A practical investigation is reported of rapid depressurization and blowdown in case of riser rupture of an offshore pipeline exporting live oil at high pressure. This work was performed to assess the environmental and safety consequences of not installing sub-sea isolation valves on the Heather-Ninian pipeline. The Heather Field is close to its economic limit, consequently, the overriding objective was to evaluate changes in operating procedure which would provide an equivalent level of safety to that of installing a sub-sea isolation valve. Three methods – simple analytical, approximate and rigorous numerical simulation were used to predict leak rates in case of riser rupture. To build confidence in the predictions, a blowdown test to the separators on the exporting platform was performed. The test results are in good agreement with the model predictions. The oil was found to have a bubble point pressure near the threshold pressure at which riser rupture could cause sustained hydrocarbon release due to solution gas drive in the sub-sea pipeline. As a result, the process facility on the exporting platform was modified to lower the bubble point pressure. The effects are compared to the installation of a sub-sea isolation valve. A blowdown system on Heather platform was shown to be effective when the leak is from a hole with a diameter of less than 2 inch (50 mm).