Drilling Riser Integrity Assurance for Deepwater Floating Drilling

Abstract

Objectives/Scope Historically, deepwater drilling riser challenges were always seen to be related to riser weight and metocean stresses. Consideration of the riser as a pressure vessel has been limited to production riser applications or smaller high pressure risers used for floating surface BOP drilling. In single gradient drilling, the riser sees pressure differentials of 3000PSI or even more in the lowest regions. A failure of the riser will create an underbalanced situation, which could result in a release of hydrocarbons to the environment. In the lower part of the riser, there is very little difference between riser stresses for conventional and managed pressure drilling applications. In the upper part, the slightly higher pressure during back pressure MPD may even help with tensile stress performance of the riser. This understanding of the riser function should also change the perception of what really is the primary barrier in deepwater drilling, where the riser and the DP system should be considered as the key primary barrier elements, holding the overbalanced fluid column. Considering the conventional drilling riser a pressure vessel, the authors look at recommended and regulatory requirements for riser integrity assurance, and find them not satisfactory for todays uses of the riser, which is another example of the "mission creep" that occurred in our industry, drilling in ever deeper waters, without changing the operating principles for floating drilling originally developed for areas just outside the reach of the jackup. Aside from offshore physical inspections every five years, no insitu method for riser integrity inspection exists at this time. The authors propose a new method for inspecting the integrity of the riser, based on ultrasonic inspection technology, which has become a vital element in pipeline integrity management. Basically, pipelines are pressure vessels which are located close under the surface and extending at great length through environmentally sensitive and often densely populated areas. To assure the integrity and safe operation of these gas and liquid systems, PHMSA (Pipeline and Hazardous Materials Safety Administration) requires these pipelines to be inspected on a regular basis for Wall Loss and Crack Detection. Internal In Line Ultrasonic Crack and Wall Thickness Inspection Systems are providing highly detailed information on the integrity of the pipeline and have become the most widely accepted and accurate technology used to assure the continuous and safe operation of pipeline systems worldwide. The Offshore Riser Inspection Systems (ORIS) has adapted this highly efficient and well proven technology to the offshore industry, providing a clean and detailed bill of health through regular, non-disruptive and insitu inspection of the marine riser system. The ORIS ultrasonic sensor carrier, controlled by a wire-line logging unit, will provide high quality digital data, through real-time surface read-out and on-site interpretation, while allowing for substantial savings when compared to standard surface inspection methods.