Mooring system reliability in tropical cyclone and North Sea winter storm environments

The characteristics of waves, winds and currents in a tropical cyclone environment differ significantly from those in a winter storm environment, like the North Sea. This can have a significant effect on the reliability of a mooring system that is designed to satisfy 100 yr conditions with specified Factors of Safety in accordance with ISO19901-7 or API RP 2SK. This paper presents reliability analysis of the mooring system of a permanently connected Floating LNG vessel, placed at two locations: (a) a tropical cyclone environment of the North West Shelf of Australia and (b) a winter storm environment of the North Sea. It is demonstrated that as a result of differences in the long term distribution of environmental parameters (waves, winds) between a North Sea environment and a tropical cyclone environment, the long term distribution of the mooring line response differs significantly in these two locations.This paper shows that a mooring system which is designed in accordance with ISO (or API), in these two environments, will achieve very different reliability levels because of the significant differences in environmental characteristics. In order to achieve the same reliability for the mooring system at these two geographical locations, Factors of Safety for use with 100 yr environmental conditions (Ultimate Limit State) were derived to achieve the same target probability of failure of 10⁻⁴/annum. It was found that for the North Sea environment, a factor of 1.5 is required for both the mooring chain and the pile, while for the tropical cyclone environment the required Factor of Safety has to be increased to 2.1. These differences are very significant and design standards need to be revised to reflect these findings.     

North sea offshore structures

Existing and planned North Sea offshore structures for exploration (drillings) and structures for exploitation (production platforms) are mentioned. Introductorily the mereorological conditions, wave climate and bottom composition in the North Sea are reviewed briefly. They are determining factors for design as well as for operation. Furthermore a brief account on present and planned activities on exploration for and exploitation of oil and gas in the North Sea is given.     

Sub-recent nitrogen-isotope trends in sediments from Skagerrak (North Sea) and Kattegat: Changes in N-budgets and N-sources?

We determined ¹⁵N/¹⁴N ratios of total nitrogen in surface sediments and dated sediment cores to reconstruct the history of N-loading of the North Sea. The isotopic N composition in modern surface sediments is equivalent to and reflects the isotopic mixture of oceanic nitrate on the one hand (δ¹⁵N = 5‰) and the imprint of river-borne nitrogen input into the SE North Sea (δ¹⁵N up to 12‰ in estuaries of the SE North Sea) on the other hand. We compare the results with δ¹⁵N records from pre-industrial sediment intervals in cores from the Skagerrak and Kattegat areas, which both constitute significant depositional centres for N in the North Sea and the Baltic Sea/North Sea transition. As expected, isotopically enriched anthropogenic nitrogen was found in the two records from the Kattegat area, which is close to eutrophication sources on land. Enrichment of δ¹⁵N in cores from the Skagerrak – the largest sediment sink for nitrogen in the entire North Sea – was not significant and values were similar to those found in sediment layers representing pre-industrial conditions. We interpret this isotopic uniformity as an indication that most riverine reactive nitrogen with its characteristic isotopic signature is removed by denitrification in shallow shallow-water sediments before reaching the main sedimentary basin of the North Sea.     

欧洲北海海域波高区间分布概率研究

为了研究欧洲北海海域的波高全区域概率分布情况,从而为海洋平台等海洋浮式结构物的选址和结构设计提供依据。首先基于Global Waves Statistics(GWS)提供的实测数据,确定典型计算工况的发生概率;同时考虑实测数据中极端波浪环境下的数据缺失导致大波高分布概率偏小的问题,利用三参数Weibull分布确定不同重现期下的极值风速,作为典型计算工况的补充。以不同风速、风向的定常风场为输入项,利用第三代海浪数值模型SWAN模型,对北海全区域波高进行数值模拟。将数值模拟的稳态形式依照各工况的发生概率进行归一化累加处理,认为其结果可以表征全区域的波高概率分布情况。以波高概率分布的计算结果为依据,分析北海海域波浪环境的统计学特征,发现有效波高为7 m以上的大波高频发区在北海北部区域有大范围分布;有效波高4～5 m为北海东北区域的多发海况,极端海况下的有效波高主要分布于7～14 m区间,在地形突变区域的波高发生显著变化。     

Implications of bedform dimensions for the prediction of local scour in tidal inlets: a case study from the southern North Sea

The bedforms and the local scour at the base of a cylindrical pile were studied in a tidal inlet in the Wadden Sea, southern North Sea, using high-resolution multibeam bathymetry data from four surveys. The observed changes in scour and bedform dimensions were interpreted in terms of hydraulic forcings varying periodically at different time scales. It appears that bedform orientation reacts to changing flow conditions on a semidiurnal basis, whereas bedform height and steepness reflect the spring-neap cycle as well as seasonal signals. The scour depth carries a strong overprint of the semidiurnal tidal cycle, which is at a maximum during the strongest tidal flow. Subtler variations in scour depth can possibly be attributed to the spring-neap tidal cycle. Based on these data on bedform and scour dimensions, correlation functions were established between scour depth and dune height as well as dune length. In measuring the scour depth under mobile bed conditions, establishing the seabed level based on the trough level of the bedforms nearest to the scour proved useful. These findings suggest that the dimensions of bedforms in dynamic equilibrium with prevailing hydraulic flow conditions can be used to estimate scouring in tidal environments.     

成层海床波浪响应的状态空间解及其应用

针对成层海床的波浪响应问题,采用Laplace-Fourier变换和状态空间法求解控制方程,得到表征该问题基本状态量之间的传递矩阵。利用矩阵递推规律、边界条件及逆变换技术实现成层海床波浪响应的计算。以北海和密西西比三角洲的波浪和海床特征为参数进行算例分析,结果表明该方法具有较高的效率和精度。结合北海的工况指出在砂质海床的表面铺设一层砾石土可以增强海床的抗液化能力,起到保护海床的作用。     

渤海东侧海岸带黄土中的放射虫

阐述了海岸带黄土中放射虫的种类及组合特征，并对其来源进行了探讨。 初步认为：放射虫最初是在间冰期高海面时被黄海暖流带入渤海的，后来在冰期低 海面时，裸露的海底受强大的西北向低空气流作用，致使一些放射虫壳体伴随沙尘 而沉降到附近海岸黄土之中。     

Probabilistic analysis of offshore wind turbines under extreme resonant response: Application of environmental contour method

Offshore wind turbines can exhibit dynamic resonant behavior due to sea states with wave excitation frequencies coinciding with the structural eigenfrequencies. In addition to significant contributions to fatigue actions, dynamic load amplification can govern extreme wind turbine responses. However, current design requirements lack specifications for assessment of resonant loads, particularly during parked or idling conditions where aerodynamic damping contributions are significantly reduced. This study demonstrates a probabilistic approach for assessment of offshore wind turbines under extreme resonant responses during parked situations. Based on in-situ metocean observations on the North Sea, the environmental contour method is used to establish relevant design conditions. A case study on a feasible large monopile design showed that resonant loads can govern the design loads. The presented framework can be applied to assess the reliability of wave-sensitive offshore wind turbine structures for a given site-specific metocean conditions and support structure design.     

中国南海极端风暴海况下固定式平台结构可靠性分析

基于中国南海海域风暴环境条件,研究分析南海海域固定式导管架平台结构整体性和可靠性,以及新建固定式平台结构设计准则。采用海洋环境数据后报方法,得到南海海域1972~2011共40年的风、浪、流联合数据,从中抽取风暴环境条件;利用通用荷载模型,将40年间的风暴环境数据转变成结构的荷载数据,即基底剪力或倾覆力矩;并计算得到风暴环境荷载的短期及长期分布,以及任意风暴下荷载的概率分布;根据结构可靠性模型,结合荷载的长期分布,研究基于结构暴露等级及失效概率的固定式平台结构强度储备比。根据计算,得到了不同暴露等级下中国南海平台的强度储备比,并与墨西哥湾及北海海域进行了比较,为新建平台提供设计参考。     

Ecological performance indicators in the North Sea: development and application

Different signals of distress from the North Sea ecosystems started a discussion on the protection of the ecosystem at the third North Sea Ministers Conference in 1990. This was followed by a number of workshops on ecological indicators under the auspices of the North Sea Task Force and the Oslo and Paris Commissions (OSPAR). In 1997 the member countries around the North Sea agreed to develop and apply an ecosystem's approach in the management programs for both the North Sea fisheries and the marine environment. Following this agreement the identification of the ecological qualities objectives for the North Sea ecosystem started. Further work in this direction has lead to the setting up of various national projects geared towards creating a better understanding of the North Sea ecosystem.In the Netherlands, two departments (Water management and Nature and fisheries management) from the Ministry of Transport, Public works and Water Management (V&W) and the Ministry of Agriculture, Nature and Fisheries Management (LNV) agreed to integrate their policies for the management of the Netherlands section of the North Sea. The results from these projects (started in 1996) should enable the policy makers and managers to formulate policies that could lead to the creation of a balance between the effects of human activities and the preservation of the natural qualities of the North Sea ecosystem.One of the projects is strictly designed to develop ecological indicators for the Dutch part of the North Sea. These indicators are based on various monitoring data and related in one way or another with human activities. In this way it is possible to evaluate the effects of human impacts on the ecosystem. Long-term changes in the ecological performance of the ecosystem can be elucidated by the use of the ecological indicators. Thereby making it possible for the policy makers and the managers of the NCP to evaluate the effects of their policies and management recommendations on the quality of the ecosystem.This paper describes the process and the steps taken by the authorities in the countries around the North Sea to develop a set of ecological indicators for the management of the North Sea ecosystem. Focus will be made on the efforts made in the Netherlands to develop indicators that did provide the basis for the development of Water and Nature conservation policies and new management strategies for maintaining the sustainable use of the Dutch section of the North Sea.     

Foundation engineering for offshore gravity structures

This is the third in a series of three state‐of‐the‐art papers concerning the geotechnical engineering aspects of the installation of gravity platforms in the North Sea. Andresen et al. (1979) discuss the planning and execution of offshore site investigations and the procedures used to obtain soil engineering parameters. Eide et al. (1979) review the details of the different platform designs and the installation procedures for the thirteen gravity structures that so far have been placed in the North Sea in 70 m to 153 m of water. In this paper, the foundation engineering for gravity structures installed in the North Sea is reviewed including recommended analyses of permanent and cyclic foundation deformations, dynamic behavior, foundation stability, skirt penetration resistance and base contact stresses, potential piping and erosion problems, and stresses in oil wells and risers. Field performance observations are reported, and it is concluded that gravity structures may safely be installed in offshore areas with less favorable platform foundation conditions than so far encountered in the North Sea.     

Soil parameters used for design of gravity platforms in the North Sea

The foundation soils for most of the North Sea gravity platforms installed so far have been very stiff over-consolidated clays and dense sand. Two actual platform sites are described that are representative of these soil conditions. It is shown how the design soil parameters required for a geotechnical analysis of the platform foundation have been selected for these two sites. The results of in situ testing, laboratory tests and experience from existing platforms are used, with the main emphasis placed on procedures used at the Norwegian Geotechnical Institute.     

On the joint distribution of heights and periods of sea waves

In the design of offshore structures, a consistent method of choosing the height and period for the adopted design wave may be crucial for a reliable design. With this objective, the joint probability density function for the height and period is considered in this paper. At first the adequacy of a simple theoretical model for the joint distribution under stationary conditions is investigated, using measurements achieved during storms in Northern North Sea. In a slightly modified form the model is found to be of a reasonable accuracy as far as the highest waves are of interest. Design curves regarding simultaneous values of heights and periods are estimated by means of this model and their possible impact for design is discussed.     

北黄海盆地区域地质特征

北黄海盆地位于黄海的北部，由于勘探程度低，研究投入少，使得它成为我国迄今为止尚未发现油气的惟一海区。随着我国在南黄海盆地和朝鲜在北黄海盆地东部发现油气，如何实现北黄海盆地油气突破，成为我们研究的焦点。结合该区现有资料和前人研究成果，对北黄海盆地的地球物理场和地质构造特征进行了全面的论述，并在此基础上，对该区油气资源远景作了初步评价。     

Autumn atmospheric preconditioning for interannual variability of wintertime sea-ice in the Okhotsk Sea

Relations in year-to-year variability between wintertime Sea-Ice Concentrations (SICs) in the Okhotsk Sea and atmospheric anomalies consisting of zonal and meridional 1000-hPa wind speeds and 850-hPa air temperatures are studied using a singular value decomposition analysis. It is revealed that the late autumn (October–November) atmospheric conditions strongly influence sea-ice variability from the same season (late autumn) through late winter (February—March), in which sea-ice extent is at its maximum. The autumn atmospheric conditions for the positive sea-ice anomalies exhibit cold air temperature anomalies over the Okhotsk Sea and wind anomalies blowing into the Okhotsk Sea from Siberia. These atmospheric conditions yield anomalous ocean-to-atmosphere heat fluxes and cold sea surface temperature anomalies in the Okhotsk Sea. Hence, these results suggest that the atmospheric conditions affect the sea-ice through heat anomalies stored in sea-ice and oceanic fields. The late autumn atmosphere conditions are related to large 700-hPa geopotential height anomalies over the Bering Sea and northern Eurasia, which are related to a stationary Rossby wave propagation over the North Pacific and that from the North Atlantic to Eurasia, respectively. In addition, the late autumn atmospheric preconditioning also plays an important role in the decreasing trend in the Okhotsk sea-ice extent observed from 1980 to the mid-1990s. Based on the lagged sea-ice response to the late autumn atmosphere, a simple seasonal prediction scheme is proposed for the February–March sea-ice extent using four-month leading atmospheric conditions. This scheme explains 45% of the variance of the Okhotsk sea-ice extent.     

Interannual thermohaline changes in the northern North Atlantic 1991–1996

In the period 1991–1996 the WOCE hydrographic section A1E/AR7E between Greenland and Ireland was repeated five times. The observed thermohaline changes altered the baroclinic structure along the eastern margin of the subpolar gyre significantly. Between June 1995 and August 1996 an overall increase of the temperature and thickness and a decrease of the density of the Subpolar Mode Water (SPMW) layer were observed, accompanied by an increase of its salinity east of the Reykjanes Ridge and a decrease of its salinity in the Irminger Sea. The changes were most pronounced in the Iceland Basin, where the Subarctic Front retreated westwards, coinciding with a strong weakening of the Westerlies as determined by the North Atlantic Oscillation. They are related to a local reduction of the Ekman upwelling and the ocean-to-atmosphere heat flux on the one hand and to the advection of anomalies from the subtropics on the other hand.The eastward spreading of the different Labrador Sea Water (LSW) vintages led to a corresponding cooling of the LSW in the Irminger Sea and in the Iceland Basin in the period 1991–1996. The renewal of the LSW in the Rockall Trough occurred more sporadically, indicating that the North Atlantic Current (NAC) impedes the southward spreading of LSW in the eastern Atlantic. The changes in 1996 seem to have also counteracted this spreading.     

Labrador Sea convection and subpolar North Atlantic Deep Water export in the SODA assimilation model

Labrador Sea convection was most intense and reached the greatest depths in the early 1990s, followed by weaker, shallower, and more variable convection after 1995. The Simple Ocean Data Assimilation (SODA) version 2.0.2/2.0.4 assimilation model is used to explore convective activity in the North Atlantic Ocean for the period from 1992 to 2007. Hydrographic conditions, which are relatively well observed during this period, are used to compare modeled and observed winter mixed-layer depths and water mass anomalies in relation to Deep Western Boundary Current transports and meridional overturning circulation (MOC) changes at the exit of the subpolar basin. The assimilation differs markedly from local observations in the March mixed-layer depth, which represents deep convection and water mass transformation. However, mean MOC rates at the exit of the subpolar gyre, forced by stratification in the mid-latitudes, are similar to estimates based on observations and show no significant decrease during the 1992–2007 period. SODA reproduces the deep Labrador Sea Water formation in the western North Atlantic without any clear indication of significant formation in the Irminger Sea while the lighter upper Labrador Sea Water density range is reached in the Irminger Sea in the 1990s, in agreement with existing assumptions of deep convection in the Irminger Sea and also supported by computed lag correlations with the Labrador Sea. Deep Water transformation mainly takes place in the eastern North Atlantic. The introduction of CFC-11 into the SODA model as a tracer reproduces the mean and multiyear variations of observed distributions.     

The Decline of the North Sea Whelk (Buccinum undatum L.) Between 1970 and 1990: A Natural or a Human-Induced Event?

Abstract. Recent surveys in the southern and central North Sea and the Dutch Wadden Sea revealed that the whelk Buccinun undarum L. can no longer be found in areas where whelks were abundant twenty or more years ago.
The decline of whelk populations tends to coincide with areas of various, intense human activities, which may cause either pollution or physical damage. Potential causes for the decline of whelks from the southern North Sea and coastal areas are discussed and considered in view of North Sea ecology and protective policies for the North Sea.     

The Aleutian Low, storm tracks, and winter climate variability in the Bering Sea

Previous studies have found inconsistent results regarding how wintertime conditions in the Bering Sea relate to variations in the North Pacific climate system. This problem is addressed through analysis of data from the NCEP/NCAR Reanalysis for the period 1950–2003. Composite patterns of sea-level pressure, 500 hPa geopotential heights, storm tracks and surface air temperature are presented for four situations: periods of strong Aleutian Low, weak Aleutian Low, warm Bering Sea air temperatures, and cold Bering Sea air temperatures. Winter temperatures in the Bering Sea are only marginally related to the strength of the Aleutian Low, and are much more sensitive to the position of the Aleutian Low and to variations in storm tracks. In particular, relatively warm temperatures are associated with either an enhanced storm track off the coast of Siberia, and hence anomalous southerly low-level flow, or an enhanced storm track entering the eastern Bering Sea from the southeast. These latter storms do not systematically affect the mean meridional winds, but rather serve to transport mild air of maritime origin over the Bering Sea. The leading indices for the North Pacific, such as the NP and PNA, are more representative of the patterns of tropospheric circulation and storm track anomalies associated with the strength of the Aleutian Low than patterns associated with warm and cold wintertime conditions in the Bering Sea.     

Long-term simulation of the eutrophication of the North Sea: temporal development of nutrients, chlorophyll and primary production in comparison to observations

The ecosystem model ERSEM II has been used to hindcast the development of the ecosystem of the North Sea during the years 1955 to 1993. The simulation was driven by the box-aggregated output from a general circulation model of the North Sea of corresponding duration; radiation, river inputs, atmospheric input and boundary conditions at the borders to the Atlantic Ocean and to the Baltic Sea were applied as realistically as possible. The general features of the eutrophication process are reproduced in the hindcast: the coastal areas show strong changes in nutrient concentrations in the hindcast as well as in the observations. Eutrophication not only shows up in the nutrient concentrations, but also in primary production. The simulated spatial distributions of phosphate, nitrate and primary production compare well with the observed ones. In addition, the hindcast simulates considerable trend-like changes of the nutrients in the southern part of the North Sea, where the nutrients are transported from the continental coastal strip to the southern central North Sea. The line from the river Humber to southern Norway separates the region of noticeable anthropogenic influence of riverine and atmospheric input from the northern area, which is mainly influenced by the Atlantic nutrient inflow. The observed annual cycles in the central and northern North Sea are quite well reproduced by the hindcast. The comparison of the hindcast with the long-term observations at two sites in the continental coastal zone of the North Sea shows that the long-term behaviour of phosphate, nitrate and silicate is simulated well. Primary production is increased in summers during the main period of eutrophication, 1975 to 1989, in the hindcast and in the observations. The flagellates at Helgoland, however, experience much more pronounced annual cycles with much less interannual variability in the hindcast than in the observations.