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* Launching

* Jacket structures that are too heavy to be lifted can be launched into the sea

off a launch barge. A launch barge is a flat top cargo barge equipped with

skid beams, a rocker arm, launch winches and a suitable ballasting system.

Jackets are designed to be either self-upending or upended with the

assistance of a crane vessel. Launched jackets need to have sufficient reserve

buoyancy in order to ensure they float at the end of the launch sequence. The

jacket legs are made buoyant by the use of rubber diaphragms at their

bottom ends and steel caps at the top. Additional buoyancy located

appropriately is sometimes required to achieve the required level of reserve

buoyancy or to ensure the jacket will upend itself at the end of the launch

sequence. Launching operations require the jacket to be fitted with a launch
truss. The launch truss is an integral part of the jacket structure and serves to

transfer the weight of the jacket into the skid beams and the rocker arm
during the launching operation. The weight of the launch truss normally

constitutes a significant part of the jacket weight.
* The rocker arms are two beams that are installed at the stern of the barge in

line with the skid beams. They are connected to the stern through hinges. The

rocker arms serve to support the jacket weight as it rotates over the barge stern and

dives into the water. As such, the rocker arms and the supporting hinges can be

substantial structures. Figure 4.4 shows a typical launching sequence of a jacket

that was designed to be self-upending.

* 4.2.2 Compliant Towers

* Compliant towers are made of several rigid steel sections joined together by

hinges such that the tower can sway under environmental loads. A compliant

tower’s mass and stiffness characteristics are tuned such that its natural period

would be much greater than the period of waves in the extreme design

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