Engineering design methods across the upstream industry have a common foundational objective of assuring safe and reliable wells, facilities, and operations. Even with these common goals, methods vary in their sophistication across disciplines. In some areas, deterministic so-called working stress design methods remain in use. In select areas, engineering methods have advanced in their application of probabilistic-based methods such as the load resistance-factor design. In high-pressure/high-temperature (HP/HT) well and equipment design, the industry is moving forward with the development of more-sophisticated design methods, which are both appropriate and needed.
One element of these enhanced design methods is the ability to account for the probability of a design scenario taking place and for the distribution of the equipment’s ability to resist the load. The design of offshore structures for varying strengths of storms and hurricanes is an example.
In addition, some disciplines have advanced criteria based on the nature of the design scenario. The criteria for service loads that will occur with certainty are appropriately conservative and account for exposure to the loads, possible cyclic effects that can result from repeated exposure during service life, environmental loads, and well-specific characteristics such as reservoir depletion.
Design criteria for extreme and accidental loads are different from those for service loads because these loads are not likely to occur and, in fact, should not occur. Nevertheless, if an extreme or accidental load does occur, the equipment must survive the exposure. Survival-based designs are distinct from conventional designs for known service loads because it is acceptable for design margins to be less conservative and it is acceptable to have limited yielding as long as containment is maintained and the system survives the event. Examples are found in the design of offshore structures in terms of the consideration of ship collisions.
Additional advancements in design methods are being realized in the more-thorough accounting of environmental conditions. A key example here is accounting for the beneficial effect of hydrostatic pressure on subsea equipment. The hydrostatic pressures subsea impose a uniform set of compressive stresses in all equipment, which offsets the tensile stresses induced from wellbore pressure containment. In this area, an important new American Petroleum Institute standard (API SC17 TR12) that addresses this consideration is nearing completion and publication.
Technologies in our industry are constantly advancing from more-thorough consideration for improving designs and from findings that emerge from focused research-and-development activities. HP/HT design and operations are key areas where the most-sophisticated engineering methods are warranted. These important and innovative industry advancements are driven by the unchanging principles of ensuring fit-for-purpose, safe, and reliable HP/HT well designs and operations.
Recommended Additional Reading
SPE/IADC 163557 Annular-Pressure-Buildup Analysis and Methodology With Examples From Multifractured Horizontal Wells and HP/HT Reservoirs by Jonathan Bellarby, Canmore Consulting, et al.
SPE 168321 HP/HT Cement-Sheath-Integrity-Evaluation Method for Unconventional Wells by Arash Shadravan, Texas A&M University, et al.
SPE 171865 Making An Undrillable HP/HT Well Drillable Using Mud-Cap Drilling by Farhaad Khaled Al-Awadhi, ADNOC, et al.
