FLOWLINES & PIPELINES
New installation methods may facilitate
ultra-deepwater pipelay
Asle Venas
DNV
Since the 1970s, offshore oil and gas development has gradually proceeded from shallow-water installations up to around 400 m ( 1,312 ft) to the ultra- deep waters around 3,000 m ( 9,842 ft)
that represent the maximum today. The question is whether the curve will fatten at 3,000
m, or if this is just a temporary pause on the
way to even greater depths. There have been
plans for a gas trunkline from Oman to India
at 3,500 m ( 11,483 ft) depth, but it is yet to be
seen if there will be many such projects in the
near future.
Pipe wall thickness
The main design challenge for development
beyond 3,000 m is related to the high external
pressure that may cause collapse of the pipeline. From depths of 900 m ( 2,953 ft) onwards,
external over-pressure is normally the most
critical failure mode for pipelines. The risk of
collapse is typically most critical during installation when the pipe is empty and external over-pressure is at its maximum.
In addition, the pipe will be exposed to large
bending deformation in the sag bend during
installation that may trigger collapse, and collapse may also be relevant for operational pipelines subject to signifcant corrosion.
The main manufacturing processes relevant
for larger-diameter, heavy-wall line pipes are
UO shaped, welded and expanded/compressed
(UOE/C, JCOE) and three roll bending. These
processes provide a combination of excellent
mechanical properties, weldability, dimensional
tolerances, high production capacities and relatively low costs compared to seamless pipes.
There are at least six pipe mills that regularly supply heavy-wall, welded line pipe for
offshore projects based on the UOE process:
Tata Steel, Europipe, JFE, Nippon Steel, Sum-itomo, and Tenaris. Research into further improving manufacturing techniques continues
in the industry, and we also see several “
newcomers” that can produce good quality pipes
for deepwater.
This potential failure mode is normally
dealt with by increasing the pipe wall thickness. But at ultra-deepwater depths, this
may require a very thick walled pipe that becomes costly, diffcult to manufacture, and
hard to install due to its weight. Currently,
there is a practical limit on wall thickness
that limits the maximum water depth for 42-
Many of the world’s offshore pipelines are designed and constructed to DNV’s pipeline standard
DNV-OS-F101, and new concepts such as pipe-in-pipe may easily be accounted for by adjusting the
relevant failure modes. (Photo courtesy DNV)
in. pipes to around 2,000 m ( 6,562 ft) while
for a 24-in. pipe, this limit is approximately
doubled to 4,000 m ( 13, 123 ft).
Three factors have a major infuence on
the fnal compressive strength of the pipeline: quality of plate feedstock, optimization
of compression and expansion during pipe
forming, and light heat treatment. By focusing on these factors together with improving
the ovality of the fnal pipe, it is possible to
obtain a collapse resistance comparable to
that of seamless pipes.
X-Stream
X-Stream is a novel pipeline concept developed by DNV that aims to solve the collapse
challenge by limiting and controlling the external over-pressure. In a typical scenario, the
pipeline is installed partially water-flled, and is
thus pressurized at large water depths. Then, to
ensure that the internal pressure does not drop
below a certain limit during the operational
phase when it is flled with gas, it is equipped
with a so-called inverse HIPPS (i-HIPPS).
This system also includes some inverse
double-block-and-bleed (i-DBB) valves. It is
inverse in the sense that instead of bleeding
off any leakage to avoid pressure build up in
standard DBB systems, any leakage and loss
of pressure is avoided by a pressurized void
between the double blocks. This is needed
to avoid unintended depressurization by a
leaking valve which may not be 100% pres-
sure tight when the pipeline system is shut
down. Studies undertaken during the devel-
opment of X-Stream show that the weight
increase due to fooding is more or less bal-
anced by the reduction in steel weight.
