Taking a Fresh View of Riser Margin for Deepwater Wells Potentially Boosts Safety

Abstract

This article, written by Editorial Manager Adam Wilson, contains highlights of paper OTC 22889, ’A Step Change in Safety: Drilling Deepwater Wells With Riser Margin,’ by Robert Ziegler, SPE, Petronas, prepared for the 2012 Offshore Technology Conference, Houston, 30 April-3 May. The paper has not been peer reviewed. Riser margins were abandoned when fluid columns in risers became too long with increasing water depth accessed when floating drilling units with subsea blowout preventers (BOPs) moved to deep water. Removing risers in disconnect events means wells go underbalanced and rely on BOPs to contain wellbore pressure. With the advent of a simple retrofit dual-gradient system based on partially evacuating the riser by pumping mud returns back to surface from an outlet in the riser, riser margin in the form of an overbalanced well can be reintroduced in many cases. Setting the Scene Deepwater drilling has a multitude of unique challenges. Water depth: Cold temperatures combine with long fluid columns with high pressures at the seabed most often sitting squarely in the hydrate-formation zone. Ocean forces put mechanical stresses on the riser system. Perhaps most challenging, a highly complex hydraulic system, the subsea BOP, must be operated remotely in a very hostile environment. Deepwater depositional environment: This leads in most cases to a very tight margin between formation strength and pore pressure. Existing emergency procedures: In the case of a loss of position of the rig, a complex sequence of disconnecting from the BOP must be initiated. The BOP also needs to be closed before the lower marine riser package is disconnected and must reliably seal the wellbore because, by removing the riser, the well becomes underbalanced because primary well control by mud weight is lost. Status Quo The solutions that the industry has embarked on to address these challenges have often been overly complex evolutionary developments instead of clean-sheet designs, leading to high costs and an unavoidable increase of mechanical failure modes. The best example for this approach is the current generation of subsea BOPs. In their functionality, these devices remain close to the original patent in 1929; but, today, they are very large, complex, and heavy (350 tons) devices that require an enormous amount of maintenance and testing. Even if a BOP is closed on an inflow in time so the flow is entirely contained below the BOP, circulating out this kick by an established “driller’s method” very often proves to be difficult if not impossible. This is mainly related to two factors. One is the risk of hydrate formation. The moment gas and free water are mixed in the BOP area, hydrates will form. The second factor is the narrow margin between fracture gradient and pore pressure. Very often, the formation is not able to support the backpressure created by the choke line friction, and circulation without losses can never be established.