Abstract

A recirculation flow loop was designed, constructed and operated to emulate the transportation of light condensate oils in subsea pipelines under hydrate forming conditions. The main objective of the study was to evaluate the formation and settling characteristics of hydrates around orifice plates, valves and fittings, and in straight sections of a pipe. A horizontal "see-through" section enabled direct visual observation of the flowing particles. Two mixtures were prepared in situ by mixing light hydrocarbons, N2 and CO2 with a field stabilised mixture from North Sea. The true vapour pressures (TVP) of the mixtures were 2 and 4 MPa at 311 K. Experiments were carried out with both the mixtures at about 1.5 MPa above their TVP with water contents of 0.5 - 5% v/v in the temperature range of 276 to 278 K.

In this paper we focus our attention on the effect of hydrate inhibitors. In particular, methanol was added to the test loop at two different concentrations and its effect on hydrate formation and deposition was quantified through (i) pressure drop measurements across pipe sections and (ii) indirect measurement of total moles of water that have gone into solid hydratephase at any point in time (calculated on-line). In addition the effect at velocity was examined by circulating the mixture in the main observation section at velocities ranging from 0.9 to 2,5 mls.

Introduction

One potentially serious problem in the design and operation of offshore unstabilised oil and condensate transportation pipelines is the formation of gas hydrates. The accumulation of these ice-like crystalline solids in the horizontal sections, depressions and around fittings such as elbows, flow meters etc. could lead to complete or partial blockage of the pipeline. In addition, the design and operating procedure should also address scheduled and unscheduled shutdown of the pipeline, during which hydrate formation and accumulation may continue. The problem attains enormous proportions when the pipelines considered are several hundred kilometres long [1].

The traditional solution to this problem is to operate the pipeline outside the hydrate formation conditions. This is achieved (i) by the removal of the water from the oil/gas stream before entering the pipeline; (ii) by the use of inhibitors that prevent hydrate formation; or (iii) by maintaining the contents of the pipeline warm until it reaches a reheating station or a processing facility. Obviously, these methods are either prohibitively expensive or unsuitable for offshore pipelines in cold waters. Huge platforms need to be installed for housing the dehydration facilities, or the continuous supply of inhibitors is required through a parallel pipeline from an onshore processing unit. We believe that a more economical solution is to allow the hydrate particles to form and consider their transportation in the form of a slurry where the formed particles neither deposit nor agglomerate. This approach is now receiving attention from the international oil and gas community involved in cold water exploitation.

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