09-02-2013, 11:09 AM
DESIGN OF MULTIPLE-PIPE SLUG CATCHERS
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INTRODUCTION
SCOPE
This new DEP specifies requirements and gives recommendations for the design of
multiple-pipe slug catchers to be installed on land. Vessel-type slug catchers and parking
loop slug catchers are only briefly described.
DISTRIBUTION, INTENDED USE AND REGULATORY CONSIDERATIONS
Unless otherwise authorised by SIOP and SIEP, the distribution of this DEP is confined to
companies forming part of the Royal Dutch/Shell Group or managed by a Group company,
and to Contractors and Manufacturers/Suppliers nominated by them (i.e. the distribution
code is “F”, as described in DEP 00.00.05.05-Gen.).
This DEP is intended for use in gas plants, exploration and production facilities and
supply/marketing installations.
If national and/or local regulations exist in which some of the requirements may be more
stringent than in this DEP, the Contractor shall determine by careful scrutiny which of the
requirements are the more stringent and which combination of requirements will be
acceptable as regards safety, environmental, economic and legal aspects. In all cases the
Contractor shall inform the Principal of any deviation from the requirements of this DEP
which is considered to be necessary in order to comply with national and/or local
regulations. The Principal may then negotiate with the Authorities concerned with the object
of obtaining agreement to follow this DEP as closely as possible.
General definitions
The Contractor is the party which carries out all or part of the design, engineering,
procurement, construction, commissioning or management of a project, or operation or
maintenance of a facility. The Principal may undertake all or part of the duties of the
Contractor.
The Manufacturer/Supplier is the party which manufactures or supplies equipment and
services to perform the duties specified by the Contractor.
The Principal is the party which initiates the project and ultimately pays for its design and
construction. The Principal will generally specify the technical requirements. The Principal
may also include an agent or consultant authorised to act for, and on behalf of, the
Principal.
Vessel-type slug catcher
The geometry of the vessel-type slug catcher could range from a simple knock-out
vessel to a more sophisticated lay-out (Figure 1). Since the overall length of vessel-type
slug catchers is relatively short for a given volume this type is preferred in the case of
limited plot sizes (e.g. offshore platforms). When larger liquid volumes have to be
accommodated, say of more than 100 m3, either the multiple-pipe or the parking-loop
type should be used.
Multiple-pipe slug catcher
Examples of typical multi-pipe slug catchers are shown in Figures 2, 3, 4 and 5.
Multi-pipe slug catchers can be made of standard pipeline materials. Besides having a cost
advantage over the high-pressure vessel design, this type allows for a greater flexibility in
design possibilities to cope with wide ranges of flow situations and geometrical
configurations with different shapes of the available ground area.
A definition and description of the main parts of this type of slug catcher is given in (1.3.2).
More information concerning their specific hydraulic functions is given in (3).
Parking loop slug catcher
In this DEP the parking-loop slug catcher is only mentioned for general information.
The problem of counter-current flow is avoided in the parking-loop type slug catcher. This
concept is illustrated in Figure 7. In this concept the separating and storage parts are
virtually disconnected: it consists basically of a large separator with the liquid outlet
connected to a long single pipe-loop. The incoming gas/liquid stream is separated in the
vessel. When the liquid level rises rapidly, indicating that a slug rather than a gas-liquid
mixture is arriving, gas flow from the vessel is restricted forcing the liquid to flow into the
pipe-loop in which a sphere separates the liquid entering from the gas present. With the
other end of the loop now opened to the downstream facilities the gas is driven out in a
direct co-current manner. The stored (parked) liquid can be discharged as a single slug by
using high pressure gas if the location is at a booster compressor station as shown in
Figure 7 or the liquid can be discharged gradually to a downstream treating plant. This type
of slug catcher is particularly suited to offshore applications where the separator can be
placed on the platform and the parking loop on the sea-bed. All valves and controls would
be on the platform. The advantage of the concept is the possibility to save space. A
disadvantage of the method is the high reliance on strict operational procedures.