Fabrication and Installation - Offshore Structures: Design, Construction and Maintenance - page 346

umbilical, the anchor should be securely stowed on the deck of the anchor-
handling vessel. In circumstances where either gravity anchors or closed stem
tugs are used, and anchors cannot be stowed on deck, the anchors should be
double secured through the additional use of a safety strap or similar method.
The vertical clearance between any anchor line and any subsea asset should
be not less than 20 m in water depths exceeding 40 m, and 50% of water depth
in depths of less than 40 m.
Clearance between any mooring line and any structure other than a subsea
asset should be more than 10 m.
When an anchor is placed on the same side of a subsea asset as the crane
vessel, it should be placed more than 100 m from the subsea asset.
When the subsea asset lies between the anchor and the crane vessel, the final
anchor position should be more than 200 m from the subsea asset.
During lifting operations, crossed mooring situations should be avoided
wherever practical. Where crossed moorings cannot be avoided, the separation
between active catenaries should be not less than 30 m in water depths exceed-
ing 100 m, and 30% of water depth in water depths less than 100 m.
If any of the clearances are impractical because of the mooring configuration
or seabed layout, a risk assessment should be carried out and special precautions
taken as necessary.
Figure 5.28 presents the procedure for calculating the lifting weight; the pro-
cedure for calculating the loads on the padeye is illustrated in Figure 5.29 .After
obtaining the padeye loads, the structural members and padeye are required to be
checked through a procedure presented in the flowchart in Figure 5.30 . Figure
5.31 presents the procedure for checking the rigging facilities. The barge operator
should have a chart that defines the crane

'

s capability to lift the structure to or
from the barge; a sample chart is shown in Figure 5.32 .
Figure 5.32 presents the lifting capacity for the crane on a barge, which is
the relation between boom radius, hook height and the lifting capacity. This
chart should be included in the lifting procedure, which is usually delivered
from the construction company and reviewed by the engineering firm and the
company representative.

5.8.5 Lifting Calculation Report
Calculations should be presented for the structure to be lifted, demonstrating its
capacity to withstand, without overstress, the loads imposed by the lift opera-
tion, with the load and safety factors, and the load cases.
The calculation package for lifting should contain the following minimal
requirements:

a. Plans, elevations and sections showing main structural members
b. The structural model. This should account for the proposed lifting geometry,
including any offset of the lift points.