Undrained capacity of a surface circular foundation under fully three-dimensional loading

Circular foundations are widely employed in offshore engineering to support facilities and are generally subjected to fully three-dimensional loading due to the harsh offshore environmental load and complex operational loads. The undrained capacity of surface circular foundations on soil with varying strength profiles and under fully three-dimensional loading is investigated and presented in the form of failure envelopes that obtained from finite element analyses. The combined ultimate limit state of circular foundations is defined as the two-dimensional failure envelopes in resultant H-M loading space accounting for the vertical load and torsion mobilisation. The effects of vertical load and torsion mobilisation, soil shear strength heterogeneity and loading angle from moment to horizontal load on the shape of normalised H-M failure envelopes are explored. A series of expressions are proposed to describe the shape of failure envelopes obtained numerically, enabling essentially instantaneous generation of failure envelopes and optimisation of a circular foundation design based on constraint of any input variable through implementation in an automated calculation tool. An example application is ultimately provided to illustrate how the proposed expressions may be used in practice.     

Basement tectonics and flexural subsidence along western continental margin of India

The Paleocene-recent post-rift subsidence history recorded in the Mumbai Offshore Basin off western continental margin of India is examined. Results obtained through 2-D flexural backstripping modelling of new seismic data reveal considerable thermo-tectonic subsidence over last ca. 56 Myr. Reverse postrift subsidence modelling with variable β stretching factor predicts residual topography of ca. 2000 m to the west of Shelf Margin Basin and fails to restore late Paleocene horizon and the underlying igneous basement to the sea level. This potentially implies that:(1) either the igneous basement formed during the late Cretaceous was emplaced under open marine environs; or(2) a laterally varying cumulative subsidence occurred within Mumbai Offshore Basin(MOB) during ca. 68 to ca. 56 Ma. Pre-depositional topographic variations at ca. 56 Ma across the basin could be attributed to the extensional processes such as varied lower crustal underplating along Western Continental Margin of India(WCMI). Investigations about basement tectonics after unroofing of sediments since late Paleocene from this region support a transitional and heavily stretched nature of crust with high to very high β factors. Computations of past sediment accumulation rates show that the basin sedimentation peaked during late Miocene concurrently with uplift of Himalayan-Tibetan Plateau and intensification of Indian monsoon system. Results from basin subsidence modelling presented here may have significant implications for further studies attempting to explore tectono-climatic interactions in Asia.     

Observed dynamic soil–structure interaction in scale testing of offshore wind turbine foundations

Monopile foundations have been commonly used to support offshore wind turbine generators (WTGs), but this type of foundation encounters economic and technical limitations for larger WTGs in water depths exceeding 30 m. Offshore wind farm projects are increasingly turning to alternative multipod foundations (for example tetrapod, jacket and tripods) supported on shallow foundations to reduce the environmental effects of piling noise. However the characteristics of these foundations under dynamic loading or long term cyclic wind turbine loading are not fully understood. This paper summarises the results from a series of small scaled tests (1:100, 1:150 and 1:200) of a complete National Renewable Energy Laboratory (NREL) wind turbine model on three types of foundations: monopiles, symmetric tetrapod and asymmetric tripod. The test bed used consists of either kaolin clay or sand and up to 1.4 million loading cycles were applied. The results showed that the multipod foundations (symmetric or asymmetric) exhibit two closely spaced natural frequencies corresponding to the rocking modes of vibration in two principle axes. Furthermore, the corresponding two spectral peaks change with repeated cycles of loading and they converge for symmetric tetrapods but not for asymmetric tripods. From the fatigue design point of view, the two spectral peaks for multipod foundations broaden the range of frequencies that can be excited by the broadband nature of the environmental loading (wind and wave) thereby impacting the extent of motions. Thus the system lifespan (number of cycles to failure) may effectively increase for symmetric foundations as the two peaks will tend to converge. However, for asymmetric foundations the system life may continue to be affected adversely as the two peaks will not converge. In this sense, designers should prefer symmetric foundations to asymmetric foundations.     

Seismic dynamics of offshore breakwater on liquefiable seabed foundation

Offshore structures, such as composite breakwaters, are generally vulnerable to strong seismic wave propagating through loose or medium-dense seabed foundation. However, the seismically induced failure process of offshore structures is not well understood. In this study, seismic dynamics of a composite breakwater on liquefiable seabed foundation is investigated using a fully coupled numerical model FSSI-CAS 2D. The computation results show that the numerical model is capable of capturing a variety of nonlinear interaction phenomena between the composite breakwater and its seabed foundation. The numerical investigation demonstrates a three-stage failure process of the breakwater under seismic loading. In this process, the far-field seabed can become fully liquefied first, inducing excessive settlement of the structure, followed by significant lateral movement and tilting of the structure when the near-field soil progressively liquefies. The study demonstrates great promise of using advanced numerical analysis in geotechnical earthquake design of offshore structures.     

埕北油田聚合物的筛选与性能评价

埕北油田具有原油黏度高、油层渗透能力强和非均质性严重等特点,水驱开发效果差,剩余油储量大。目前,埕北油田综合含水90%,采出程度30%左右,而生产平台工作时间已达到设计使用寿命,急需高效、快速的开发技术。针对埕北油藏地质、流体性质和开发现状,利用室内仪器检测和理论分析方法,开展了聚合物筛选和聚合物性能评价,利用模糊综合评判方法对聚合物进行了优选,推荐使用中国石油大庆炼化公司生产＂抗盐＂和＂超高＂部分水解聚丙烯酰胺。     

Salt structures in the Laizhouwan depression, offshore Bohai Bay basin, eastern China: New insights from 3D seismic data

Using the new high-quality 3D seismic data, this paper addresses the salt structures in the KL11 area of the Laizhouwan depression in the southern offshore Bohai Bay basin. In the study area, the salt in the Sha-4 Member of the Paleogene Shahejie Formation thickened, and then formed an S–N trending salt wall, which changes shape regularly along its trend from salt diapir to salt pillow. The change in thickness of the suprasalt layers record five growth phases of the salt wall from the Eocene to the Quaternary: (1) early diapirism, (2) active diapirism, (3) passive diapirism, (4) relative structural quiescence, and (5) arching. The evolution of the salt structures was mostly governed by the multi-phase compression induced by the dextral strike-slip of the Tan–Lu fault, which formed a restraining bend in the study area. There was an original passive stock in the south, which was later tectonically squeezed by E–W compression and became a diapir. As the shortening propagated to the north from the original stock, the salt pillow was created in the north. Relative structural quiescence then followed until the next phase of compression, which arched the thick roof of the salt wall.     

1992-2011年夏季南海西部离岸流区涡流相互作用特征

本文利用南海海洋再分析产品REDOS（Reanalysis Dataset of the South China Sea）和风场资料CCMP（Cross-Calibrated,Multi-Platform）,从能量学角度探讨了1992-2011年夏季（6-9月）越南离岸流区域涡-流相互作用特征,并通过能量收支方程诊断评估了风应力、压力梯度、正压不稳定以及平流的相对贡献。以越南离岸流的强度作为分类标准,对1992-2011年划分为正异常年、负异常年和正常年。结果表明,在正异常年,涡动能EKE（Eddy Kinetic Energy）和涡势能EPE（Eddy available Potential Energy）极大值主要分布在越南离岸流附近;在负异常年,EKE极大值向南北两侧分散,EPE极大值向北延伸;在正常年,EKE和EPE的极值空间分布介于正负异常之间。斜压不稳定是EPE年际变化的主要因素,越南离岸流影响周围海域的速度和密度分布,是斜压不稳定的主要原因。而影响EKE年际变化的因素较为复杂,压力做功是最主要的影响因素,风应力做功和平流做功次之,正压不稳定最小,其中正压不稳定依赖于流速大小和由风应力旋度扰动引起的上层水平流速剪切。     

Behavioral responses of a harbor porpoise (Phocoena phocoena) to playbacks of broadband pile driving sounds

The high under-water sound pressure levels (SPLs) produced during pile driving to build offshore wind turbines may affect harbor porpoises. To estimate the discomfort threshold of pile driving sounds, a porpoise in a quiet pool was exposed to playbacks (46 strikes/min) at five SPLs (6 dB steps: 130–154 dB re 1 μPa). The spectrum of the impulsive sound resembled the spectrum of pile driving sound at tens of kilometers from the pile driving location in shallow water such as that found in the North Sea. The animal's behavior during test and baseline periods was compared. At and above a received broadband SPL of 136 dB re 1 μPa [zero-peak sound pressure level: 151 dB re 1 μPa; t₉₀: 126 ms; sound exposure level of a single strike (SEL_ss): 127 dB re 1 μPa² s] the porpoise's respiration rate increased in response to the pile driving sounds. At higher levels, he also jumped out of the water more often. Wild porpoises are expected to move tens of kilometers away from offshore pile driving locations; response distances will vary with context, the sounds' source level, parameters influencing sound propagation, and background noise levels.     

Frequent locations of oceanic fronts as an indicator of pelagic diversity: Application to marine protected areas and renewables

Frequent locations of thermal fronts in UK shelf seas were identified using an archive of 30,000 satellite images acquired between 1999 and 2008, and applied as a proxy for pelagic diversity in the designation of Marine Protected Areas (MPAs). Networks of MPAs are required for conservation of critical marine habitats within Europe, and there are similar initiatives worldwide. Many pelagic biodiversity hotspots are related to fronts, for example cetaceans and basking sharks around the Isle of Man, Hebrides and Cornwall, and hence remote sensing can address this policy need in regions with insufficient species distribution data. This is the first study of UK Continental Shelf front locations to use a 10-year archive of full-resolution (1.1 km) AVHRR data, revealing new aspects of their spatial and seasonal variability. Frontal locations determined at sea or predicted by ocean models agreed closely with the new frequent front maps, which also identified many additional frontal zones. These front maps were among the most widely used datasets in the recommendation of UK MPAs, and would be applicable to other geographic regions and to other policy drivers such as facilitating the deployment of offshore renewable energy devices with minimal environmental impact.