A New Technique to Solve Gas Hydrate Problems in Subsea Christmas Trees

Abstract

Summary The paper is aimed at describing the series of actions taken to re-enter a well, the (wet) Christmas-tree cap of which was clogged by solid hydrate and therefore could not be released by means of a regular releasing tool. A deepwater well located in Campos Basin, offshore Rio de Janeiro, Brazil, was the location for this unique job. To our best understanding, the kind of approach used to solve this hydrate problem had never been used in the industry before. A patented thermochemical method—the so-called self-generated nitrogen (SGN) method—was applied to heat and dissociate the crystallized hydrate, which was clogging the piston chamber of the tree-cap locking pin. Heat application around the body of the Christmas tree enabled us to dissociate the hydrate and release the tree cap by means of its regular retrieving tool. This paper describes all the steps taken to perform this pioneer workover job. As a matter of fact, it was a multidisciplinary task that encompassed the following points: Development of a "tailor-made" environmentally friendly SGN formulation. Heat exchange simulations to design the optimum volume for the SGN treatment batch. A review of the theoretical aspects of hydrate formation/dissociation. Mechanical modifications of the (regular) tree cap releasing tool. A meticulous review of the job and safety procedures.