Cassie Schott 0000-00-00 00:00:00
The flurry of hurricane activity off the Texas and Louisiana coasts in the last decade has created complex challenges for the offshore salvage industry in the Gulf of Mexico. From 2004 to 2008, five major hurricanes (Ivan, Katrina, Rita, Gustav, and Ike) left hundreds of structures either damaged or destroyed, elevating the demand for removal. Salvage operations of toppled platforms typically required extensive diver involvement, numerous subsea cuts, and many separate lifts to the surface to remove the wreckage piece by piece. In 2007, Versabar provided a breakthrough in offshore salvage operations with the development of new catamaran lifting technology, named the “Bottom Feeder.” The dual barge mounted truss system’s ability to perform single-piece topside retrievals provided a safe and efficient alternative to time consuming and hazardous piecemeal recovery. With the Bottom Feeder’s initial success and a large stream of hurricane-related cleanup remaining in the Gulf, the demand for a second system became apparent. Versabar decided to build a larger system that could not only handle heavier lifts of toppled platforms, but could also perform above-water decommissioning of damaged or abandoned platforms as well. The VB 10,000 was completed in a 12-month span and launched in October 2010 as the largest lift vessel built in the U.S. After retrieving dozens of sunken platforms with the Bottom Feeder and VB 10,000 lift systems, Versabar engineers found themselves with a new set of challenges. In a number of cases, the sunken topsides were too fragile to remain intact during the lift to the surface. Versabar engineers designed and fabricated large steel “baskets” with multiple lift points to assist in these lifts. Lowered to the seafloor adjacent to the damaged structures, these baskets provide a stable base to enable the damaged topside to be retrieved intact. As subsea work progressed, Versabar continued to search for more efficient, safer recovery methods that would minimize diver involvement in subsea operations. Diving is inherently dangerous under normal conditions. Sending divers to the seafloor to attach hooks to unstable structures in a debris field with swirling currents and poor visibility exposes divers to significantly greater risks. Challenged to remedy the situation, Versabar president, Jon Khachaturian, came up with a solution that would require minimal subsea preparation in topside retrievals: a new lift device that would eliminate the need for using hooks as lift points. Named, “The Claw,” the new device would scoop up the sunken topsides from the sea floor and deposit them on baskets to be retrieved using the VB 10,000 lift system. The Claw project began in December 2010 with a series of rough sketches as Khachaturian communicated his concept to his team of engineers and draftsmen. The draftsmen then went to work to produce a series of drawings that would evolve into the final documents that fabricators would use for construction. Fabrication began in March 2011 as welders began cutting and fabricating the primary elements of the Claw structure at Gulf Marine Fabricators in Aransas Pass, Texas. By April, assembly had begun, and the large structures began to take shape as the pipe sections were joined. Meanwhile, four identical baskets to be used in conjunction with the device were engineered and fabricated at Versabar and C&C Marine in Belle Chasse, Louisiana. The baskets, each with a support capacity of 1,250tons, were designed with built-in outward-facing hooks, in order to be lifted from underneath by a steel pipe. The new design would allow for a more efficient recovery procedure and gave the baskets the flexibility of being lifted either by the Claw, or a customized rigging solution. As work progressed at Gulf Marine Fabricators, the Claw began to resemble the drawings and models. By August, the two identical Claw assemblies were ready to be installed on the VB 10,000 lift system and undergo testing prior to deployment. The Claw design consists of two identical grappling devices suspended from each of the gantries of the VB 10,000. Each Claw assembly operates independently from the other, but they can be used in tandem for a double Claw lift. Each Claw is made up of two halves, joined at the top by a pin connection to a girder. Each Claw girder is attached to the VB 10,000 by two 9-part slings. This sling connection allows the Claw to easily attach to the VB 10,000 and detach for lift operations when it is not used. The main hoist blocks of the VB 10,000 lift system control the raising and lowering movement of the Claw assembly. A second set of blocks, operated by 100ton hydraulic winches, control the opening movement. These blocks are connected to each Claw half by using a long-throat pelican hook and a 3- part sling. The pelican hook can easily attach to the sling, eliminating the need for manpower in assembling the rigging.When each Claw block is raised, the Claw half rotates on the pin at the girder, resulting in the opening of the Claw. When the block is lowered, gravity pulls the jaws of the Claw back to a closed position. As a backup to real-time surveys, an angle indicator located on the girder allows an ROV to determine the exact position of the Claw when it is underwater. The jaws of each Claw assembly have eight steel tines weighing 11tons apiece. The tines are connected in an offset fashion, allowing them to interlock when the Claw is in the closed position. When retrieving a topside from the seafloor, the blocks open each Claw half, allowing the tines to be positioned around the sunken topside. The tension in the blocks is then released, the Claw moves back to a closed position, and the topside is retrieved in a scooping motion in the process.Depending on the structure, the tines are either inserted into the topside or interlock beneath it, securing it for lifting. The main hoist blocks then raise the Claw assembly as it clutches the topside, and the GPS thrusters of the VB 10,000 position the system above the basket that has previously been lowered to the seafloor. The Claw deposits the topside on top of the basket, and the entire lift package is now ready to be brought to the surface. In a 4-day span in September, the VB 10,000 used the Claw to retrieve five sunken topsides from the Gulf of Mexico. One gantry was used to perform the underwater lift of each topside with the Claw, while the other gantry of the VB 10,000 used a counter weighted lift bar rigging setup to lift each basket, resulting in the quick, efficient recovery of approximately 3,500tons of hurricane debris.
Published by Technology Systems Corporation. View All Articles.