Fugro Seacore Ltd recently completed a challenging contract on the volcanic island of La Reunion in the Indian Ocean. La Reunion is a small island with an outline budget of €1,000 million to potentially invest in a new and dynamic transport link 13km long, which will remove the risk of landslides from vertical cliffs which challenge the existing route.
The ground investigation was jointly carried out with Fugro Geotechnique SA, of an agreed contract value of approximately €5 million. The scope comprised over 50 drill locations including drilling for pressuremeter testing and coring at 37 individual sites along the strike length of the proposed new transport link. The purpose of the contract was to complete a preliminary phase of geotechnical ground investigations on behalf of the client - Region Reunion/Direction Departmentale de I'Equipment de la Reunion (Region Reunion), in order to reach the most economically viable design solution to the "Nouvelle Route du Littoral".
Because of the inaccessible shoreline, and shallow water which the tug could not access, it was decided to use Skate 4 in its ‘walker’ mode. This allowed the platform to walk into deeper water to gain access to the tow from the tug. The walking mode is achieved by two sets of four legs operating as independent systems with the movement controlled by two main hydraulic rams. The platform is approximately 200t displacement and has two moon pools fitted to allow a rig to be configured for each type of drilling.
Operations were carried out over seven days per week, on twenty four hour shifts to optimise the efficiency of the platform. Prior to commencement of the works, an environmental report of the seabed was commissioned by diver survey to assess the potential impact to the local ecosystem. During this visual survey large boulders were identified at each borehole location, these were notified to the client and required increased potential tolerance when locating the platform. The bargemaster avoided jacking up on top of a boulder to prevent any sudden movement of the barge legs in adverse weather conditions.
Drilling for core sample recovery was achieved using the Geobor wireline coring system attached to a Comacchio PX1000 track-mounted top drive drill rig which produces 165kN/m of torque and a maximum pulling capacity of 110kN. The Geobor system was used with a variety of diamond impregnated bits for optimum core recovery. Core recovery was in excess of 90% to-date with the very hard basalt proving a good medium to core through.
Casing of 100mm OD allowed drill bits of between 60-64mm to be used for the formation of the rock socket, depending on the nature of ground encountered. Where ground conditions proved to be unstable, such as deep erosion features, the casing was installed at several locations to a maximum of 22m below the seabed. Drilling mud was also used to assist with borehole stabilisation and in-hole cleaning.
Pressuremeter testing took place at specified increments of 1m throughout the length of the borehole. The rock socket was advanced between 3-5m each time as required and drilled outside of the Symmetrix casing, as the test had to be conducted with only the instrument making contact with the ground. The pressuremeter system operated to a maximum of 50 bar pressure in soft ground and 80 bar pressure within the hard basalt layers. The results were recorded in real-time on an Apageo Geospad data recorder - the stronger the surrounding ground, the higher the resistance.