Hydrate Plug Remediation: Options and Applications for Deep Water Drilling Operations
Authors: Yousif M.H. , Dunayevsky V.A. , Hale A.H. 
Published: December 2007

Gas hydrate formation during deep-water offshore drilling is a well-recognized operational hazard. Plugging the Blowout Preventor (BOP) stack, choke and kill lines with hydrates can cause a serious well control problem.

In this paper, we examined the available options to remove a hydrate blockage from the choke and kill lines. The options included radial heat tracing, pipe warm-up, and hot water circulation through coiled tubing. A complete mathematical formulation of the energy balance of the hydrate melting process is presented.

We examined the effects of heat flux, hydrostatic pressure over the plug, insulation thickness and quality, water circulation rate, and inlet water temperature on the melting process. The results showed that the controlling parameters are the heat flux, the quality of the insulation material, and the water circulation rate.

The results also indicated that heat tracing is a viable technique to either melt a hydrate plug or to keep the choke and kill lines warm enough not to form hydrates. Furthermore, if coiled tubing intervention is permissible, hot water circulation can also be an effective method to remove hydrate blockages. In both cases, the choke and kill lines must be insulated. Otherwise, the heat losses to the environment will consume the bulk of the delivered energy.

Introduction

Hydrate formation incidents during deep-water offshore drilling are rarely reported in the open literature. To the authors' knowledge, only two such incidents have been well documented. The first incident took place while drilling offshore the U.S. west coast at a water depth of 1150 ft (350 m) and mudline temperature of 45 F (7 C) at the mudline. The second case took place while drilling a well offshore in the Gulf of Mexico at a water depth of 3100 ft (945 m) and mudline temperature of 40 F (4 C). In both cases hydrates plugged the choke and kill lines of the bottom joints of the riser during the containment of a gas kick. In deep-water drilling rigs, the risers are partially insulated with the floatation material attached to them, while the BOPs and choke and kill lines are exposed to sea water. As a result it is more likely for hydrates to form inside the BOPs and the choke and kill lines.

In general, hydrate melting can be achieved by using four basic schemes. These are:mechanical,depressurization,chemical andthermal.

In the first scheme, the hydrate blockage is removed by applying direct mechanical force such as drilling or differential pressure.

In the depressurization scheme, the pressure over the hydrate plug is reduced below the hydrate equilibrium pressure at the prevailing temperature. Consequently, the hydrate blockage starts to dissociate at the boundary subjected to the pressure reduction.

The third scheme involves bringing inhibitors like methanol, salts or glycol into direct contact with the hydrate blockage to destabilize the hydrate.

In the thermal scheme, an external source of energy is utilized to raise the hydrate plug temperature above the hydrate equilibrium temperature. The last three schemes are schematically illustrated on the hydrate phase equilibrium diagram shown in Fig. 1.

This paper presents an evaluation of the potential options available under the thermal scheme to remove a blockage of hydrate from the choke or kill lines. The evaluation includes radial heat tracing and hot water circulation through coiled tubing. The evaluation of those two options was done using two mathematical models that we developed as part of this work. The mathematical formulation for the hydrate melting process using these options is presented. A brief discussion of other potential options is also given in this paper.

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