Danish firms collaborate on development of new heave compensation-enabled drillship
The growth of offshore wind necessitates a more flexible fleet of vessels – but that flexibility can pay dividends to oil and gas too. The latest evidence of that comes in the form of a new collaboration between engineering consultancy Geo and the shipping company NCT Offshore.
The two firms have just launched Freja, an S-Class ROV Support DP2 drillship, which, they claim, is “the only drill ship of its calibre and type in the Nordics.” The 75m vessel offers a flexible range of geotechnical tools, with a particular focus on offshore wind development.
The drilling setup aboard Freja is based on a unique system which provides optimised, heave-compensated drilling conditions. The active heave compensated working platform (HCP) allows the entire drilling floor – including the drill rig and drillers – to move according to the movement of the ship: the vessel follows the movement of the waves, while the deck remains stationary.
This system can compensate for up to 6m of vertical movement by the vessel at the drilling location – typically at the mid-ship moon pool – allowing for around 3m of drilling compensation.
Winds of change Geo Marine Survey department director Jens Brink Clausen discussed the vessel’s development with InnovOil. “For Geo to be able to supply our geotechnical drilling/sampling services, we needed a suitable vessel to be able to service projects offshore, at water depths of 20-60 m. The main target is offshore wind. In order to be able to supply high-quality drilling/coring, as is possible on shore, the concept of the HCP was developed and combined with our drilling system,” he explained.
On most conventional vessels, typically only the drill string itself is heave-compensated. The addition of the HCP enables drillers to use techniques which would otherwise be limited to onshore operations. “The drilling system is a dedicated geotechnical rig, enabling standard drilling – for example using piston/hammer sampling in soil formations,” Clausen added. “However, the main advantage is that we have introduced high-quality triple tube coring system (GeoBor-S) in floating/heave compensated mode, which enables us to recover large-diameter core samples from firm clay formations up to chalk and rock.”
This enables the more complex system to be used to its full effect. GeoBor-S ensures much higher sample quality than that usually attainable with traditional drill ships, which apply the so called ‘piggy back’ approach. A number of moon-pool configurations are available – either 1,200 x 1,200 mm or 3,580 x 3,580 mm – and moon-tubes with installed high-precision acoustic positioning (HiPAP).
Freja Is also outfitted with a range of CPT (Cone Penetration Tests) and Vibrocore rigs, and prepared for a suite of geophysical tools including side scan sonar, sub-bottom profiler, magnetometer and hull-mounted multi-beam echo sounder. Geo and NCT have positioned the ship as a multi-purpose vessel to offer the flexibility to take on a wide range of different projects, ranging from large offshore drilling campaigns with core drilling or sampling, handled on the vessel’s on-board laboratory, to projects that only require shallow CPT and Vibrocore.
The cost structure is also set up to reflect the technical capability required. Clausen added: “This should help keep the overall usage up and thereby keep the vessel competitive on a pure cost basis.” That benefits operators too: the less of the vessel that is needed, the lower the rate of hire.
Better geotechnical investigations are crucial for the design and optimisation of foundations for the planned offshore wind farm – which itself affects total construction costs, and the final price for the produced power. Freja was mobilised at the beginning 2017 and has already completed its first two projects in Danish waters. Its next mission at an offshore wind farm project is scheduled to begin later this spring, running until summer.