Assessment of Existing Structures and Repairs - Offshore Structures: Design, Construction and Maintenance - page 519

intervention, due to instability of the platform. The recommendation was to prepare
the structure for a controlled toppling. Inaddition, it was determined that diver opera-
tions were not possible due to significant overhead and dropped object hazards.
Once the platform is resting on the bottom, the wells can be secured using
methodology similar to that applied before to numerous demolished platforms
and wells projects. The main principle of the toppling process is to disconnect
the risers near the sea floor and to create hinges in the conductors and north
legs, thus creating the potential for the platform to fall in a general southerly
direction. All subsea operations would be conducted by ROVs. Tugboats
would then be utilized to pull the platform over for final toppling.
The dynamic nature of this type of work means that the work plan may
change due to weather, safety or operational restrictions to ensure continued
work-flow efficiencies.
Also, the platform could fail with continued regular weather forces that
affect the structure, and it is possible that collapse could occur after any struc-
tural cutting. So it is important to prepare for this potential situation by pro-
posed tooling packages that can be used for toppling, post-toppling debris
removal, and any intermediate situation.
A toppling analysis was developed through basic overturning and structure
analysis calculations. The analysis revealed that the bracing had to be cut in
defined locations, as shown in Figure 7.25 . Analysis showed that the platform
would require around 85 to 120 tons of force for toppling after proposed cuts to
First step, cutting the bottom;
second step, cutting the upper
FIGURE 7.25 Locations for cutting the bracing.