Tubular must be properly deigned to cover all anticipated load cases during the life of the well. Engineers must select the appropriate tubular grade and weight, which will withstand the loads and be economic for the project. High grade to tubular can lead to excessive cost, which may not be economic viable. However, if the selected tubular is very close to the anticipated load, it might not be safe to operate the well. Therefore, engineers must fully understand the concept about design factors in tubular design.
Design factors represent the design degree to ensure that tubular will have the extra load to cover all load cases. The design factor can be described in the equation below;
Design factor of 1 is what the tubular can withstand before it starts to yield.
Anticipated load cases can be categorized into several categories as shown below;
- Tensile loads
- Burst loads
- Collapse loads
- Tri Axial loads
- Drilling loads
- Production loads
- Axial loads
- Running and cementing loads
- Service loads as injection/stimulation load
Safety factor has a similar meaning as a design factor, but safety factor can be more than or equal to the design factor. Only minimum safety factor will be equal to the design factor. The relationship between the design factor and the safety factor is described below;
Each company may have their own standard to meet a company standard. The table below shows general design factors for design factors.
The design factor will account for uncertainty in all cases, which can happen, such as uncertainty of pipe manufacture, assumptions, etc.
Figure 1 demonstrates tri axial load with a design factor of 1.25. The red circle is 80K psi strength and the green dotted line is 64K psi strength (80/1.25).
Jonathan Bellarby, 2009. Well Completion Design, Volume 56 (Developments in Petroleum Science). 1 Edition. Elsevier Science.
Wan Renpu, 2011. Advanced Well Completion Engineering, Third Edition. 3 Edition. Gulf Professional Publishing.
Ted G. Byrom, 2014. Casing and Liners for Drilling and Completion, Second Edition: Design and Application (Gulf Drilling Guides). 2 Edition. Gulf Professional Publishing.