Lankelma has introduced a new downhole CPT system for nearshore investigations, which the company says is capable of delivering high quality data, reliably and quickly.
The system, based on the Wison-APB downhole CPT, can be disassembled to fit inside a standard 40ft (12.2m) shipping container and can carry out testing and sampling in conjunction with rotary coring from fixed jack-up platforms.
The system comprises a double-acting cylinder with the cone attached to the end of a piston rod. As well as performing CPTs and vane tests, it can take soil samples in water depths of up to 3,000m. Seismic, magnetometer and conductivity CPTs can be carried out in water depths of up to 1,000m.
Kim Hodgson, Lankelma nearshore projects manager, explained the system was a viable alternative to top-push CPT systems. “As well as providing CPT data of the same high quality, downhole systems can test up to 40m a day, double that of top-push, in boreholes more than 30m deep,” she said.
She added that set up times were significantly faster because there was no need to withdraw and then re-assemble the CPT rods whenever a drill out was needed.
Downhole systems were particularly well suited for coping with harder, more variable geology, she added.
“A major challenge when carrying out CPT over water is that marine sediments, in particular, can comprise very soft alluvium over highly variable materials of differing strength, including rock,” said Hodgson.
“The upper soft material does not give enough lateral support to prevent rods bending if hard ground is encountered. This means all the rods have to be withdrawn and the casing advanced by drilling, before running the rods back in. This can be very time-consuming at greater depths.”
Downhole systems, on the other hand, are deployed via an umbilical cable, so set-up times are significantly reduced. “If hard layers are encountered, the tool can be withdrawn quickly to allow drilling or coring.”
The Wison-APB system was used alongside top-push CPT on the nearshore investigation for the Puente Nigale, a new bridge over Lake Maracaibo in Venezuela, allowing Lankelma to compare the performance of both methods.
A total of 1,150m of testing, 555m using top-push and 595m using the Wison downhole system, was carried out through the mudstone of the lake bed from Lankelma’s Sandpiper jack-up platform, working up to 10km offshore in 10m to 15m of water.
“The maximum rate achieved by the downhole system was 16% higher than the highest achieved by the top-push,” Hodgson said, “while the average meterage rate achieved by the downhole system was about 50% higher than the top-push CPT on boreholes over 40m deep.
“This demonstrates how variable ground plays a significant role in the production rates of the top push system. The wireline CPT system is more consistent, allowing for better management and planning of investigations.”
She added that, while this was a relatively small data sample, and ground conditions, weather, equipment and operative performance all affected production, results demonstrated that – given the right conditions – downhole CPT can be a reliable way of producing high resolution data for use in modelling.