Semisubmersible MODU deck mated with hull - onshore

Aug. 1, 1999
Sliding hull sections under deck

R&B Falcon's Deepwater Nautilus (RBS-8M) semisubmersible was assembled onshore, without the need to build the deck atop the hull offshore or float the deck over. Hyundai Heavy Industries (HHI) completed assembly of the drilling unit in its Ulsan, South Korea fabrication yard by jacking the deck and assembling completed hull sections underneath.

The fabrication technique is called the "onshore deck mating method." The method allowed faster delivery, simpler final commissioning, and the use of a smaller fabrication workforce, compared to traditional methods.

Semisubmersible drilling units generally are fabricated in a shipyard drydock by stacking unit blocks sequentially from the lower to upper level. The problem with this method is that the extended use of a drydock can cause schedule and cost problems with the overall material flow to the shipyard.

To alleviate this problem and free up the drydock, some fabrication yards use a different method where the structures are mated offshore. In this method, the lower and upper structures of a rig are completed separately and loaded onto a transportation vessel for mating offshore. However, this offshore deck mating requires mobilization of an expensive marine spread and a detailed plan to reduce the risk of a marine operation.

Onshore deck mating

To overcome the rare availability of slots in a drydock, the disadvantages of a sequential block stacking method, and meet tight delivery schedules, HHI developed the onshore deck mating method and applied it to the fabrication of the Deepwater Nautilus.

This new method involved lifting the upper structure 38 meters with 24 strand jacks on six lifting towers. The lower structures, which were fabricated simultaneously on the grounds of the yard, were then skidded underneath the lifted upper structure. Connections between the structures were then welded out for final assembly of the rig. This onshore deck mating method consists of three major steps:

Lifting upper structure

The upper structure of the rig is 74 meters long, 61 meters wide, 73 meters high, and weighs 11,000 tons. The upper structure was lifted from ground level to 38 meters using a hydraulic lifting system on four steel framed temporary lifting towers near to the forward and aft sides of the rig, and two temporary cylindrical lifting towers at the center of the rig.

The forward and aft lifting towers were designed for a 30-knot horizontal wind force in addition to the vertical lifting loads. In addition, 24 560-ton lifting capacity strand jacks were leased from a British company for the lifting. During the procedure, deflections of the upper structure, lifting towers, ground foundations, and loads of the hydraulic strand jacks were continuously monitored and fed back to the operation control system to ensure the safety of the project.

Skidding lower structures

The two lower hull structures were 114 meters long, 39 meters wide, 33 meters high, and weigh 6,000 tons each. The two sections were skidded underneath the lifted upper structure on four skidways covered with lubricated synthetic plastic to reduce friction resistance.

Hydraulically compensated active jack support systems were used to maintain an even level during the side skidding at the bottom of the lower structures to prevent any structural damage caused by unexpected settlement of skidway foundations.

Structures connection

Welding connections of upper and lower structures required high technology to meet the 5 mm fit-up tolerances. The upper deck structure was aligned with the lower hull structure to minute horizontal tolerance by position guide systems at the forward and aft lifting towers.

The vertical gaps along the 240-meter-long weld connection between the upper deck structure and the lower hull structure were fine tuned within allowable tolerance using 24 strand jacks and16 hydraulic jacks.

Deepwater Nautilus

The completed Deepwater Nautilus is a fifth generation RBS-8M deepwater semisubmersible rig measuring 114 meters in length by 78 meters in width by 106 meters high. The rig is capable of drilling in 8,000 ft water depth to a total depth of 30,000 ft.

The rig weighs 25,500 tons and has accommodations for 130 crew members. The vessel will be delivered to R&B Falcon in November. HHI plans to apply the onshore deck mating method to a second R&B Falcon rig, the RBS-8D, and has filed for a local patent on the method.