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This paper investigates the effect of gas density and surface tension on flow pattern transitions in horizontal and near-horizontal pipes. Experiments were conducted at atmospheric conditions in a 12.75-m long pipe with a diameter of 0.024 m and downwardly pipe inclinations of 0°, 0.25°, and 1°. The effect of gas density was examined using CO 2 and He gases and the effect of surface tension was examined using aqueous solutions of normal butanol. The various transitions were identified visually and by statistical analysis of film height measurements. Gas density strongly affects the transition to 2-D and K-H waves, whereas the transition from stratified to slug flow remains rather unchanged. Both wave transitions can be described satisfactorily by existing models in the literature with some modifications. A reduction in surface tension causes the transitions to 2-D waves to be shifted to much lower gas rates. For downward flows, as previously reported in the literature, even a small inclination can cause an expansion of the stratified flow regime. In this regime two different types of waves can be identified, which retain the 2-D and K-H wave characteristics observed in horizontal flow.

Electrically Traced Heated Pipe in Pipe (ETH-PiP) technology has been developed to overcome some of the challenges associated with deeper and more remote offshore oil and gas production. This active heating technology applies power to achieve a production fluid temperature above the wax or hydrate appearance temperature either continuously, during normal production, or intermittently, during shutdown periods. Concerning hydrate management, the contractor Company in collaboration with Major Operators conducting experimental and modelling studies to investigate hydrate dissociation in heated flowlines through a Joint Industry Project (JIP) kicked-off in 2012. The main objective of these investigations is to demonstrate that a long, non-permeable hydrate plug can be dissociated in a safe and controlled manner with the ETH-PiP technology. Large hydrate plugs (approximately 200 kg each) are formed in an 18m ETH-PiP 6? OD prototype, using a water and gas system equipped with DTS fiber optics systems for temperature monitoring, pressure and temperature sensors, and high accuracy gas flow meters. Different heating strategies are tested to investigate the best active heating procedure for safe hydrate plug dissociation, using temperature, pressure and released gas flow rate monitoring along the entire length of the prototype. Hydrate plug dissociations are performed in open or closed volumes for various conditions during the 2nd phase of the experimental campaign, which started at the end of 2013. High pressure differentials are applied across the hydrate plugs; non-uniform longitudinal heating profiles are applied to reproduce operating conditions similar to direct electrical heating; and three-phase dissociation experiments are conducted to simulate the influence of oil present in the hydrate pores on the plug dissociation. The paper gives an overview of the experimental set-up and measuring techniques used. It describes the hydrate plug formation, location, and characterization, as well as the successful dissociation of hydrate plugs. Preliminary simulation results based on a specifically developed "in-house" simulator are presented, as well as extrapolation of the results to real subsea conditions. This test program demonstrated that large non-permeable hydrate blockages in single line field architectures could be dissociated without local pressure build-up or plug run-away using ETH-PiP technology.