Showing papers on "Submarine pipeline published in 1991"

Gas-related sedimentary features from the eastern Canadian continental shelf

Abstract: The first discovery of pockmarks (gas-escape craters) was in muds of the basins of the central Scotian Shelf off eastern Canada. Since then, pockmarks have been found in many continental shelf environments of the world. They may be used as a hydrocarbon exploration tool and recently their role as foci of intense biological chemosynthetic activity has attracted considerable attention. Pockmarks occur in the Gulf of Maine, Passamaquoddy Bay, eastern Scotian Shelf, Scotian Slope, Laurentian Channel, Gulf of St Lawrence, Halibut Channel and on Labrador Shelf. Their formation in Passamaquoddy Bay may be enhanced through earthquake activity as their greatest density occurs in association with a large fault zone. Pockmarks on Labrador Shelf and in Laurentian Channel may have been formed preferentially as a consequence of disturbance of the seabed by grounded moving icebergs. Pockmarks in Placentia Bay, Newfoundland occur in an area of large megaflutes at the seabed and the pockmarks may have provided an initial seabed roughness for subsequent formation of the megaflutes by a tsunami-generated turbidity current. Subsurface zones of gas-charged Quaternary sediments are often associated with areas of pockmarks. Gas-charged sediments in the nearshore frequently appear as thick Holocene deposits occurring in areas overlying buried channel systems. In the nearshore, biogenic methane is the probable source. In other areas, such as Downing Basin on the Grand Banks of Newfoundland, the gas-charged sediments contain ethane, butane and methane, suggesting a deeper hydrocarbon source. Active gas venting has been observed at the seabed in Downing Basin without the formation of pockmarks. Seabed observations from research submersibles and bottom photography have identified areas of white filamentous bacteria on sandy sediments sometimes associated wtih circular patches of dense benthic communities. Seismic reflection profiles from these areas also indicate the presence of shallow gas (bright spots) within Tertiary age sediments. Taken in their entirely, the broad distribution of gas-related features suggests that the major basins of the eastern Canadian continental shelf vent gas, but pockmarks or other morphologic venting features are only produced where soft cohesive sediments are present at the seabed. The chronic release of gas from the seabed may contribute to the biological productivity of the offshore areas as well as provide a significant, but as yet unquantified, volume of methane and other gases to the ocean and atmosphere.

Prediction of Maximum Scour Depth at Submarine Pipelines

Abstract: This paper examines and reviews published results relating to local scour around submarine pipelines. It highlights the limitations of existing methods for estimating scour depth at the pipeline. Based on experimental results, the study proposes an empirical function relating the amount of gap flow through the scour hole for given flow conditions. With the aid of this function, it is possible to predict the maximum Scour depth at submarine pipelines for given flow and geometric boundary conditions. Published results suggest that the maximum equilibrium scour depth occurs when the pipeline is just lying on a plane bed and subjected to a pure unidirectional current. The undisturbed bed shear stress is equal to the critical shear stress for sediment entrainment. This condition implies that there is no general sediment transport away from the pipeline. The predicted maximum scour depth using the iterative method proposed in this study compares well with experimental results.

Wave‐induced instability of seabed: Mechanism and conditions

Abstract: The instability of the seabed constitutes an important consideration in the planning and design of various offshore facilities. The stresses and the pore water pressure in the sediments, induced by the action of waves during a storm, may cause them to fail, leading to seabed instability. In this article the possible mechanism of such failures are discussed and the conditions necessary for them are formulated through simplified analyses. These provide the combinations of wave, site, and sediment parameters likely to cause instability, which may be used to identify potentially unstable sediments in a given offshore region during a storm. The numerical results are presented in convenient graphical form. Some illustrative field studies are also presented.

Offshore and onshore liquefaction at Moss Landing spit, central California—Result of the October 17,1989, Loma Prieta earthquake

Abstract: As a result of the October 17, 1989, Loma Prieta (Santa Cruz Mountains, California) earthquake, liquefaction of the fluvial, estuarine, eolian, and beach sediments under a sand spit destroyed the Moss Landing Marine Laboratories and damaged other structures and utilities. Initial studies suggested that the liquefaction was a local phenomenon. More detailed offshore investigations, however, indicate that it occurred over a large area (maximum 8 km{sup 2}) during or shortly after the earthquake with movement of unconsolidated sediment toward and into the head of Monterey submarine canyon. This conclusion is supported by side-scan sonographs, high-resolution seismic-reflection and bathymetric profiles, onshore and sea-floor photographs, and underwater video tapes. Many distinct lobate features were identified on the shallow shelf. These features almost certainly were the result of the October 17 earthquake; they were subsequently destroyed by winter storms. In addition, fresh slump scars and recently dislodged mud debris were found on the upper, southern wall of Monterey submarine canyon.

Effect of seafloor instability on offshore pile foundations

Abstract: This paper employs a modified nonlinear boundary element approach to analyze the response of offshore piles subjected to external soil movements arising from submarine slides. Theoretical solutions for a realistic hypothetical offshore pile computed by the approach are presented and discussed. The analysis is then used to analyze the behaviour of two full-scale offshore piles in submarine slide areas, and reasonable agreement is found between observed and theoretical behaviour. Key words: seafloor instability, submarine slides, offshore piles, theoretical solutions, field tests.

System pinpoints leaks on Point Arguello offshore line

Abstract: A new system for detecting pipeline leaks has been installed on 10 miles of the 17-mile onshore segment of Chevron Pipe Line Co.'s Point Arguello natural gas pipeline. The system, known as the Leak Alarm System for Pollutants (LASP), has an advantage over many other systems by being a "direct" system instead of inferential, i.e., the leaking liquid or gas is detected, not a condition caused by the leak, such as pressure loss or volume imbalance. Because of its response time, however, Chevron sees the LASP as a supplement to more conventional systems, rather than a substitute. This article, based on a presentation to the 1991 Pipeline Conference (API Pipeline Transportation Committee), Dallas, April 23-24, discusses the system's evolution, two-part design, components, installation, and operating experience.

Regular and random wave pressures around large diameter submarine pipeline near ocean bed

Abstract: The submarine pipeline fixed horizontally near a simulated ocean bed and parallel to the crests of the oncoming regular waves is formulated as a boundary value problem utilizing Green's second identity formulation for potential functions. Numerical results are obtained for the hydrodynamic pressures that act around the circumference of the pipeline in a finite water depth for salient pressure ports by solving the resulting integral equation. The normalized maximum pressure at each pressure port is correlated with scattering parameter, gap ratio of the pipeline from the ocean bed and depth parameter. An experimental investigation has also been carried out on the regular and random wave induced hydrodynamic pressures around a horizontal submarine pipeline placed at various elevations or gaps from the simulated ocean bed. In the random wave pressure experiments, the pipeline was subjected to random waves with Pierson- Moskowitz spectrum (P-M spectrum) at various energy levels to achieve different significant...

Emergency situation in the shoreline reach of an offshore oilfield pipeline and remedial measures

Abstract: This paper describes the critical situation experienced in the northeast coast of Brazil by an oil and a gas pipeline, in their shoreline reach, due to the growth of a spit, and the remedial measures adopted, which a voided an environmental and economical disaster.

Submarine landslides: Elements of analysis

Abstract: The stability of seafloor sediments constitutes an important consideration in planning and design of various offshore facilities. The stresses induced either by waves during a storm or by shaking d...

Phase behavior of glycol in gas pipeline calculated

Abstract: Transportation of wet, CO{sub 2}-containing gases in pipelines requires special measures to prevent corrosion of the pipe wall. A relatively new option uses injection of glycol, the presence of which in the water phase mitigates corrosion. A method has been developed for calculating the phase behavior of glycol in such wet CO{sub 2} pipelines. Composition of the glycol-water condensate in the pipeline is one essential in determination of the corrosion rate. The effect of natural-gas condensate on the mass transfer of glycol and water between liquid glycol-water at the bottom of the pipeline and the gas phase is approached by means of two dispersion criteria. The composition of glycol-water is shown for the various cases considered for the various cases considered for the operation of the 66-km, 36-in. trunklines between the Troll platform of the Norwegian coast and the offshore processing plant on the Troll natural-gas pipeline system.

Alternative Approaches To Pipeline Survey: The Pipeline Engineer's View

Abstract: Annual ROV pipeline survey specifications tend to place considerable emphasis on visual records and the commentary made by the survey crew. However, the latter is based largely on the former which are, in turn, restricted by the field of view and the uncertainty of orientation of the boom cameras. Thus a slow and laborious process yields only limited and subjective results. In addition, re-visiting old records is not only extremely time-consuming, tedious and costly, but does not facilitate any comparison from year to year. This paper, which is based upon three recent studies, looks at the limitations of current survey techniques and explores, from the point of view of the pipeline engineer, some alternative and innovative approaches aimed at improving pipeline survey. By taking advantage of recently-developed techniques, it will be possible to obtain and present more pertinent data. The way these data can be used to advance the understanding of the dynamic interactions between pipelines and the seabed are discussed.

Route selection key first step in avoiding ocean floor spans

Abstract: Pipeline spanning is becoming a greater concern in the U.S. Gulf of Mexico as field development moves into deeper waters. Two factors combine to create the problem. First, seafloor topography becomes more irregular as pipelines move from the relatively smooth continental shelf to the steeper, irregular bathymetry of the continental slope. Second, pipeline installation tension increases in deeper water. This first of two articles looks at how careful route selection may preclude spanning and how that selection process may avoid or reduce costly offshore surveys.

Analysis of Community Risk Resulting from Rupture of a Sour Gas Pipeline

Abstract: Natural gas pipelines present fire and explosion hazards to nearby communities. These hazards are compounded if significant amounts of hydrogen sulfide (H2S) are present in the gas. This paper presents the methodology and difficulties encountered in analyzing a long sour gas pipeline (H2S present) from an offshore field in southern California. A number of issues were studied in depth in order to present community hazards in the form of risk profiles. More specifically, these issues encompassed: the rate of gas released from a pipeline rupture; the value of parameters such as gas temperature and density as the gas flashes to atmospheric pressure; the failure frequency of the pipeline; the population distribution; the effect of varying weather conditions; toxic hazard levels; and such concerns as the impact of shielding, terrain and age of potential recipients. Some of these issues were particularly difficult to analyze. For example, the rupture rate for sour gas lines is highly correlated with the potential for internal corrosion, which is influenced by such factors as H2S and CO2 content in the gas stream. This problem is further influenced by other factors such as pipeline steel metallurgy and gas wetness. This study presents the approach taken to analyze the impact of the more important issues affecting the problem. Because most of the input parameters display a fair amount of uncertainty, the analysis places bounds on the results. An upper and lower bound on both casualties and fatalities is presented for the community risk profiles.

Loss Control by Subsea Barrier Valve for Offshore Gas Pipeline

Abstract: Platform-mounted shut-down valves for offshore pipeline systems, are vulnerable to fire and exploration hazards. Should this primary barrier fail, the full inventory of the pipeline could be consumed in the fire. A secondary subsea barrier would reduce substantially the loss of pipeline inventory, but also expose the platform to a new hazard. Valve failure could lead to the formation of gas or oil plumes which could be carried towards the platform. The choice of subsea barrier location, requires a trade-off between these safety concerns. The paper presents one prerequisite for such a trade-off; and analysis of the potential gas loss for different secondary-barrier positions and valve-closing rates when a guillotine break occurs at platform deck level. The paper gives a detailed study of the flow characteristics close to the valve and over the length of the segment between the barrier valve and the rupture. Particular attention is paid to the boundary conditions at the valve. These have been formulated based on the method of characteristics. Computer simulations using the simplifying assumption of isothermal flow have been compared to the adiabatic case. The differences between these cases have been found to be small.