Page 240 - Pipelines and Risers

Installation Design 213

500 1000 1500 2000 2500 3000
Water Depth (rn)
It010 t016 *024“ +036’ I

Figure 12.30 Installed submerged weight, Pipeline installed flooded.
When the line is installed empty a direct consequence of the wall-thickness required in deep
water is the large submerged weight. This becomes significant when water depth is deeper
than lOOOm where the submerged weight doubles every lOOOm (Figure 12.29). As would be
expected, the submerged weights are still lower than having a flooded line - until water depths
of circa 2000m are reached. At 2000m the submerged weight of a flooded line can be less
than an empty line because hydrostatic collapse is not a failure mode.

Figure 12.30 illustrates the associated pipeline submerged weights for a range of pipeline
diameters when installed wet. The on-bottom stability requirements benefit from the increase
in the submerged weight due to the heavier wall thickness. This example has not accounted
for thermal insulation coating that would reduce the submerged weights while still satisfying
stability requirements.

12.6.5 S-Lay vs. J-Lay
The offshore pipeline industry is familiar with experienced in installing air-filled pipelines by
the S-Lay method. An indication of the absolute minimum lay tensions are illustrated in
Figure 12.31, which is generated on the basis that no additional weight coatings are required
for stability purposes. On the basis that existing spreads have a maximum lay tension capacity
of between 400 and 500 tones then the deepest a 016” line can be installed is 2000m (Figure
12.31).

It is interesting to note that the lay tension in 2000m depth would be the same (or less) to
install the 016” pipeline flooded as opposed to dry (Figure 12.32), but the associated cost
would be less as the required pipe steel would be approximately half of that installing the line
dry. The difference is even more dramatic for a 010” line, which can be installed in over
3000m of water with existing spreads (when flooded) compared to 2500m when dry.