Page 209 - Pipelines and Risers

182 Chapter 12

General Principle
Pipelay semisubmersibles are effectively a floating factory which weld line pipe joints
together and installs the pipe accurately on the seabed. Pipelay barges used originally in the
Gulf of Mexico experienced difficulty in the North Sea in laying pipelines quickly and
without damage. Consequently, pipelay semisubmersibles were developed. These vessels
perform the pipelaying in the following sequence (see Figure 12.2).

Handle 12 m pipe joints onto deck, using vessel cranes;
Handle joints onto conveyors for beveling and joining joints into pairs (double joints);
Storing double joints;
Join double joints onto main firing line;
Lay pipeline onto seabed without overstressing the line.

To perform these tasks the vessel should exhibit the following capabilities:

Stable platform and constant tension tensioners to pennit the line to be “S laid into the
sea (see Figure 12.3). Should the vessel move too much (i.e. due to weather) the pipeline
may overstress and possibly buckle.
Method of handling joints quickIy. These vessels can install up to 5km of pipeline a day
(one joint every 3.5 minutes);
Method of welding and nondestructive testing of joints with sufficient speed to average
3.5 minutes per joint. The time given to weld a joint is assisted by the double joints, and
having up to 4 welding stations on the firing line.

This means that each weld can actually take (3.5 x 4 x 2) 28 minutes to complete.

Installation CapabititiedConstraints
Pipelay semisubmersibles can install in a wide range of diameter pipelines (6” to 40”) in
water depths from 10 to 1500m deep, the deepest to date is 600m.

The main constraints of the pipelay vessels is the cost, they typically require 400 personnel,
two anchor handling vessels, a survey vessel and the supply vessels for transporting linepipe.
The total cost of the spread varies annually depending on the equipment rates etc.

12.2.2 Pipelay Ships and Barges
General Principle
Pipelay ships install pipelines in the same manner as the pipelay semisubmersibles. The
principal difference is that these vessels are monohulls, and hence do not have as good sea
keeping abilities as the semisubmersibles. Flat barges have worse sea keeping abilities than
the ships and are used only in the calmer wave climates.