Compact drillship configured for stable, low power service

The proposed deepwater drillshipGSP Venus is based on the XDS 3600 design.

The design confers numerous other advantages compared to a deepwater semisubmersible, the company adds, notably lower construction costs and a higher payload. The latter makes the vessel less dependent on nearby support infrastructure, which can be critical for exploration or development drilling in remote areas.

Among the XDS 3600’s main dimensions are an overall length (over the waterline) of 208 m (682 ft), a beam of 35 m (115 ft), an operating draft of 9 m (29.5 ft), and a displacement at that draft of around 46,500 metric tons (51,257 tons). Station-keeping is maintained via three 4,500 kW aft azimuthing thrusters, two 3,500 kW forward azimuthing thrusters, and two 2,500 kW tunnel thrusters. Power is provided by six 5,530 kW main generator sets and one 1,500 kW emergency generator.

As with all USOS designs, the mission equipment to be integrated is the client’s preferred makes and as such the client’s choice. A typical drilling equipment package for which the XDS 3600 is designed, includes a 2,500 kips derrick, four 2,200 JOP mud pumps, an active heave-compensated drawworks, and an 18¾-in., 15,000 psi BOP. The vertical racking system can handle drill pipe in the range 3½ to 6 5/8-in., and there is storage available for up to 12,000 ft (3,657 m) of 75-ft (23-m), 21-in. (53-cm) risers. There is also accommodation for up to 200 personnel.

According to USOS Managing Director Bob Rietveldt, the overall design “ensures full integration between vessel performance and drilling operations. A good example is the innovative handling of marine risers and the BOP outside the drill floor. Furthermore, the risers can be stored below deck, creating a much larger free deck area compared to existing drillships.”

At the same time, he points out, “the total power requirement is reduced by 15-20% compared to existing designs and the DP thrusters are smaller. The XDS 3600 is therefore very cost-effective to build and operate.”

Safety considerations led to all accommodation and marine systems, including the engine room, being located closer to the bow, freeing up a larger deck space area. “All spaces aft of the accommodation are dedicated to drilling operations,” Rietveldt says, “the areas becoming more hazardous towards the aft of the vessel. The aftship is dedicated to well testing and is where the flare booms are located.”

The vessel’s X-BOW shape ensures a transit speed of 14 knots, but moreover provides a much better motion behavior. “Because of its bow, the wave drift forces are reduced and the waterline is increased, resulting in higher speeds by the same power requirements. And as a result, fuel economy is optimized,” he adds.

“Drillships also weathervane during operations, facing wind and waves as much as possible head on,” Rietveldt observes. “Models test in the water basin at the MARIN center in Wageningen, the Netherlands, with a customized version of the XDS design, have shown that due to the bow shape, the motion and acceleration levels are significantly reduced. This not only results in an enhanced operational profile, but also creates a safer and more comfortable working environment.”

Another feature is a new derrick design developed by USOS, which measures 50 x 50 ft (15 x 15 m) x 180 ft (55 m) high. “In contrast to other new derrick solutions as an alternative to the commonly used lattice derricks, our starting point has been that our derrick should be able to cope with currently used drilling equipment and should not have an impact on the current way of working on board,” Rietveldt notes. “This resulted in a box-structure derrick, which allows for a lighter and stiffer structure, while at the same time protecting cabling and hydraulics against operational or weather-induced damage.”

For theGSP Venus, USOS is performing basic design to the client’s requirements, including a dual derrick designed to pass underneath the Bosporus Bridge when in transit to and from the Black Sea. USOS is working on alternative versions of the vessel for various other contractors.

Rietveldt claims the XDS 3600 would also work well in far northern regions. “The shape of the X-BOW is perfectly suited for Arctic conditions, as it does away with open forecastles, staircases and accommodation walkways. As such, mooring equipment and crew are sheltered against spray water and icing. And because of its round shape, high bow and large freeboard, the vessel is less prone to ice build-up and as such the danger of loss of stability.”

USOS has developed two further drillship designs targeted specifically at the Caspian Sea. TheCDS 1500 is similar to recent deepwater drillships, but has been designed for relatively low-cost, modular construction to ease passage through the canal system into the landlocked sea. The concept features a fully-contained mid-body section taking the vessel’s complete longitudinal strength (the overall length is 145.6 m, or 476 ft), and housing power generation, drilling and marine systems. Its wing tanks are detachable by means of a bolted connection.

TheDB 1000 is a drilling barge, suited for shallow (down to 4 m, or 13 ft) and deepwater work in the Caspian Sea, again with an emphasis on modular construction. This vessel can handle tender-assist drilling, using a self-erecting derrick, and it can also be used for high pressure wells in water depths up to 1,000 m (3,281 ft), in spread-moored mode.