On the biological connectivity of oil and gas platforms in the North Sea

Tests are made of the conjecture that the platforms in the North Sea are biologically connected, with organisms originating from some platforms reaching and substantially augmenting those at others so that, together, the platforms effectively form a sort of artificial reef. The M₂ tide results in a relatively rapid transfer of organisms between neighbouring platforms. Some 60% of platforms in the southern UK Sector are directly connected by tidal flows. Such connection in the northern Sector is relatively rare, about 23% of platforms being connected. Mean flows connect platforms in ‘strings’ sharing a common streamline spread by turbulent dispersion. Metrics are devised to indicate how well a particular platform is connected to others. Strings are broken when contributions to the concentration of organisms from platforms fall below detection limits. Many platforms are likely to be connected in strings in the southern UK Sector, but relatively few in the northern Sector.     

Modelling the relative dominance of wave erosion and weathering processes in shore platform development in micro‐ to mega‐tidal settings

In this paper we use a numerical model to explore the relative dominance of two main processes in shore platform development: wave erosion; weathering due to wetting and drying. The modelling approach differs from previous work in several aspects, including: the way that it accounts for weathering arising from gradual surficial intertidal rock degradation; subtidal profile shape development; and the consideration of a broad erosion parameter space in which, at either end of the erosion spectrum, shore platform profiles are produced by waves or weathering alone. Results show that in micro‐tidal settings, wave erosion dominates the evolution of (i) shore platforms that become largely subtidal and (ii) sub‐horizontal shore platforms that have a receding seaward edge. Weathering processes dominate the evolution of sub‐horizontal shore platforms with a stable seaward edge. In contrast, sloping shore platforms in mega‐tidal settings are produced across the full range of the process‐dominance spectrum depending on the how the erosional efficacy of wave erosion and weathering are parameterized. Morphological feedbacks control the process‐dominance. In small tidal environments wave processes are strongly controlled by the presence/absence of an abrupt seaward edge, but this influence is much smaller in large tidal environments due to larger water depths particularly at high tides. In large tidal environments, similar shore platform profile geometries can be produced by either wave‐dominant or weathering‐dominant process regimes. Equifinality in shore platform development has been noted in other studies, but mainly in the context of smaller‐scale (centimetre to metre) erosion features. Here we draw attention to geomorphic equifinality at the scale of the shore platform itself. Progress requires a greater understanding of the actual mechanics of the process regimes operating on shore platforms. However, this paper makes a substantial contribution to the debate by identifying the physical conditions that allow clear statements about process dominance. © 2018 John Wiley & Sons, Ltd.     

基于赤平投影法的西源岭边坡稳定性分析

通过对西源岭320、350、380、410m4个台阶边坡的优势结构面进行赤平投影分析,得到410平台有5种组合情况、380平台有1种组合情况、350平台有3种组合情况、320平台有1种组合情况可能发生楔形体破坏,以及410平台1处、350平台1处、320平台1处可能发生单平面破坏。然后,运用刚体极限平衡方法对各种实际的潜在破坏面进行天然及暴雨2种工况下的稳定性分析,得到了380平台F380-01与F380-02所控制的楔形体破坏在天然状态和暴雨状态下小于安全系数、350平台F350-03与F350-04控制的楔形体破坏以及320平台F320-11所控制的单平面破坏在天然工况下大于安全性系数、暴雨工况下小于安全性系数的结论。对比分析可知,赤平投影法可以很快速地搜索到可能滑动的结构面组合方式,但是要判断实际结构面组合是否滑动还需要进行定量分析。     

The formation of beaches on shore platforms in microtidal environments

Beaches are common features of many rocky shorelines and can be considered to be constrained by the underlying geology. In mesotidal to macrotidal areas the slope of the substrate and sediment supply are the primary factors in constraining the size and development of beaches on shore platforms. In microtidal settings it is not known if these factors are wholly responsible for determining the presence of beaches on shore platforms, nor the contribution of other factors such as hydrodynamics. The microtidal coast of Victoria, Australia, is surveyed in this study in order to quantify the morphological boundary conditions that constrain beach development on semi‐horizontal shore platforms. An ample sediment supply indicates that the underlying geology is controlling the presence and absence of beaches. Where beaches occur they always overlie a rock ramp which is the steepest part of the platform. The two most important morphological constraints were platform width and height both of which significantly correlated with beach volume. An elevational threshold exists at just over +1.0 m where beaches cannot accumulate. Below this threshold, platform width appears to be the principle constraining factor in beach accumulation. An evolutionary model is inferred which suggests that dissipation of wave energy associated with platform widening plays an important role in allowing beaches to accumulate. The model suggests beaches on platforms will be particularly sensitive to sea level rise. Copyright © 2014 John Wiley & Sons, Ltd.     

Parametric earthquake response of torsionally coupled buildings with foundation interaction

A study is made of the effect of soil-structure interaction on the coupled lateral and torsional responses of asymmetric buildings subjected to a series of historical free-field earthquake base motions. It sh shown that for particular classes of actual buildings the equivalent rigid-base responses are significantly increased for structures founded on medium-stiff soils, and hence the assumption of the major building codes that a conservative estimate of response is obtained by considering the structure to be fixed rigidly at its base is shown to be inconsistent with the presented dynamic results. It is shown that foundation interaction produces greatest amplification of torsional coupling effects for structures subjected to a particular class of European strong-motion earthquake records, identified by similarities in their spectral shape, for which the vibrational energy of the ground motion is distributed approximately uniformly over the range of frequencies which are of interest for real structures. It is recommended that provision be made in the torsional design procedures of building codes for the increase in the coupled torsional response due to soil-structure interaction as indicated in this study. Such provision should be based on the results of comprehensive parametric studies employing a wide selection of earthquake records and accounting for expected variations in localized soil conditions.     

地震-台风耦合作用下近海导管架平台动力响应分析

针对地震-台风耦合作用下的近海导管架海洋平台,运用Morison方程将台风对导管架平台的拖曳力及波浪对导管架平台的拖曳力和惯性力施加在结构上,并在基底施加地震动,建立地震-台风耦合作用下的运动方程。通过模态分析,确定结构的基本自振频率,进而选取卓越频率与该频率较为接近的海底地震动进行输入。对通过数值模型计算得到的导管架平台动力响应,参考相关文献中的限值,对耦合作用下的平台进行安全评估,给出了近海导管架海洋平台在地震-台风耦合作用下的损伤状态评定标准。本文关于导管架平台动力响应的统计结果,对导管架结构性态设计具有一定参考意义。     

Lacustrine shore platforms at Lake Waikaremoana,North Island,New Zealand

This paper examines the morphology and processes governing the development of shore platforms at Lake Waikaremoana, North Island, New Zealand. Shore platforms at Lake Waikaremoana are recent features, and were formed when a new sequence of shoreline development was initiated, due to lowering of the lake by 5 m in 1946 for hydroelectric power development. Three predominant platform morphologies were identified around the lake. These include gently sloping platforms (c.1·5 to 3·9°), ramp platforms (c.6·8 to 9·2°), and concave ramp platforms (c.7·9 to 12°). Platform widths ranged from 11 to 31 m, with the gently sloping platforms characterized by the widest morphologies. Erosion rates were estimated using perched sandstone boulders and were found to range from 3·4 to 12·5 mm a^?1, with a mean erosion rate of 5·9 mm a^?1. Higher rates of erosion were identified at lower platform elevations, due to a greater frequency of wetting and drying cycles coincident with storm waves, while lower erosion rates were identified at higher elevations. Field evidence suggests that shore platforms at Lake Waikaremoana were likely initiated and continue to develop as a result of subaerial wetting and drying cycles. Waves, coincident with fluctuating lake levels, play an important role by removing the weathered material from the platforms, and appear to control the width of the platforms. A conceptual model of platform development is presented, and analogies are drawn between this model, and the formation of shore platforms in oceanic environments. Copyright © 2002 John Wiley & Sons, Ltd.     

On tuned mass dampers for reducing the seismic response of structures

This paper presents an energy‐based theoretical model for a two degree‐of‐freedom mechanical system. After a general formulation in Appendix A, the model is specialized to study tuned mass dampers as a means to substantially increase modal damping in order to induce a consequential decrease of the seismic response of the structures thus provided. Although approximate since it neglects coupling due to damping, it is shown that the model yields a first‐order approximation to the exact frequencies, providing values of optimum damping that closely match exact results proposed by others. In view of this, it is proposed that the model be applied through an iterative numerical procedure that identifies the pertinent optimum parameters. It is also shown that for certain particular benchmark cases the model provides closed‐form equations for the parameters defining the dynamic states related to these special conditions. Despite its approximate nature the model presented in this paper is rational, and due to its explicit consideration of energy balance and overall simplicity, it provides a convenient platform for the study of tuned mass dampers, as well as for other methods of structural passive control. Copyright © 2005 John Wiley & Sons, Ltd.     

多筒型基础平台结构基础单体刚度分析方法

本文针对多筒型基础平台结构复杂的基础地基边界条件，提出了一种筒型基础边界条件简化方法——筒型基础单体刚度分析法。以位于渤海湾锦州地区的JZ9—3筒型基础系缆平台为对象，引进筒型基础单体刚度，对平台整体结构进行了的动力响应分析。将分析结果与实测结果进行对比，得知该分析方法是可行的。     

海洋石油平台的工程地震问题

对于海洋石油平台,结构抗震设计要求提供地震动峰值、反应谱、持续时间以及速度时程等参数,由于海洋石油平台的结构和形式的特殊性,以及海域地震环境特征,考虑海洋石油平台地震动作用时要有不同的方法及侧重点,本文对海洋石油平台地震作用分析中涉及的一些工程地震问题研究进行了讨论,包括设防地震动水准、设定地震、地震动衰减关系、长周期地震动及其相关问题。     

某高精密设备平台基础隔振措施分析

结合某高精密设备平台基础隔振实际工程,基于UPFs(User Programmable Features)二次开发特点,提出考虑基础-地基-基础动力相互作用计算基础隔振分析的三维有限元方法。该方法首先将动力机器、周围地基以及高精密设备平台基础作为完整的研究对象,通过引入三维紧支黏弹性边界单元模拟半无限地基辐射阻尼效应和远场介质的弹性恢复性能,并在通用有限元软件ANSYS中实现。进而基于最小二乘方法回归分析得到振动衰减关系曲线,通过其与数值分析结果的对比验证计算模型的合理性。最后针对此工程场地条件建立隔振沟减振隔振措施的有限元模型,并对隔振沟影响因素的减振效果进行分析。计算结果表明,隔振沟的位置、深度以及长度是实际工程中需要考虑的重要因素,对其进行合理的选取可获得较好的隔振效果。     

Hydrodynamic constraints and storm wave characteristics on a sub‐horizontal shore platform

Few studies of wave processes on shore platforms have addressed the hydrodynamic thresholds that control wave transformation and energy dissipation, especially under storm conditions. We present results of a field experiment conducted during a storm on a sub‐horizontal shore platform on the east coast of Auckland, New Zealand. Small (<0.5 m) locally generated waves typically occur at the field site, whereas during the experiment the offshore wave height reached 2.3 m. Our results illustrate the important control that platform morphology has on wave characteristics. At the seaward edge of the platform a scarp abruptly descends beneath low tide level. Wave height immediately seaward of the platform was controlled by the incident conditions, but near the cliff toe wave height on the platform was independent of incident conditions. Results show that a depth threshold at the seaward platform edge > 2.5 times the gravity wave height (0.05–0.33 Hz) is necessary for waves to propagate onto the platform without breaking. On the platform surface the wave height is a direct function of water depth, with limiting maximum wave height to water depth ratios of 0.55 and 0.78 at the centre of the platform and cliff toe, respectively. A relative ‘platform edge submergence’ (water depth/water height ratio) threshold of 1.1 is identified, below which infragravity (<0.05 Hz) wave energy dominates the platform energy spectra, and above which gravity waves are dominant. Infragravity wave height transformation across the platform is governed by the relative platform edge submergence. Finally, the paper describes the first observations of wave setup on a shore platform. During the peak of the storm, wave setup on the platform at low tide (0.21 m) is consistent with measurements from planar sandy beaches, but at higher tidal stages the ratio between incident wave height and maximum setup was lower than expected. Copyright © 2014 John Wiley & Sons, Ltd.     

Feedback–feedforward control of offshore platforms under random waves

This study investigates the control of jacket‐type offshore platforms. The deck displacement of jacket‐type offshore platforms can be controlled using both passive and active control mechanisms. Among the passive control mechanisms, a tuned mass damper concept is studied in this paper. Active control mechanisms considered here include the active mass damper, the active tendon mechanism and the propeller thruster. An optimal frequency domain approach to active control of wave‐excited platforms is used in which the H₂ norm of the transfer function from the external disturbance to the regulated output is minimized. In this study, the hydrodynamic drag force is evaluated using the JONSWAP wave spectrum. Unlike conventional linearization approaches, the influence of non‐linearity in the drag force is retained in this scheme by expressing the non‐linear force components in terms of higher‐order convolutions of the water‐particle velocities. To demonstrate the effectiveness of this scheme, the platform performance with and without control devices under different sea states is evaluated. It is demonstrated that the control devices are useful in reducing the displacement response of jacket‐type offshore platforms, especially when the wave forces are concentrated at frequencies close to the natural frequencies of the platform. This becomes especially significant in deep waters because the natural frequencies of jacket‐type platforms fall closer to the dominant wave frequencies in deep waters. Adding control devices to deep water platforms will ensure a reduction both in the global response of the platform and in localized effects, such as the fatigue of welded joints. Copyright © 2001 John Wiley & Sons, Ltd.     

多筒基础平台冰荷载作用下基础反力分析

本文将边界层模型用于吸力桩(筒型基础)这种特殊的桩体，对位于渤海湾锦州地区的JZ9—3筒型基础系缆平台冰力作用下的土压力进行了计算分析，并将计算结果与实测数据进行了对比。结果表明计算值基本上反映了基础土-筒相互作用的应力状态，边界层模型可以有效地分析吸力桩-土相互作用问题。     

Shore platform morphology on a rapidly uplifting coast,Wellington, New Zealand

The coast of Wellington, New Zealand, is tectonically active and contains a series of uplifted and contemporary shore platforms that are developed in Triassic Greywacke. The platform profiles are rugged with relief of metre scale common. The surveyed platforms were formed at, and at two distinct levels 1–1·5 and 2–2·5 m above, mean sea level. They range in width up to 70 m and are highly fractured with fracture densities in excess of 20[sol ]m² common. The rate of development of these platforms is rapid, with lateral erosion rates of up to 0·15 m[sol ]yr calculated, allowing platform development to occur over centennial scales. Even given this rapid development, continued instantaneous uplift of the coast has meant they are unable to reach an equilibrium state, whereby the effectiveness of wave processes in removing material is reduced by platform extension. The co‐seismic uplift means that the rear of the platforms is raised beyond the limits of marine process and has become an area of deposition. Although no direct process measurements were made the highly fractured nature of the bedrock appears to play a major role in platform evolution, with wave processes being easily able to pluck blocks as evidenced by fresh erosion scars and active gravel beaches at the rear of many platforms. This coast therefore represents an extremely dynamic youthful shore platform environment, where the processes of marine abrasion can be observed over historical timescales. Copyright © 2005 John Wiley & Sons, Ltd.     

插桩过程对临近平台桩基础的影响研究

由于大型自升式钻井船的插桩位置通常距离海上钻井与采油平台较近,桩靴的插入过程可能会对临近平台的桩基础承载力和稳定性产生不利影响。以实际工程为背景,运用球孔扩张理论推导挤土效应产生的水平附加应力大小及其范围;提出一种近似考虑动力挤土效应的拟静力数值模拟方法,分析桩靴下沉到不同深度处时的桩基承载力、桩身应力和最大水平位移,并与静力分析结果进行对比。研究表明,桩靴插入过程对周围土体产生巨大的挤压和扰动作用,使得桩基承载力降低,桩身应力变大,最大水平位移增加;与静力法计算数值相比,由挤土效应导致的单桩承载力下降6%~8%,桩身应力增大30%~80%,桩身最大变形量增长1倍。     

Torsionally coupled dynamic performance analysis of asymmetric offshore platforms subjected to wave and earthquake loadings

The dynamic equations of motion of asymmetric offshore platforms under three different environmental conditions:seismic action,wave action and their combination are established in this paper. In establishing these motion equations,three typical eccentricity types including mass eccentricity,rigidity eccentricity and their combination were considered,as are eccentricities that occur un-idirectionally and bi-directionally. The effects of the eccentricity type,the dynamic characteristics and the environmental conditions on the torsional coupling response of platforms are investigated and compared. An effort has also been made to analyze the inffluence of accidental eccentricity on asymmetric platforms with different eccentricity in two horizontally orthogonal directions. The results are given in terms of non-dimensional parameters,accounting for the uncoupled torsional to lateral frequency ratio. Numerical results reveal that the eccentricity type has a great inffluence on the torsionally coupled response under different environmental conditions. Therefore,it is necessary to consider the combination of earthquake and wave action in the seismic response analysis of some offshore platforms.     

Identifying mechanisms of shore platform erosion using Structure-from-Motion (SfM) photogrammetry

Shore platforms control wave energy transformation which, in turn, controls energy delivery to the cliff toe and nearshore sediment transport. Insight into shore platform erosion rates has conventionally been constrained at millimetre-scales using micro-erosion metres, and at metre-scales using cartographic data. On apparently slowly eroding coasts, such approaches are fundamentally reliant upon long-term observation to capture emergent erosion patterns. Where in practise timescales are short, and where change is either below the resolution or saturates the mode of measurement, the collection of data that enables the identification of the actual mechanisms of erosion is hindered. We developed a method to monitor shore platform erosion at millimetre resolution within metre-scale monitoring plots using Structure-from-Motion photogrammetry. We conducted monthly surveys at 15 0.25 m² sites distributed across the Hartle Loup platform in North Yorkshire, UK, over one year. We derived topographic data at 0.001 m resolution, retaining a vertical precision of change detection of 0.001 m. We captured a mean erosion rate of 0.528 mm yr^-1, but this varied considerably both across the platform and through the year. We characterized the volume and shape of eroded material. The detachment volume–frequency and shape distributions suggest that erosion happens primarily via removal of shale platelets. We identify that the at-a-point erosion rate can be predicted by the distance from the cliff and the tidal level, whereby erosion rates are higher closer to the cliff and at locations of higher tidal duration. The size of individual detachments is controlled by local micro-topography and rock structure, whereby larger detachments are observed on more rough sections of the platform. Faster erosion rates and larger detachments occur in summer months, rather than in more energetic winter conditions. These results have the potential to form the basis of improved models of how platforms erode over both short- and long-timescales. © 2019 John Wiley & Sons, Ltd.     

Lithological control on the elevation of shore platforms in a microtidal setting

The shore platforms on Shag Point, southern New Zealand, are quasi‐horizontal surfaces and are developed between supratidal and low water spring levels. A range of morphologies occur, with more exposed platforms having a distinct low‐tide cliff, in contrast to low‐tide surfaces where the seaward edge is buried beneath rubble and macro‐algal growth. The platforms range in width from 20 to 80 m and are eroded into Late Cretaceous/Early Tertiary fine marine sandstones and mudstones. Shore platforms have formed in two principal lithological units: a homogeneous unit that is characterized by few discontinuities, and a fractured unit with joints spaced about 0·5 m apart. Rock hardness is low in both units (L‐type Schmidt hammer rebound values of 31 ± 4), and there is little systematic variation in values between the two units in which platforms have developed. Case‐hardened concretions within the sandstone are significantly harder than surrounding rock and cause local relief of metre scale as the spherical diagenetic features are eroded from the bedrock. They do not, however, appear to affect broad‐scale platform geometry. Joints within the bedrock are a primary control on platform elevation. Platforms formed in jointed rock occur at the lower portion of the intertidal zone, in contrast to platforms formed in unjointed bedrock, in which horizontal surfaces occur at or above mean high water spring tide level. Rock structure, therefore, appears to be the primary determinant factor of platform geometry at Shag Point. Copyright © 2006 John Wiley & Sons, Ltd.     

Defining shore platform boundaries using airborne laser scan data: a preliminary investigation

As an initial evaluation of the potential of digital elevation models (DEMs) and geographic information systems (GISs) for geomorphic characterization of rocky shorelines, airborne laser scan (ALS) data have been used to characterize shore platforms around Shag Point, southeastern New Zealand. The platforms have been characterized using field‐based techniques in previously published research, and therefore offer an ideal site for evaluation purposes. The main challenge involved the delineation of the shore platform area in terms of landward and seaward extents. The cliff top and landward edge of the shore platform was readily mapped, whereas the seaward edge of platforms was mapped with lesser precision due to difficulties associated with tidal inundation and the interference of wave action and surface water. In the central region of the study area (～0·1 km²) higher platform elevations and dense point cloud data enabled the generation of a high‐resolution (1 m) DEM. In analysing the DEM, ALS offered an advantage over the previous field survey in respect of the ability to assess continuous topography in plan‐view. The extent and form of two distinctive erosional surfaces is clearly apparent and was revealed through classifications based on slope and elevation. The spatial continuity of the upper surface implies that, in addition to the role of rock structure described in previous work, sea level and wave exposure may have been important factors in the generation and preservation of platform morphology at Shag Point. Copyright © 2007 John Wiley & Sons, Ltd.