Nearshore wireline CPT shows its colours

Wireline cone penetration testing is a viable alternative to traditional top-push techniques for nearshore projects.

Kim Hodgson, Lankelma Nearshore Projects Manager.

While wireline downhole Cone Penetration Testing (CPT) systems have proved popular for deep water investigations, when it comes to nearshore projects, contractors tend to opt for land-based top-push CPT rigs deployed on jack-up platforms or shallow water vessels.

The reasons are clear: top-push equipment, methodology and the data produced are familiar to engineers, operatives and clients and it is relatively simple matter to adapt the approach for overwater investigations. However, downhole CPT systems can deliver data to a high quality similar to that obtained by top-push and, in some cases, at double the production rate.

Traditionally, nearshore investigations involve drilling a conventional borehole to take samples and SPTs, with a CPT carried out within a metre or so of each borehole location. Multi-purpose downhole tools mean testing and sampling can be carried out in the same borehole, allowing SPTs to be replaced by the more accurate and detailed CPT. If required, the CPT data can be converted back to a SPT n value, although it should be noted that CPT data can be used directly in design.

Overall, this means downhole systems are far quicker than top-push in deep boreholes (more than 30m), with production rates of up to 40m a day – double that of top-push – and show a marked improvement over top-push in being able to cope with harder, more variable geology. So, while technique is perhaps more expensive, the additional costs are far outweighed by the savings achieved in shortening investigation programmes.

The Wison-APB downhole CPT system is basically a double-acting cylinder, with the cone attached to the end of a piston rod. The system can perform CPTs and vane tests, as well as taking soil samples using a Shelby push sampler, in water depths of up to 3,000m. Seismic, magnetometer and conductivity tests can be carried out in water depths of up to 1,000m.

Suitable for both offshore and nearshore investigations, it operates using a specially-designed 15m high derrick and can be disassembled to fit inside a standard 40ft (12.2m) shipping container.

Click on the photo below to watch a short film of the system on our Sandpiper jack-up platform being readied for deployment on a recent project.

A major challenge when carrying out CPT over water is that marine sedimentary sequences, in particular, tend to comprise very soft alluvium overlying highly variable materials of differing strength, including rock strata.

The upper soft material does not give sufficient lateral support to prevent rods bending in a top-push system. If a hard layer is encountered, then the entire drill string needs to be withdrawn and the supporting casing advanced by drilling before running the CPT rods back to the base of the hole. This is particularly time consuming at greater depths. Top-push CPT also often requires the installation of two sets of casing through the water to the sea, river or lake bed to ensure that the rods are not affected by water flow.

On the other hand, as downhole systems are deployed through a single drill string, set-up times are significantly reduced. The tool is simply lowered into the borehole via an umbilical cable (including data exchange cabling) with an electrical constant tension winch and carries out the test. And, if hard layers are encountered, the tool can be withdrawn quickly to allow drilling or coring.

The entire operation is controlled from topside and, as with top push CPT, viewable in real time on deck by the engineer.

The ability to withdraw the downhole tool quickly minimises downtime in the event of a breakdown and makes it easier to check, maintain and replace CPT cones, improving data quality. In fact, data quality in downhole systems is comparable to top-push. Even though the test is not continuous, tests can overlap to gather continuous data for the entire investigation depth.

Lankelma has used the Wison-APB downhole CPT on a number of projects around the world and has achieved average metreage rates up to 50% higher than top-push CPT on boreholes more than 40m deep.

Currently we are carrying out investigations for a proposed undersea project in northern Europe, where 23 combined sampling and CPT boreholes up to 40m deep are being drilled in water depths of 16m. Every CPT is over-sampled, as well as after the push.

While clearly ground conditions, weather, equipment and operative performance all have an effect on production rates, the results of these investigations demonstrate that, given the right conditions, downhole CPT can achieve metreage rates nearly double those of top-push.

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