Offshore Oil and Gas Pipe Laying Vessel Jobs.
In offshore oil and gas field at times a difference made between a
flowline and a main pipeline. The first is an intra-field pipeline, which is used
to join subsea wellheads, manifolds and the platform within
an offshore oil and
gas field. The second, is an export pipeline, is used to bring the resource to
shore generally known as main pipeline. Many aspects are taken into
consideration for offshore pipeline construction developments like the offshore
ecology, geohazards and environmental effects.
Offshore pipelines are used mostly to transfer oil or gas, but carrying
of water is also done where it is essential like where land pipeline crosses water expanses, such as small seas,
straights and rivers. An offshore pipeline which is also called as marine pipeline,
subsea pipeline or submarine pipeline is a pipeline that is laid by offshore pipe
lay ship/barge on the ocean floor or underneath it inside a dugout called trench.
Trenching: An offshore pipeline may be laid by a pipe lay ship or
barge within a trench for protection of
pipeline against fishing gear (e.g. anchors) and trawling movements. Trenching
of pipeline is required in shore approaches to protect the pipeline against under
water currents and wave movements when it crosses the surfing zone.
Trenching work can be carried out either by pre-trenching method i.e,
before to lay pipeline, or later by post trenching by digging seafloor from underneath
the pipeline. In the post trenching, the trenching machine rides on top of, or
straddles, the pipeline.
A number of methods are utilised to dig trenches in the sea bottom
for offshore pipelines: Dredging or excavation: A Dredger or an excavator is
deployed for digging seabed in shallower water before laying a pipeline. This is
carried out by different process, especially with a ′cutter-suction′ arrangement,
with the use of buckets or a with a backhoe.
Plowing: The plowing method, which was primarily used for
pre-trenching, has developed into successful method which is lighter in mass
for faster and safer action.
Mechanical cutting: This method uses chains or cutter disks to dig
through and take out harder soils, as well as boulders, from underneath the
pipeline.
Jetting: This is a post-trenching process in which the soil is
removed from below the pipeline by means of powerful pumps to blow water on
each side of pipeline.
Burial: In an open trench a pipeline is not so much protected, but
when pipeline is buried it very much more safer. This is usually done either by
covering the pipeline with rocks extracts from a close by shoreline. Otherwise,
the soils dig out from the seafloor while trenching can be used as refill. The
main disadvantage with burial is that if a leak occurs in pipeline it is
difficult to find it out and for the subsequent repairing processes.
