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Proceedings of Onshore Pipelines Conference 2000 IBC’s 4th International Event October 2000, Paris FITNESS-FOR-PURPOSE ASSESSMENT OF ENCIRCLEMENT SPLIT-TEES C S Jandu, S Wheat, A Goodfellow, D N Bramley BG Technology Ltd, Loughborough, UK ABSTRACT Hot-tap tees of the full encirclement split-tee type design are currently being used in UK gas industry to provide connections onto existing high pressure gas transmission pipelines and above ground installations. The fitness-for-purpose of these types of tees are not covered by the UK design code for above ground gas installations IGE/TD/9. A fitness-for-purpose methodology has been developed by BG Technology Ltd to determine the integrity of the fitting and attachment welds. The fitness-for-purpose assessment addresses the following; •

The compliance of the fitting to plastic collapse, shakedown and fatigue design criteria.

The integrity of the attachment welds onto the carrier pipe.

The use of Engineering Critical Assessments in conjunction with existing procedures to ensure overall integrity.


INTRODUCTION In-service welding or hot-tapping is a technique used to connect pipes or

pipelines into other items of equipment without shutting the pipeline down. The need arises when a connection needs to be made to an existing pipeline for gas operators to meet a specific demand. Hot-tap tees of the full encirclement split-tee (referred to hereafter as split-tee) type design are currently used by the gas operator BG Transco plc to provide an

economic connection to the UK High Pressure Gas National Transmission System (NTS). BG Transco plc is responsible for the design, construction, operation and maintenance in accordance with current legislation and appropriate standards to ensure fitness-for-purpose. BG Technology Ltd has been performing the fitness-for-purpose assessments of split-tees on behalf of BG Transco plc, and have developed a methodology, Figure 1, for ensuring the integrity of the split-tee for its intended design life.


assessments, integrated with welding and inspection procedures, are used to ensure safe operation of the fitting. The intention of this paper is to briefly review the design and integrity issues that the methodology in Figure 1 is intended to address.




Welding Procedures The welding of fittings onto gas pipelines under pressure within the BG Transco

plc transmission system is covered by the P9 (Ref. 1) procedure. This procedure was originally developed by the British Gas Engineering Research Station and has been used successfully for over 25 years. The key elements of the procedure include the use of full encirclement split-tees fittings and the precautions to avoid the potential problems of burn through/blow out, hydrogen cracking and lamellar tearing. In recent years the demand for the installation of split tee fittings onto large diameter, high pressure pipelines has increased.

This has coincided with a

requirement for split-tees to be designed for higher gas pressures (>70bar). This has raised two specific issues, namely: •

There is a design requirement for heavy-wall split tees which according to the welding codes should be subjected to post weld heat treatment (PWHT) in order to relieve the residual stresses. This is potentially a hazardous process and PWHT should be avoided on in-service pipelines, if possible.

The large volume of gas flowing through the pipelines causes an increase in the cooling rate and hence a reduction in time available for welding between preheating cycles.

Ultimately the integrity of the split-tee largely depends on well-established welding procedures such as P9, and for that to continue the design of split-tees and the integrity of these geometries need to be reviewed. 2.2

Split-Tee Design The type of split-tee used by BG Transco plc is shown in Figure 2. The BG

Transco plc split-tee specification F4 (Ref. 2) specifies the use of the ‘area replacement’ method in ANSI (ASME) B31.3 piping code (Ref. 3) to design the fitting. The ‘area replacement’ methodology is used to determine the fitting wall thickness and reinforcement length based on internal pressure alone. The method has been used for many years and the concept requires that the metal cut out for the opening be replaced by reinforcement within a prescribed zone around the opening. The concept is relatively simple and the vast majority of vessels and piping with openings conforming to this concept have given satisfactory performance in the past. The area replacement method is based on internal pressure being the only design load. However, for piping components such as the split-tee, system loads at above ground installations can be significant. The following section reviews the current area replacement rules used in the design of split-tees and provides some recommendations to manufacturers to account for system loading. 2.3

ASME B31.3 Area Replacement Method The rules governing the area replacement method are simple (Figure 3 is used

for reference). Where the area provided is greater than the area to be replaced then the design is considered to be satisfactory for internal pressure. No account of the fillet weld areas and carrier pipe is taken into consideration as stipulated in the BG Transco plc specification F4.

Although these rules are simple to apply, there is no guidance in F4 with respect to where the reinforcement should be added. The implications are that some of these split-tees will be connected to other pipelines, and possibly be buried, thereby inducing significant system loadings. Using the area replacement philosophy, it is possible to have a design with a branch thickness that just meets the pressure requirements but makes no allowance for system loading. Therefore, there is a strong possibility that subsequent system loadings on the branch may overstress the component and possibly cause the component to fail by either plastic collapse, incremental plasticity (ratchetting) or fatigue. This is one issue that F4 and the manufacturer’s could address at the design stage if the magnitude of the system loading for which the split-tee is to be designed for was known. However, this information is not known until a flexibility analysis of the layout has been performed. A fitness-for-purpose assessment method of split-tees is therefore required to determine the significance of the system loads, once the design of the split-tee to internal pressure has been confirmed and once the flexibility analysis results are available. 2.4

Avoidance Of Post Weld Heat Treatment (PWHT) For large diameter (>36in), high pressure (>70bar) gas transmission pipelines,

the area replacement methodology requires large wall thicknesses of the run and branch pipes, typically 50mm.

The ASME code would require welds of these

thicknesses to undergo PWHT. The F4 specification requires the fitting ends to be locally reduced by chamfering to a thickness (Figure 4) which is twice the carrier pipe wall thickness at the locations where the circumferential fillet welds are to be made. It is considered that, not only is this used to control the welding operation by reducing the number of weld deposits, but also as a method of ensuring the fillet welds avoid the requirement of any PWHT. By reducing the throat area of the weld, for large diameter, high pressure pipelines, these welds can become highly stressed due to the operational loadings.

Therefore, the integrity of these welds requires further assessment and is addressed in this paper.


STRESS ANALYSIS BG Transco plc require the conformance of any above ground installation (AGI)

piping system to IGE/TD/9 (Ref. 4) which subsequently requires a pipe stress analysis to be performed in accordance with IGE/TD/12 (Ref. 5). The design criteria contained within IGE/TD/12 ensures that any fitting does not fail by plastic collapse (sustained), incremental plastic collapse or ratchetting (shakedown) and fatigue. The problems associated with this type of split-tee in performing the IGE/TD/12 assessments are the following; •

Geometrical stress concentration factors for split-tees designed to F4 are not contained within IGE/TD/12 or ASME B31.3.

The internal pressure in the annulus between the fitting and the carrier pipe causes the circumferential fillet welds to be highly stressed. Figure 5 shows the opening up of the carrier pipe and split-tee fitting with the circumferential welds exhibiting a highly localised stress region. This response of the split-tee, cannot be adequately modelled using the simple beam element pipe stress model. The pipe stress model does serve to provide system forces and moments to

which the split-tee would be subjected. This is achieved by modelling the stiffness of the split-tee similar to the encirclement type geometry in IGE/TD/12. Using the forces and moments from the pipe stress analysis a more detailed analysis is performed using a 3-dimensional finite element model (Figure 2) of the split-tee. 3.1

Modelling Figure 2 shows a half model of a typical split-tee. The split-tee mesh is

constructed from between 30,000 to 50,000 20-noded reduced integration brick elements of ABAQUS (Ref. 6) type ‘C3D20R’ with 4 elements through the thickness. To date all the analyses performed by BG Technology Ltd have been linear elastic with solution times of approximately 1 hour. In determining an adequate mesh,

particular attention is given to the circumferential fillet welds and the crotch location around the fitting. No gap is modelled between the carrier pipe and the fitting. However, one of the main verification aspects of the analysis is to determine whether any contact occurs between the carrier pipe and fitting. Where contact is predicted then a nonlinear contact analysis should be performed. The pre and post-processing is carried out using PATRAN Version 9.0 (Ref. 7) and the analysis code is ABAQUS Version 5.8. 3.2

Design Conditions Split-tees on the NTS are designed for at least 70bar design pressure.

IGE/TD/9 specifies that the maximum design temperature is +60oC and the minimum design temperature is –20oC. Piping expansion and contraction is based around a thermal stress-free datum temperature referred to as the tie-in temperature. For the plastic collapse analysis, pressure and dead weight loadings are applied on the split-tee model. If the split-tee is to be buried then the soil effects should be included by extracting the forces and moments from the flexibility analysis. For the incremental plastic collapse or ratchetting (shakedown) assessment, both primary (pressure) and secondary (thermal) loadings are required. For this assessment the loadings giving rise to the maximum stress range are required. For the fatigue assessment, BG Transco plc require a fatigue life of 40 years that comprise the following fatigue cycles; 

Commissioning/recommissioning full design pressure and temperature cycles,

Compressor operation, pressure and temperature fluctuations,

Winter diurnal, pressure and temperature fluctuations and

Summer diurnal, pressure and temperature fluctuations.

The above analyses are conducted using the PD 5500:2000* (Ref. 8) & ASME VIII (Ref. 9) stress categorisation design-by-analysis criteria. These are used to determine the conformance of the fitting to the required plastic collapse, shakedown and fatigue assessment. This is discussed in the following section.


ASSESSMENT CRITERIA IGE/TD/12 code allowable criteria are specific to pipe stress analysis results.

When a more detailed finite element model of a fitting has been created, the onset of failure can be more accurately understood using more appropriate design assessment rules. A method is given in PD 5500 and ASME VIII and is known as the ‘stress categorisation route’. The method requires the categorisation of stresses from a finite element analysis as primary, secondary and peak categories.

These stresses are then

combined and assessed to the appropriate allowable design stress limits. For the split-tee fitting, appropriate locations are chosen through the thickness of the fitting where the stress categorisation is to be undertaken. These locations are termed stress classification lines (SCLs).

Figure 6 shows typical SCLs and they are

selectively chosen in regions of the highest Tresca stress. It must be noted that SCLs should be taken at various angles around the split-tee fitting for a comprehensive assessment. It is important to note that these rules were designed to guard against the failure mechanisms using linear elastic finite element analysis. It is essential to appreciate that plastic collapse and incremental plastic collapse failure mechanisms cannot be dealt readily with using an elastic analysis, as the failure mechanism is inelastic. The elastic analysis approach does not make use of the ductility of the material and results in a conservative margin of safety. With the continuing development of computer hardware and software, performing an inelastic finite element analysis is becoming the norm. A new design *

PD 5500:2000 has replaced BS 5500:1997 which is now withdrawn because its status as a national standard will be incompatible with BSI’s obligations to CEN when the European Standard EN 13445, Unfired Pressure Vessels, is published. This European Standard is expected to be published early in 2002.

route incorporating inelastic analysis has been developed and is included in the new design-by-analysis manual (Ref. 10).

The analysis being proposed is to be

incorporated in the draft CEN unfired pressure vessel standard pr EN13445 (Ref. 11). 4.1

Plastic Collapse Plastic collapse falls into two categories namely global plastic collapse (termed

general membrane failure) and local plastic collapse (termed local membrane failure). The difference between the two failure mechanisms is that general membrane failure is associated with piping free from any stress concentration features and local plastic collapse addresses fittings that have been badly designed or overloaded with areas of high stress concentration features. For the detailed assessment of the failure mechanisms, SCLs as given in Figure 6 can be selected for the appropriate assessment. SCLs 2, 6 & 7 are used to assess against the general primary membrane stress limit. The maximum average Tresca stress should not exceed, 0.67 of the specified minimum yield stress (SMYS), or 0.425 of the specified ultimate tensile strength (UTS), whichever is the lowest. SCLs 1, 3, 4, 5 are used to assess against the local primary membrane plus local primary bending stress limit. The maximum average Tresca stress should not exceed the lower of the SMYS or 0.64 of the UTS. Where the above criteria are exceeded then further analyses can be performed to determine the extent of yielding by performing an elastic-plastic analysis. Generally, the area replacement method provides sufficient reinforcement to discount failure due to plastic collapse. 4.2

Shakedown Shakedown is a design criterion that is required to ensure that a component

remains in an elastic condition during any cyclic loading.

The objective of the

assessment is to ensure that if any plasticity does occur, a residual stress is created that will eventually ensure that the component shakes down to a purely elastic response.

It must be noted that predicting the onset of failure due to shakedown using the results from a linear elastic analysis is not a strictly accurate assessment. However, the current design rules state that any possible incremental plasticity should be avoided and the maximum stress range at any location within the fitting should be less than twice the SMYS of the respective run or branch material. The SCLs 1, 2, 3, 4, 5 are used to determine the stress intensities at the critical stress locations within the fitting.

Once again SCLs are taken at various angular positions for a detailed

assessment to be made. Where the above criterion is exceeded then BG Transco plc are advised to take remedial action. Such an action may require the operator to reduce the level of system loads on the split-tee by possibly increasing the flexibility of the piping layout. 4.3

Fatigue The maximum principal stress (at any location) for each fatigue loadcases

should be used to undertake the fatigue assessment. For the circumferential fillet weld, the ‘Class W’ S-N fatigue curve from PD5500 should be used. Typically the circumferential welds have the highest principal stresses and in reality are the welds most susceptible to cracking. For the seam weld, the ‘Class D’ S-N fatigue curve in PD 5500 should be used. For the fillet weld at the split-tee intersection, the ‘Class F’ S-N fatigue curve in PD 5500 should be used. It is important to note that for the fatigue analysis of any fillet weld, the stresses are very much mesh dependent, and likely to be artificially high at that location due to numerical singularities. Hence, principal stresses are extrapolated to the weld toe to determine a more accurate stress and geometrical SCF. For all the fatigue duty cycles, the maximum principal stress should be determined and the number of allowable cycles evaluated. A Miner’s Law summation can then be performed to determine the fatigue usage. Where the Miner’s Law summation is > 1.0, then the largest damaging fatigue cycle is identified and loadings contributing the most damage identified for possible reduction. Where loadings cannot be reduced then the design is recommended to be modified.


Circumferential Fillet Weld Integrity The stresses in fillet welds are complex and are not covered specifically in the

previous design criteria. BS 7910 (Ref. 12) provides some guidance on the maximum allowable shear stress on the net minimum throat area. It stipulates that this maximum shear stress should not exceed 0.48 of the SMYS of the weld metal. For the welding of linepipe of material grades X52 to X70, P9 states that basic coated low-hydrogen electrodes are suitable.

Data from the FILARC electrode

handbook for 27P electrodes (which BG Transco plc use for P9 welding) quotes an SMYS of 460 MPa for the weld metal. The sustained assessment for the fillet welds should consider loads from the design pressure plus dead weight (sustained load case) forces and moments. Where stresses exceed the allowable stress then the fillet weld is required to be re-designed. 4.5

Stress Analysis & Code Assessment Concluding Remarks

Under internal pressure, the pipe stress analysis does not address the behaviour of these types of split-tees.

IGE/TD/12 or other piping codes do not contain any relevant geometrical stress concentration factors for F4 designed split-tees.

Current philosophy is to ensure code compliance to PD 5500 design-byanalysis criteria that cover plastic collapse, shakedown and fatigue. It must be noted that the IGE/TD/12 allowable stress criteria are inappropriate to use for detailed finite element analysis.

The integrity of the fillet welds is determined by limiting the maximum shear stress to 0.48SMYS.

The split-tee is considered to be fit-for-purpose when all of the above assessments have been satisfied.

Where the split-tee fails to comply with the above criteria, recommendations are made to the operator to either reduce the system loadings by increasing the flexibility of the piping arrangement or to alter the design of the split-tee.


ENGINEERING CRITICAL ASSESSMENTS & NDE One of the areas of concern is possible cracking at the fillet welds attaching the

split-tee to the carrier pipe. Since the site welds are unlikely to be subjected to any PWHT or site hydrotest that can act as a source of stress relief, Engineering Critical Assessments (ECA) can be used to determine what size defect is likely to cause failure and whether the non-destructive examination (NDE) inspections would detect these reliably. 5.1 Engineering Critical Assessments (ECA) To perform any ECA calculation, information from the detailed stress analysis and any fracture toughness testing are required as inputs to the assessments as shown in Figure 7. The ECA evaluates the proximity to failure of a defective weld due to both brittle fracture and plastic collapse and the assessment result provides the analyst with some guidance on theoretical size of defect that could cause failure. The evaluated defect size is termed the critical defect size. The calculations are conducted using the BS 7910 defect assessment procedure, with a Level 2A failure assessment diagram (FAD). Minimum specified material tensile properties are used, along with no partial safety factors. For the assessment of fillet/girth welds, stresses normal to two planes through the carrier pipe thickness will be extracted from the finite element analysis (FEA) as shown in Figure 8. The planes of interest lie at the toes of the fillet weld, both outer and root. For the assessment of the axial seam welds, defects are postulated for both internal and external surfaces. Welding residual stress distributions are estimated using the recommendations of Annex J of BS7910. Lower bound critical defect calculations are carried out which include the following: 1. Critical part-penetrating circumferential defects for the fillet/girth welds. 2. Critical part-penetrating defects for the axial seam welds.

3. Critical part-length, fully-penetrating defects. These defect sizes are then used to envelope the extent of real defects and are used to set an appropriate inspection policy as part of the welding procedure. 5.2

Fracture Toughness The ECA assumes that fracture toughness data can be estimated for the carrier

pipe and weldments. If toughnesses are not known, then a target toughness is normally set, based on limiting the defects to sizes detectable by NDE.


toughness may be sufficiently low that it can be assumed that it will be met, otherwise it will have to be demonstrated by the operator that the actual toughness exceeds this target value. This can involve either performing a literature review to obtain better toughness data or material testing to generate CTOD toughness data at the minimum design temperature. 5.3

Development of NDE Procedures The scope of any NDE procedures will be based on the findings of the ECA. If

the critical defect size cannot be reliably detected, then an attempt to improve the reliability of the inspection will be reviewed. Where no further improvements can be made, then further fracture toughness tests, possible PWHT of welds, or modifying the design may be recommended.


SUMMARY This paper details a methodology developed by BG Technology Ltd that is used

to determine the fitness-for-purpose of encirclement split-tees.

The paper has

highlighted numerous assessments that gas pipeline operators should perform when implementing such split-tee designs. The paper has highlighted the shortcomings of the ‘area replacement’ methodology. For heavy-wall split-tees, manufacturing and weldability may become a problem and more up-to-date design techniques possibly using finite element stress analysis should be considered.

The author would like to thank the Stress Analysis Team at BG Technology Ltd in the development of this methodology.




P9: ‘The Welding of fittings to Gas Pipelines Under Pressure, and Having a Wall Thickness Not Less Than 5mm’, BG Transco plc, June 1984.


F4: ‘Hot Tap and Stopping Off Connections (For Operating Pressures 7 barg to 70 barg inclusive), BG Transco plc, January 1994.


ASME Code for Pressure Piping B31.3: ‘Chemical Plant & Petroleum Refinery Piping’, ASME B31.3-1990, American Society of Mechanical Engineers.


Institution of Gas Engineers: ‘Offtakes and Pressure – Regulating Installations For Inlet Pressures Between 7 and 100 bar’, Recommendations on Transmission and Distribution Practice’, IGE/TD/9:1986.


Institution of Gas Engineers: ‘Pipework Stress Analysis for Gas Industry Plant, Recommendations on Transmission and Distribution Practice’, IGE/TD/12:1985.


ABAQUS/Standard Version 5.8, Hibbitt, Karlsson & Sorenson, Inc., 1998.


MSC/PATRAN Version 9.0, MacNeal-Schwendler Corporation.


PD 5500:2000: ‘Specification For Unfired Fusion Welded Pressure Vessels’, BSI 2000.


ASME VIII, ‘Boiler & Pressure Vessel Code’, American Society of Mechanical Engineers.


European Pressure Equipment Research Council: ‘The Design by Analysis Manual’, EUR 19020 EN, 1999.


Draft CEN prEN13445: ‘Unfired Pressure Vessel Standard’.


British Standard BS 7910: ‘Guide to Methods For Assessing The Acceptability of Flaws in Metallic Structures’, December 1999.

Hot-tap Tee Design Develop Welding Procedures


Undertake Finite Element Analysis

Pass Code? Yes

Engineering Critical Assessment Critical Defect Size

Target Toughness

Detectable defect size Fracture Toughness

Estimate Fracture Toughness

Is Fracture Toughness Reasonable?

No Yes

Develop NDT Procedures Check actual fracture toughness data against assumptions


Qualify Weld Procedures Ready for site welding


Severa l Options Available