Page 156 - Pipeline Pigging Technology

The Zeepipe challenge

An internally-coated pipeline can be expected to contain substantially less
debris than an uncoated line. In addition, suitable measures will be taken to
minimize the introduction of debris during construction. The cleaning re-
quirements are therefore, at this stage, assumed to be minimal.
If, however, excessive build-up of debris occurs in front of the cleaning
pigs or if the seal/guide discs wear down, the cleaning effect will be reduced.
In addition to precautions taken prior to and during pipelaying, cleaning pigs
are included in the intermediate testing of each section, and thereby informa-
tion about pipeline cleanliness will be available prior to the final design of the
precommissioning cleaning train.
The present philosophy is that cleaning will be performed using a single
train of pigs equipped with magnets to remove ferrous debris. Although it is
not planned, gel could be used during the cleaning operation to act as a
lubricant, if this should prove to be necessary.

Dewatering

Dewatering and subsequent drying of a gas pipeline is required in order to
avoid hydrate formation during the initial start-up phase and to be able to
deliver sales gas according to specification.
The dewatering train will basically consist of batches of methanol. For the
longer sections, a leading water-based gel and a trailing diesel-based gel have
been chosen for the following reasons:

to improve the sealing effect of the leading pigs and to prevent
methanol slug depletion;
to lubricate the pigs to avoid excessive wear of the discs; and
to ensure proper sealing between the propelling gas and the methanol
batches.
The dewatering train for the 810-km Sleipner to Zeebrugge pipeline will
be launched from Zeebrugge, and propelled by dry gas. Propulsion speed will
be between 0.6 and 0.8m/s; gas supply will be by pressure control, and the
speed control of the train will be performed by the flow control system
installed on the dumpline at Sleipner.
The use of an "incompressible" liquid (water) between the dewatering
train and the flow-control station, and having the gas supply on pressure
control, will ensure a smooth and stable pig travel.
At least four to five methanol batches will be included. Each of the front
and rear gel batches will be split in two by a pig; this will ensure that at least
one pig in each batch is fully surrounded by gel, and thereby secure the long-

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