Machar deployment rationalizes CAPEX, OPEX costs
- Subsea launcher and manifold interface frame. [29,207 bytes]
- ROV deployed pig launcher cassettes (4). [55,396 bytes]
- Machar manifold site integration tests. [18,711 bytes]
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 launcherGD 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.
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.
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 mechanismThe 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 versionFor 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.
Copyright 1998 Oil & Gas Journal. All Rights Reserved.