Seeing double in the deep

Elaine Maslin takes a look at Fincantieri’s latest ultra deepwater drillship design – Proxima, sporting twin cylindrical drill towers and two moonpools.

Fincantieri’s Proxima drillship design. Images from Fincantieri Offshore.

Since the first drillships were built in the 1950s, with the CUSS I, capable of drilling in 400ft of water, drillship design has come a long way – and so has what we consider deepwater.

Today, technology development continues apace as much of the offshore industry’s future profits are set to be gleaned from deep waters and from high-pressure, high-temperature reservoirs.

Italy’s Fincantieri Offshore, part of the Trieste-based Fincantieri group, recently unveiled what it hopes will be a drillship to meet the future deepwater drilling market needs. Working with its Norwegian subsidiary Castor Drilling Solutions, the firm created Proxima, an innovative drillship provided with two drilling towers.

Fincantieri was founded in 1959, bringing several existing shipyards, including one in operation in Naples since 1780, under one organization.

The new design consolidates some of the previous concept, Overdrill, – introduced in 2013 – where the drilling systems were integrated into the lean body vessel with greater capabilities and performances. The new design goes beyond Overdrill, substituting the conventional lattice-design derrick structure for enclosed cylindrical drilling towers, based on wind turbine support structures. The design allows for an extended, open drill floor, by reducing the potential for dropped objects and simultaneous and safer drilling operations.

The towers, with hydraulically powered low-speed winches, controlled by digital hydraulic valves, are provided with a passive and active motion compensation system also integrated into the structure of the vessel. Other innovations have also been included in the design, including the spaces and equipment to carry on board in future two, 20,000 psi blowout preventers (BOP) and the option to make the vessel LNG-fueled.

“We had three main drivers in our idea: the performance, to drill wherever possible; safety; and added value for our clients,” said Gianni Scherl, Fincantieri, while describing the new 208m-long, 40m-wide, design at the Offshore Mediterranean Conference (OMC) in Ravenna earlier this year.

Scherl said that the vessel will deliver a cost saving of about 10% of the overall well costs per well, shared between the drilling contractor and the operator.

Fincantieri set out with a number of goals for Proxima: an ability to drill in up to 12,000ft water depth and to 50,000ft of total drilling depth, transit at more than 14 knots, accommodate up to 250 people, carry two 20,000 psi BOPs and have a variable deck load up to 32,000-tonne. Scherl said the vessel will be safer, due to the cylindrically-shaped, enclosed, drill towers, situated over two well centers, spread 26m apart. Compared to few hundred bolts in the new tower concept, developed by Norway-based Fincantieri subsidiary Castor Drilling Solutions, a standard, lattice design tower has some more than 5000 bolts, Scherl said.

The top drive, rated at 1500-tonne and suspended from six pre-cut wires, which connect directly to a hydraulic winch, has been configured so that it doesn’t have to be connected to a traveling block or pulley system, cutting the need for cut and slip operations, Scherl said. An auxiliary top drive is also included, rated at 1150-tonne. The winch, installed at the bottom of the tower, is driven by hydraulic motors and can be used for tripping as well as active heave compensation. However, the cylinders underneath the winch also provide passive and active heave compensation.

“We worked closely with Castor Drilling Solutions to integrate an innovative rig into the ship to have the total benefits – for the equipment but in particular for the efficient operations of the ship,” Scherl said.

Fincantieri’s Proxima drillship design.

The 2000-tonne capacity setback (where stands of drill pipe or tubing are set back and racked) is embedded in the ship’s hull on the opposite side of the vessel to the towers. The variable load is then compensated using a pair of water ballast tanks dedicated to heeling service.

The towers may also be telescopic, which means they can be lowered for safer maintenance using the onboard cranes and also should they need to be lowered to go under a bridge. For this operation they will employ a rack and pinion system.

The drill floor itself is open and extended, running continuously from aft, where stands for the drill pipe are stored, to the accommodation deckhouse, allowing easy access for dedicated onboard service cranes.

To accommodate the 26m distance between the two drill centers, Fincantieri opted to incorporate two circular-shaped smaller sized moonpools into the design, helping to increase deck space as well as reduce hydrodynamic resistance. In tow tank testing, the company compared the three configurations, no moonpool, one traditional big moonpool and with two smaller moonpools. Scherl said this confirmed that the resistance with two smaller moonpools was less than that with one bigger, traditional configuration moonpool. The firm also found that this reduction in residual resistance is linked to the oscillations of the water inside the moon pools, developed through sloshing or piston mode water oscillations.

“Where for the rectangular moonpool (L/B > 2) the natural sloshing mode starts at around Fn = 0.1 and increases with speed, while for the squared or circular moonpool (L/B = 1) only the natural piston mode is present and well developed around Fn = 0.1 and decreases to zero at around Fn = 0.14,” Fincantieri said. The water oscillations inside the rounded or squared moonpools is of the piston mode, by far lower in absolute value respect to the sloshing mode typical of rectangular shapes lowering the residual resistance in transit and allowing to reach the 14 knots of transit speed with the power delivered by the three aft thrusters.

The vessel also has an automated, fully redundant riser handling system, Scherl said, so risers are always held in clamps and never hanging, during transferal from the riser hold to the catwalk and vice versa, removing the risk of damage to the risers. This solution also helps to reduce the amount of space needed inside the riser hold when they are outfitted with standard overhead cranes.

With its extended drill floor design, with two elevators, fore and aft, which connect the service corridor under the main deck with the drill floor deck, Fincantieri also said that use of forklifts could be extended, because it would enable continuous circulation of the forklift in all operative areas, reducing the use the onboard cranes. Proxima will also have an extended helipad so that, if required, two helicopters could be landed.