Marco van der List, Frank Berrens, Jan de Jonge, Melanie Struben - Dockwise
In 2008, Exmar contracted Dockwise to transport the 7,500-metric ton (8,267-ton) hull of its Opti-EX 6000 platform from Korea to the Gulf of Mexico. Opti-EX is a deep-draft, ring-pontoon semisubmersible developed by Exmar to produce marginal fields in various water depths. The company has been in dialogue with two national oil companies over an initial offshore application.
The Opti-EX hull pre-skidded to theSwan’s side. Photo courtesy of Dockwise.
Samsung Heavy Industries fabricated the hull in Geoje, Korea. Dockwise’s brief was to support the load-out of the hull and to mobilize the structure to the Kiewit Offshore yard in Ingleside, Texas, for topside integration.
TheSwan departs Samsung’s yard for the anchorage with the Opti-EX sitting on the skidbeams. Photo courtesy of Dockwise.
The hull consisted of a squared ring pontoon measuring 76 x 76 m (249 x 249 ft). Each of the four corners was fitted with columns which towered 48 m (157 ft) above the pontoon baseline. Two of the four pontoons that made up the ring pontoon were erected directly onto two skidbeam tracks to provide a continuous skid track onto the vessel.
When Dockwise and Samsung began designing the vessel skidbeams, several construction issues had to be taken into account. Quayside skidbeams are supported by solid foundations on the rock bed under the quay, whereas the vessel skidbeams had to be constructed to distribute the weight of the cargo over the webframes inside the vessel.
To ensure vessel stability during the 16-hour load-out operation, a 165-metric ton (182-ton) grillage consisting of 30 girders was installed for each skidbeam to distribute the cargo weight over two non-adjacent webframes onboard the vessel. The skid track then was installed on top of these girders and connected to their quayside counterparts in order to keep the transport vessel properly aligned during the load-out.
To prevent frictional forces compressing the vessel against the quay side fendering during the load-out, Dockwise and Samsung installed strand jack system anchors on the quayside instead of onboard the vessel. This was made possible by the hull’s massive overhang. A two-hinged link beam also was introduced to mechanically connect the vessel with the landside skidbeams in the horizontal direction. As a result, the vessel would retain freedom of vertical motion, further eliminating any friction-induced vertical forces.
The open team effort between discipline experts at Exmar, Samsung, and Dockwise allowed engineers to predict the forces acting on the vessel and the cargo during the load-out, to elevate the sensitivity of the ballast operation, and to reduce risks.
Torsion suppression
Originally, Dockwise had proposed a diagonal transport configuration for the Opti-EX hull, with two columns directly on the centerline of the transport vessel and two columns hanging over the side. This configuration meant that around 60% of the hull’s weight would be directly supported by the vessel’s deck, with the remaining 40% hanging over the vessel’s side. However, when Exmar’s engineers analyzed this proposal, they concluded that significant structural reinforcements would be required to make the hull compatible with the bending and torsional forces in the pontoons.
TheSwan is submerged while the Opti-EX is ready to be pulled into position above the cribbing. Photo courtesy of Dockwise.
Therefore, Exmar requested Dockwise to investigate a square transport configuration with all four columns hanging over the vessel’s sides, which meant suspending 80% of the weight outside the vessel. This eliminated both the torsion forces in the pontoons and the need for structural reinforcements inside the cargo. However, it also significantly reduced the total length of webframes and bulkheads available inside the cargo for support by the wooden cribbing, yielding contact pressures very close to the limit of the wood.
To exploit other strong points in the ring pontoon and to address the webframes and bulkheads inside the hull, Dockwise developed special cribbing, which was pre-installed, together with the vessel’s skidbeams before the actual skidded load-out operation began. This allowed the hull to be floated directly from the vessel skidbeams onto the wooden cribbing in a single operation, substantially reducing risk and vessel time.
With the Opti-EX secured on its transportation cribbing, theSwan prepares to depart for a 15,000-mi (24,140-km) voyage. Photo courtesy of Dockwise.
Immediately after the load-out, temporary seafastenings were installed to allow shifting to the anchorage a few miles from the yard. Upon arrival at the anchorage the temporary seafastenings were removed and the vessel was pre-ballasted to float the cargo aft. The Opti-EX hull was then refloated from the vessel skidbeams, and cargo winches were used to pull the hull aft until it was positioned over the wooden cribbing.
In concert with the seafastenings, the wooden cribbing properly restrained the cargo during the ocean voyage.
On Oct. 25, theSwan began her 15,000-mi (24,140-km) voyage to the GoM.
Motion analysis
For transport, an Octopus decision support and motion monitoring system onboard theSwan measured wave-induced motions and acceleration forces on the cargo. This system is designed to collect a range of key data, including the position of the ship, which is relayed to a central database, enabling operations and engineering to monitor the journey precisely and remotely.
The collected data keep the client informed of progress on a daily basis, and can help evaluate the entire voyage after the discharge. Based on Ship Performance Optimization System (SPOS) weather forecasts, the system predicts motion responses such as roll, pitch, and accelerations on the cargo’s center of gravity. The data are compared with the engineering limits set for the voyage. If these limits are exceeded, the captain can decide in good time to change course or to adjust the vessel’s speed. Finally, the measured motion response data are provided to the client for the life-time assessment of their property, and area also available on-line.
On Dec. 15, theSwan arrived at Ingleside and the Opti-EX 6000 hull was successfully floated off and redelivered to Exmar at the topside fabrication yard. This project provided a useful learning curve for all parties involved, which should also help to optimize future projects.
