Offshore cranes rotate on pedestals. API 2C requires that pedestals have high strength
requirements to ensure the crane does not overturn or separate. The top portion of the pedestal
attaches to the crane and is usually provided by the crane manufacurer. The lower portion
of the pedestal connects to the crane pedestal to the supporting base (platform, vessel, etcetera).
The connecting structure between the crane pedestal and the crane base is the responsibility of the
purchaser and must be properly engineered. The top portion of the pedestal is
often field-welded to the lower portion.
There are many types of crane bases, but they can be
divided into two categories - floating and bottom-supported.
A bottom-supported crane base is stationary (somehow
affixed to the earth). Floating crane bases undergo motions
due to wave action. The motions tilt the crane, impart velocity
to the crane boom tip and also impart forces on the crane due
to acceleration of self weight. This affects both on-board and
off-board lift capacity.
Bottom-supported bases include fixed platforms, docks, jack-ups and inland barge rigs (which lie on the marsh bottom).
API Spec 2C provides data for certain categories of vessels. These include: TLP, Spar, Semisubmersible, Drillship, FPSO and "Ship or Barge in Calm Water". All categories account for crane motion except the "Ship or Barge in Calm Water". This category is equivalent to the "shipboard crane" of classification agencies and assumes that no motion is present, but the crane base undergoes a list of 5 degrees and trim of 3 degrees.
If a floating crane base does not fall under one of the provided
categories, the purchaser must provide "vessel specific" data,
which indicates how the vessel and the crane boom tip
will respond to wave conditions. The vessel designer or a naval
architect/marine engineer can perform this analysis. For more