Civil Engineering Reference
In-Depth Information
effective reach of the crane and its load capacity are diminished by the distance
from the boom seat to the stern or side of the barge.
The main advantages of the derrick barge is its close control of positioning,
allowing it to be able quickly to reach any point in three-dimensional space, and
its ability to be oriented in the most favorable direction to minimize boom tip
displacements and accelerations
Note that a load suspended from a boom tip is a pendulum, and, while the
load line length is usually too long for direct resonance, the load may tend to get
dynamic amplification from lower-frequency energy. Therefore, the practical
solution is to raise or lower the load quickly through those positions that
develop an amplified response.
5.10.10 Jack-up Construction Barges
In cases of high wind speed and higher wave height, in turbulent sea areas, or
with breaking waves, such as in shoal or coastal water, the jack-up barge is the
suitable vessel for use in construction.
The jack-up barge is more expensive than the other construction barges, but
its main advantage is that it incurs no stand-by time, because its crew can work
all the time.
The typical sequence starts with the barge moving to the site with its legs
raised. Upon arrival at the site, it is moored with a spread mooring. Construction
jack-ups can operate only in relatively shallow water, 30
-
60 m, with 150 m as
an extreme, so the use of a taut mooring is practicable, although up to 300 m
may be adopted for drilling rigs.
With waves and swells usually of less than 1 m, the barge
s legs are lowered
to the sea floor and are allowed to penetrate under their own weight. In some
soils, penetration can be aided by jetting and vibration. Using the jacks on
one leg at a time, the bearing capacity of the soil should be checked. With
all legs well embedded, the barge is jacked up clear of the water. This is the
most critical phase, since wave slap on the underside of the barge may cause
impact loads on the jacks and may shift the barge laterally, bending the legs.
To cushion the impact, special hydraulic cushioning may be connected to nitro-
gen-filled cylinders; alternatively, neoprene cushioning may be employed. Once
well clear, the barge is raised up to its working height. Then the legs may be cut
loose, one at a time, and a pile hammer used to gain even greater penetration.
A jack-up barge is shown in
Figure 5.42
.
Since uneven settlements may take place as a result of time, operations and
wave energy input into the legs, the jacks have to be periodically reactivated to
equalize the load on each jack. This is especially necessary during the first few
days at a site. The mooring lines are reattached, slack. The barge is then jacked
down until it is afloat. Once again, the critical period is when the waves are hit-
ting the underside. The mooring lines are tightened. Then the legs are jacked
free, one at a time.
'
Search WWH ::
Custom Search