Published on 02 Oct 2016 by Tadd Pham
All offshore pipelines in water depths shallower than 200 feet below mean sea level should be removed unless buried to a minimum of 3 feet below the seabed surface, e.g. below the natural seafloor, in which case abandonment-in-place may be an alternative.
Reburial to the 3 feet minimum below the seabed surface may also be an acceptable alternative to removal. Buried pipelines in water depths from 200 feet to 300 feet are candidates for abandonment-in-place regardless of burial depth, provided that they do not present a hazard (obstruction) to navigation, commercial fishing (trawling) activities or future use. Where national, regional or local regulations and requirements, concession terms and contractual agreements, or permit requirements allow abandonment-in-place at depths shallower than 3 feet a risk assessment should be performed to confirm that abandonment-in-place is sufficiently protective of the Company’s interests from all perspectives, including HSE, business, social, and reputation. If the risk assessment confirms that abandonment-in-place is sufficiently protective of the Company’s then an exception procedure is not required.
- Plan the pipeline abandonment, including isolation, contents disposal, removal versus abandonment-in-place, and impacted areas. Perform site visits, surveys, contents sampling, survey map reviews, etc., as needed, as well as identifying in-line valves, repair locations, anchors, anchor mats, pipeline crossings, power or communications cable crossings, dead legs or other features that may require mitigation procedures. During the site visit, meet with field engineers and operators to discuss the history and condition of the pipeline (e.g., last time pigged, paraffin plugs, corrosion issues, repair details, and number of repairs) and identify potential concerns such as areas of high scour, sea bottom mudslides, and environmentally sensitive areas.
- Pipeline agreements (e.g. right-of-way, crossing, access to third-party facilities, fluid handling) should be reviewed for special requirements, and to verify that the agreement will not be violated or that Company rights will not be unintentionally relinquished. Particular attention should be given to retaining rights-of-way at shore crossings where future use is of concern.
- Where seabed or environmental conditions (e.g., mud flows, erosion, currents, storms, mudslides, sediment movement, seismic events) may have caused a pipeline to move or shift, consider surveying affected pipelines with sonar equipment or by diver with a GPS pinger. The intent is to verify the line location, identify damage and locate impediments to abandonment (e.g., pipeline crossings, electrical power or communications cable crossings, valves, repair clamps, auger anchors, and concrete mats), and to determine the type and location of debris (shipwrecks, containers, etc.) on or near the pipeline. This information will be critical for planning pipeline abandonment.
- Sample the pipeline to determine the contents, then check contents for hazardous chemicals, materials or wastes (e.g., NORM, mercury or benzene) and check external coatings such as paint and insulation for potentially hazardous wastes (e.g., lead, asbestos, PCBs) that might require special handling procedures, disposal permits, or both.
- De-energize and isolate the pipeline from energy sources. Where a pipeline to be abandoned is connected to other pipelines (or segments) that are to remain active (live), inactive or idle, the abandoned pipeline should be isolated by physical removal, e.g. “water gapped” or “air-gapped”. All abandoned connections to active, inactive or idle pipelines should be permanently closed with flanges and blinds (or weld caps) per the pipeline material classification or specification. All welds and flanged joints installed to achieve isolation should be inspected and tested per the pipeline material classification or specification. Lockout and tag-out all sources of energy (such as pumps, rectifiers, cathodic protection systems, and valves) that are not physically disconnected.
- If possible, pig the pipeline to be abandoned. Recover all fluids and dispose of them in accordance with the waste management plan, taking into account waste characteristics.
- Flush or purge the pipeline to the cleanliness level specified by national, regional and local regulations and requirements, concession terms and contractual agreements, and permit requirements. If unspecified by regulations or contractual agreement, the project team and management should set the minimum cleanliness level, which for example may be specified in parts per million or “flushing until clear returns”, e.g., until there is no sheen on receiving waters in a static sheen test. The minimum cleanliness requirement should be flushing until clear returns if no other requirements have been established. The minimum pipeline flushing volume to achieve clear returns should be 150% of the pipeline volume, but flushing efficiency is also dependent upon achieving adequate flow velocity, so higher volumes/flowrates may be required.
- Pipelines should not be cut, severed, sheared, drilled or welded upon unless it can be positively identified along its entire length (with no risk of misidentification) and there is a reasonable assurance that it has been flushed, depressurized to an equilibrium state, and is free of hazardous liquids and gases, including hydrocarbons. Suitable safeguards should be employed to mitigate risk (i.e., drilling a pilot hole and checking contents, setting a pollution dome, etc.) and to protect against releasing hazardous materials to the environment.
- Where there is any doubt or question relative to the identity, depressurization or cleanliness of a pipeline, and the decision has been made to proceed with cutting, severing, shearing, drilling or welding, suitable precautions should be taken to mitigate the higher risk. One proven industry practice is to use “hot tap” methods to verify that the pipeline is depressurized and to confirm that the pipeline has been flushed and cleaned, and the contents are suitable for release to the environment. The “hot tap” provides a means of controlling flow and sampling/testing the contents, and if the contents are not clean it provides a means of localized flushing to further clean the line. The “hot tap” provides pressure and flow control mitigation against cutting into the wrong pipeline, which could be pressurized with hazardous contents.
- Whether severed in place for removal or abandonment-in-place, precautions should be instituted to protect divers, surface personnel, equipment, and marine vessels, and the environment during subsea work. Containment domes (hoods) are one mitigation method against inadvertent fluid releases during subsea severing of pipelines.
- On conventional fixed structures the pipeline and tube turn are often severed and removed together, and the riser left in place for removal with the jacket as part of facility decommissioning. In this case, the pipeline riser should be severed inside the jacket footprint, and sufficiently supported by the jacket to allow safe lifting with the jacket. A plug should not be placed in the bottom of a riser as it can present a projectile hazard during jacket lifting and removal.
- Abandoned pipeline segments and related appurtenances such as risers, equipment, and piping that are cleaned and abandoned, but not removed should be identified as such with nameplate tags. The tags should include the pipeline number (or segment number), with date abandoned, and facility “to/from” information. Nameplate tags are typically fabricated from brass (other materials may be acceptable) and should be attached with nylon tie-wraps if subsea, and with stainless steel wire if on surface facilities.
- Pipelines (or segments) that are abandoned in place in water depths of 300 feet or less should meet the following requirements unless noted otherwise:
- Each end of the pipeline should be buried to a depth of 3 feet or greater. In areas with evidence of seabed scour, additional precautions should be taken to mitigate erosion resulting from abandonment activities.
- Each end of the pipeline should be sealed with a blind flange or a locking plug regardless of water depth, i.e., even in water depths greater than 300 feet. Examples of suitable locking plugs include a “dogging type internal plug” on pipelines 6 inches and larger and a “dogging external sealing cap” on pipelines 4 inches and smaller. A rubber plumber’s plug is not an acceptable long-term locking plug.
- Each end of the pipeline should be stabilized at the burial movement. Stabilization should be accomplished with an engineered solution based on seabed conditions and likely loads, or by utilizing proven industry practices. In shallow water, hurricane (typhoon) prone areas (such as the United States – Gulf of Mexico) a proven practice utilizes an auger-type anchor and steel cable to temporarily secure the pipeline end, followed by permanent stabilization with cement bag coverage.
- Where an auger-type anchor is used to temporarily stabilize the end of a pipeline, it should be fully embedded and externally attached or looped to the pipeline ends with a 1/2-inch cable. Cement bags (not simple sandbags) should be placed over the end of each pipeline to extend 6 feet in front of, 3 feet behind and 3 feet to each side past the end of the pipe as a minimum. If allowed by regulations, requirements, and agreements, concrete mats may be used in lieu of cement bags to cover the ends of the pipeline.
- The resulting surface should be even with natural seabed surface and should not present an obstruction to navigation or commercial fishing (trawling) activities, e.g., the top of cement bags, concrete mats, etc. should not protrude above the natural seabed surface.
- Pipelines (or segments) that are removed should meet the following requirements:
- Pipelines on the seabed surface may be severed in place, and then lifted or pulled to the surface, and transported to shore facilities for clean-up and disposal or salvage (material recycling).
- Pipelines buried below the seabed surface should require excavation, severing, and removal. Some seabed soil conditions may allow pulling of the pipeline out of the seabed to the surface without excavation, but caution must be used as obstructions (such as other pipelines or cables) may be hidden and could pose significant risks.
- Pipelines are typically severed on both sides of crossings (other pipelines or cables) to eliminate the risk of damage at crossings during pipeline lifting or pulling to the surface, and then the pipeline segment under the crossing carefully removed.
- If reverse-lay operations are employed, the subsea end of the pipeline segment should be sealed with a locking plug or cap to contain the contents during pulling to the surface. Plugs or caps should be mechanically attached and able to withstand the forces encountered without releasing. Examples of suitable locking devices include a “dogging type plug” on pipelines 6 inches and larger and a “dogging cap” on pipelines 4 inches and smaller. A rubber plumber’s plug is not an acceptable locking plug.
- Associated pipeline subsea infrastructure and appurtenances (such as pipeline manifolds, jumpers, pipeline end manifolds [PLEMs] and terminations [PLETs], anchors, pilings, concrete mats, riprap, cement bags, etc.) should be removed as required by
national, regional and local regulations and requirements, concession terms and contractual agreements, and permit requirements, and as follows:
- Subsea infrastructure and associated appurtenances in water depths of 200 feet and less should be removed, unless flush with or buried below the seabed surface.
- Subsea infrastructure and associated appurtenances in water depths from 200 feet to 300 feet should be removed, unless flush with or buried below the seabed surface, unless they can be demonstrated through a risk assessment to not present a hazard (obstruction) to navigation, commercial fishing (trawling) activities or future use.
- Subsea infrastructure and associated appurtenances in water depths greater than 300 feet do not have to be removed solely due to surface hazard (obstruction) considerations.
- Remove all concrete foundations, footings, and other subsurface infrastructure (such as drains and sumps) in coastal sites (onshore) that are included in offshore pipeline abandonment.
- Remove all pipeline surface minor equipment, piping, and appurtenances associated solely with pipeline operation and safety, unless they will be removed during facility decommissioning or have future economic utility.
- Document final site condition and retain all records (including, for example, “as abandoned” drawings, before-and-after photos, videos or sonar scans, regulatory submittals, permits and approvals, contractual records, route maps and end points of abandoned-in-place pipelines, sampling reports, and inventory data).