Improving the Safety of Marine Pipelines (1994) / Chapter Skim
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5 Avoiding Outside Interference with Pipelines
5 Avoiding Outside Interference with Pipelines
Pages 66-85
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From page 66... ...
In others such as vessels that operate in shallow waters adequate pipeline burial is the only satisfactory measure. For this reason, regulatory standards and sound engineering practice require pipelines to be sunk below the bottom in shallow waters (generally those less than 200 feet deep)
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From page 67... ...
They work offshore of the Louisiana coast within 10 miles of land, where the menhaden schools are found in waters less than 80 feet deep. With drafts of about 14 feet, these vessels commonly maneuver in very shallow waters, to the point of stirring up bottom sediments with their propellers (Gulf States Marine Fisheries Commission, 19884.
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From page 68... ...
per year along the western Louisiana and eastern Texas coasts. Storms can cause wave- and current-induced movements of nearshore bottom sediments, barrier islands, and shorelines that can affect the depth of burial and integrity of pipelines laid in waters less than about 60 feet deep (Tubman and Suhayda, 1976~.
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From page 69... ...
Thicker Holocene sequences have filled valleys carved across the shelf by the Trinity and Sabine Rivers during lower sea level stands and are associated with large sand and shell banks thought to be relict coastal barrier islands (Siringan and Anderson 1991~. Shoreline dynamics are relatively predictable.
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From page 71... ...
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From page 72... ...
Muddy sands cover the shoreface and shelf, because most of the mud associated with discharge from the modern Mississippi is deposited off the continental shelf, while that from the Atchafalaya is retained relatively near shore and moves mainly to the west. Extensive oyster reefs, no longer alive, front the coast immediately offshore of Atchafalaya Bay, but most of the rest of the shoreline outside of the bird foot delta itself is characterized by more or less continuous sandy barrier islands.
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From page 73... ...
Relatively low rates of shelf sedimentation may prolong natural filling of pipeline trenches and expose unburied pipe to storm forces. PIPELINE AVOIDANCE TECHNIQUES FOR VESSELS At present and for the foreseeable future, it will be impossible for moving vessels to avoid pipelines by detecting them at a distance.
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From page 74... ...
Ultimately, all coastal waters of the United States are expected to be covered (U.S. Coast Guard, 1992~.
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From page 75... ...
California authorities have quite accurate information on pipeline locations for that state's small offshore pipeline mileage, but those pipelines are fewer, with less intricate interconnections, and do not present the same risks to vessels as the network of pipelines in the shallow waters of the Gulf. Pipeline location data could be gathered relatively cheaply during periodic surveys, using data from the Global Positioning System (GPS)
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From page 76... ...
(Crews of offshore service and fishing vessels are not considered passengers; nor are personnel being transferred from shore to offshore rigs considered passengers for hire under the Passenger Vessel Safety Act of 1993.) The persons in charge of uninspected towing vessels and uninspected passenger vessels are required to be licensed as Operators.
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From page 77... ...
and that licensing requirements for offshore service operators are considerably more stringent than uninspected passenger vessel licenses. Uninspected vessels in the offshore energy industry, such as offshore tugs, are generally operated, for insurance reasons, by persons with Master or Mate licenses, which are normally required only for operators of inspected vessels.
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From page 78... ...
Engineering Considerations in Installation Installation of a marine pipeline must take into account a variety of local conditions in addition to the minimum regulatory requirements, including soil characteristics, currents, vessel traffic, and the potential for erosion of the shoreline at shore crossings. These factors determine the initial burial depth, the amount of weight coating, and the need for any additional stabilizing features such as pipeline anchors or backfill.
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From page 79... ...
Prudent engineering practice involves thorough bottom surveys along pipeline routes, with soil core samples taken at regular intervals. The samples are analyzed for design parameters relevant to specific gravity, grain size, shear strength, resistance, and potential for fluidization.
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From page 80... ...
Table 5-1 summarizes the characteristics of the shoreline and seabed dynamics that are encountered in various regions of the Gulf of Mexico and identifies the pipeline safety issues associated with these dynamics. Table 5-2 identifies the types of inspection programs that are appropriate to these regions.
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From page 81... ...
Occasional exposures at shoreline; deposition on seabed Storm-induced cover loss; gradual cover loss. TABLE 5-2 Depth-of-cover inspection needs for different shoreline and seabed regimes Shallow water inspection program Region Without occurrence of storm With occurrence of storm Nondeltaic Chenier plain and barrier islands Periodic monitoring of shoreline crossing.a If shoreline changes, then investigate near-shore depth-of-cover.
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From page 82... ...
The transmission lines, however, will most likely continue to serve other fields, and possibly even newer, deeper water production as well. The extent of abandoned platforms and pipelines correlates directly with the original progression of oil and gas field development from coastal marshes to shallow waters to OCS waters.
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From page 83... ...
Adequate burial of a new pipeline requires a thorough bottom soil survey in advance, to determine the best route, the proper depth of soil cover, and the appropriate weight coating to keep the pipeline from floating upward in soils that may be fluidized by wave action. From the human safety standpoint, it is particularly important to maintain pipelines at their intended depths of cover in water less than 15 feet deep (a depth that can accommodate the drafts of large fishing vessels)
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From page 84... ...
The committee is not aware of any systematic study of whether the currently required initial burial depths and procedures are either adequate or inadequate. Regulators will need to assess Me matter farther, taking into account local variations in shoreline and seabed dynamics and the results of the periodic depth-of-cover inspections recommended in Chapter 7.
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From page 85... ...
1989. Pipelines, Navigation Channels, and Facilities in Sensitive Coastal Habitats: An Analysis of Outer Continental Shelf Impacts, Coastal Gulf of Mexico.
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