The dynamically positioned drillship, Chikyu, has performed riser drilling operations twice in high-current environments in the Nankai Trough, offshore Japan. The drilling site is located in a water depth of approximately 6, 361 ft (1, 939 m) and regularly experiences the Kuroshio Current. The mean surface current at the site is 3.5 knots, and the 1-year extreme current is 7.0 knots. In addition, the current is not seasonal and typically changes frequently during the day. The Nankai Trough site also experiences intense typhoons during the months of July to October and frequent cold fronts from November to January. Thus, all operational phases need to be analyzed thoroughly to account for these harsh environmental conditions.
The following operational issues need to be carefully considered: riser deployment and retrieval, BOP/LMRP landing approach with vessel drifting, determination of watch circles, typhoon evacuation plans, hang-off operational criteria, evacuation plans for cold fronts, allowable riser tension range, and riser fatigue measurement with vortex-induced vibration (VIV) monitoring.
Assessment of deepwater/high-current riser drilling systems provides a means for identifying critical stages of drilling operations and enables their successful execution. Feedback from two drilling campaigns (2012/2013) conducted in high currents are described in this paper. The actual riser response matched riser analysis results to a significant degree. Extreme operations including typhoon evacuation, drilling operations in up to 5.2 knot currents, BOP landing in 4 knots, and emergency disconnect sequences (EDS) due to a cold front were performed in 2012. VIV fatigue damage was extremely low, with a maximum of 1.5% fatigue damage in the entire riser assembly in 2012. In 2013, operational criteria for cold front events were reviewed again since the number of these weather events experienced in 2012 was 3 to 4 times greater than in a typical year.
General guidelines and associated criteria are provided to evaluate a drilling riser system under high currents and approaching typhoons. This paper focuses and discusses the major operational concerns in these harsh environments. In addition, a riser fatigue monitoring system is described that has been used to track fatigue damage on riser joints both from VIV fatigue and wave loads.