Riserless Dual Gradient Drilling (DGD) using a specialized subsea pump placed on the seafloor during top hole drilling has been widely used on offshore subsea wells prior to installing the blow out preventer (BOP). Riserless DGD systems, kown as riserless mud recovery (RMR), have been developed to allow riserless sections to be drilled with weighted mud while taking returns back to surface. This allows the operator to set surface casing strings deeper, thereby reducing the total number of liners/casing strings in the well.

This paper addresses the deepest RMR operation on the Norwegian Continental Shelf to date. The water depth was 854m and the 26″ hole section was successfully drilled to 1972mMD (1913mTVD). The riserless mud recovery system enabled the 20″ shoe to be set at a sufficient depth to allow elimination of the 16″ liner, thereby reducing casing strings, cost, well complexity and most importantly setting up the 13-5/8″ string cementation for success. This was mission critical from a well integrity perspective in the context of a big bore 9-5/8″ upper completion. Following the successful drilling of the 26″, the 20″ casing was installed and cemented. Foam cement was pumped using the riserless mud recovery system in cuttings transfer mode to prevent accumulation of returned cement at the well head. The 20″ casing was successfully tested after installing the BOP and marine riser.

The paper describes the riserless mud recovery equipment and the planning of the well, plus the engineering of a multi-stage subsea pump and the subsequent seabed deployment in order to drill the longest and deepest 26″ section on the field. A 26″ directional BHA with a 1.30sg surface mud weight (1.18sg downhole mud weight) KCL glycol mud at flow rates of up to 4800 lpm was used to drill to TD. At TD the well was circulated at a lower rate to 1.38sg surface mud weight at 3200lpm to provide trip margin over the effective mud density required for wellbore stability.

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