Subsea pig launcher
option on marginal, deepwater fields
Machar deployment
rationalizes CAPEX, OPEX costs
Brian Smith
GD Engineering
GD Engineering
The rapidly
expanding development of deepwater marginal fields using subsea production
systems with long flow lines has led to the need to consider routine pigging operations
as an integral part of the pipeline maintenance program.
To maintain pipeline
operating efficiency, wax and liquid removal may be required on a continuous
basis using conventional pigging and/or chemical treatments. Until now, subsea
pig launchers have been technically inflexible and not always reliable. As a
result, they have only been installed where there was no real option, their use
being mainly restricted to commissioning operations.
Reliable pigging
facilities are critical to the development of marginal fields which use subsea
production systems. Many of these fields are located some distance from the
production platform, requiring long flow lines to be laid. The ability to
reliably and economically pig these lines from the subsea installation, without
the need to lay a second line to provide a round trip pigging facility, can
result in substantial overall cost savings when full account is taken of the
CAPEX and OPEX costs.
Even when the field
layout does allow round trip operations, the problems inherent in pushing
solids and wax to the wellhead before returning it to the platform may
eliminate this as an option. Pipeline insulation costs can impact significantly
achievement of a favorable cost trade between CAPEX and OPEX for dual lines.
Temporary
launcher
GD Engineering in
Worksop, UK has developed a new subsea pig launching unit which combines
economic and technical flexibility with positive pig launching. Two basic
systems have been developed around the need to match system deployment and
operation with the field's operational philosophy.
The recent provision
of a subsea multiple pig launching system for BP ETAP is an example of a
temporarily installed launcher deployed subsea only when pigging operations are
stipulated. ETAP is the largest North Sea development for a decade and also one
of the most complex. The pig launching system was originally developed to meet
the demanding requirements for continuous pigging of the 22-mile, 16-in.
multiphase flow line from the Machar Field subsea manifold to the Marnock
central processing facilities platform.
The length of this
pipeline and the resulting temperature drop from the 120! well temperature
meant that heavy wax deposition could be expected in the pipeline. Process
studies indicated that a continuous program of mechanical pigging would be
needed through the field's life in order to maintain maximum operating
efficiency.
Two pigging
philosophies were considered:
- Round trip, two-line pigging using surface launchers and receivers
- A single-line subsea pig launcher then installed on the Machar manifold.
Comparisons between
the two systems showed that the single line subsea pig launcher would be most
cost-effective when CAPEX/ OPEX, pigging philosophy and operational factors
were fully evaluated. But the overriding factor was the prohibitive cost of
providing an additional flow line to the manifold for the total round trip
pigging distance of 44 miles.
Brown & Root,
which performed ETAP development engineering, contracted GD for the launcher
system, which comprises the following elements:
- Vertically deployed pig launcher with a capacity for three conventional pigs or a single intelligent pig
- High pressure cap structure to provide positive sealing of the pipeline when the launcher is not installed
- Test stand to allow on-site pressure and function testing
- Manifold interface framework to provide terminations for the flowline and pig kicker line
- Conventional guide wire deployment system to allow deployment/retrieval of the launcher using a standard diving support vessel
- Pig stop and bypass (PSB) mechanism to provide positive pig launching.
This equipment,
operated by a work class ROV using standard API tooling interfaces, was
developed by GD Engineering to meet the continuous demand for reliable pig
launching at pre-determined intervals throughout the field's operating life.
A standard DSV is
required for installation of the launcher using guide wire alignment (guide
post and funnel) and heave-compensated lifting equipment. Two hydraulic subsea
connectors (16-3/4-in. nominal size for the pipeline and 5-1/8-in. nominal size
for the kicker line) would provide the interface between launcher and manifold.
Installation of the launcher demanded simultaneous makeup of both connectors to
their respective hubs, installed on the manifold structure.
Pig launcher
installations are anticipated to be performed four times annually, assuming
current predictions of wax deposition are accurate. On each occasion, three
pigs will be deployed, each removing up to 10 tons of wax.
The pigs' sealing
discs form a tight fit with the launcher bore, which provides a positive launch
situation when kicker fluid is introduced behind the pigs. The launcher barrel
is long enough to hold three pigs or a single intelligent pig.
Each pig launcher will
require the high pressure cap assembly to be retrieved from the manifold, after
first establishing pipeline sealing integrity. Deployment of the launcher and
subsequent fill and pressurization with manifold product (multiphase
hydrocarbon) would follow.
Pig release
mechanism
The mechanism
developed by GD Engineering for pig release comprises a pressure balanced spool
mounted in a rigid housing. This arrangement provides the integrated function
of a pig stop and bypass (PSB) facility. In operation, the pigs are loaded into
the line-sized launcher barrel to predetermined positions.
The PSB mechanism
spools are extended to provide positive retention of the pigs should they slip
during installation of the launcher. The PSB mechanisms are interconnected by pipework
to provide a continuous flow path for the kicker fluid. Connection of this
pipework to the manifold kicker line is achieved through the 5-1/8-in.
connector.
Following
pressurization with hydrocarbon, flow from the kicker line will pass through
the mechanisms to the front and back of each pig, and between the sealing discs
via the pigs' bypass facility, giving a pressure-balanced situation.
To launch the first
pig, the spool of the first PSB mechanism is retracted. As the spool is
withdrawn level with the inside bore of the launcher barrel, the kicker flow
passing through the spool is restricted and full flow is diverted through this
mechanism to the adjacent PSB mechanism. A pressure differential is created
that causes the first pig to be pushed along the barrel into the pipeline.
Launching of
subsequent pigs follows the same procedure. The PSB mechanism design ensures
that the pig stop is fully retracted before full bypass occurs to prevent the
pig from creeping under the stop as pressure differential increases.
The selected
configuration contains a blend of proven subsea technology with new
innovations, where required. By its nature, new technology carries some
technical risk until proven in service. To offset this, detailed test
procedures have been introduced to determine, as far as is practical, the
likely performance of such equipment.
The Machar manifold
pipelay was completed in March 1997, with site integration testing of the
complete structure last September. GD Engineering manufactured the equipment
described, which was integrated into the manifold structure this February.
Pigging operations are due to begin in October.
Deepwater version
For deepwater
applications, an alternative to the temporary installation of the launcher uses
a pig cassette system, the pig launcher being permanently located on the subsea
manifold. Instead of deploying the pre-loaded launcher, a lightweight cassette
containing the pigs is used to re-load the subsea launcher with pigs.
Both ROVs or
conventional guide wire systems can be used to deploy the cassette, which is
loaded into the launcher through a subsea closure. Sequential release of the
pigs is achieved by operation of pig release latches mounted on the cassette.
Kicker flow is directed to each pig in sequence, in a similar manner to the PSB
mechanism on ETAP. This method is especially economic for large diameter
pipelines requiring subsea pigging operations or when continual ROV
interventions are required on the manifold system.
The cassette system
incorporates numerous design features to suit different operating philosophies:
- A lightweight cassette (reduces installation needs)
- No requirement for multi-make/break and aliagnment of connectors for launcher barrel
- Deployment by conventional guidance systems or ROV
- Horizontal or vertical launcher orientation
- Control and operation by ROV or umbilical
- Launcher barrel of simple construction - pig release mechanism forms integral part of the cassette and is recovered to the surface for routine maintenance
- Intelligent pig launching and pipeline intervention tool capability with same cassette replenishment of pigs from subsea storage when availability of surface vessels is limited.
In conclusion, the
single line pig launcher can provide a cost-effective solution for marginal and
deepwater applications, whether the requirement is for frequent routine pigging
or infrequent intelligent pig inspections. The system's basic building blocks
are designed to provide a standard interface to other subsea equipment and may
allow equipment pooling, leading to further cost savings.
Copyright 1998
Oil & Gas Journal. All Rights Reserved.
04/01/1998
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