Fabrication and Installation - Offshore Structures: Design, Construction and Maintenance - page 336

assembled into a jacket in the fabrication yard. Analysis of such assembly load-
ing conditions requires sequential simulation of jacket geometry and loads and
knowledge of the jacket assembly plan and procedures.
During transportation of the jacket to the site on the barge, the jacket and tie-
down braces, their connections and the transportation barge are subjected to
significant dynamic accelerations and inclined self-weight loads. These motions
and resulting dynamic loads must be simulated in incremental loading
sequences to determine the highest stressed components. Some bracing may be
needed only for the jacket transportation phase, and some of these braces may
have to be removed before the jacket is installed on site, to reduce in-place
wave loads.
During its launch to the sea, the jacket will be subjected to significant inertia
and drag loadings. In general, the most critical loading occurs as the jacket starts
tilting around the launch beam and rapidly descending to the sea. At this position,
the tilting beams exert high concentrated loads on the stiff bracing levels. These
require a launch bracing system specially designed to distribute and reduce the
launch forces. As the jacket hits the water plane and rapidly descends into the
sea, the leading jacket braces may experience high drag and inertia forces.
It is worth mentioning that another critical case of loading is crane lift of the
deck or jacket from the transportation barge. In such lifting operations, deck and
jacket members and connections may be loaded in directions that differ from
their in-place loading directions. Additionally, lifting slings that are redundant
or shorter/longer than planned may result in loads that are substantially different
from those calculated for idealized conditions: in the case of a four-sling lift, if
one sling is shorter than planned, three instead of four slings may carry the
entire deck loads. An unplanned load distribution may also be caused by a cen-
ter of gravity that is at a location somewhat different than calculated. Further-
more, lifting padeyes and lugs are components with high consequences of
failure. A single padeye failure may result in the loss of the entire deck and
jacket and the crane. Such critical components and their connections to the
structures lifted must be designed for higher safety factors. Safety factors of
4 or more against ultimate capacity are commonly used for padeyes, their con-
nections to the structure, and the associated lifting gear.
5.8 LIFTING PROCEDURE AND CALCULATIONS
Lifting forces are functions of the weight of the structural component being lifted,
the number and location of lifting eyes used for the lift, the angle between each
sling and the vertical axis and the conditions under which the lift is performed.
All members and lifting point connections for the lifted component must be
designed for the forces resulting from static equilibrium of the lifted weight and
the sling tensions. Moreover, API RP2A recommends that, in order to compen-
sate for any side movements, lifting eyes and the connections to the supporting
structural members should be designed for the combined action of the static