EP 2000-9073 5-1 Restricted to Shell Personnel Only

Shell Casing and Tubing Design Guide

Chapter 5

Design Factors

5. DESIGN FACTORS..............................................................................................................2

5.1 Historical Origin of Design Factors................................................................................2

5.2 Load and Resistance Design Factors ..............................................................................2

5.3 Alternative Design Factors .............................................................................................3

Adjustment of the Design Factor to Account for Lower or Higher Load Uncertainty.................. 3Adjustment of the Design Factor to Account for Probable Rupture Capacity .............................. 4Adjustment of Design Factor for Particular Pipe Materials .......................................................... 4Adjustment of Design Factor to Account for Likelihood of Events.............................................. 4Adjustment of Design Factor to Account for Consequences ........................................................ 4Adjustment for the Depth of Engineering Preparation.................................................................. 4Experience with Other Burst Design Factors in Level Two/Three Design Practice ..................... 5Probabilistic Approach to Collapse Pressure ................................................................................ 5Burst Design Factor for Injection .................................................................................................. 5Completion Components and Design Factors ............................................................................... 7

EP 2000-9073 5-2 Restricted to Shell Personnel Only

5. DESIGN FACTORS

5.1 Historical Origin of Design FactorsThe purpose of the design factor is to address uncertainty in both the resistance (capacity) of thepipe and the loads applied to the pipe. The design factor is intended for the unexpected loading(e.g., stuck pipe, higher production rate to higher temperature than anticipated) or the unexpectedperformance of the pipe (low end of the strength distribution curve). The design factor is notintended to compensate for failing to engineer parts of the well design: the design factor is notintended to cover the effects of temperature on yield strength or thermal stresses, to cover theimpact of casing wear, or to cover the neglect of corrosion or the use of an unintended lightweightor light-grade joint in the string. That is, the design factor is intended to address the uncertainty ofbackground events within reasonable tolerances; the factor is not intended to compensate forrogue pipe or large operational mistakes.

The design factors used for decades by Shell have their origins in experience taken over a hugenumber of wells drilled and produced. The design factor is not based on any particular advancedstress analysis, limit calculation, or probability assessment. Instead these design factors are basedon average experience. In Level Two design practice, the values of design factors used by aparticular operating company are derived from the particular extensive operating experience ofthat operating company. Based on historical operating experience, the following design factorsare recommended:

Recommended “Combined” Design Factors for Level One Design

Triaxial Burst 1.25Collapse 1.0Tensile 1.3

These are referred to as combined design factors because they are single, net values which addressboth uncertainty of load and uncertainty of pipe (or connection) resistance to withstand a givenload. The triaxial burst design factor is applied to the pipe yield strength in the triaxial stresscalculation, and the yield strength is also separately adjusted for temperature. The collapse designfactor is applied to the rated pipe collapse pressure listed in API 5C3 (ISO 10400 pending). Thiscollapse strength is a function of the pipe yield strength (which depends on temperature), pipe D/Tratio, and pipe axial tension. The tensile design factor is applied to the yield strength of the pipe,which again depends on temperature.

5.2 Load and Resistance Design FactorsThe design factor which gets used in the design software and gets applied to the pipe yieldstrength or pipe pressure is a “combined” design factor which represents the combination ofuncertainty about loads applied to the pipe and uncertainty of the resistance (capacity) of the pipeto withstand the loads. Tubulars design is executed by balancing the resistance of the pipe withthe load which acts on the pipe during different well operations. Because of the two differentsources for uncertainty, there really are two different design factors which are combining into thesingle “combined” design factor: one resistance design factor dealing with the uncertainty of thepipe or connection to contain pressure, and one load design factor dealing with the uncertainty of

EP 2000-9073 5-3 Restricted to Shell Personnel Only

the load or pressure which will be applied in the well. The combined design factor is the productof the load and resistance design factors. Table 5-1 presents a notional breakdown of load andresistance design factors contributing to an overall combined design factor. Furthermore, designfactors do not need to be the same for all strings, because both the load and the resistanceuncertainty may be different for different strings.

Table 5-1

CONCEPTUAL DISTINCTION BETWEEN LOAD AND RESISTANCEDESIGN FACTORS COMBINING INTO A “COMBINED” DESIGN FACTOR

String Load Uncertainty(approximate)

Resistance Uncertainty(approximate)

CombinedDesign Factor

General Surface andProtective Casing

1.15 1.10 1.25

Production Casing 1.05 1.10 1.15Production Tubing 1.05 1.10 1.15CRA Tubing 1.05 1.15 1.20

Usually, unless QRA is done, there is little quantitative information, so you need to use prudentintuitive and experience information to evaluate these uncertainties and select design factors.

The notion of resistance uncertainty represented by the design factors in Table 5-1 applies only togood-quality pipe (good toughness, inspected free of large defects; Chapter 7). Using a largeresistance design factor is not adequate to compensate for use of poor-quality, brittle pipe. Forbrittle pipe, much of the traditional stress analysis breaks down and the risk becomesunacceptably high under burst loading.

5.3 Alternative Design FactorsLevel Two and Three design practices are intended to facilitate flexibility in the value of thedesign factor based on local experience or based on use of risk assessment. The sections belowprovide some guidance on possible reasons that design factors might be adjusted based on anevaluation of the driving sources for risk in the design.

Adjustment of the Design Factor to Account for Lower or Higher Load UncertaintyOne should be careful in thinking that there is no load uncertainty when it actually exists. Forexample, in a production casing or tubing, you may exactly know the reservoir pressure.However, there still is some uncertainty in the packer column pressure or the mud pressure due tothe effect of temperature and pressure on the mud/packer fluid density. During production, theremay be uncertainty in the distribution of temperature along the well, and this impacts the pressuregradients of completion fluids and lighter-weight muds.

EP 2000-9073 5-4 Restricted to Shell Personnel Only

Adjustment of the Design Factor to Account for Probable Rupture CapacityTo guide the choice of the burst design factor, the engineer may take account of the limit loadrepresenting the true rupture capacity of the pipe (Appendix 6). This is not to say that the pipeshould be designed on rupture instead of on yield. However, the choice of the design factor andhow close the pipe is loaded to yield can be based on the insight derived by knowing with goodaccuracy the actual limit load. To do this, you must have pipe with good toughness and you mustaccount for inspection practices used to eliminate large flaws. This is outlined in Appendix 6.

Adjustment of Design Factor for Particular Pipe MaterialsFrom a likelihood-of-event point of view (this should dominate), there is a case to be made for ahigher design factor on some (not all) CRA’s which have low work-hardening compared withcarbon steels. The risk of resistance failure of CRA strings can be calibrated to the experiencedused of carbon steels, which usually have higher work-hardening and higher ratios between actualrupture and nominal yield.

Adjustment of Design Factor to Account for Likelihood of EventsHere the design factor would be higher for intermediate strings where there is more uncertainty ofthe load conditions, i.e., more risk of unexpectedly severe loading. The design factor would besmaller for the production strings where the load is known with much greater certainty. Thisshould be tried only if experience supports this or if risk assessment indicates that the risk isacceptable. Usually, it is important to include and balance the consequences of failure, not justthe likelihood of events.

Both the load uncertainty and the pipe resistance uncertainty contribute to the likelihood of anevent occurring. Both of these are different from the consequences of failure. Qualitative andquantitative risk assessment can be used to rationalize the choice of an appropriate design factordifferent from the historical standard.

Adjustment of Design Factor to Account for ConsequencesHere the design factor would be higher for the inner strings, the tubing, and the production casing,because the consequence of failure is much higher. Once the well is completed, there is moreinvested in the well, there is higher investment impact to a collapse failure, and there is very muchhigher impact to a burst (containment) failure with a completion in place.

Both the likelihood and consequence aspects of the impact of the design factor can be combined ina risk assessment.

Adjustment for the Depth of Engineering PreparationOne may consider the option of using a higher design factor when a smaller amount ofengineering manpower is applied to the design and developing a lower design factor (for lowerload uncertainty) to use in cases where larger amounts of engineering manpower can be applied tothe design. In this case, either you pay for the excess margin in the well or you pay for theengineering manpower needed to remove the excess margin from the well judiciously. While thisshould be done with caution to balance risks, this does point to the desire to push designs wherethere is the most reward for doing so.

EP 2000-9073 5-5 Restricted to Shell Personnel Only

Experience with Other Burst Design Factors in Level Two/Three Design PracticeSeveral operating companies have extensive and successful experience using triaxial burst safetyfactors of 1.15, 1.10, and even 1.00 with high-pressure, even sour gas, wells. This is an importantbenchmark, because it indicates that wells can be drilled, completed, and produced verysuccessfully while the values of design factors are pushed. However, these cases have been basedon very thorough, targeted application of experience and risk assessment to a specific, limitedseries of designs. The use of triaxial burst design factors less than 1.25 requires experience or riskassessment to justify that this can be done with prudent management of risk.

All of the examples which can be cited have been for strings of tubing or production casing.These are strings for which there is less load uncertainty compared with protective casing.Usually (but not always), the production strings were never drilled through and so did not haveany casing wear. Where production casing has had a low triaxial burst safety factor, theproduction tubing intentionally has had a higher safety factor, and both the tubing and theproduction casing have received secondary re-inspection following mill inspection. In all cases,the consequences of pipe failure were examined and found to be manageable.

Probabilistic Approach to Collapse PressureWhen the collapse design factor of 1.0 is used, it means the engineer uses the API rating of pipecollapse pressure. However, implicit in this rating is the way in which API collapse strength isdetermined. The API collapse strength rating is derived from collapse “modes” associated withmodels and analyses of collapse. The underlying collapse strength across most of the D/T spaceis determined by the statistical scatter in observed collapse strength, based on a large amount oforiginal API testing. In the API formulation of collapse strength, the collapse rating is set tocorrespond to a target pipe reliability of 0.5% at the rated pressure. At very high D/T, this ratingceases to be statistically founded, but for most of the practical range of pipe D/T, this probabilisticformulation applies. Hence, when the engineer uses a design factor of 1.0 and the API collapserating of the pipe, the engineer actually is designing with an assumption of certainty about theload combined with probabilistic uncertainty of pipe performance, that five pipes out of athousand would be expected to fail when loaded to the rated collapse pressure.

Depending on the potential gain and potential consequences, engineers may desire to recalibratetheir collapse design to a different target probability of failure. It is more likely that gains andconsequences will be driven by the probabilities associated with the load criteria, and this also canbe put into a probabilistic framework. The API collapse pressure rating is only one example ofapproaches that are available where design is calibrated to a target risk. Both resistance and loadprobabilities and full-blown risk assessments have been conducted to guide the choice of designfactors or to bypass design factors completely with a risk-based framework. Examples are cited inthe references to Chapter 8.

Burst Design Factor for InjectionHere the load is very well known. If the fluid gradient is accurately known inside and outside thepipe, then the load is very well known and the design factor addresses only the uncertainty inresistance of the pipe. A case can readily be developed for using a lower design factor forinjection.

EP 2000-9073 5-6 Restricted to Shell Personnel Only

Design factors in pressure tests are the same as design factors for injection. The load uncertaintyis small, and one can make a case for reducing the combined design factor accordingly. However,before one dispenses with the design factor for load uncertainty, it is important either to assume themost conservative possibility for pressure gradients inside and outside the pipe or to know withvery good accuracy the actual fluid properties on each side of the pipe.

The justifications for using lower design factors for pressure test conditions are as follow:• The conditions to which the pipe is exposed are accurately known during the installation

phase of a well completion.

• The pipe is new and in good condition.

• The pipe supposedly is not exposed to hydrocarbons, so the consequences of failure are lesssevere than during production. The pipe should be mechanically isolated from the reservoir.Examples are plugs below the tailpipe or an unperforated liner/casing.

• This allows higher test pressures to be used if required. The advantage of this is that it iscommon for service loads to have different depths where stresses are at their peak comparedto test stresses. This is usually caused by different pressure gradients between test and serviceloads. If higher test pressures can be safely used, it allows the maximum test stresses to behigher than the service stresses at more points in the tubing.

Design factors for connections are discussed in Chapter 4 on connections. In general, a designfactor is appropriate on a connection to cover load uncertainty, not product uncertainty. Whenconnection performance equals or nearly equals the performance of the pipe body, the pipe (with alarger combined design factor) will drive the design. However, for connections which aresubstantially weaker than the pipe body (e.g., round thread connections), the connection can verywell drive the design and drive the choice of the pipe. In this case, the use of a connection designfactor for load uncertainty is important if (as usual) there is any load uncertainty.

Premium connections tend to be very weak in compression. They are particularly vulnerable toleakage after cycles of large compressive loading. There are a few proven exceptions to this.Sometimes engineers regard the low compressive capacity of a connection as though thisrepresents a large design factor in compression. This is one way to look at the limitation ofconnection performance and relate it to the pipe. The other way is simply to recognize that thereis a particular limit to the amount of compressive load that can be applied to a given connection,based on the qualified service envelope of the connection (Chapter 4). The latter way of thinkingabout connection performance may be a bit more clear, because for a given connection with itscorresponding limit on axial compression, there should be a design factor applied (to theconnection) for load uncertainty and not for product uncertainty. That is, it is useful not to mix upthe role and value of a design factor applied to the pipe body and the role of a different designfactor applied to the connection. The ratio of connection resistance to pipe resistance does not initself represent a design factor; it only highlights the difference between the two products.

EP 2000-9073 5-7 Restricted to Shell Personnel Only

Completion Components and Design FactorsAs with connections, there is a choice to be made for the design factor to apply to completioncomponents such as packers, PBRs, and safety valves. These are machined components with tighttolerances. Like connections, a design factor should not be applied to product performance (thisshould be set by qualification testing), but a design factor should be considered for loaduncertainty. On the other hand, a design factor for load uncertainty is not needed if the engineercan be sure that the part is designed for the very worst possible assumed loading.

In order for the completion component not to dominate the design, the resistance of thecompletion component should be chosen to equal or exceed the performance rating of the tubingand tubing connection. Great care should be taken when this is not possible. Examples includethe following:• The stated pressure rating on DST test equipment may not have the same safety factor as used

for the tubing. The safety factor may be as low as 1.0.

• Some components (especially packers and PBRs) will have a separate triaxial envelope(combined axial and burst/collapse loads). This envelope may not be the same as for thetubing. Triaxial effects on packers in particular may be large, as they can be subject to highsimultaneous compression and burst loads.

If the completion component is weaker than the tubing, then the tubing analysis should explicitlyinclude a section of pipe that approximates the strength limitations of the completion component.If the component is stronger than the pipe, then the completion component does not need to beincluded in the analysis.