Page 483 - Pipelines and Risers

450 Chapter 24

comparison to a single wall flowline. On shore fabrication site requirements will depend on
system design and local availability of resources.

Prior to offshore installation, for most pipe-in-pipe system, onshore fabrication of the
individual pipe-in-pipe joints is required. The inner pipe must be placed within the outer pipe
and the annulus filled with the insulating material, or the inner pipe precoated with the
insulation material must be slide into the outer pipe. The joint fabrication method would
depend on the pipe-in-pipe system selected and the installation method and vessel selected.
As part of the pipe-in-pipe joint fabrication, the joints could be made up as double, quad or
hex joints in single operation to suit the installation method.

Field joint is a critical area for S-lay and J-lay installation. A suitable method that allows the
welding of pipe joints in a efficient manner that maintains the integrity of the insulation and
mechanical properties is essential. There are two basic methods available. The first, which is
more applicable to J-lay is to allow the outer pipe to slide over the inner pipe after the inner
pipe field weld has been made. The outer pipe is then welded after the field joint area has
been insulated with suitable insulating material .t he technique is required and the integrity the
pipe-in-pipe system during the sliding operation needs to be closely examined. This system
may not be used for S-lay as the outer pie cannot be slid over the inner pipe in the firing line
over multiple weld stations.

242.10 Installation
Installation Methods
The total submerged weight of the pipe section suspended in the water column increases as a
faster rate than the water depth. A pipe-in-pipe system is generally much heavier than its
single wall counterparts, therefore the tension capacity of the installation vessel becomes an
important design factor given the generally low tension capacities of the existing installation
vessels available on the market.

The methods for the installation of deepwater pipelines are S-lay, J-lay, reeling and towing.
Detailed accounts on these methods have been made by various authors. A brief summary is
given here to capture some the key characteristics of each method.

The S-lay method is tall active with the use of S-lay vessels with dynamic positioning and
with stinger capable of very deep departure angles. With its long firing line and many work
stations, an S-lay vessel can be reasonably productive. Limitations o the use of the S-lay
technique are tension capacity and potential high strains in the overbend region, hence
restrictions on combination of large pipe diameter and water depth.

The J-lay method results in a reduction in lay tension requirements. Also, large J-lay vessels
have better motion characteristics and hence lower dynamic pipe stress especially at the
stinger tip as compared to S-lay. However, productivity can'be low due to limited number of
work stations and rather confined working space. This shortcoming may be offset somewhat
by the use of pre-fabricated quad or even hex joints. J-lay is generally not suitable for shallow
water applications.