The main physical control is through the use of steel having sufficient thickness and of sufficient strength to resist penetration by the design threat. However in 1997 the Standard did not provide guidance on the methods it considered as sufficient for penetration resistance.
To fill this need, APIA’s Research and Standards Committee commenced a research program to develop a technical basis that would ensure a common approach across the industry in achieving pipeline safety through the provision of external interference by resistance to penetration.
This work allowed the development of equations and supporting data that were generally consistent with the results of overseas research, and these were incorporated in the draft revision.
Use of these equations by industry showed that the approach incorporated into the draft standard was overly conservative.
Following some additional testing, discussion with CSM and rational thinking, the approach was substantially revised and the revised approach has been incorporated in the Standard.
Industry application
In the interim period following publication of the public comment draft of AS 2885.1 in December 2004 the industry applied the resistance to penetration requirements with various degrees of compliance.
Industry appeared generally comfortable with the approach until the requirements were applied to a DN 550 (22 in.) pipeline designed for installation in metropolitan Sydney. The initial design for this pipeline selected API 5L Grade X42 line pipe with a wall thickness of 12.7 mm, and a maximum allowable operating pressure of 3.5 MPa on the basis that this would satisfy the key parameters of the draft Standard of resistance to penetration, no rupture and maximum energy release rate. In reaching the decision in relation to resistance to penetration, the designers recognised work by others that considered pipe wall thickness greater than 10 or 12 mm as being resistant to puncture.
During detailed design, the designers applied the equations in the draft Standard and concluded that these showed the design threat (a 35 t excavator fitted with tiger teeth) could puncture the pipe, while much smaller machines could puncture the pipe if fitted with single pointed “penetration” teeth.
This result – using a maximum bucket force multiplier of 2 required in the 2004 draft standard – while computationally correct, was contrary to all experience.
If a bucket force multiplier of 1.0 was applied, the calculation showed that the 12.7 mm thick pipe would resist puncture from machines up to 40 t gross mass fitted with penetration teeth, a result much more consistent with experience.
Clearly there was a need for more research.
To provide pipeline specific data, the pipeline owner instigated a series of two field trials, one using the DN 550,
12.7 mm Grade X42 pipe, and one using DN 550, 9.53 mm Grade X42 pipe (see figure 1).
These trials showed conclusively that:
1. Conventional experience that pipe thicker than about 10 mm will resist
2. puncture by all excavators typically used in construction and maintenance activities near pipes is soundly based.
3. Steel pipe provides a very large level of resistance to penetration.
4. The calculation method in the draft Standard was unnecessarily conservative, and if retained, would significantly increase pipeline capital costs.
Reassessment
General
Throughout the APIA research program there was an appreciation that in any excavator contact event there is are multiple outcomes – from minor coating damage, through to metal surface damage, dent, dent and gouge and in the most severe case, puncture.
The first consideration was whether the force applied by an excavator as determined from laboratory and theoretical analysis was real.
APIA was considering a fourth stage to their penetration resistance research to test the laboratory analysis through a research program using an instrumented excavator to measure the actual force applied to a pipe. Fortunately the APIA results were discussed with Mr Gianlucca Mannucci of CSM who indicated that CSM had undertaken specific research projects concerning the study of excavator mechanical aggression towards buried pipelines.
This research4 was designed to undertake comprehensive analysis of real crawler excavators in-field working methodology and through the execution of full scale aggression tests (both static and dynamic) using fully instrumented real excavators.
In order to identify how a real excavator acts with respect to a buried pipeline, real excavators have been studied and both static and dynamic digging tests have been carried out by CSM using excavators fully instrumented. The results were used to understand the real behaviour of an excavator during pipe damaging and to better design the full-scale testing facility.
Force Applied by a Real Excavator – Static Analysis
Figure 2 shows clearly why using a theoretical maximum force analysis is wrong. All excavators are capable of exerting sufficient force at their bucket to destabilise the excavator by lifting its tracks off the ground, and most are capable of exerting sufficient pulling force to overcome the frictional resistance between their tracks and the ground.
The correct analysis should have developed a relationship based on a statics loading analysis, using the machine weight and the typical excavator dipper arm dimensions and compared this with the computed maximum force.
Using data obtained by instrumented excavator’s tests, an analytical model for predicting the maximum static force that an excavator can exert in a given position has been developed by CSM from which the maximum static force in both vertical and horizontal direction can be calculated. Figure 3 shows an example of the model’s capacity to capture real excavator behaviour.
Force Applied by a Real Excavator – Dynamic Analysis
Assessment of the maximum impact force requires consideration of the complex dynamic interaction between excavator and pipe during impact. CSM developed a numerical model which is based upon instrumented excavator’s test data.
In order to estimate the maximum impact force that the excavator could exert on the pipe under consideration, both pipe and excavator main characteristics are needed. It is worth noting that, unlike static force, larger working distances and larger the tooth speeds increase the kinetic energy; as a consequence, higher impact force is reached.
The model tends to slightly overestimate the maximum impact force recorded during real excavator tests, essentially because the calculation considers in all cases the maximum speed at impact.
Puncture Resistance Requirements
The other factor was to re-assess the requirements incorporated in the Standard for situations where there is a loss of integrity.
The draft Standard requires design for protection against external interference by the identified threat.
However the Standard does recognise that there are situations where a pipeline can be punctured, and it applies limits to the consequence of such action.
Clearly, if a pipeline was required to be designed to resist penetration in every possible instance the requirements in the Standard would not be necessary. More importantly, in a country like Australia where transmission pipelines are long, generally their diameter is small, and the threats are generally low to negligible, the cost of the pipelines would become unreasonably high.
Revised approach
Considering the above the Committee revised the requirements for resistance to penetration to performance based ones meeting objectives defined by the designer for each pipeline. In particular the mandatory requirement for minimum resistance to penetration was removed.
The Standard now requires that:
“Where resistance to penetration is selected as a physical threat control at a location, the design methodology and requirements for resistance to penetration shall be defined.
In R1 and R2 areas there is no mandatory requirement for penetration resistance beyond that provided by the pressure design wall thickness although it may be selected as a physical method of protection if required by the safety management study.
Where a pipeline route is deliberately chosen so that isolated buildings occur within the 12.6 and 4.7 kW/m2 radiation contours in R1 and R2 areas, localized increased protection against external interference should be provided, including increased penetration resistance where appropriate. The objectives for increased protection and the methods adopted shall be defined within each radiation contour and shall be considered in the safety management study.
In T1 and T2 areas, and in secondary location classes S and I, penetration resistance shall satisfy the requirements of Clause 4.7 (high consequence areas) for the respective locations.
The effectiveness of resistance to penetration may be determined using one of the following methods:
1. Calculation in accordance with Appendix M or other approved method Physical testing.
2. Comparison with previous physical tests, provided the tests were performed on pipe of similar or lower grade and wall thickness and with a similar or larger test machine.
3. Where resistance to penetration is to be calculated (in accordance with Appendix M), the acceptance test is:
PBucket R > B “¢ F
Where RP and FBucket are calculated using the bucket tooth dimensions nominated in the Standard.
The constant B is a recommended multipurpose factor that takes account of the likelihood of an excavator contact being sufficiently severe to puncture the pipe and the consequence of a hydrocarbon release in a location. Table 1 illustrates the values recommended for B.
It must be stressed that the values of B are set on the basis of an assessment of the inputs to this reassessment and not on specific research outcomes. However testing the requirements against experience with pipelines having a range of diameters and thicknesses, installed throughout Australia, and with reported data from overseas has led to a conclusion that the requirements are reasonable.
Conclusion and future direction
While there is a high level of confidence that the revised approach is reasonable the decision process in reaching the acceptance levels is based on assessment and rationalisation, not on sound research.
Research will be undertaken in the future to better underwrite (or provide the basis for revision) of the resistance to penetration requirements of the Standard, and to better understand whether some modification is required to refine the limits to ensure that dent and gouge combinations resulting from static or dynamic excavator attack are addressed with sufficient conservatism for the assumption that delayed failure will provide sufficient time for management by procedural methods to be acceptable.
References
1. Brooker, D., Pipeline Resistance to External Interference. Phase III Final Report.January 2005, APIA.
2. Roovers, P., et al. EPRG Methods for the Assessing of the Tolerance and Resistance of Pipelines to External Damage. in Pipeline Technology Conference. 2000. Brügge, Belgium.
3. Driver, R. and T. Zimmerman. A Limit States Approach to the Design of Pipelines for Mechanical Damage. in 17th Int. Conf. on Offshore Mech. and Arctic Eng. 1998.
4. Mannucci, G., Malatesta, G., Demofonti, G., Barsanti, L., “An Experimental and Analytical Approach for Predicting the Formation of Dent and Gouge Damage on Gas Pipeline by an Excavator”, Proceedings of 4th International Conference on Pipeline Technology, Ostend, Belgium May 9-13, 2004