowable Stress as per defined in the ASME B31.3 code for the thickness calculation. 3. Piping Stress Analysis 3.1 List of calculation reports A list of the calculations reports shall be issued and regularly updated. The lines which are subject to approximate or comprehensive analysis methods (levels 2 and 3) shall be registered on this list which shall include the following information: · Line number, including revision and date · Calculation number · Type of analysis · Revision and date of issue. 3.2 Screening or hand calculations All level 1 and level 2 lines shall be checked by piping and/or supporting engineers. Screening or hand calculations shall be used to verify that enough inherent flexibility exists in the piping system to prevent excessive thrusts, stresses and distortion. Differential displacements shall be considered. All piping stress calculation notes of level 2 lines shall be submitted to COMPANY for review. 3.3 Computerized analysis Comprehensive computer analysis shall be performed for critical lines level 3 (or assimilated level 3). The calculation program used shall be CAESAR II, 4.50 or 5.0 version, developed by COADE Inc. (Houston - Texas USA). The lines shall be calculated, including branch lines up to the first anchor or acceptable boundary condition when branch diameter ³ half header diameter.

When small size lines (branch) connected to main lines (header) are to be calculated separately (when branch diameter < half header diameter) the linear and angular movements of the junction point shall be calculated or estimated for proper analysis of the branch line. Axis for piping stress analysis shall be the same as for the structure calculation, normally: Onshore site or offshore platform FPSO unit X = North direction X =Horizontal axis parallel to the FPSO longitudinal axis (forward)Y = Transversal axis (toward West)Y =Horizontal axis perpendicular to X (toward portside)Z = Vertical Axis (upward)Z =Vertical Axis (upward) All piping stress calculation notes of level 3 (or assimilated) shall be submitted to COMPANY specialist for acceptance once the CONTRACTOR has approved them. Input data for all Piping Stress Calculation Notes (in electronic files) will be transmitted to COMPANY for check. The computer outputs shall be submitted to the COMPANY for review. 3.4 Units Unless otherwise specified by the Project requirements, the unit system shall be the metric system. 4. Design Conditions The following paragraphs detail the design conditions, the Caesar II model, the load cases and the acceptance criteria to be considered. 4.1 Design Pressure The design pressure of each component in the piping system shall be equal to the most severe design conditions expected, indicated in the Critical Line List. In particular cases, the design pressure specified in the Critical Line List may be replaced by the maximum operating pressure upon COMPANY approval. 4.2 Calculation Temperature The basic temperature for the flexibility analysis shall be the Design temperature; the temperature for the occasional conditions such as start-up, cool-down, shutdown, regeneration, depressurization and other special conditions shall also be considered. This temperature shall be taken from the process and utilities list and shall be specified on the Critical Line List. The following rules shall be taken into account: 4.2.1 Nozzles and supports The maximum operating temperature shall be considered for nozzle loads on rotating equipment and air coolers. This temperature is taken from the process and utilities lines list.

The thermal displacement of the equipment nozzles shall be calculated using the relevant line temperature. The temperature homogeneity shall be met between piping and equipment. The operating temperature may be applied for the calculation of the loads on the supports. 4.2.2 Solar heating temperature The solar heating temperature shall be used for the design of the exposed un-insulated lines when the operating temperature is lower than the solar heating temperature. The value of the solar heating temperature to be considered shall be specified in the Project environmental conditions (i.e. 60°C for exposed pipes in Africa). 4.2.3 Dry out and steam out Dry out and steam out temperatures (if any) shall be taken from the process and utilities list. In case of steam traced lines, the operating temperature of the traced line or 70% of the steam temperature, whichever is greater, shall be considered. For jacketed lines, the operating temperature of the heated line or the steam temperature, whichever is greater, shall be considered. 4.2.4 “Dead branch” For the lines which do not have flow, such as piping connected to the spare pumps, by-pass, etc., the following value shall be considered: · Insulated line: 50% of the operating temperature in the dead branch · Non-insulated line: plant temperature in the dead branch · Steam traced line: 70% of the operating temperature of the steam line in the dead branch · Warm-up by-pass of the spare pumps: 70% of the operating temperature of the line. 4.2.5 Plant temperature The plant temperature is defined by considering the Project environmental conditions: · For cold piping stress, the installation temperature is the maximum air temperature · For hot piping stress, the installation temperature is the minimum air temperature. 4.2.6 Line with a dual operating temperature For lines subject to dual operating temperature (both above and below the installation temperature), the thermal effects shall be checked by considering the full thermal range: · From the installation temperature to the hot temperature and · From the cold temperature to the installation temperature. 4.3 Thermal Effects The following thermal effects, combined with loads and forces from other cases, shall be taken into account for the design of piping: · Thermal loads due to restraints or anchors · Loads due to temperature gradients (bowing effect)

· Loads due to difference in expansion characteristics. ASME B31.3 code requires to check the “Displacement Stress Range” including the stresses induced by both the thermal effects (range) and the support displacements. 4.4 Snow loads - Ice The analysis for snow or ice loads depends of the climatic conditions specified in the Project environment conditions. For un-insulated cold lines where ice formation is anticipated, the additional sustained load due to ice shall be considered for relevant stress evaluation and piping support design. Note: heat traced lines shall be considered too (in case of a possible failure of the heating system). 4.5 Dynamic Effects 4.5.1 Surge effects The loads generated by overpressure in the lines due to water hammer effect shall be taken into account where applicable. They shall be based on Process data (transient rate). Data obtained from a process simulation model which can predict valve Cv as a function of time shall be used as input to the CEASAR model. The method and the model used for the surge effects study shall be submitted to the COMPANY acceptance before calculation. An equivalent static stress analysis shall be performed where the force due to the pressure surge is applied at appropriate locations (changes of direction). The forces shall be modelled acting on straight part of pipe (at the inlet node of the elbows for example). Piping supports and their attachment to the main structure shall be designed to withstand these loads. A force spectrum dynamic analysis shall be performed when high surge pressure fluctuations and high stress level (or high loads on supports) are obtained from the static stress analysis. The lines submitted to the surge effects shall be identified on the Critical Line List. 4.5.2 Slug effects The slug force shall be applied at change of direction (in the middle of the elbows for example) for the calculation of the stresses and pipe support loads. Piping supports and structure shall be designed to withstand these loads. The lines submitted to the slug effects shall be identified on the Critical Line List. Calculations of slug loads on 90° bends shall be based on the following simplified calculation which assumes equal inlet and outlet velocities and no change in pressure FR = r.a.V2.DLF Where: FR is the resultant force as slug strikes elbow r is the density of the fluid a is the internal surface of the pipe

V is the slug velocity DLF is the Dynamic Load Factor (DLF = 2). 4.5.3 Wind The effect of wind loading shall be taken into account in the design of an exposed piping system. Wind will be taken in X or Y axis whichever is the more stringent according to the prevailing wind direction. Maximum wind speed shall be taken from the Project environment conditions. The wind shape factor shall be 0.8 unless otherwise specified by the Project specifications. Generally wind loads are derived from API RP2A WSD rules. 4.5.4 Earthquake 4.5.4.1 Accelerations Piping shall be designed for earthquake-induced forces. Generally the accelerations correspond to the seismic site conditions. Accelerations to be considered shall be defined by the structure studies (platforms and racks). Envelop values can be used on a first step however they shall comply with the structure final calculations. The structural deflections induced by the earthquake shall also be considered. 4.5.4.2 Deflections Structural deflections and equipment displacements due to earthquake accelerations shall be introduced in case of significant relative displacement. Estimated values can be considered at the stage of the Basic Design in accordance with the Project environment conditions. However at final stage, the estimated values of the structures displacements shall be compared with the results of the global calculation performed by the Structure discipline. If the final values are higher than the preliminary one, piping stress analysis shall be rerun. Otherwise the estimated values can be kept. Differential displacements due to earthquake shall be considered. 4.5.5 Discharge reactions - Safety valves Forces and moments due to Pressure Safety Valve discharge shall be taken from the Instrumentation PSV data. If such data are not available estimation can be made according to: · ASME B31.1 Appendix II using a Dynamic Load Factor (DLF) of 2.0. · API 520 Part ll section 2.4.1 using a Dynamic Load Factor (DLF) of 2.0 This hypothesis shall be confirmed then by the PSV data sheet. In case of several PSV connected on the same upstream header, the DLF shall be applied on one valve at a time, in order to avoid the over design of the structure.

Acoustical induced vibrations shall be checked according to the CONCAWE report n° 87/59 or to another relevant rule to submit to COMPANY. 4.5.6 Inertial accelerations 4.5.6.1 Accelerations during the transport The accelerations corresponding to the transport shall be considered (towing, transport of modules, etc.). As far as possible, the piping supports shall be designed in order to avoid additional sea- fastening during the tow-out. The cooling water lines and the fire water lines are to be calculated full of water during the transport. The other lines shall be empty. 4.5.6.2 Accelerations during operation (Offshore conditions) These accelerations correspond to the swell or hull motion on site. The accelerations values shall be defined by the Naval or Structure discipline and shall comply with the structure final calculations. Envelop values can be used on a first step however they shall be conservative. For the lines connected on sensitive equipment (i.e. compressors) more accurate local accelerations can be used. In that case, they shall be clearly indicated in the calculation report. 4.6 Weight Effects The following weight effects, combined with loads and forces from other causes, shall be taken into account in the design of piping. 4.6.1 Live loads These loads include the weight of the medium transported or the medium used for test. Snow and ice loads due to both environmental and operating conditions shall be considered. 4.6.2 Dead loads These loads consist of the weight of piping components, insulation, and other superimposed permanent loads supported by the piping like valves, flanges, etc. 4.7 Effects of Supports and Structural Deflections The effect of movements of piping supports, anchors, structure and connected equipment shall be taken into account in the design of the piping. These movements may result from the flexibility of equipment, supports, or anchors; and from settlement, tidal movements, or wind sway. All imposed deflections applicable to the loading conditions shall be considered: 4.7.1 Deflections due to inertial accelerations Structural deflections and equipment displacements due to accelerations shall be introduced in case of significant relative displacement.

4.7.1.1 Basic Design Estimated values of horizontal and vertical deflections to be used for preliminary calculations shall be defined in early stage with the Structural Analysis Group. These values shall be compatible with the current displacements imposed to the piping system. An integrated model considering both the structure and the piping may be necessary for the estimation of these values. In case of a pipe installed through several structures, the worst case to be considered is “first structure” moving, “second structure” without displacement, “third structure” moving, and so on… For the design of the piping attached on the hull (risers, etc.), the deflection of the topside floor shall be introduced in the calculation. 4.7.1.2 Detailed Design The same procedure may be used for the Detailed Design, but at final stage, the estimated values of the structure displacements shall be compared with the results of the global calculation performed by the Structural Analysis Group. If the final values are higher than the preliminary one, piping stress analysis shall be rerun. Otherwise the estimated values can be kept. 4.7.2 Deflections due to hogging and sagging (Floating Units) Differential displacements between the deck and the topsides modules structures (or between two modules) due to the swell, the hull motion, and the differential tank filling shall be taken into account for the maximum displacement stress range check. In the case of a Floating Unit, the longitudinal differential displacements (X axis) due to the hogging and sagging shall be considered. The transverse differential displacements (Y axis) due to the hull hogging and sagging are usually not significant, but should be considered if piping is rigidly supported transversely from the hull. Consideration shall also be given to the construction methodology: construction on-shore followed by load-out to a floating condition can result in larger deflections of the hull than would occur in tow or in service. CONTRACTOR shall propose a method for the combination of the different displacements (in phase or out of phase). 4.7.2.1 Envelope values for Basic Design The following values can be used at the Basic Design stage: · Sagging deflection at the middle of the hull/platform = -400 mm on a flexural length of 200 m · Hogging deflection at the middle of the hull/platform = +400 mm on a flexural length of 200 m · Longitudinal strain of the hull deck should assume 1 mm/m. Other values can be considered if justified but they shall be submitted to the COMPANY for acceptance.

4.7.2.2 Detailed Design The values used for the Basic Design are normally conservative and shall be updated for the Detailed Design. For base-anchored elevated equipment, rotations shall be taken into account. Final values of displacements shall be evaluated from the global model calculation (deck + structure + pipe) by Structure discipline and compared with preliminary one. If the final values are higher than the preliminary ones, used at the basic engineering stage, piping stress analysis shall be rerun. Otherwise the preliminary values can be kept. The cases to be analyzed shall consider tow, 1-year and 100-year operating cases (on site). Accident conditions, including accidental inclination of the hull of a floating structure shall be considered. 4.7.3 Differential displacements Inter and intra platform (deck or module) relative movements are required for the design of the concerned piping. The differential displacements for seismic effect, swell (or hull) effect and operating effect shall be considered between platforms (decks or modules) and between each structure (or module) within the same area (air coolers structure, pipe racks, etc.). CONTRACTOR shall provide a method explaining the differential displacements considered with the justification of their value. These hypotheses shall be defined in the Piping Stress Design Basis. 4.8 Hydrostatic test conditions For the hydrostatic test, the test pressure shall be taken as follow: · For rating 600# and below: 1.5 time the Piping Class Maximum Pressure · For rating 900# and above: 1.5 time the Maximum Design Pressure. It will be specified on the calculation note that spring supports and expansion joints are “locked-out” on the field to prevent any excessive deflection and over-stressing of the system. 5. Caesar II model 5.1 Boundary conditions A level 2 line or a level 1 line connected to a level 3 line shall be included in input data of the level 3 line calculation note with partial routing at the appreciation of Pipe Stress Engineer for boundaries conditions only. These partially calculated lines shall not be included in the Critical Line List. 5.2 Piping supports Except the case of the spring supports, piping supports shall be included in the piping stress analysis by considering them as rigid elements into CAESAR II model. Characteristics of the spring supports shall be specified in the relevant calculation note (movements, operating & calibration loads, spring rate, hydrotest loads & travel stop).