Riser drilling access to deep subseafloor science

Riser-based drilling became available to scientists for the first time when the Japanese drilling vessel (D/V) Chikyu began operations in 2005. The introduction of a vessel that could take advantage of riser drilling technology was a key advancement as scientific ocean drilling transitioned from the Ocean Drilling Program to the Integrated Ocean Drilling Program. Because riser drilling enables control of downhole pressure, Chikyu opened a new frontier for scientists, allowing for deeper drilling and sampling of the subseafloor to at least 7 km depths. Riserless drilling uses seawater as drilling fluid, and technical difficulties may arise when the borehole becomes unstable. In contrast, riser drilling uses riser pipes that connect the ship to the wellhead at the seafloor (Figure 1). Circulation of weighted drill mud through the riser pipe stabilizes the borehole during deep drilling operations, allowing continuous sampling and data collection, including from cuttings, mud gas, and downhole logging. These samples and data allow characterization of in situ physical properties (e.g., stress, fluid pres-sure) and gas and fluid chemistry. Riser drilling operations in the Nankai Trough and off Shimokita Peninsula in Japan demonstrate the value of this technology to achieving new understanding of the processes occurring deep in seismogenic zones.