Assessment of Existing Structures and Repairs - Offshore Structures: Design, Construction and Maintenance - page 522

second tug will pull to the southeast. During this time, tugs will be directed
based on the reaction of the global structure, to ensure that once the platform
gets momentum it is guided to continue falling.
During the toppling work on any platform requiring decommissioning, there
is a potential for pollution from pipeline/risers (if they have not been recently
pigged), trapped annular and other well-bore fluids, deck storage tanks and
the like. Pollution prevention/control entails necessary response vessels, con-
tainment boom, sorbent boom and skimmer/separator equipment. All equip-
ment should be ready to be deployed.
After the platform is toppled, it will be necessary to survey and to depict the
new orientation and altitude of the platform on the seabed. An ROV visual sur-
vey and single side band sonar data will be collected to create a revised three-
dimensional model of the platform resting on the seabed. Once the platform has
been surveyed in its new orientation, any additional structural debris removal
required to clear the direct well bay area should be completed.
It is anticipated that the bell will be kept approximately 100 feet FSW. The

structure will need to be surveyed for any high points that are above this depth
and these items will need to be removed as well.
Toppling analysis should determine the pulling force required to topple the
platform after the cutting and notching take place on the legs. Then the location
of each tug that will be pulling against the single rigging point can be defined in
the analysis. In addition, the analysis should provide the required towline dis-
tances and compare them with the fall path to enable smooth toppling without
problems.
Figure 7.28 shows the rigging arrangement that will be used for each
tugboat.

−

1 × 4” 4-ply nylon
sling arranged in
a basket
configuration for
the breakaway
member on the tug

1-1/2” bolt-type shackle

250' of 1-3/4” wire rope

ROV hook for recovery of rigging in case of
unwanted break away

FIGURE 7.28 Rigging arrangement.