Abstract

n oil pipeline ruptured in May 1994 and 2 700 tons of crude oil leaked into the São Sebastião Channel, affecting several neighboring areas. A program for the monitoring of hydrocarbons in sediments, using the gas chromatography / flame ionization detector methodology, was being undertaken in the area at the time. The data obtained were compared to those of samples collected after the accident to determine the fate of the oil spilled and ascertain its contribution to the environment. The earlier results showed that hydrocarbons were introduced from two different sources: biogenic, mainly from terrestrial plants, and anthropogenic, as oil, in sewage and from shipping. The later data indicated that the site closest to the pipeline rupture had been the most affected. Following that, two stations located at the north entrance of the channel presented the highest n-alkane concentrations, suggesting that the northeasterly wind-driven currents had carried the oil northward. Seven months later, one of these stations, a high-energy site, showed some signs of recovery, but this process was not observed at the other, which seemed to be a low-energy site. In conclusion, the data showed that the aliphatic hydrocarbon analyses were powerful tools for the assessment of the fate of the oil spill and that the northern part of the São Sebastião Channel is more subject to the effects of oil spills. A ruptura de um oleoduto em maio de 1994 introduziu 2.700 tons de óleo cru no Canal de São Sebastião, contaminando algumas áreas. Na ocasião um monitoramento de hidrocarbonetos em sedimentos estava sendo realizado na região, utilizando o método de cromatografia em fase gasosa/ detecção por ionização de chama. Os dados obtidos nesse monitoramento foram comparados aos resultados das amostras coletadas logo após o acidente, com o objetivo de verificar a extensão da contaminação por óleo e os lugares mais atingidos. Os dados prévios mostraram duas fontes de introdução de hidrocarbonetos: biogênica, com predomínio de plantas superiores, e antrópica, causada por atividades náuticas e esgotos. Os dados posteriores mostraram que a estação de coleta mais próxima ao oleoduto foi a mais atingida. Na sequencia, as duas estações localizadas na entrada norte do canal apresentaram as maiores concentrações de n-alcanos, sugerindo que as correntes NE, impulsionadas pelo vento, transportaram o óleo no sentido norte do canal. Sete meses depois uma destas estações, aquela situada em local de alta energia, mostrou sinais de recuperação, o que, contudo, não foi observado na outra, considerada de ambiente deposicional. Em conclusão, pode-se afirmar que análises de hidrocarbonetos são ferramentas poderosas para avaliar o destino de um óleo e que a parte norte do Canal de São Sebastião tem maior probabilidade de ser afetada na ocasião de um derrame de óleo. Document type: Article

Abstract

This paper presents a comprehensive review on subsea jet trenching technology via a critical analysis of its principle, mechanism, devices, modeling approaches, as well as properties of subsea sediments. This review shows that the success of jet trenching operations is closely related to some key factors including the soil conditions, trencher specifications, and characteristics of pipelines or cables. Three case histories are presented to demonstrate the importance of these key factors and their interrelationships. This paper also points out a number of challenges pertaining to the implementation of the jet trenching method in carbonate sediments, as well as some limitations and gaps in the existing modeling approaches. Future perspective researches that are recommended to develop: (1) promising theories, like turbulent submerged flow, and (2) robust numerical approaches, such as the smoothed particle hydrodynamics (SPH) and material point method (MPM) to substantiate the experimental experience and reveal possible hidden mechanisms. Document type: Article

Abstract

Freak waves are an extreme marine environment factor in offshore structure design and become a potential risk, particularly for laying oil-gas pipelines in deep waters. The objective of this study was to reveal the freak wave effects on dynamic behaviors of offshore pipelines for deepwater installation. Thus, a dedicated finite element model (FEM) for deepwater pipeline installation by the S-lay method was developed with special consideration of freak waves. The FEM also took pipelay vessel motions, pipe&ndash stinger roller interactions, and the cyclic contacts between the pipeline and seabed soil into account. Real vessel and stinger data from an actual engineering project in the South China Sea were collected to obtain an accurate simulation. Moreover, an effective superposition approach of combined transient wave trains and random wave trains was introduced, and various types of freak wave trains were simulated. Extensive numerical analyses of a 12 inch gas pipeline being installed into a water depth of 1500 m were implemented under various freak wave conditions. The noticeable influences of freak waves on the pipeline and seabed responses were identified, which provides significant awareness of offshore pipelines for deepwater installation design and field operation monitoring. Document type: Article

Abstract

Submarine pipelines, as arteries for offshore oil and gas transportation, play a particularly important role in the exploitation of offshore oil and gas resources. Since the world’s first submarine pipelines were laid in the Gulf of Mexico, numerous failures have been caused by pipeline free-spanning. This paper provides a review of the causes and treatment measures for the free span of submarine pipeline. Various factors cause the free span of submarine pipelines, including wave flow scouring, fluctuations in seabed topography, residual stress or thermal stress of pipelines, and human activities. The scour of the wave current is the main factor affecting free span; the research on sediment starting and equilibrium depth during scour is reviewed in-depth. For the span treatment of submarine pipelines, the main measures available at present include the re-digging trench burying, structural support, covering bionic water plants, and choke plate self-burying. For each, the principle, advantages, disadvantages, and research are discussed. This review provides a convenient resource for understanding the causes of submarine free-spanning pipelines and choosing suitable treatment measures. Document type: Article

Abstract

The evaluation of the wave-induced seabed instability around a submarine pipeline is particularly important for coastal engineers involved in the design of pipelines protection. Unlike previous studies, a meshfree model is developed to investigate the wave-induced soil response in the vicinity of a submarine pipeline. In the present model, Reynolds-Averaged Navier-Stokes (RANS) equations are employed to simulate the wave loading, while Biot&rsquo s consolidation equations are adopted to investigate the wave-induced soil response. Momentary liquefaction around an offshore pipeline in a trench is examined. Validation of the present seabed model was conducted by comparing with the analytical solution, experimental data, and numerical models available in the literature, which demonstrates the capacity of the present model. Based on the newly proposed model, a parametric study is carried out to investigate the influence of soil properties and wave characteristics for the soil response around the pipeline. The numerical results conclude that the liquefaction depth at the bottom of the pipeline increases with increasing water period (T) and wave height (H), but decreases as backfilled depth ( H b ), degree of saturation ( S r ) and soil permeability (K) increase. Document type: Article

Abstract

With the accelerated development of marine engineering, a growing number of marine structures are being constructed (e.g., seabed pipelines, drilling platforms, oil platforms, wind turbines). However, seismic field investigations over recent decades have shown that many marine structures were damaged or destroyed due to liquefaction. Seismic liquefaction in marine engineering can have huge financial repercussions as well as a devastating effect on the marine environment, which merits our great attention. As the effects of seawater and the gas component in the seabed layers are not negligible, the seabed soil layers are more prone to liquefaction than onshore soil layers, and the liquefied area may be larger than when liquefaction occurs on land. To mitigate the impact of liquefaction events on marine engineering structures, some novel liquefaction-resistant marine structures have been proposed in recent years. This paper reviews the features of earthquake-induced liquefaction and the mitigation strategies for marine structures to meet the future requirements of marine engineering. Document type: Article