PIPELINE PRE-COMMISSIONING (INCL. PIPELINE CLEANING, GAUGING & TESTING, DEWATERING, DRYING & INERTING)
A) GENERAL
Upon the completion of all construction works on the pipeline, the pipeline must be cleaned internally to remove any debris, mill scale and rust. The pipeline is also gauged to ensure that there are no ID reductions or buckles. Following this, a hydrotest is done to ensure that there are no leaks at the flanged connections, and to prove the strength of the pipeline. Pre-commissioning then follows, where the pipeline is dewatered, dried and made ready to accept the intended final product. It should be noted that before pigging can commence the pipeline should be surveyed for free spans to confirm that the pipeline can be flooded without exceeding permissible stress criteria (refer to Section 8).
B) PIPELINE CLEANING AND GAUGING
Pipeline cleaning and gauging is achieved by running a specified number of pigs through the pipeline. These pigs are constructed with two or four sealing discs and a pair of guiding discs, mounted on a steel body. These discs are normally made of polyurethane. The sealing discs are of a slightly large diameter than the pipeline ID, and allow the pigs to be pushed through the pipeline by a pressure differential. The guiding discs are slightly smaller than the pipeline ID, and provide support for the pig, ensuring that it remains central in the pipeline as the sealing discs wear out.
The pigs may be equipped with brushes to clean the internal walls of the pipeline, and gauging plates to check that the internal diameter of the pipeline has not been compromised. A train of three to four pigs is normally run at one time. The first pig is normally a brush pig, followed by alternate gauging and brush pigs as specified by the Client. The brushes fitted are usually of spring steel construction, although nylon brushes should be used where the pipeline is internally coated. Spacing between pigs is usually around 100 metros or as specified by the customer
Gauging plates are sized as a specified proportion of the pipeline diameter, and are normally made of ¼” thick mild steel plate or high tensile aluminum. The pipeline should be accepted if a minimum of one gauging plate be recovered undamaged. Common practice is to install at least one gauging plate on each pig in the train. The decreases the risks of requiring another pig train run if any of the plates installed are inadvertently damaged during loading or unloading.
As a contingency measure, an acoustic pinger is normally fitted to the last pig in the train. This allows the detection of the last pig should the train not arrive at the other end of the pipeline after the required volume of water has been pumped. The pig train is normally driven using clean filtered seawater. Pumps and strainers will be set up adjacent to the pig launcher to supply the required water to the pipeline. In most cases, the water will be drawn directly from the sea. The pumping system is normally required to have a flowmeter / totalizer to determine the flowrate and the total volume pumped. The flowrate will indicate the speed of the train in the pipeline. This is normally specified as a minimum of 0.5m/s. The volume of water pumped into the pipeline will allow the calculation of the distance the pig train has traveled.
A chemical injection pump with similar instrumentation will also be required for introducing the required pipeline packing chemicals. The chemicals used normally comprise a cocktail of corrosion inhibitor, biocide and oxygen scavenger elements and a fluorescent indicator dye. The quantity of packing chemical diluted into the seawater depends on the storage period of the pipeline, before it is dewatered. The dye will assist in detecting any leaks in the pipeline or flanged connections when the pipeline is hydrotested. The chemicals are introduced into the pigging water through a manifold that ensures complete mixing before the water enters the pipeline.
A pig launcher is used to load the pigs into the pipeline. This will consist of a pipe barrel with a larger ID than the pipeline, with a smooth taper to the pipeline ID. A door is provided for loading the pig, as well as inlets for the water propellant. The pig receiver at the other end of the pipeline will be of a similar design, except that it will have a much larger discharge outlet for the water. This is necessary to prevent any pressure build-up at the end of the pipeline, which will impede the progress of the pig train. A provision for the water to bypass pigs in the receiver must also be provided, so that a pig may be removed as soon as it has arrived.
These structures need to be designed in consideration of the anticipated pressures required for driving the pigs, and some Clients will require pressure testing. Variations to these structures also include provisions for launching or receiving pigs subsea, channeling of the water to a disposal location, etc.
Before inserting the first pig into a dry pipeline, a slug of water, a minimum of 100m long, is pumped into the pipeline to provide lubrication. Similar slugs of water are also introduced between subsequent pigs. The last pig in the train will have chemically treated seawater pumped behind it to propel the train to the other end of the pipeline. The loading and recovery of the pigs are typically exercises that must be witnessed by the client. The Client representative will check to see that the gauging plates on the launched pigs conform to specification, and confirm at the recovery that the plates are not damaged.
Disposal of pigging effluent is sometimes a significant issue particularly at beach site locations. Often the direction of pigging will be mandated as onshore to offshore to avoid this problem although customers have differing priorities. Where pigging from onshore is required access to a source of adequately clean water is often a problem that must be addressed.
C) PIPELINE HYDROTESTING
The purpose of the pipeline hydrotest is to ensure that the pipeline, as well as all flanged connections and appurtenances meet or exceed the design specifications. This is done by introducing a pressure typically 125% of the design operating pressure of the pipeline. The pressure is introduced by pumping additional water into the pipeline system, and maintaining the pressure for a minimum of 24 hours, without additional pumping or unaccountable losses.
When the pigging run has been accepted by the Client, the pig launcher and receiver assemblies are removed and blind flanges installed. A pumping rig comprising a high pressure pump, chemical injection pump and a flowmeter / totalizer will be set-up adjacent to the end of the pipeline. Addition instrumentation for monitoring the test will also be set-up in a sheltered location nearby. This will normally comprise a dead weight tester, pressure recorders and gauges, subsea temperature monitors, temperature recorders, etc.
The pressure in the pipeline is raised gradually when all the entrapped air in the pipeline is purged. Frequent volume and pressure monitoring is done for the first 30% of the pressure increase to observe for signs of air entrapment. If this is not exceeded, then pumping will continue, otherwise the pressure will be bled off and further purging done to remove the entrapped air.
Stabilization hold points are normally observed at 75% of test pressure, and thence at 100%. The test period will begin when the pressure in the pipeline stabilizes over a reasonable length of time. When the test is deemed to begin, new charts will be placed in all the recording instruments. Any pressure loss will need to be correlated to temperature fluctuations. When the Client is satisfied that the test is acceptable, after holding for a period of 24 hours or more, then the pressure can be bled off gradually.
Care must be taken when preparing the hydrotest to ensure that all the pipeline components are rated to or above the test pressure. This may be a problem when hydrotest limits extend to process pipe work on topside decks or refinery equipment onshore.
D) PIPELINE DEWATERING, DRYING AND INERTING
Pipeline pre-commissioning comprises the dewatering, drying and inerting of the pipeline prior to the introduction of product. Dewatering involves the running of pigs equipped with sealing discs with compressed air to remove the bulk of the water in the pipeline. Further runs with polyurethane foam pigs will then continue to swab and remove free water from the pipeline. This process also further cleans the internal walls of the pipeline. Strict criteria may exist governing the internal cleanliness of the pipeline that may require the running of many such swabbing pigs.
When this is completed, the pipeline will be dried, normally using either a vacuum to reduce the dewpoint internally and thus evaporate the remaining water, or by running super dry air through the line to absorb the free water. When this is done, the pipeline may then be inerted by packing with nitrogen gas. The pipeline is then ready to accept product. Pipeline pre-commissioning is a very specialized field, requiring specialized equipment and specially trained personnel. This procedure is normally sub-contracted
Source: J. RAY McDERMOTT, S.A.EASTERN HEMISPHERE