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21.
海洋平台复杂节点阴极保护电位分布的有限元法计算 总被引:1,自引:0,他引:1
了解阴极保护电位分布是合理设计阴极保护工程的必要条件和评价阴极保护效果的重要依据。本文应用有限元素法(FEM)计算海洋平台复杂节点表面的阴极保护电位分布。在实验室通过边界条件试验建立数学模型,计算海水中被保护的I,Y,K型复杂节点电位分布情况。计算结果与测量结果吻合得很好,说明FEM能够应用于海洋平台复杂节点阴极保护电位分布的计算,所获得的边界条件合理。 相似文献
22.
The ridge located between 31° S and 34°30′S is spreading at a rate of 35 mm yr−1, a transitional velocity between the very slow (≤20 mm yr−1) opening rates of the North Atlantic and Southwest Indian Oceans, and the intermediate rates (60 mm yr−1) of the northern limb of the East Pacific Rise, and the Galapagos and Juan de Fuca Ridges. A synthesis of multi-narrow beam,
magnetics and gravity data document that in this area the ridge represents a dynamically evolving system. Here the ridge is
partitioned into an ensemble of six distinct segments of variable lengths (12 to 100 km) by two transform faults (first-order
discontinuities) and three small offset (< 30 km) discontinuities (second-order discontinuities) that behave non-rigidly creating
complex and heterogeneous morphotectonic patterns that are not parallel to flow lines. The offset magnitudes of both the first
and second-order discontinuities change in response to differential asymmetric spreading. In addition, along the fossil trace
of second-order discontinuities, the lengths of abyssal hills located to either side of a discordant zone are observed to
lengthen and shorten creating a saw-toothed pattern. Although the spreading rate remains the same along the length of the
ridge studied, the morphology of the spreading segments varies from a deep median valley with characteristics analogous to
the rift segments of the North Atlantic to a gently rifted axial bulge that is indistinguishable from the shape and relief
of the intermediate rate spreading centers of the East Pacific Rise (i.e., 21°N). Like other carefully surveyed ridge segments
at slow and fast rates of accretion, the along-axis profiles of each ridge segment are distinctly convex upwards, and exhibit
along-strike changes in relief of 500m to 1500 between the shallowest portion of the segment (approximate center) and the
segment ends. Such spatial variations create marked along-axis changes in the morphology and relief of each segment. A relatively
low mantle Bouguer anomaly is known to be associated with the ridge segment characterized by a gently rifted axial bulge and
is interpreted to indicate the presence of focused mantle upwelling (Kuo and Forsyth, 1988). Moreover, the terrain at the
ends of each segment are known to be highly magnetized compared to the centers of each segment (Carbotte et al, 1990). Taken
together, these data clearly establish that these profound spatial variations in ridge segment properties between adjoining
segments, and along and across each segment, indicate that the upper mantle processes responsible for the formation of this
contrasting architecture are not solely related to passive upwelling of the asthenosphere beneath the ridge axis. Rather,
there must be differences in the thermal and mechanical structure of the crust and upper mantle between and along the ridge
segments to explain these spatial variations in axial topography, crustal structure and magnetization. These results are consistent
with the results of investigations from other parts of the ridge and suggest that the emplacement of magma is highly focused
along segments and positioned beneath the depth minimum of a given segment. The profound differences between segments indicate
that the processes governing the behavior of upwelling mantle are decoupled and the variations in the patterns of axis flanking
morphology and rate of accretion indicate that processes controlling upwelling and melt production vary markedly in time as
well. At this spreading rate and in this area, the accretionary processes are clearly three-dimensional. In addition, the
morphology of a ridge segment is not governed so much by opening rate as by the thermal structure of the mantle which underlies
the segment. 相似文献
23.
桩式离岸堤保滩促淤工程消浪效果试验研究 总被引:9,自引:0,他引:9
在上海奉贤南北港保滩促淤工程中,采用了一种新型结构型式-桩式离岸堤,并通过物理模式试验进行了桩式离岸堤消浪效果研究。针对离岸堤通常建于近岸水区破波带的特点,重点研究水深,堤高以及堤身结构对波浪衰减的影响,同时对桩式离岸堤堤后水域的波浪底流速 分析探讨。研究结果表明,桩式离岸堤不仅具有良好的消浪效果。而且可在较大范围内改变波态,即由引起水体剧烈紊动的破波转变为浅水推进波,从而有效地改善海滩上的动力条件,促进海滩免受侵蚀,是一种具有广泛应用前景和新型保滩促淤结构。 相似文献
24.
W. R. Roest J. J. Dañobeitia J. Verhoef B. J. Collette 《Marine Geophysical Researches》1992,14(1):1-24
The data from a recent magnetic compilation by Verhoefet al. (1991) off west Africa were used in combination with data in the western Atlantic to review the Mesozoic plate kinematic evolution of the central North Atlantic. The magnetic profile data were analyzed to identify the M-series sea floor spreading anomalies on the African plate. Oceanic fracture zones were identified from magnetic anomalies and seismic and gravity measurements. The identified sea floor spreading anomalies on the African plate were combined with those on the North American plate to calculate reconstruction poles for this part of the central Atlantic. The total separation poles derived in this paper describe a smooth curve, suggesting that the motion of the pole through time was continuous. Although the new sea floor spreading history differs only slightly from the one presented by Klitgord and Schouten (1986), it predicts smoother flowlines. On the other hand, the sea floor spreading history as depicted by the flowlines for the eastern central Atlantic deviates substantially from that of Sundvik and Larson (1988). A revised spreading history is also presented for the Cretaceous Magnetic Quiet Zone, where large changes in spreading direction occurred, that can not be resolved when fitting magnetic isochrons only, but which are evident from fracture zone traces and directions of sea floor spreading topography.Deceased 11 November 1991 相似文献
25.
Submarine groundwater discharge (SGD) into a coastal lagoon off Perth, Western Australia, contains nitrate and silicate in concentrations two orders of magnitude higher than those of the receiving waters. This discharge delivers enough nitrate to replace that dissolved in the lagoon water mass about every eight days and enough silicate to replace the lagoon silicate in about 48 days. The delivery rate of nitrate nitrogen by SGD is equal to about 48% of that required for observed growth rates of lagoon macrophytes. Surface salinity is lower close to the shore as a result of SGD. During calm conditions a salinity front was observed in the lagoon, with a nearshore pool of nutrient-enriched water floating above the more saline ocean water. 相似文献
26.
Beach nourishment is an environmentally preferred method of shore protection, but the annual sand requirement may lead to substantial maintenance costs. The shoreline processes, involving the surf zone, beach and dune, are reviewed with the aim of reducing the annual sand requirement of eroding shorelines. It is shown that surf zones with equilibrium profiles, on which the wave energy conversion is evenly distributed across the surf zone, from experience for given conditions indicate least loss of sand. On steep, eroding shorelines it may be difficult to establish an equilibrium profile. For such cases, the use of perched surf zones is recommended, which are supported at the seaward limit by an underwater sill. For reduction of littoral transport, the use of pervious pile groynes is recommended. These are arguably more efficient than impervious groynes. The sand loss from a usually dry beach by raised water levels is shown to be a function of the beach slope and is least when the storm waves at raised water levels do not cut an erosion escarpment. The loss of sand from a dune by infrequent severe storm tides can be prevented with the aid of a built-in membrane. These sand losses are usually large and constitute an uneconomic use of this sand resource. The proposed concepts and measures are linked to existing knowledge, augmented by data from the large wave flume (LWF) in Germany and field data from the North and Baltic Sea coasts. 相似文献
27.
本文根据胜利二号极浅海步行坐底钻井平台的作业和工作环境特点,进行了防腐措施研究,设计了切合实际的防腐措施。该平台在海上作业近四年进坞修理,经观测防腐情况良好。 相似文献
28.
中国红树林生态系统的植物种类、多样性、功能及其保护 总被引:28,自引:0,他引:28
本文从4个方面对中国红树林生态系统的植物进行论述:(1)中国红树林生态系统的红树植物、半红树植物、伴生植物、红树林区的大型藻类和浮游植物种类;(2)中国红树林生态系统植物的物种多样性、红树植物生理和形态适应的多样性、产物的多样性;(3)红树林的生态功能;(4)中国红树林生态系统的主要保护问题及其保护措施。 相似文献
29.
长白山自然保护区是我国建立较早的和典型的自然保护区之一,其主要功能是进行自然生态环境的保护。但是,为了克服保护资金短缺境况,长白山自然保护区将发展旅游业作为生态环境保护工作资金支持的一条途径,而旅游业曾长期被认为是对生态环境影响微乎其微的产业,但自长白山开始发展旅游业以来,经济效益的实现似乎掩盖了原本的生态环境保护目标。本文在分析了长白山旅游发展对生态环境造成的负面影响之后,提出了一系列在长白山旅游发展过程中生态环境的保护措施,以期兼顾长白山自然保护区发展和保护两大目标的实现。 相似文献
30.