Subsea Geotechnical Investigation

Cone penetrometer testing of lake bed on Lake Wakatipu for Frankton Marina


Diver operated drill rig deployed on stern of offshore work vessel.  Client New Zealand Diving and Salvage

Diver operated drill rig deployed on stern of offshore work vessel.  Client New Zealand Diving and Salvage

OCEL are well experienced in subsea soil investigation work having worked on a number of subsea projects to gather data for the design of subsea pipelines, anchor systems and foundations/piles for fixed structures. Much of this has been direct hands on experience rather than only contractor supervision. The Practice is unusual to the extent of owning and operating specialist drilling/sampling support frames and support pontoons. The three directors of the company are trained commercial divers. The necessity in New Zealand of working in remote areas – remote in terms of access of specialist marine support and equipment – means that the Practice has had to develop its own specialist lightweight equipment. A prime example of this is the lightweight frame developed for the seabed investigation work carried out for the Christchurch and Dunedin ocean outfalls. The frame was used to carry out diver assisted standard penetration testing down to 5 m penetration in sand seabeds to enable a liquefaction assessment to be undertaken. The frame can either be pulled off the beach and along the bottom on its wheels or lifted by helicopter direct to the location. A hydraulic power pack carried by the dive support boat is used to power the hydraulic winch on the frame. The borehole casing is advanced by water jetting.

OCEL has also built a specialist wide beam inflatable catamaran to carry A water jet pump and hoses for water jet probing investigation work in the surf zone.

Based on the experience of the directors, OCEL in conjunction with NZ Diving and Salvage Limited, (NZDS) designed a diver operated drill rig to carry out geotechnical investigation work in water depths up to 50 m depth. The drill rig floor is supported up clear of the seabed to reduce diver decompression time and to keep above the seabed turbid layer.

The drill rig has been used in water depths up to 48 m at the Kahu location offshore Taranaki. The drilling work was undertaken to determine the depth to which the spud cans of a jackup drill rig would penetrate the seabed. It has also been used extensively for sampling of offshore ironsand resources.