Slug flow is commonly observed in gas production offshore fields. At high operation pressure only short hydrodynamic slugs are observed. However, as the offshore fields become older, the operation pressure becomes lower and long slugs may form. At near atmospheric pressures the long slugs may reach a size of 500 pipe diameters or more. Such slugs can cause serious operational failures due to the strong fluctuating pressure. Identifying the operation pressure conditions at which the long slugs appear, may reduce or prevent these negative effects. In this paper we process and analyse gas-liquid flow measurements in order to investigate the different slug types and their sensitivity to the operation pressure. The measurements were performed in a 103 m long pipe with an internal diameter of 0.069 m and an inclination of -0.1 degrees from the horizontal. Three types of slugs were categorized according to the difference in liquid levels (liquid excess) between the slug front and tail. The long slugs were found to have the largest liquid excess after formation, whereas the hydrodynamic slugs had no liquid excess. The analysis of the measurements provides a detailed overview on the effect of pressure on the long slug length, and a safe operation region where long slugs will not appear. (C) 2009 Elsevier Ltd. All rights reserved.