June 25, 2008
Japan Agency for Marine-Earth Science and Technology
The Deep-sea Drilling Vessel Chikyu, operated by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC: Yasuhiro Kato, President), carried out a Drydock Survey, including repair and maintenance work from February until April this year at the Sasebo Shipyard, Sasebo Heavy Industries Co., Ltd in Sasebo City, Nagasaki Prefecture. During the survey, some cracks and chips were discovered on the teeth of some of the Azimuth Thruster (360º swiveling propellers used to maintain ship’s position during drilling operations: see Fig.1) gears.
JAMSTEC and outside experts investigated the cause and found that several factors are involved, including design and manufacture, the gear material, and the assembly and adjustments made to the thrusters. Projected future preventative measures include the use of improved materials, designing, and manufacturing, and more careful assembly and adjustment. Therefore, all six gears will be completely replaced with new ones.
We anticipate that the gear change will be completed by January 2009, and Chikyu will conduct IODP (the Integrated Ocean Drilling Program)*1 riser drilling after completing test drilling in the Sea of Kumano, off the coast of the Kii Peninsula.
Damage Assessment and projected solutions
Damaged areas were carefully analyzed with technical advisers who identified the following areas:
The following measures will be taken, in consideration of other possible factors:
Therefore, all six existing gears will be replaced by new ones with the new modified design.
The CHIKYU installs a computer system called Dynamic Positioning System (DPS) to control the power and direction of thruster so that the CHIKYU can maintain a constant position for the deep-sea drilling with 6 Azimuth thrusters (3 on the bow and 3 on the stern: propeller diameter is 3.8m) which is used worldwide.
Figure1: Identified damage is shown in the red squared part in the bevel gear (gear used when it turn right angle).
Damage has been identified on 3 out of 6 wheel gears and one of those was replaced (Picture 1 and 2). It was the first overhaul inspection of Azimuth thruster since the CHIKYU’s delivery in July 2005.
Picture 1: Wheel gear
Material of gears on the CHIKYU is nickel-chrome molybdenum forged steel treated with carburizing and case-hardening.
Picture 2: Enlarged picture of the gear