This paper provides a background on remotely operated seafloor drill technology and its benefits to the offshore industry. It also presents a bearing capacity and breakout force analysis for Fugro's Seafloor Drill I at a deepwater location. Both Seafloor Drill I and Seafloor Drill II are used to complete offshore geotechnical and geohazard site investigations.
The Seafloor Drills are sophisticated data gathering devices rather than simple coring tools and can operate in water depths up to 4,000m (13,000 ft) and on seafloor slopes up to 25°. They are capable of performing rotary drilling, coring, sampling, and in situ testing including piezocone penetration tests with real-time observation to 150m (490 ft) below the seafloor. Custom-built containers are used to ship the Seafloor Drills globally where they are mobilized onto vessels of opportunity. Also, controlling the Seafloor Drills via an umbilical maximizes personnel safety while mitigating shallow gas events.
Distributing the Seafloor Drills' submerged weight over a larger bearing area is an effective approach to reduce bearing pressures; however by expanding the bearing area, higher pullout loads are required to recover the Seafloor Drills back to the vessel. Proper consideration needs to be given to this inverse relationship between bearing pressures and pullout loads when using seafloor drill technology.
In order to reach deeper water depths with conventional drill ships, aluminum drill pipe was introduced through the water column. This method decreased the hook load on the drill rig rather than necessitating larger drill rigs and vessels. Seafloor drilling systems are the latest innovative technology to explore the next frontier of ultra-deepwater locations. Since the Seafloor Drills use neutrally buoyant umbilicals, bearing pressures and ultimately breakout forces do not increase with water depth. By using a patented braid design, these umbilicals also serve as the strength members for the Seafloor Drills.
