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installation method specifically aimed at deepwater and ultra-
deepwater projects. This method is characterised by a steep ramp,
typically 65" or higher departure angle, so that the pipe has a
suspended J-shape.
The stresses and strain close to the top are minimised, as
well as the horizontal tension component at the top and the
horizontal tension at the mudline [Langner and Ayers, 1985].
S-lay
S-lay is utilised to install the vast majority of all offshore
pipelines. Allseas have configured its flagship, the Solitaire, with a
stinger that can reach very steep departure angles. As a result, it
was able to install, a 10-in. pipeline at 5400ft water depth.
S-lay is a very efficient lay method, since all welding is
done with pipe in an horizontal position.
Reel-lay
The reel method was patented in the USA by Gurtler
(1968), who makes reference to a British Patent of 1948. The
patent [Gurtler, 19681 has very detailed drawings of a horizontal
reel, as well as a pre-bending apparatus and straightener.
Towed Pipelines
In this installation method, the pipeline is constructed
onshore and towed into place. There are different ways to tow the
pipeline string to site: surface tow, mid-depth tow or bottom tow.
In the surface tow the pipe is positively buoyant, towed to location
on the surface, and sunk in position by flooding. Wave action is a
factor; therefore this method is used typically where rough seas are
not likely. In the mid-depth tow typically the pipe or pipe bundle is
negatively buoyant, suspended above the seabed and towed by a
lead tug with a tail tug at the end of the pipe string. If the pipe is
positively buoyant, mid-depth tow may be achieved by
incorporating the use of drag chains at specified intervals along the
pipe string, so that the pipe string assumes an equilibrium position
above the seabed. For the bottom tow method, the pipeline rests on
the seabed, and a tug pulls it. The length of the towed string is
limited to about ten miles in the most favourable conditions.
The tow methods are challenging due to the effects of the
environment such as waves action, oscillations during pull or
abrasive effects of the seabed during bottom tow.
However, the onshore construction may significantly
reduce cost when compared to the installation methods described

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