Have you ever climbed a staircase in the dark, only to expect one more step that wasn’t there?

It’s an unpleasant surprise, your foot falls through the empty air and lands with a thud. In a similar way, unexpected soft ground can cause unpleasant surprises when drilling for core samples at sea.

Geotechnical exploration is the preliminary phase of most marine development projects. Whether preparing to install foundations for construction of an offshore wind turbine or looking for rare minerals, taking core samples is key to understanding the makeup of the seafloor.

However, various geological processes can create a stratified sequence of different materials in the seabed. For example, in areas of tectonic activity, such as subduction zones or mid-ocean ridges, layers of soft sediment like clay may be interspersed with harder igneous rock, like basalt, due to volcanic eruptions or the movement of tectonic plates.

Punchthrough

When drilling in such stratified areas, the drill can ‘punch’ through soft layers unexpectedly, leading to a sudden drop until hitting competent rock again. A driller might easily overlook the downward motion, as the constant up-and-down movement of the boat due to heave makes it hard to distinguish subtle changes in the drillpipe's behaviour.

The first indication of punchthrough often comes only after the core is recovered and data is missing. The core’s length will be shorter than the actual distance drilled, because the core barrel was unable to collect material during the punchthrough.

Traditional solutions

Vessel owners have tried to address punchthrough using various methods. For example, by using a frame fixed to the seabed that feeds down the drillpipe at a constant rate. However, these frames are expensive and impossible to land in some topographies, like in parts of the western Pacific where cliffs and boulders pock the seabed.

On board, passive heave compensation makes drilling easier by providing a constant weight on bit (WOB), but if a passive system encounters soft ground, it will punch through. System friction also generates WOB fluctuation of around two to four tonnes, depending on the wave height, complicating the issue.

Active heave compensation, on the other hand, counters swell using a computer to measure the position of the vessel and then moving to compensate, for example by taking up or paying out on a winch. This creates a near-stationary drill bit but cannot control WOB. This can lead to damaged equipment and downtime, for instance if touchdown is too forceful.

Removing risk

The Government has set ambitious renewable energy targets of producing 50GW of offshore wind and 5GW of floating offshore wind by 2030. To meet those targets, the industry must ramp up project delivery. At the same time, the low hanging fruit has gone, and future projects will need to take place in more challenging locations, such as very soft ground or extremely hard ground.

The bottom line is that failure is both more likely and less acceptable. It is therefore crucial to obtain the most complete understanding possible of the subsurface conditions. That requires quality cores and a system that prevents punchthrough can help.

Any solution must make use of tried and tested technology and failsafes to minimise the likelihood of downtime from breakdowns or steep learning curves. Vessel owners could combine the benefits of traditional heave compensation systems by using an active computer system that assists a passive compensator.

Such a system would control two key parameters: target weight on bit and maximum feed rate. Once the drill bit touched down and WOB increased, the system would automatically adjust the feed rate, eventually coming to a stop as it reached target WOB.

Then, once drilling began, the drill would advance again. If it encountered a softer layer, the system would only allow the bit to accelerate up to the preset maximum feed rate. This would prevent the bit from suddenly dropping and damaging the core or equipment. If something went wrong, such as a power loss, the system would revert to passive mode and automatically balance itself, preventing unpleasant surprises. By preventing just one day of downtime, the system could pay for itself.

Although punchthrough is almost as easy as treading on a step that isn’t there, its solution is far more complex than simply turning on the lights above the staircase. Thankfully though, despite the complexity of the challenge, engineering solutions do exist.