To meet the evolving demands of the geotechnical drilling and subsea operations industries, design engineers at Cornwall-based specialist engineering firm MintMech have developed a new heave-compensation system. Active Heave Assist (AHA) offers heave compensation for vessel motion within less than ten centimetres, ensuring the drill bit remains stable relative to the seabed even during challenging conditions.
Traditionally, offshore geotechnical drilling operations have relied on passive heave-compensation systems, which, although effective in some conditions, come with limitations due to friction and vessel motion. Passive systems are often unable to maintain drill string stability in soft ground conditions, during delicate downhole logging or when working off-bottom, such as during tool changes.
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Traditional passive heave compensation (PHC) systems use a pneumatic spring to stabilise the drill string but require a reaction force from the seabed – weight on bit – to properly counteract vessel motion. When the seabed geology is soft or the drill bit isn’t touching the seabed, this reaction force is lost, causing the PHC to stop compensating for vessel motion.
By retrofitting the AHA system, whereby the drill position is no longer dependent on the seabed reaction force, the drill bit position can be controlled regardless of the ground conditions, improving both quality and consistency of cores. Similarly, the ability to maintain the drill bit position when the drill bit isn’t touching the seabed improves borehole stability during downhole tool changeovers.
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Minimising heave motion is important because the vertical motion increases the risk of inconsistent weight on bit (WOB) — a critical variable while drilling. If WOB is too high, the driller risks damaging the bit, deforming the drill string or crushing the formation and reducing core quality, especially when working with soft or fractured geologies. Excessive WOB can also stress drilling equipment, leading to higher maintenance costs and operational inefficiencies.
‘Active Heave Assist fundamentally changes the way we can work offshore,’ said Jack Berryman, cofounder of MintMech. ‘By retrofitting AHA to the existing passive system, we can actively control the position of the bottom hole assembly (BHA) and enable reliable core and data collection in ground and metocean conditions that previously made it practically impossible. Our ability to combine accurate BHA positional control with automated weight on bit limits is transformative.’
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The AHA system has already been successfully deployed in a commercial campaign in the West Pacific. Operating in rocky, uneven seabed in deep-water conditions where traditional jack-up barges and seabed frames couldn’t be used, the system enabled advanced downhole logging operations that required precise positional control of the bottom hole assembly.
‘High-quality cores and accurate downhole data will be very useful to the European offshore wind sector, where developers are urgently seeking to optimise foundation designs and reduce costs,’ said MintMech cofounder Laurie Thornton. ‘AHA can enable more efficient foundation design and significant savings on steel, time and embodied carbon.’
MintMech continues to develop AHA enhancements including automatic weight-on-bit (AWOB) and auto-core-run control systems to further improve drilling performance across a range of seabed conditions. An AWOB system immediately removes the variability in WOB. The driller uses their field experience to determine the appropriate WOB for the situation, then sets the AWOB to a specific target value, which it won’t exceed, 2.1 tonnes, for example. An AWOB system not only frees up the driller’s attention for other dynamic variables, but also improves the consistency of the core by maintaining constant WOB throughout its length.
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‘In offshore wind, the low hanging fruit is disappearing fast,’ said Theo Cleave, commercial manager at MintMech. ‘As more projects are completed, developers must revisit areas previously considered unattractive or unviable – like very soft ground, rocky ground or deeper water – having a better understanding of the seabed conditions could open up these locations for development.’
AHA can be retrofitted to a vessel’s existing passive-heave-compensation system with minimal disruption to a normal mobilisation. Multiple fail-safes and an intuitive control panel make the system easy to use for the driller, who can choose the level of automation based on their experience and override it if necessary. AHA and AWOB can also be deployed to large-diameter marine drilling and construction applications, for example the installation of FLOW foundations.
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Footage of AHA in action can be viewed here.