The launch of the 27,000-ton Lena jacket was a critical and challenging aspect of the Lena Guyed Tower installation. The Lena jacket was the longest one-piece jacket ever launched, and is the industry'sfirst side launch of a major offshore structure. Although the side launch offered advantages over a conventional end launch, this technique presented unique problems that required extensive analysis and innovative solutions. In addition to requiring significant launch barge modifications, the side launch increased the potential for jacket skewing relative to the barge due to differential friction and caused higher-than normal slamming loads due to high entry velocities.
This paper discusses the challenges that were faced and the solutions that were developed. Computer analyses and scale model launch tests are also discussed. Particular emphasis is placed on the jacket skewing solution which utilized a special release system similar to that used in the Space Shuttle Program.
Figure 1 depicts the Lena guyed tower jacket on the launch barge. The 1079-foot jacket rested on four transverse skidways on the 580-foot long by 580-foot wide launch barge and overhung the stern and bow of the barge by 194 feet and 305 feet, respectively. As shown in Figure 1 the jacket was offset 20 feet from the barge center line during the transportation and pre-launch operations.
A two-step sequence of the side launch is depicted in Figure 2. The upper diagram shows the barge ballasted to a predetermined starboard heel angle of 7 degrees. At launch, the jacket was freed from the holdback, release mechanisms and began sliding toward the starboard side of the barge. The jacket slid approximately 60 feet, as shown in the lower diagram, before the jacket's center of gravity passed over the tilt pins of the four starboard tilt beams. At this point the tilt beams rotated, and the jacket slid off the tilt beams into the water.
The side and conventional end launch methods were investigated and compared using extel1sive computer analyses the results of which indicated that both methods were feasible. However, the side launch method was selected because it offered several advantages.
The side launch saved approximately 3000 tons of structural steel that would have been necessary to sustain the cantilevered loads experienced in an end launch.
Because of this reduced weight, the eight main pile 1060-foot lead sections were preinstalled, which reduced offshore installation time.
The risk of damage to the jacket and or barge was minimized during the side launch because the time of exposure to winds, seas, and currents was substantially reduced. The duration of the side launch was less than to seconds as compared with 40 seconds for an end launch.