Currents and cold fronts

Jerry Lee looks at how to deal with deepwater drilling riser operation in high currents, typhoons and cold fronts offshore Japan.

The drilling riser deployment from the Chikyu drillship. Image from Stress Engineering Services.

Offshore drilling operations are by nature more complex and challenging than onshore. Add in typhoons, cold fronts, and high currents, and the challenge is compounded.

In 2012 and 2013, JAMSTEC (Japan Agency for Marine-Earth Science and Technology) undertook the Integrated Ocean Drilling Program expeditions 338 and 348, respectively, in the Nankai Trough, a mega-earthquake site. The drill site, C0002, is 80km southeast of Japan’s Kii peninsula, 1939m below mean sea level, in the Forearc basin.

Using the Chikyu, a dynamic positioning (DP) capable drillship, the target depth was 5200m below sea floor, where cores were taken and sensors installed to monitor and collect earthquake information.

Before reaching the target, however, the Chikyu had to contend with the Kuroshio current, which can vary daily, typhoons, and cold fronts. To mitigate these issues, JAMSTEC partnered with Houston-based Stress Engineering Services to optimize operations.

Kuroshio current

“The magnitude and depth of the Kuroshio current poses a number of problems for riser deployment, controlling flex joint angles while drilling and vortex induced vibration of the drilling riser system,” says Kenneth Bhalla, principal at Stress Engineering Services.

At the site, the Kuroshio current not only changes speed throughout the day, but also direction, and has a non-uniform profile, meaning the current at the surface is much higher than near the sea floor.

For example, currents greater than 3.5 knots at the surface are typically reduced to 1 knot at 400m below mean seal level and to 0.5 knot at 600m below mean sea level.* These currents can have significant effect during riser deployment and retrieval, and blowout preventer (BOP)/lower marine riser package (LMRP) landing operations.

To cope with these factors, an offsite deployment procedure was adopted to deploy the risers, BOP, and LMRP. Like the name suggests, an offsite deployment involves the deployment of equipment away from the high currents at the drill site, in a location with lower current. The equipment will then transit to the drill site.

For this operation, the Chikyu uses a method called drift on site, whereby the Chikyu deploys its equipment and drifts down current to the drill site. Using this method, however, puts the equipment at risk of running into shallow ridges or missing the drill site. As a result, detailed bathymetry charts were used to define the approach and evacuation routes, with remotely operated vehicles used to survey areas where clearance is less than 50m.

Another concern, due to the currents, is the change in angle of the flex joint; as the current increases, so does the angle. API RP 16Q address this issue and limits the flex joint angle to 2° during drilling mode when surface currents exceeds 5 knots. However, operations were allowed to continue if the current exceeded 5 knots as long as the angle remained 2° or less. Thus, tensioners attached to telescopic joints were utilized to help absorb some of the bending stress caused by the currents, managing the effects of high currents and sea state and allowed drilling operations to continue.

A further issue induced by high currents is fatigue damage to the riser caused by vortex induced vibration (VIV). To address VIV, fairings are used to mitigate the effects, though the fairings are only implemented on the top sections of the riser which will face higher current due to time constraints.

Additionally, to prevent equipment damage and alleviate VIV concerns, operations are restricted to currents traveling 2.5 knots or less. Accordingly, the currents are monitored using two acoustic current doppler profiler (ADCP) equipped supply boats at the drill site and 5mi southwest of the Chikyu.

Typhoons and cold fronts

The Chikyu drillship. Image from JAMSTEC.

With drilling occurring between autumn and winter, operations can be affected by cold fronts and seasonal typhoons.

“Typhoons and cold fronts can develop quickly at this location and cause the vessel to be “pushed off” location, thus halting all drilling operations,” Bhalla says.

Cold fronts, which usually occur from November-January, can cause strong wind gusts, and sudden changes in direction causing significantly more drag, thereby forcing the ship out of position.

The procedure in such cases is to first direct the vessel heading to face southwest by south-southwest, to respond to the strong winds that come from the south and the high currents flowing from the west-east direction. However, once the direction of the wind changes to northwest by north-northwest, drag forces increase significantly, causing the vessel to lose position and drift. To prepare for this, the drilling assembly is raised above the BOP, the blind shear rams are closed, the riser displaced to sea water, and emergency disconnect sequences (EDS) are enabled to disconnect the LMRP from the BOP. Also, a rubber joint is installed to prevent damage caused by the intermediate flex joint contacting the keel during the force off.

With a narrow operational window, the expeditions need to spend as much time being productive as possible. Specific operational criteria were established for dealing with the cold fronts, which are dependent on forecasted conditions. When wind gusts are forecasted to be more than 18m/s, the operations are put on standby, whereby drilling operations stop, and the bit is pulled back above the BOP. When winds gusts are forecasted above 28m/s, or if the current exceeds 5 knots following the passage of the cold front, disconnection is planned.

Additionally, controlling the vessel in high winds and currents is also challenging. So the DP operators prepare for these force off situations on simulators for position recovery and mainlining speeds below 2 knots to minimize VIV and the risk of the riser contacting the keel.

Typhoons, which usually occur from July-October, can produce wind gusts as high as 50 m/s which can affect DP controls. As a result, when a typhoon is forecasted within 200mi from the site, EDS is enabled, disconnection and riser retrieval is planned, and the site is evacuated.

However, all typhoons are not dealt with the same way. The procedure for disconnection and riser retrieval is a lengthy process that requires 5.5 days for full evacuation. So, for small typhoons, which can reach the site within four days of forming, a hard hang-off with the riser, can be performed, whereby the riser is effectively locked into the drill floor rather than a full riser retrieval. The benefit to this procedure is the rig time saved deploying and retrieving the riser. Though the risk involves momentary compression near the top of the riser, shock loading on the equipment when tension is restored, and the riser hitting the keel. Large typhoons on the other hand, can take 6-14 days to reach the site, allowing enough time for full retrieval. The decision for which option to deploy is dependent on the sea state conditions.

For both cold front and typhoon scenarios, forecasting plays an important role in decision making and operational safety. As a result, special typhoon information services, weather forecasting services, and regularly scheduled meetings with weather information providers are necessary.

Execution

During the 2012 and 2013 expeditions, the initial drift on site deployment of the risers was successfully executed. With surface current at 3.5-4 knots, the Chikyu deployed the riser 10-15mi up current from the drill site, maintained vessel speed between 0.8-1.0 knots. The following BOP deployment and the 2013 LMRP deployment were similarly successful. VIV fatigue was kept low using the fairings and tensioners with 0.15% damage in 2012, and 0.25% in 2013. In total, VIV fatigue damage was kept down to 1.5% for the entire riser assembly in 2012.

Cold fronts and typhoons were also successfully dealt with utilizing the prescribed procedures. A typhoon and a cold front each required a planned EDS, while another cold front caused a forced EDS, though all three were deemed successful.

In spite of the harsh environments and the unique set of challenges presented by the NT3-01 site, the drillship Chikyu was able to successfully execute the planned drilling activities with minimal downtime.

(This article is based on the 2014 Offshore Technology Conference paper “Planning and Feedback for Deepwater Drilling Riser Operation in High Currents, Typhoons and Cold Front”). Saruhashi, T., Sawada, I., Kyo, M., Miyazaki, E., Yamazaki, Y., Yokoyama, T., Bhalla, K., Stahl, M.J., Ganpatye, A., Han, Y., Gong, L. (2014, May 5). Planning and Feedback for Deepwater Drilling Riser Operation in High Currents, Typhoons and Cold Front. Offshore Technology Conference. doi:10.4043/25182-MS