TESTS IN SIMULATED TANKS
To simulate conditions inside a ship’s tank, a drone was put through its paces in a land-based, steel test tower. The experiments, using a DJI Matrice100 drone, studied the impact on performance due to compass unreliability, intermittent or complete lack of GPS and poor lighting both for flight control and photography.
Take-off was possible when the drone was placed more than 50cm away from magnetic materials, i.e. the steel walls. There was no difference in attaining stable flight between GPS and non-GPS mode, revealing that drones are operable with only basic inputs from gyro and accelerometer. Piloting a drone that has relatively low control precision is a good way for operators to accumulate experience.
When flying near walls, there was a slight shaking in the drone motion, owing to the wind from the drone’s own propellers. To prevent collision with walls, it may be necessary to attach a guard or collision prevention device.
Images taken inside the tower were generally of low quality, due to a combination of poor light conditions, small camera sensor size, and inappropriate ISO settings. Artificial light sources, both from ground level and mounted on the drone itself, resulted in improved images. When the photo subject contains highly polished or metallic surfaces, it will be necessary to experiment with different arrangements in order to produce diffused light that minimizes reflection. Night-vision cameras may also be considered as an option. It was also noted that videos transmitted from the camera were frequently corrupt on the receiving terminal such as a smartphone.
TESTS ONBOARD SHIPS
The final phase of the study was directed at testing the performance of drones inside a cargo hold of a bulk carrier and tank of an oil tanker and was carried out with the cooperation of Kanda Shipbuilding and Orient Marine in the first instance and Tsuneishi Shipbuilding and Technos Mihara in the second. Drones used were the 4.5kg DJI Matrice 210 and the 3.5kg DJI Matrice 100.
On the bulk carrier, the trial took place with the hatch half open, providing an intermittent GPS signal and good daylight for photography, so additional lighting was not required. The flight route was based on the path of an actual internal audit. Because the Matrice 210 is an industrial grade drone with sophisticated machine-vision based self-localization, its flight was stable regardless of GPS availability.
Photography was carried out at a distance of 5m, as the drone itself is almost 1m wide. The high- quality camera benefited from both optical and digital zoom. Zoom photography is generally sensitive to vibration or movement, but good results were obtained thanks to the drone’s high stability when hovering at a fixed point.
However, at higher altitudes the downward facing vision system found it harder to capture landmarks on the hold’s uniform ground surface. This inhibited the self-stabilization, especially when GPS was unavailable. In such conditions, manual flight by the operator is preferable.
At one point during the trials both GPS and the vision system did not function at the instant that a compass error occurred. Although the drone drifted, the operator, who had more than 500 hours flight experience responded calmly and kept the flight stable. The exercise demonstrated that safe flight in a hold is possible but highlights the importance of having a skilled operator and choosing a drone with high redundancy for magnetic
materials. Furthermore, as the size of Matrice 210 prohibited flight in close proximity to members, there will inevitably be blind spots in surveys with this type of drone.
On the oil tanker, as with the bulk carrier, the flight path was selected according to locations for photography needed in an actual close-up survey. The pilot worked together with an assistant in charge of the camera. A third surveyor checked the images as they came in, requested close-ups when needed, and signaled when to move to the next location. Additional illumination was provided by lights mounted on the drone. This configuration delivered good results for still images but video images suffered from noise.
Fixed-point hovering was difficult because flight was performed in non- GPS mode. The resulting movement caused blurring in photos of members and made pinpoint checks somewhat stressful. This degradation in image quality was exacerbated in zoom photography. During close-up photography, results were heavily dependent on the operational skill of the drone pilot, especially when flying close to walls or other structures. The pilots in these tests had at least 100 flying hours experience.
Compass error occurred and prevented take-off when attempting to fly the Matrice 100 after an extended period of time. Although it is hard to draw definitive conclusions about the effect of magnetic materials, it is thought that the accumulation of error due to the presence of magnetic materials was a factor.
The procedures and best practice given in ClassNK’s guidelines combine the understanding of drone performance gained in these and other trials with its decades of experience conducting class surveys. The society will continue to actively work with innovative technologies like drones and will continue to make efforts to contribute to the further development of the maritime industry.
34 | The Report • March 2019 • Issue 87
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