Reconciling methane emission measurements for offshore oil and gas platforms with detailed emission inventories: Accounting for emission intermittency

Comparisons of observation-based emission estimates with emission inventories for oil and gas production operations have demonstrated that intermittency in emissions is an important factor to be accounted for in reconciling inventories with observations. Most emission inventories do not directly report data on durations of active emissions, and the variability in emissions over time must be inferred from other measurements or engineering calculations. This work examines a unique emission inventory, assembled for offshore oil and gas production platforms in United States coastal waters, which reports production-related sources on individual platforms, along with estimates of emission duration for individual sources. Platform specific emission rates, derived from the inventory, were compared to shipboard measurements made at 72 platforms. The reconciliation demonstrates that emission duration reporting, by source, can lead to predicted ranges in emissions that are orders of magnitude broader than those based on annual average emission rates. Total emissions reported in the inventory for the matched platforms were 20-30% larger than total emissions estimated based on observations. The distributions of emissions were similar, with 75% of platform total emission rates falling between 0 and 49 kg/hr for the observations and between 0.59 and 54 kg/hr for the inventory.