Hydrate Plug Dissociation by Method of Depressurization
Authors: Bollavaram P. , Sloan E.D. 
Published: October 2008

In this work, we concentrated on hydrate dissociation by the method of depressurization (two-sided and one-sided). Experiments focused on formation of hydrate plugs and subsequently their dissociation by both the methods to develop a database. The experiments lead to broadening our limited knowledge on hydrate dissociation by depressurization. The work was conducted with both structure I/structure II type of hydrates. The mechanism of hydrate dissociation (radial or axial) was studied in this work.

Work on hydrate dissociation modeling is very limited. Peters (1999) had modeled the two-sided hydrate dissociation as a radial moving boundary model. The model is capable of predicting the hydrate dissociation time and the total time for plug melting. A one-sided depressurization model was developed that predicts the time to re-start the flow. The re-start time depends on the downstream pressure, length, porosity and permeability of the hydrate plug. A safety model was developed that assesses the hydrate plug movement when subjected to pressure gradients. The safety model provides the user a safety optimum downstream pressure for one-sided depressurization.

The one-sided model was verified with laboratory and Tommeliten field plugs. The model predicted that the Tommeliten field plugs were re-started when the annulus spacing was 8% of pipeline radius. The safety model compared to the simulations of Xiao et al. (1998), and predicted higher, but comparable plug velocities.

As a result of this work, operators should be able toConduct one-sided depressurization safelyPredict time when flow can be re-started A user-friendly Visual Basic front-end program (CSMPLUG) was developed that incorporates one-sided depressurization, two-sided depressurization and safety simulator.

Experiments

The purpose of the experiment was to form hydrates from ice and to dissociate the hydrate plug. The experiments were conducted in an unstirred batch reactor under isobaric and isothermal conditions. The formation experiments were conducted under variable temperature and pressure. Experimental conditions were limited to pressures between 0 and 3000 psig and temperatures between -10C and 5C.

Experimental Equipment

In the present work, a new long cell was built in order to study effects of length/diameter on plug dissociation times. Figure 1.1 shows the schematic of the long cell. The stainless steel cell is 0.92 m long, has an internal diameter of 0.0254 m and an internal volume of 0.00047 m3. The new long cell had a threaded end caps sealed by a rocket seal?.

Figure 1.1: Schematic of the Long Cell (not to scale) (AVAILABLE IN FULL PAPER)

All other parts of the apparatus (Figure 1.2) were connected with 0.006 m stainless steel tubing connected with Swagelock fittings. An Omega/Ashcroft pressure transducer, with a pressure range between 0 and 5000 psig, was used to continuously monitor the pressure in the cell. The accuracy of pressure transducer was + 25 psig. The temperature inside the cell was determined with the use of five type T (Copper-Constantan) thermocouples. The location and numbers of the thermocouples is also detailed in Figure 1.1. The temperature of the gas just outside each end of the cell was also measured using type T thermocouples. The temperature of the bath was monitored using a platinum resistance temperature detector (RTD).