A Viking DHC-6-400 Twin Otter suffered a runway excursion after landing on runway 14 at Kerema Airstrip, rolled into a drainage ditch which runs along the left side of the runway and impacted the embankment. The flight was transporting eight Total Energy workers returning from Purari. During the landing roll the aircraft continued to decelerate while tracking along the runway centerline. However, according to the PIC, when he attempted to apply the brakes to further reduce the aircraft’s speed, it began to gradually drift to the right of the centerline. Tyre markings identified by the onsite investigation showed that the nose wheel as well as the main landing wheels departed the runway, and the aircraft tracked on the grass surface along the runway edge. According to the PIC, asymmetric power was applied together with rudder input, to get the aircraft back onto the centerline. The aircraft subsequently turned back to the left towards the centerline. The PIC stated that once back on the runway centreline the aircraft continued to veer further left of the centreline towards the left edge of RWY 14. Recorded data showed that the PIC applied full reverse thrust, however, the aircraft continued onto the grass surface past the runway edge and into an adjacent drainage ditch.