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IMPROVING THE SAFETY OF MARINE PIPELINES 5 Avoiding Outside Interference with Pipelines Marine pipelines' most serious risks to human life and the environment have to do, not with corrosion, but with larger scale damage from anchors, fishing nets, and storm-induced failure of the supporting sediments (see Chapter 2). In shallow waters (less than about 15 feet), collisions of vessels with inadequately buried pipelines can kill (as shown by the Northumberland and Sea Chief cases, in which fishing vessels burned and crew members died after the vessels struck exposed pipelines in shallow water). In deeper waters, anchor and net damage has been responsible for more than 90 percent of the pipeline oil pollution recorded on the OCS between 1967 and 1990; the four largest spills—all caused by anchor damage, and totaling 211,000 barrels—accounted for 85 percent of the volume of pollution from pipelines during this period (see Chapter 2). At a minimum, the vessel may lose valuable gear, such as snagged nets or anchors. No available sensor technology allows moving vessels to detect pipelines at a distance, and thereby avoid them. Satellite-based location technology is improving rapidly, but it is neither accurate enough nor widely enough used to be relied on. (In any case, pipelines—and especially older pipelines—are often not accurately charted.) It is therefore incumbent on the operators of pipelines to protect against interactions with vessels. In some cases, such as the anchoring of supply boats near platforms, improved communications between platforms and vessels can provide protection. 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). The minimum depth of cover is set at 3 feet (18 inches in rocky soils). While loosely referred to as “burial,” this procedure generally does not include covering the pipeline with sediments; currents are relied on to cover the pipeline in time. The problem of burial is complicated in some parts of the Gulf of Mexico by the area's coastal dynamics, which feature large movements of sediments and a general pattern of shoreline erosion and retreat, modulated by storms. Pipelines near the shore, or crossing the shore, may become exposed as a result of this sediment movement. In some cases they may even work their way up from their original depth of cover unless they are adequately weighted or otherwise stabilized.
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IMPROVING THE SAFETY OF MARINE PIPELINES COEXISTING ACTIVITIES The intensity of all types of offshore use increases near the coast, where water is shallowest and pipelines most densely distributed. A network of large- and medium-diameter transmission pipelines move production ashore from both nearshore and deepwater fields, and a profusion of smaller flowlines and gathering lines interconnect wells and platforms. Except for large cargo vessels and tankers, and in the immediate vicinity of maintained channels, traffic in this area operates outside of any marked channels or fairways, traveling courses governed by the skills, prudence, and experience of individual skippers, who are often—usually, in the case of fishing vessels—unlicensed and without formal training, although they may be highly skilled and experienced. Oil and Gas Field Activities Serving the oil and gas fields in the Gulf at any one time are several hundred oil field supply vessels from 60 to 200 feet long; the numbers roughly parallel the level of drilling activity. These vessels work closely around rigs and other structures and must anchor in areas with high concentrations of pipelines (Reed, 1987). Self-propelled and barge-mounted mobile drilling and workover rigs with extendable “jack-up” legs operate in shallow waters; in deeper waters, rigs use multiple-point mooring systems and anchors. The footings and anchors of these rigs can also crush or snag pipelines (Reed, 1987). Fishing Fishing vessels in even greater numbers ply the same waters. About 20,000 shrimping vessels, 20 to 200 feet long, operate bottom trawls that can make direct contact with up to a square mile of seabed in a single day (Baron-Mounce, 1991). Large shrimping vessels can trawl in water up to 500 feet deep, but the vast majority of shrimpers are smaller vessels that operate in waters less than 50 feet deep and can work in coastal bays less than 5 feet deep (Gulf States Marine Fisheries Commission, 1988). About 50 menhaden purse seiners up to 200 feet long (commonly referred to as pogey boats) operate in shallow coastal waters. (There were about 80 operating during the late 1980s). Menhaden seiners are limited to daytime operations and excluded from bays and inlets. 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, 1988). Smaller longliners and other fishing vessels, both commercial and recreational, operate throughout the Gulf, often congregating around offshore oil and gas structures, which attract some species of fish. Fishing vessels frequently snag their nets, trawls, and other gear on unseen obstructions on the bottom. A recent atlas of the sites of such “hangs” reported by shrimpers in the northern Gulf of Mexico includes the approximate locations of more than 7,500 sites where shrimpers have lost or damaged gear in water depths up to 300 feet (Graham, 1988). Of the reported hangs, 89 percent had no identified cause; 3 percent were thought to be caused by natural formations; and 4 percent were attributed to lost cargoes, ship and plane wrecks, anchors, and a variety of other human-made debris, exclusive of pipelines.
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IMPROVING THE SAFETY OF MARINE PIPELINES Fewer than 1 percent were attributed to pipelines. Of course, these figures give no real indication of the frequency with which pipelines damage fishing gear, because most pipeline hangs, fortunately, do not result in retrieval of recognizable pipeline sections or fittings or visible product loss. To compensate for such losses, the state of Louisiana and the National Oceanic and Atmospheric Administration (NOAA) of the U.S. Department of Commerce each dispenses funds collected from the offshore oil and gas industry to fishermen who make plausible claims. The federal “hang fund” is in effect on the OCS, and the Louisiana fund in state waters. NOAA publishes the locations of these sites in the weekly Notice to Mariners; once a hang has been reported and noticed from a single location, no further compensation of fishermen for damage at this location is permitted. The Louisiana Fishermen's Gear Compensation Fund, operated by the Louisiana Department of Natural Resources, maintains no database of hang locations; claims are limited to $5,000 per incident, and to no more than three per year per claimant. From the fund's inception in 1979 until February 1993, 5,252 claims were filed, of which 4,084 were approved for payment, totaling about $6.8 million (Hinojosa, 1993). Cargo and Other Traffic Finally, the area is traversed daily by up to 20 large self-propelled cargo vessels and tankers, and a greater number of barge tows moving to and from ports on the Mississippi River, the Louisiana Offshore Oil Port (LOOP), Galveston, Beaumont, Lake Charles, Pascagoula and Mobile. Subsurface oyster shell reefs are also mined by dredging in central Louisiana bays and coastal waters. Vessel traffic statistics collected by the U.S. Army Corps of Engineers are difficult to interpret but provide an overall indicator of the level of activity in Corps-maintained waterways and at locks; in 1985 it was estimated that as many as 3 million vessel trips occurred in 24 northern Gulf of Mexico waterways between Pascagoula, Mississippi, and Beaumont, Texas (Reed, 1987). NEARSHORE AND COASTAL DYNAMICS OF THE GULF OF MEXICO The Gulf shoreline is constantly shifting. The northern Gulf, where both pipelines and vessel traffic are most concentrated, experiences rapid shoreline erosion in places and frequent severe storms, which may expose pipelines that were originally safely buried. Avoiding interference by vessels requires careful engineering and well-developed inspection and maintenance programs, in addition to care and skill on the parts of vessel operators. Patterns of erosion and shoreline retreat vary dramatically. Along the Louisiana coast, shoreline retreat is generally rapid, as shown in Figure 5-1.1 Along the Gulf Shore, however (Figure 5-1) retreat averages 60 feet (18 meters) per year on the barrier islands south of Houma and 12 to 18 feet (3.7 to 5.5 meters) 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). As shorelines retreat, sediment is excavated from the shoreface, which can expose or undermine pipelines and other fixed structures (Williams et al., 1989). 1 Figure 5-1 gives erosion or accretion rates in meters per year.
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IMPROVING THE SAFETY OF MARINE PIPELINES Areas that are more dynamic in terms of shoreline and sea bed sedimentation and erosion require the most attention from pipeline builders and operators and from vessel operators. The variability of shoreline retreat provides one means of classifying the northern Gulf of Mexico into geological regions that are meaningful to pipeline safety. Wicker and colleagues (1989) followed this approach, dividing the area into four regions, from west to east: the Texas Barrier Island System, the Strandplain-Chenier Plain System, the Mississippi Delta System, and the North-Central Gulf Coast System. Texas Barrier Island System From the Mexican border to Rollover Pass on Galveston Bay, extensive lagoons with relatively little riverine freshwater and sediment input lie behind large, continuous, sandy barrier islands. Shoreline retreat exceeds 12 feet (3.7 meters) per year only along the headland adjacent to eastern Matagorda Bay, except at the ends of barrier islands. Offshore, Holocene sediments, generally of sandy muds, form a veneer less than 3 feet thick overlying a highly oxidized and cemented Pleistocene clay formation (Anderson et al., 1992). 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. The oxidized clay sea bed may be difficult to trench to uniform depths. Relatively low rates of shelf sedimentation may prolong natural filling of pipeline trenches and expose unburied pipelines to storm forces. Strandplain-Chenier Plain System Between Rollover Pass and Vermilion Bay, Louisiana, much of the shoreline consists of an eroding marsh scarp fronted by sandy and shelly beaches. Where beaches are well developed they are perched on marsh deposits. Inland, marshes are interrupted by older beach ridges with oak trees, known as cheniers (Russell and Howe, 1935). Shoreline retreat along much of this stretch averages 30 feet (9.1 meters) per year, although it is more stable at its eastern and western ends. Recently deposited sediments consist of silty clays in a seaward thinning wedge extending out to about the 30-foot depth contour (which lies as far as 20 miles from shore in the eastern section. As much as the upper 3 feet of these sediments can consist of unconsolidated fluid mud in a gel-like clay suspension, deposited by the sediment discharge of the Atchafalaya River and carried westward by coastal currents (Kemp and Wells 1987). Elsewhere offshore, the shelf is relatively stable and receives little new sediment. The Pleistocene surface either outcrops or lies just beneath the mud line, as is true off of Texas. Deepwater corals and carbonate sediments are found locally on salt domes that rise above the sea bed in 200- to 300-foot water depths. Rapid retreat of the shoreline and lowering of the shoreface can affect pipeline burial in this area. The presence of fluid mud deposits near shore may make it difficult to determine a true bottom position using traditional acoustic instruments, whether for the pipeline operator attempting to locate a permanent soil surface and comply with burial requirements or for the mariner seeking to ensure adequate clearance under the keel. In addition, wave loadings can fluidize muddy nearshore deposits, causing inadequately weighted pipelines to float upward from their buried positions, and reducing the shear strength of covering soils.
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IMPROVING THE SAFETY OF MARINE PIPELINES
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IMPROVING THE SAFETY OF MARINE PIPELINES FIGURE 5-1 Map of the Gulf of Mexico shoreline, showing rates of shoreline erosion and accretion (meters per year), (Louisiana Geological Survey, 1991)
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IMPROVING THE SAFETY OF MARINE PIPELINES Mississippi Delta System This system is a composite, including the Mississippi and Atchafalaya river mouths and the barrier island chains that outline the rest of the Louisiana coast east of Vermilion Bay. Except for short segments, virtually all of this shoreline is retreating at more than 18 feet (5.5 meters) per year. Land-building at the river mouths is occurring in Atchafalaya Bay, 5 to 10 miles inland of the Gulf coast, and to a lesser extent at the heads of some passes in the “bird foot” Mississippi delta. Land and inner shelf sand banks built during delta-building sequences of the Mississippi in the past 5,000 years take up much of the continental shelf, so that the mouth of the modern Mississippi is perched on the edge of the continental slope. Because of erosion associated with ancestral Mississippi River channels during lower sea level stands, and a general downwarping of the crust, the Pleistocene surface, which is largely exposed on the continental shelf to the west, lies 200 to 900 feet below the modern seabed throughout this area (Kolb and van Lopik, 1958). 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. The Chandeleur and Breton Islands east of the river mouth are moving landward, while the islands east of the delta, defining the seaward boundaries of the Terrebonne and Barataria estuaries, are narrowing and breaking up in place. Much of this deterioration is attributed to the high rate of subsidence of the deltaic plain, as the thick underlying column of Holocene sediment consolidates and compacts (Penland et al., 1985). New passes are formed through these islands with every major storm, and often do not seal in fair weather, because sand supplies are so limited. Around the margins of the bird foot delta of the Mississippi, at the edge of the continental shelf, special conditions apply that can greatly affect pipeline safety in this area of concentrated oil and gas development. Unconsolidated mud accumulations, rapidly deposited during high river flows, move downslope in mass movements when disturbed by storm waves. Pipelines and structures in the path of such movements can be moved and otherwise exposed to severe stresses, which may cause failure. Pipelines in this so-called mudslide area are laid on the surface, so as to ride over the moving sediments, and are equipped with breakaway joints and check valves to limit damage and loss of hydrocarbons. (Because these areas are at depths of more than 200 feet, there is no requirement for burial.) Inland of the barrier islands and bird foot delta, the highly organic sediments that make up the deltaic plain marshes are also experiencing subsidence that, when combined with global sea level rise, can total between 0.5 and 1 inch per year of “relative sea level rise” (Penland and Ramsey, 1990). As a result, coastal marshes are being converted to open water in Louisiana at rates as high as 40 square miles per year, and averaging about 28 square miles per year over the past 50 years (Dunbar et al., 1992). Pipelines and other structures that were constructed in the marsh are now exposed to wave forces. Mariners must find channels and navigate through waterways that retain few of the visible boundaries shown on charts. Most navigation channels crossing Louisiana's coastal plain have experienced substantial bank erosion, widening in some cases to more than twice the authorized channel dimensions (Wicker et al., 1989). Pipelines that were once buried
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IMPROVING THE SAFETY OF MARINE PIPELINES beneath the land surface can protrude from the eroding banks of widening navigation channels and pose a hazard. The Mississippi Delta System presents a variety of problems to pipeline engineers. Barrier island migration, breaching, and erosion on both bay and Gulf shorelines are unpredictable. Fine sand shoals offshore are prone to liquefaction. Mudslides and fluid mud complicate pipeline installation and maintenance around the bird foot delta. Loss of marshes in the interior of the coastal plain and generally unstable soils make channel margins unstable and prone to erosion. North-Central Gulf Coast System This system extends east from the mouth of the Pearl River along the Mississippi and Alabama coasts into Florida and shares many of the features of the Texas coast, including its relative stability. It is characterized by white sand beaches and large continuous barrier island chains. Shoreline dynamics can be relatively dramatic in the vicinity of tidal inlets, but despite a relatively high wave energy regime, shoreline retreat is generally less than 6 feet (1.8 meters) per year. Nearshore and shelf sediments are sandier than in the other provinces, and include shell, gravel and carbonate rubble (Shultz et al., 1990). Sediments in the shallow subsurface of the inner shelf were deposited by fluvial and coastal systems that developed on the shelf during Pleistocene sea level fluctuations and were reworked during subsequent sea level rise (Parker et al., 1992). Modern sedimentation on the inner shelf has been minor and is restricted to the vicinity of coastal bays and inlets. Shoreline dynamics are relatively predictable. The sea bed may be difficult to trench to uniform depths where hard bottoms are found. 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. The next best method, highly accurate position location for vessels, is on the horizon. However, one must also know the positions of all pipelines in the vicinity, and pipelines in general do not appear on navigational charts with the precision necessary for vessels to thread their ways among them. Moreover, few coastal charts have been produced using recent highly accurate positioning offered by GPS (Global Positioning System). For vessel operators, adequate training can help avoid the more obvious hazards. Better communication between vessels and pipeline operators about the locations of some pipeline hazards could also be achieved. As a practical matter, avoiding interactions between pipelines and vessels will remain the task of the pipeline operator (generally by ensuring that pipelines are not exposed to vessels and their gear). The Role of Technology in Pipeline Avoidance No sensor technology is available for detecting pipelines at a distance. (Side-scan sonar is routinely used to locate pipelines under water, but is incapable of identifying a pipeline that lies ahead of a moving vessel.) Eventually, a forward-looking metal detector suitable for avoiding collisions with exposed pipelines might be approached by tech-
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IMPROVING THE SAFETY OF MARINE PIPELINES niques being developed by the military. The perfection of a device that can distinguish a pipeline valve from a discarded appliance on the seafloor is certainly not at hand, and a device that produced too many false alarms would be useless, and would simply be disabled by operators. Further development by the military, followed by system simplification and major cost reduction, will be necessary before commercial applications are practical. Position Location Techniques The state of the art in position-finding equipment is GPS, which uses satellite signals for determining a vessel's position at sea. This military system provides two levels of accuracy: the Precise Positioning Service (accuracy 17.8 meters) for military purposes and the Standard Positioning Service (accuracy 100 meters, owing to certain errors deliberately introduced into the satellite signal) for all other users. Besides indicating a vessel's location (latitude and longitude), most GPS terminals are combined with simple calculators or computers that provide information on future positions (or way points), speed (based on elapsed time from the last location), and —when properly linked with the vessel's compass—the effects of currents and winds on the vessel's actual progress. The U.S. Coast Guard's Differential GPS (DGPS) program, expected to be available in some areas by the mid-1990s, will offer nonmilitary users an accuracy of 8 to 20 meters (25-70 feet). DGPS terminals process the standard low-accuracy GPS signal in combination with the signals of terrestrial marine radiobeacons to correct errors inherent in the GPS service. The technique is already in proprietary use, for example by hydrocarbon exploration vessels. An accuracy of 8 to 20 meters is considered sufficient for cargo ships—the main intended beneficiary of the program —to navigate in harbors and harbor approaches. Ultimately, all coastal waters of the United States are expected to be covered (U.S. Coast Guard, 1992). The GPS position of a vessel, of course, may be more accurate than the chart available to the mariner or the plotted position of a platform or pipeline. Improperly charted islands, headlands, and other fixed objects are a recurrent problem for mariners, but not surprising when one realizes that the surveys on which a chart is based may have been made a century or more ago, when celestial observations and accurate time were the sole basis for position determination. In any case, most charts used by mariners do not show pipelines on the seafloor. Large-scale charts of harbors, estuaries, or rivers indicate pipeline and cable crossings, but seldom show the path of a pipeline or cable across a bay or along a channel. Detail and harbor charts often clearly indicate areas that are designated anchorages or prohibited zones, but generally do not indicate pipelines. Integration of vessels' position-keeping system—presumably DGPS—with an electronic chart system that had a clear and nonfatiguing display would permit a warning light or buzzer to alert the operator to the vessel's arrival in a dangerous area. Current systems are limited to point, rather than zone, hazards, but “fuzzy logic” systems are being developed to correct this limitation. Danger zones would, of course, include platforms and/or rigs, but could also include pipeline junctions, fittings, or exposed sections of pipe. The electronic charts must be prepared and maintained, software written to integrate the charts with the DGPS signals, operators trained, and guidance provided for evasive or preventive actions. Sophisticated GPS or DGPS systems with electronic charts will be used by contract service vessels only if they are required by OCS operators as conditions of obtaining
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IMPROVING THE SAFETY OF MARINE PIPELINES contracts. It is unlikely that such systems will be economically justified for fishing vessels in the foreseeable future. Avoiding Pipelines when Mooring or Anchoring at Platforms The use of anchors by supply and service vessels in areas of dense pipelines or flowlines adjacent to offshore installations presents particular safety issues. In many cases, it may be difficult or impossible for a vessel operator to ensure that a dropped anchor will not strike a pipeline or flowline. In such conditions, permanent mooring equipment can often reduce the risk of damage substantially. In other situations, where there is enough clear bottom area to make anchoring feasible, platform operators can give vessel operators detailed and timely information on the local pipeline and/or flowline network, with preferred anchoring areas clearly marked. To simplify pipeline avoidance in these congested areas over the longer term, future pipeline installations can be placed together in well-defined “corridors,” to the extent practiced. Pipeline Location Data Adequate maps of pipeline locations would, in principle, make it possible to inform vessel operators of areas in which pipelines might be encountered. Collisions of vessels with pipelines are confined to shallow waters, mainly under state jurisdiction, where data on pipeline location, ownership and condition have not been systematically collected and put in accessible and comprehensive data bases. Some of the necessary information has been assembled by the Minerals Management Service, and some by operators under Office of Pipeline Safety regulation, but it is incomplete, particularly in the shallow waters where it would do the most good. MMS keeps a detailed data base of all pipelines on the OCS (including OPS-regulated pipelines) and their construction details. It is in the process of digitizing the as-built maps, and plans to incorporate accident data, net hang sites, abandoned lines, and other information on that geographical database. To carry out its responsibilities under the Oil Pollution Act of 1990 (see Chapter 6), the agency will add data on pipelines in state waters to this data base (personal communication, Alexander Alvarado, Minerals Management Service, February 3, 1993). The boundaries of MMS jurisdiction under the Oil Pollution Act, however, do not extend shoreward of the coastal barrier islands, so that bays and channels within that boundary will not be covered (personal communication, E. P. Danenberger, Minerals Management Service, December 2, 1993). OPS requires pipeline operators to maintain their own detailed maps and records, of gas and hazardous liquid pipelines. These maps have not been incorporated in any central data base. State requirements regarding mapping and facility data vary widely, but are generally inadequate for this purpose. Louisiana and Texas, the two states with the overwhelming majority of pipeline mileage, rely on operators to keep as-built drawings. 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). Elsewhere in this report the com-
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IMPROVING THE SAFETY OF MARINE PIPELINES mittee recommends periodic depth-of-cover surveys for pipelines in shallow waters, keyed to GPS locations. Such surveys, if required by all state and federal regulatory agencies, would in a few years produce the necessary location data. Assembling the data in a central data base would be a straightforward task. The existence of a data base that accurately locates all pipelines would not in itself prevent collisions or pipeline damage. Effective use of this information could, however, reduce the likelihood of serious accidents. For example, a “one-call” notification system, modeled on those used for years by pipeline operators onshore to prevent damage by excavators, could use the data base to inform vessel operators planning operations in a given area of any local pipeline hazards. This notification system could be integrated eventually with the recommended pipeline leak notification system (see Chapter 4), which in turn would benefit from the pipeline data base. Standards for Vessel Inspection, Licensing, and Training: Implications for Improving Pipeline Avoidance The United States is a member of the International Maritime Organization (IMO), with the U.S. Coast Guard as its representative. The Coast Guard is responsible for negotiating and administering international conventions and enforcing the maritime laws of the United States with respect to the international conventions for inspecting seagoing vessels such as tankers, freighters, and passenger ships. The international conventions do generally apply to seagoing ships but uninspected vessels such as fishing boats and most of the smaller vessels that serve offshore oil and gas fields are subject to domestic standards. The convention articles, regulations, and resolutions, are designed to protect ships, their crews and passengers, the public, and the natural environment. They serve as guidelines for most national and state laws (although the states may impose more rigid, and in some cases slightly different, requirements in specific areas). The need for uniform standards for vessels on the high seas that call at foreign ports is obvious. Conformance with these standards is strongly supported by the insurance industry, which uses economic forces to encourage the use of proper functioning equipment and the licensing of operators. Self-propelled drill ships, semisubmersible rigs, or other mobile offshore units, and most U.S. vessels operating in the world's offshore oil and gas fields are inspected vessels. The various IMO agreements do not necessarily apply in their entirety to these vessels, although lights, sound signals, flares, flotation devices, and fire prevention and fighting equipment usually do conform. Generally, however, domestic U.S. laws and U.S. Coast Guard regulations do conform with the IMO Convention on Standards of Training, Certification, and Watchkeeping for Seafarers (STCW). Fishing boats and several classes of offshore service boats, and other vessels carrying fewer than six passengers for hire, are not required to be inspected. (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. To be licensed as an Operator of an uninspected passenger vessel, an individual must pass a written examination and document experience and citizenship. These licenses frequently limit the operator to a specific vessel size and operational distance from shore. At the very least, obtaining the license requires the mariner to learn the Rules of the Road and be able to answer various open-book ques-
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IMPROVING THE SAFETY OF MARINE PIPELINES tions. Some companies, as well as the U.S. Power Squadron and the U.S. Coast Guard Auxiliary (both volunteer organizations), provide courses and training to help license applicants. It should be noted, that each license requires specific sea service (time served aboard a vessel) 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. At minimum, persons operating these uninspected vessels must possess a U.S. Coast Guard-issued Operator's license. Fishing vessels are not required to be inspected, and their operators currently require no license. Congress and the U.S. Coast Guard, however, are considering means of ensuring that these vessels meet certain equipment and stability standards, and that their operators are competent. There are no data showing that a person who can pass a license examination for uninspected vessels is a better seaman than a thoroughly experienced mariner with years of experience. The skills needed to anchor a work boat near a platform in the Gulf of Mexico without snagging a pipeline on an exposed fitting or the skills needed to successfully drag a double-rigged shrimp trawl across 20 miles of seabed while avoiding many miles of pipeline do not relate to those needed to pass a College Board exam. However, to take full advantage of new technology for vessel positioning, additional, possibly costly training may be necessary; but this training should be encouraged. PIPELINE BURIAL For structural stability and protection from outside forces such as vessels and storms, offshore pipelines in waters less than 200 feet deep are sunk beneath the bottom, in trenches. While loosely referred to as burial, this practice does not include covering the pipelines; the pipeline is lowered into the bottom by jetting, dredging, or plow methods, depending on local conditions. Generally currents are relied on to sweep sediments over the pipelines in due time. In cases in which natural sedimentation is inadequate, the pipeline may be covered by mechanical backfilling. Valves, lateral pipeline tie-in assemblies, and other pipeline appurtenances are protected from snagging trawls, nets, and anchors with pyramids of bags of concrete or other protective structures, or by lowering them as needed to prevent snags. The depth of burial required depends on local vessel traffic, soil and shoreline dynamics, and other engineering considerations. Regulatory Requirements for Depth of Cover OPS and MMS requirements for pipeline burial differ. OPS requires that pipelines installed offshore in water less than 12 feet deep must be placed at least 3 feet below the bottom in soil (18 inches in consolidated rock); in deepwater ports or navigable rivers and inlets, these depths are doubled (49 CFR 192.327, 195.248). The MMS requires that pipelines originating on the OCS be placed at least 3 feet beneath the bottom in waters less than 200 feet deep. (These regulations could conceivably leave a gap with no burial requirements between a 12-foot water depth and the beginning of the OCS, but normal pipeline construction practice involves burial from the shore to depths of 200 feet. In any case, pipelines that originate on the OCS, whether OPS- or MMS-regulated, are subject to the requirements of their MMS permits, which specify MMS burial requirements.) Neither agency has a requirement for maintaining the required depth of burial. How-
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IMPROVING THE SAFETY OF MARINE PIPELINES ever, in 1990, Congress reacted to the Northumberland and Sea Chief accidents with amendments to the Natural Gas Pipeline Safety Act of 1968 (49 U.S.C. 1671 et seq.) and Hazardous Liquid Pipeline Safety Act of 1979 (49 U.S.C. 2001 et seq.), directing the U.S. Department of Transportation to require operators to inspect all pipelines in less than 15 feet of water to identify and mark with buoys sections that were “exposed” or constituted a “hazard to navigation” (P.L. 101-599, “Improving Navigational Safety and Reducing Vessel-Pipeline Collisions”). In addition, the agency was required to establish a program of mandatory inspections. OPS issued the required regulations in December 1991, requiring operators to survey the pipelines and, by November 1993, to rebury to a depth of three feet any found with one foot or less of cover (49 CFR 192.612, 192.3). By March 1993, with more than 95 percent of the survey completed, only 24.4 miles of such pipe had been found, or 1.7 percent of the 1,456 miles surveyed (personal communication, Cesar DeLeon, Office of Pipeline Safety, March 11, 1993). OPS will consider the results of this survey in determining the need for a mandatory continuing inspection program. The Pipeline Safety Act of 1992 (P.L. 102-508) extended inspection and reburial requirements to all offshore areas and inland navigable waters less than 15 feet deep (not only those in the Gulf of Mexico, as originally provided). The committee has no information leading it to believe that the currently required initial cover depths and procedures are either adequate or inadequate. There has been no systematic study of the problem, to the committee's knowledge. Anecdotal evidence suggests that it may be not inadequate initial cover, but rather the loss of cover through erosion or fluidization of surrounding soils, that most often exposes pipelines to interference by vessels. Such was the case in the Northumberland and Sea Chief accidents. Regulators will need to assess the matter further, perhaps in conjunction with the periodic depth-of-cover surveys outlined later in this chapter. A sophisticated approach, taking into account local variations in shoreline and seabed dynamics, is likely to yield the safest and most cost-effective solutions. Pending the results of such a study, the currently specified initial depths must be considered adequate. 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. Figure 5-2 is a schematic drawing of the decision process involved. The pipeline must be designed and constructed to maintain its initial depth of cover throughout its lifetime. It is recognized that a buried pipeline may tend to float up or sink down from its initial placement depending on its weight (including contents) and on the density and shear strength of the soil. As explained earlier in this chapter, certain current and wave conditions may “fluidize” soils in much of the northern Gulf to the degree that pipelines may float upwards if they are not adequately weighted or anchored. In major storms, susceptible soils may fluidize enough to present such problems at depths of 60 feet or more. Erosion by ocean currents can cause other problems with pipeline cover and stability. Pipelines placed in trenches that run parallel to currents may be covered and stabilized by
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IMPROVING THE SAFETY OF MARINE PIPELINES FIGURE 5-2 Decision process for burial and stability evaluation of a marine pipeline. sediments very slowly compared with those that run more nearly perpendicular to currents (Mousselli, 1979; personal communication, A. H. Mousselli, July 20, 1993). 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. The information derived from such surveys helps in choosing pipeline routes, burial depths, and weight coatings. Pipelines are inspected at the determined depth of cover by divers using water pressure gauges accurate to within 3 inches of depth. A gauge reading on top of the pipe in the ditch is compared with a second made to the side of the pipeline on normal firm undisturbed bottom, and readings are recorded by a crew on the surface in the dive boat. The two gauge readings tend to average the effects of surface waves. Measurements are almost continuous as the diver swims along the pipeline in the newly created trench. If adequate cover is not found, the burying equipment is required to make additional passes. Initial depth-of-cover information from the gauge readings may be correlated with Global Positioning System location data, to meet MMS and OPS requirements for “as-built” drawings. One uncertainty in determining the depth of cover in practice is the fact that divers must establish the actual firm and undisturbed bottom, which can be quite indistinct in the unconsolidated sediments found in much of the northern Gulf. Designers of pipelines
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IMPROVING THE SAFETY OF MARINE PIPELINES generally calculate the bottom's location according to a specific shear strength criterion for the soil. But divers test the soil's firmness merely by pressing downward with their gloved hands until they reach resistant soil, and rarely use any measure of shear strength. In practice, however, experienced divers are quite accurate, placing their bottom pressure gauges within about 10 percent of measurements made according to shear strength (personal communication, A. H. Mousselli, October 20, 1993). Pipelines installed across shorelines through trenches can accelerate erosion and create currents and wave conditions that remove cover from pipelines. The directional-bore installation method, by which pipelines are installed underground, without trenching was proven in the early 1980s for river and canal crossings and pipeline installations that traverse environmentally sensitive areas, highways, and other areas where disturbance of the surface is undesirable. It is today required wherever possible by permitting agencies such as the U.S. Army Corps of Engineers and state coastal zone management commissions. It generally yields lower construction and maintenance costs, in addition to its safety and environmental advantages. Overall bore distances of as. much as 5,000 feet are now commonly attainable in pipe diameters up to 36 inches. The pipeline generally crosses the beach at about 50 foot below the surface, and gradually approaches the normal design burial depth at a point well beyond the shoreline. The distance by which the shore end of the bore is set back behind the shoreline can be adjusted to take account of local rates of shoreline erosion. Periodic Depth-of-Cover Inspections Periodic surveys to ensure that adequate depth of cover is maintained are not generally made, although operators are required by MMS and OPS regulations to keep pipelines from being exposed. In many places in the Gulf, such surveys would simplify the problem of meeting that regulatory requirement. Initial depth-of-cover information, with Global Positioning System locations along the pipeline, could serve as a baseline for future depth-of-cover surveys. Because the sediment behavior and shoreline erosion are relatively predictable at different points in the Gulf of Mexico, surveys could be scheduled according to those factors, along with such other factors as the passage of major storms. 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. An accurate baseline depth-of-cover record would be established for each pipeline in less than 15 feet of water, based either on the one-time depth-of-cover inspection required by OPS in 1991 (described earlier in this chapter) or on other recent inspections. The depth of 15 feet (enough to accommodate the drafts of large fishing vessels) was defined in the act of Congress mandating the one-time depth of cover survey (P.L. 101-599), based on testimony from the National Fisheries Institute and the American Shrimp Processors Association (personal communication, Cesar DeLeon, Office of Pipeline Safety, March 11, 1994). With such a baseline, subsequent inspections can measure changes in the depth of cover for each pipeline. Intervals between further inspections could be lengthened or shortened according to whether the depth is changing, and at what rate. (One might call this method of scheduling inspections “self-adjusting.”) A second depth-of-cover inspection would be performed within, perhaps, two years
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IMPROVING THE SAFETY OF MARINE PIPELINES TABLE 5-1 Shoreline and seabed dynamics affecting pipeline depth-of-cover inspection requirements Characteristics of dynamics Region Shoreline Seabed Pipeline safety issue Nondeltaic Localized retreat. Stable Occasional exposures at shoreline; deposition on seabed Chenier plain Rapid and generalized retreat. Very dynamic top layer of unconsolidated muds, less dynamic sublayer. Storm-induced cover loss; gradual cover loss. Barrier islands Active dynamics primarily on islands and shoals. Rapid to gradual generalized siltation; localized erosion and seabed shifting. Rapidly changing shorelines and island/shoal crossings; storm-induced changes. River mouth Very rapid change; some retreat, some advance. Slumping Storm-induced slides. 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 Periodic monitoring of shoreline crossing.a If shoreline changes, then investigate near-shore depth-of-cover. Periodic inspection of depth-of-cover is not necessary. Post-storm inspection of shoreline crossing. If shoreline changes, then investigate near shore depth-of-cover. Post-storm inspection of depth-of-cover is not necessary. Chenier plain and barrier islands Periodic monitoring of shoreline crossing.a Periodic inspections of depth-of-cover. If shoreline changes, then investigate near-shore depth-of-cover. Post-storm inspection of shoreline crossing and depth-of-cover. River mouth Periodic monitoring of shoreline crossing.a If shoreline changes, then investigate near-shore depth-of-cover. Periodic inspection of depth-of-cover is not necessary. Post-storm inspection of shoreline crossing and pipeline (in mudslide areas only). a Monitored visually with biweekly route survey, but no less frequently than every three months. of this baseline survey. Its results would determine the next inspection interval. If the depth of cover had remained relatively stable or increased, the inspection interval could be lengthened to perhaps four years in the chenier plain and barrier island regions of the Gulf of Mexico, or to eight years in the nondeltaic regions. On the other hand, if depth of cover is being gradually lost, subsequent inspection intervals would be the same or, if the loss appeared significant, could be shortened. Pipeline operators would determine the necessary inspection intervals. A maximum interval of, perhaps, 10 years should be established by regulation for the chenier plain and barrier island regions and 20 years for the nondeltaic regions. As a part of the ongoing enforcement and auditing effort, the regulating agencies should review operators' shallow water inspection programs, the results of both the baseline and subsequent surveys, and the operators' plans for extending, retaining, or reducing the inspection intervals. This method would be altered in the case of significant storm activity near the pipeline. After a large storm, a post-storm inspection would be performed for those portions
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IMPROVING THE SAFETY OF MARINE PIPELINES of pipelines in the storm path, as listed in Table 5-2. The initial choice of the storm magnitude that triggers a post-storm inspection, and the path width covered by the inspection requirement, could be based on the results of the MMS inspection program carried out after Hurricane Andrew, in 1992. Again, a self-adjusting approach could be used, in which storm magnitudes and path widths are increased or decreased based on the overall inspection results. ABANDONED AND INACTIVE PIPELINES Abandoned pipelines are often blamed for damage to vessels' anchors, hulls, and fishing gear. However, from the surface it is very hard to discriminate between a pipeline—active or abandoned—and the other kinds of debris and obstructions that litter the bottom in the Gulf of Mexico, such as sunken vessels, lost cargoes, and well casings. For this reason, there is very little information on accidents involving abandoned pipelines. The U.S. Coast Guard has a system in place, and research has been conducted, to log the locations of these “ hang” sites and obstructions. Unfortunately, in the vast majority of the cases, there is no easy way to establish the nature of these obstructions. In addition, there are no readily retrievable statistics on the injuries and property damage which may have been associated with improperly abandoned pipelines in particular. Because of the broad scope of this problem and the lack of verifiable data upon which to base an analysis, this committee can only speculate on pipelines' contribution to this problem. Additional study is needed to address the larger issue of debris in offshore waters. It is known, however, that abandonment of offshore platforms and well casings is increasing, as producing fields that have reached maturity are shut-in and their structures abandoned (Francois and Barbagallo, 1992). Oil and gas wells in coastal Louisiana waters are being plugged and abandoned at rates that in recent years have ranged between about 600 and 1,400 wells annually (Aldridge, 1993). Abandoned well casings and platform legs are required to be cut 15 feet below the seabed, and removed within a set time. The production lines associated with these shut-in wells and fields have no future use and are therefore abandoned. 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. As a result, most of the lines now being abandoned are in the marshes and shallow, state waters of the Gulf of Mexico. Abandonment involves the permanent and, for all practical purposes, irreversible process of discontinuing the use of a pipeline. The physical asset is abandoned in the truest sense of the word; no future use or value is attributed to it, and no attempts are made to maintain serviceability. Pipeline systems or segments that are not abandoned, but only idled, decommissioned, or mothballed, are considered to have the potential for reuse at some point in the future. The maintenance and inspection to be performed in these cases is a function of the probability of reuse, the cost and difficulty of remediation which may be required, and the potential impact of the in-place and idled facility on human safety and the environment. Pipelines today are abandoned, under the regulations of the MMS and OPS, by removing hydrocarbons, filling the pipe with seawater, and capping and burying the ends to prevent them from snagging nets and anchors. Side-scan sonar, diver inspections, or test trawls with nets are required to ensure that burial was effective.
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IMPROVING THE SAFETY OF MARINE PIPELINES Pipelines are not physically removed unless they are identified as hazards to navigation or nuisances to the fishing or shrimping industries. Removal is costly, and has its own environmental and safety risks. Abandoned pipelines are not inspected regularly, but owners can be required to remediate any that are brought to their attention as hazards to navigation or nuisances. There is no regulatory requirement at present for the surveillance or maintenance of cover over abandoned pipelines, unless they are determined to present hazards to navigation, interfere with commercial fishing, or unduly impede other uses of the OCS—in which case the owner may be required to remove or rebury the pipeline (Joint Task Force on Pipeline Safety, 1990). However, the Pipeline Safety Act of 1992 (P.L. 102-508) requires OPS to issue regulations requiring the lowering of offshore pipelines—active or abandoned—that represent hazards to navigation, and to study, and issue regulations on, the abandonment of underwater pipeline facilities. Some states have their own specific abandonment requirements. Louisiana law, for example, requires removal of all abandoned nonburied facilities (except flow lines) in less than 20 feet of water, and the marking of unburied flow lines left in place after abandonment in less than 20 feet of water (Stolls, 1993). A properly abandoned pipeline poses no risk to public safety or to the environment. However, past abandonment procedures—especially in state waters—were often not as scrupulous as today's. The extent of the problem is unknown. Additional study is warranted. Much concern is expressed in Louisiana about the growing number of “orphaned” or abandoned production facilities in state coastal waters. Louisiana 's Commissioner of Conservation reported in early 1993 that 63 facilities ranging from single-well caissons to multiwell fields complete with flowlines, production barges and tank batteries had been identified as “orphaned”, that is, without a competent owner of record (personal communication, H. W. Thompson, February 11, 1993). An industry-sponsored fund has been established to pay for remediation of such facilities. The Louisiana authorities have not reported similar problems with transmission pipeline operators, however. Nor, to the committee's knowledge, has any other state. FINDINGS No conventionally available sensor technology allows moving vessels to detect pipelines at a distance, and thereby avoid them. Nor is satellite-based vessel positioning technology, used by itself, suitable to the task. It is therefore incumbent on the operators of pipelines to protect against interactions with vessels, through pipeline burial, and secondarily by establishing adequate communications and notification systems. 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). While there is a general regulatory requirement in some pipeline permits to ensure that pipelines remain adequately buried, there is no regulatory requirement to make the periodic depth-of-cover or “hang site” surveys necessary for this purpose. Such surveys, at intervals set by local shoreline and seabed dynamics and the passage of major storms, would improve the assurance of safety, especially on
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IMPROVING THE SAFETY OF MARINE PIPELINES the shifting shorelines of the Gulf. From the environmental standpoint, it is important to eliminate potential net “hangs,” where fishing gear may damage pipelines or pipeline equipment. 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 the matter further, taking into account local variations in shoreline and seabed dynamics and the results of the periodic depth-of-cover inspections recommended in Chapter 7. A properly abandoned pipeline poses no risk to public safety or to the environment. An improperly abandoned pipeline can damage vessels and their gear. Reports by fishermen and others of such damage are difficult to confirm, owing to the lack of systematic data. REFERENCES Aldridge, J. R. 1993. Memorandum to H. W. Thompson, Commissioner and Assistant Secretary, Louisiana Department of Natural Resources. Subject: Request for Data. February 10. Anderson, J. B., M. A. Thomas, F. P. Siringan, and W. C. Smith. 1992. Quaternary evolution of the east Texas coast and continental shelf. In C. H. Fletcher, III, and J. F. Wehmiller, eds., Quaternary Coasts of the United States: Marine and Lacustrine Systems . SEPM Special Publication48. pp. 253–263. Baron-Mounce, E., W. Keithly, and K. J. Roberts. 1991. Shrimp Facts. Louisiana Sea Grant College Program. Baton Rouge: Louisiana State University. 22 pp. Dunbar, J. B., L. D. Britsch, and E. B. Kemp III. 1992. Land Loss Rates. Report 3, Louisiana Coastal Plain. Technical Report GL-90-2. Vicksburg, Mississippi: U.S. Army Corps of Engineers Waterways Experiment Station. Francois, D. K., and M. B. Barbagallo. 1992. Federal Offshore Statistics: 1991. OCS Report MMS 92-0056. Herndon, Virginia: U.S. Department of the Interior. Graham, G. L. 1988. “Hangs” and Bottom Obstructions of the Texas/Louisiana Coast (LORAN C). Texas Marine Advisory Service, Texas A&M University. Sea Grant College Program Rep. No. TAMU-SG-88-508. College Station, Texas. Gulf States Marine Fisheries Commission. 1988. The Menhaden Fishery of the Gulf of Mexico United States: A Regional Management Plan, 1988 Revision. Pub. No. 18. Ocean Spring, Mississippi. Hinojosa, M. 1993. Memorandum to H.W. Thompson, Commissioner and Assistant Secretary, Louisiana Department of Natural Resources. Subject: Request for Data. February 9. Kemp, G. P., and J. T. Wells. 1987. Observations of shallow-water waves over a fluid mud bottom: implications to sediment transport. In N. C. Kraus, ed. Coastal Sediments '87. Proceedings of the Specialty Conference on Advances in Understanding of Coastal Sediment Processes. 1: 363–378. New Orleans: American Society of Civil Engineers. Kolb, C. R., and J. R. van Lopik. 1958. Geology of the Mississippi River Deltaic Plain, Southeastern Louisiana. Technical Report 3-483. Vicksburg, Mississippi: U.S. Army Corps of Engineers Waterways Experiment Station. Louisiana Geological Survey. 1991 Historical Shoreline Change in the Northern Gulf of Mexico. Compilers: K. A. Westphal, M. W. Hiland, and R. A. McBride. Project Coordinator: S. Penland. Prepared for the Coastal Erosion Subcommittee, U.S. Environmental Protection Agency, Gulf of Mexico Program by the Louisiana Geological Survey in coorperation with the Texas Bureau of Economic Geology, Mississippi Office of Geology, Geological Survey of Alabama, Florida Department of Natural Resources, and U.S. Army Corps of Engineers-Jacksonville District. Louisiana Geological Survey, Baton Rouge, Louisiana. Mousselli, A. H. 1979. Design criteria for offshore lines in unstable soils can cut risks. Oil & Gas Journal. February. Parker, S. J., A. W. Shultz, and W. W. Schroeder. 1992. Sediment characteristics and seafloor topography of a palimpsest shelf, Mississippi-Alabama continental shelf. In C.H. Fletcher IIIand J.F. Wehmiller, eds. Quaternary Coasts of the United States: Marine and Lacustrine Systems. SEPM Special Publication 48. pp. 243–251. Penland, S., and K. E. Ramsey. 1990. Relative sea level rise in Louisiana and the Gulf of Mexico. Journal of Coastal Research 6(2): 323-342. Reed, A. R. 1987. Summary of ports and waterways use by OCS activities. In. R. E. Turnerand D. R. Cahoon, eds. Causes of Wetland Loss in the Coastal Central Gulf of Mexico. Vol. III: Appendices. Final report submitted to Minerals Management Service. Contract 14-12-0001-30252, OCS Study/MMS 87-0121. New Orleans. pp. B1–B17. Russell, R. J., and H. V. Howe. 1935. Cheniers of southwestern Louisiana. Geog. Review . 25: 449-461. Siringan, F. P., and J.B. Anderson. 1991. Facies architecture and evolution of coastal lithosomes on the north Texas Gulf Coast and the occurrence of preserved analogues on the north Texas inner continental shelf. Proceedings of the 12th Annual Research Conference, Gulf Coast Section, Soc. Econ. Paleontol. Mineralog. Foundation. pp. 240–247. Shultz, A. W., W. W. Schroeder, and J. R. Abston. 1990. Alongshore and offshore variations in Alabama inner-shelf sediments. In W. F. Tanner, ed. Coastal Sediments and Processes. Proceedings of the 9th Symposium on Coastal Sediments, Florida State University, Tallahassee. pp. 141–152.
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IMPROVING THE SAFETY OF MARINE PIPELINES Stolls, A. M. 1993. Oil spill legislation in the coastal United States since the Oil Pollution Act of 1990. Proceedings of the 1993 International Oil Spill Conference. Washington, D.C.: American Petroleum Institute. Tubman, M. W., and J. N. Suhayda. 1976. Wave action and bottom movements in fine sediments. Proceedings of the 15th Conference on Coastal Engineering, Honolulu. 2: 1168–1183. U.S. Coast Guard. 1992. Description of the U.S. Coast Guard Differential GPS Program. Office of Navigation Safety and Waterway Services, Radionavigation Division. Washington, D.C. April. Wicker, K. M., R. E. Emmer, D. Roberts, and J. van Beek. 1989. Pipelines, Navigation Channels, and Facilities in Sensitive Coastal Habitats: An Analysis of Outer Continental Shelf Impacts, Coastal Gulf of Mexico. New Orleans. Minerals Management Service. 470 pp. Williams, S. J., B. E. Jaffe, R. R. Wertz, K. T. Holland, and A. H. Sallenger, Jr. 1989. Map Showing Precision Bathymetry of the South-Central Louisiana, Nearshore Isles Dernieres and Ship Shoal Region. U.S. Geolological Survey open file report 89-150.
Representative terms from entire chapter: