All materials removed from the sites/facilities should be required to be loaded on a material barge for transportation to an onshore facility.
Grillage beam mats should be required to support and distribute concentrated load on the barge deck. It is possible to transport shallow water (with water depth of approximately 50 ft.) jacket in a vertical position supported on a grillage mats. However, for the deeper water jackets, it would be necessary to lay the structures on its side for safe transportation.
Specially fabricated support structures mounted on the material barge that will allow the jacket structure to rotate while being pulled by a winch for laying will be required. Alternatively, the jacket can be rotated using auxiliary hook of the crane but it requires human intervention, while the structure is hanging from the hook. This operation requires a large capacity barge and is considered riskier and should be avoided.
Careful planning and preparation should be required to support the structure on a barge. Grillage mats and specialty fabricated beams, as required, should be designed, fabricated and placed on the barge at appropriate locations before the barge arrives at the site.
Baskets should be provided to store all debris collected at the site. All materials placed on the barge should be tied down using cable, clip plates or braces.
The design of the tie down braces should consider the environmental condition and barge motions during transportation for sizing the tie down members and the welds.
Once the material barge arrives at the receiving facility onshore, the salvaged structures, riser, debris, liquid etc. should be offloaded and disposed properly. Steel cutting, cleaning and disposal of contaminated water and other objects should be carried out following the accepted and specified guidelines for disposal.
Two legs of the jacket are assumed to be supported by two cradle type supports pre-installed on a material barge that allows the jacket to rotate for lying sideways. It will be necessary to install a winch on the material barge to pull the jacket from the top to perform this operation. Alternative methods for rotating and resting the jacket on its side may be implemented when supported by engineering analyses and past experiences in performing similar tasks.
The following tasks should be performed for the structure removal:
- Preliminary assessment of make-safe preparation work
- Design and sizing of temporary work platforms required for flushing of risers and removal of structures
- Platform (risers, deck, jacket and piles) lift weight estimation / verification
- Development of the lift configuration and lift rigging design
- Design of grillage beam supports and other supporting structures for platform back loading and transportation
- Design of tie-down braces
- Development of off-loading procedure at an onshore facility
Furthermore, a SACS model should be developed for the platform to be removed. The strength of the platform framing and joints and the new rigging members and supports should be verified by running lift, back loading, and transportation analyses using SACS models. Joint can punching shear check should be performed based on API RP 2A guidelines for all the existing and the new tubular connections.
Members that are corroded and or damaged to the extent non-useable for structural strength should not be included in the structural analyses unless it is shown absence of this member will result in over-stressing many other members and joints. Such member then should be strengthened to its full capacity by cleaning, welding plates, clamps etc. to fit the situation.
The constructibility issues related to lifting and back loading of the salvaged structures and miscellaneous components should be considered in the design package for offshore disposal.
Measures to mitigate the potential hazard of dropped objects should be considered in the design and analysis.