The Barlow equation for tubular internal yield pressure is widely used in American Petroleum Institute (API) and International Organisation for Standardisation (ISO) standards, but its provenance and accuracy have never been established: indeed, until very recently, the original reference had been lost to the industry. This has led to doubt and confusion about its use. This paper presents the work done by ISO TC67 SC5 workgroup 2 to remedy this, and explains the background and technical basis for the upcoming revisions to ISO TR 10400. It is shown that Barlow's 1836 derivation violates the 3D constitutive law, and the result is therefore incorrect as originally purposed (a thick wall hoop stress). Moreover, hoop stress is a uniaxial (1D) check: the modern approach is 2D or 3D checking, based on a material failure condition such as the Von Mises yield criterion. However, the result also happens to represent the thin wall approximation to the VME failure pressure for plane stress (i.e., zero axial load), which gives an accurate measure of the yield pressure. Remarkably, this does not seem to have been recognised in previous work. The derivation is given, and the assumptions and limitations explained. Present design practice is over-conservative for thick wall pipe, and potentially unconservative for thin wall. This is not the fault of the Barlow equation per se: it is caused by the difference in physics between the design equation (VME yield) and the ultimate limit state (ULS) behaviour (ductile rupture). The industry should therefore consider revising OCTG burst ratings and accompanying design practice to achieve a more uniform safety level over the full D/t range of casing and tubing.

In 2010, Nexen UK published a paper on well time estimation (Adams et al. 2010), which for the first time allowed accurate calculation of probabilistic duration for non-critical wells. The present paper extends the method and data coverage to high step-out and HPHT wells.The method has now been in use at Nexen for five years. Actual and predicted durations are given for each year's drilling, showing that the accuracy is historically within 2-3% for stable train-wreck rates.The historical well database upon which the statistics are based now stands at 190 wells, 72 more than the previous paper. To the authors' knowledge, it remains the only large-sample timings database published in the open literature. To allow others to use the method, the updated activity timings, mechanical NPT and waiting on weather data for semi-sub drilled wells are given in full.It is shown that the commonly used distributed train-wrecks model has too high a sampling uncertainty for accurate time estimation. The lumped train-wrecks approach presented here does not suffer from this limitation, and is therefore the only method that delivers the required predictive accuracy for practical dataset sizes. Data: Updates Well DatabaseThe database now stands at 190 Nexen and legacy wells. They all satisfy the following criteria:• The same general location. All the wells are in quads 12 to 39, and 88% of the wells are from two adjacent quads, 15 and 20. This ensures that all wells had much the same geology, and hence homogeneous drilling hazards; and similar metocean conditions, and thus homogeneous waiting on weather (WOW). • Conventional drilling methods only (no through tubing rotary drilling, underbalanced drilling, etc.). • Modern wells only. With a couple of exceptions for Scott platform, all the wells were drilled from 2000 onwards, and 77% were drilled within the last ten years. This ensures that broadly the same technology was used throughout (rotary steerable BHAs, durable PDC bits, etc.).

Summary In 2010, Adams et al. published a paper on well-time estimation, which for the first time allowed accurate calculation of probabilistic duration for noncritical wells. The present paper extends the method and data coverage to high stepout and high-pressure–high-temperature (HP/HT) wells. The method has now been in use for 5 years. Actual and predicted durations are given for each year's drilling, showing that the accuracy is historically within 2 to 3% for stable trainwreck rates. The historical well database upon which the statistics are based now stands at 211 wells, 93 more than the previous paper. To the authors’ knowledge, it remains the only large-sample timings database published in the open literature. To allow others to use the method, the updated activity timings, mechanical nonproductive time (NPT), and waiting-on-weather (WOW) data for semisubmersible-drilled wells are given in full. It is shown that the commonly used distributed trainwrecks model has too high a sampling uncertainty for accurate time estimation. The lumped trainwrecks approach presented here does not suffer from this limitation, and is therefore (to the authors’ knowledge) the only published method that delivers the required accuracy for practical data-set sizes.