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The effect of marine growth dynamics in offshore wind turbine support structures
... The effect of marine growth on the dynamics of offshore wind support structures has been studied by many researchers. In [94], for example, special focus was given to the effects of the zonation and thickness of marine growth on the mode shape and natural and bucking frequencies of the supporting platform. In order to estimate the wave force, the drag and inertia coefficient were calculated based only on offshore standards and guidelines [95,96]. ...
There is a need for new numerical tools to capture the physics of floating offshore wind turbines (FOWTs) more accurately to refine engineering designs and reduce costs. The conventional measurement apparatuses in tank tests, including wave probes, velocity and current profilers, and Doppler sensors, are unable to provide a full 3D picture of velocity, pressure, turbulence, and vorticity profile. In tank tests, use of the underwater stereoscopic particle image velocimetry (SPIV) method to fully characterise the 3D flow field around floating wind platforms can overcome some of the limitations associated with classical measurement techniques and provide a rich source of validation data to advance high-fidelity numerical tools. The underwater SPIV method has been widely used for marine and offshore applications, including ship and propeller wakes, wave dynamics, and tidal stream turbines; however, to date, this technology has not seen widespread use for the hydrodynamic study of FOWTs. This paper provides a critical review of the suitability of underwater SPIV for analysing the hydrodynamics of FOWTs, reviews the challenges of using the method for FOWT tank test applications, and discusses the contributions the method can make to mitigating current research gaps in FOWT tank tests.
... One of the main implications is that marine biofouling drastically lowers the lifespan of these systems (Yang et al., 2017). The colonization of renewable energy structures such as wind turbines and wave energy converters, impacts various engineering parameters which in turn affects the overall functionality and efficiency of these systems (Shi et al., 2012;Martinez-Luengo et al., 2017;Jahjouh, 2020;Arcigni et al., 2021). It is also essential that renewable energy structures are engineered to account for the additional weight from biofouling, however, this weight can be challenging to predict. ...
Biofouling in the marine environment refers to an unwanted build-up of marine organisms on subsea surfaces including harbor docks, hulls of ships and offshore installations. The first stage of marine fouling occurs as a microbial biofilm which forms via the aggregation of bacterial, algal, and fungal cells. This biofilm provides a favorable substrate for the larval settlement of larger organisms such as mussels, barnacles and hard corals which accumulate to uncontrollable extents, causing issues for the maritime industries. Since the ban of tributyltin (TBT) in 2008 by the International Maritime Organisation, alternative antifouling agents have been used such as algaecides and copper-based coatings. Recent studies are showing that these can accumulate in the marine environment and have toxic effects against non-target species. Marine microbes and invertebrates are known to be prolific producers of bioactive molecules, including antifouling active compounds. These compounds are often produced by marine organisms as a means of chemical defense to deter predators and prevent fouling of their own surfaces, making them a promising source of new antifouling agents. This article discusses the effects of biofouling on the maritime industries, the environmental dangers of currently used antifouling compounds and why natural products from marine organisms could be a source of environmentally friendly antifouling agents.
... Damage to the turbine structure is most often caused by moisture absorption, fatigue, wind gusts, thermal stress, corrosion, fire and lightning strikes (Martinez-Luengo et al. [42]). Environmental impacts, such as Wave and current forces and biofouling, corrosion and scour affect the lifetime of the turbine support structure (Martinez-Luengo et al. [43], Klijnstra et al. [35]). Multiple distributed monitoring systems, such as the system suggested by Wu et al. [63], can measure several aspects of the structural health simultaneously using conventional 1/4 bridge single dimension strain gauges. ...
Expanding reliance on offshore wind as a renewable energy source requires a thorough consideration of the role of remote monitoring in improving operations & maintenance of offshore wind turbines to reduce costs, increase reliability of access and improve health and safety for repair workers. Real-time collection, transfer and analysis of data relating to the sea state, and monitoring the integrity of the wind turbine structure and its individual systems and components have significant potential to reduce the Levelised Cost Of Electricity (LCOE). Advances in computational capabilities and the increasing connection of sensor networks through the Internet of Things (IoT) have allowed for an expansion in the use Wireless Sensor Networks (WSNs), capable of monitoring the condition of individual components of a wind turbine, such as temperature and vibrations or the system as a whole. This research provides a review of past experience installing WSNs to monitor several aspects influencing offshore wind energy, such as the turbine structure, components and local environment and discusses WSN technology and computing requirements. Although the experience in installing and utilising WSNs is extensive, there is a lack of coordination and standardisation for WSN systems in offshore locations. This paper seeks to clearly define the steps to follow when setting up a WSN connected by IoT, based on an example gas turbine from oil & gas and introduces suggested guidelines for implementing these systems.
... An OWF is an artificial reef during its lifetime as marine organisms colonize it. This can be observed in the biofouling of buoys (Smyth et al. 2015;Tao et al. 2000), functioning communities around shipwrecks and oil rigs (Church 2007;Zintzen et al. 2006), and growth of the epibiota, such as mussels and barnacles, on man-made structures as well as natural materials (Martinez-Luengo et al. 2017;Smyth et al. 2015). There is extensive research regarding the process of habitat colonization in the marine environment. ...
With many offshore wind turbines (OWTs) approaching the end of their estimated service life, there is an increasing demand for developing and evaluating end-of-life strategies that can maximize these assets’ value while at the same time satisfying the requirements of the stakeholders involved. This study aims to perform a detailed review and develop a framework that will consider multiple criteria in the decision-making process, presenting and discussing available technologies and strategies, as well as influencing factors such as schedule, cost and environmental impact. Service life extension, repowering and decommissioning are included in this review as the main end-of-life strategies considered by asset owners, and these are translated into four processes that are applicable to offshore wind farms through a generic decision tree. A SWOT analysis is also conducted which aims to compare the different characteristics of the proposed processes. The factors contributing to the uncertainty of the processes as well as lessons learnt from the oil & gas industry are also discussed.
... Marine growth affects the mass as well as the surface roughness height, thereby affecting structural mass as well as damping due to changed fluid dynamics [48][49][50]. As with corrosion, the values are set based on depth. ...
The modal response of a four-legged jacket structure to damages are explored and resulting considerations for damage detection are discussed. A finite element model of the Wikinger (Iberdrola) jacket structure is used to investigate damage detection. Damages, such as cracks, scour, corrosion and more, are modelled in a simulation environment. The resulting modal parameters are calculated, these parameters are compared to those from an unaltered structure and metrics are calculated including frequency change, modal assurance criterion and modal flexibility. A highly detailed design-model is used to conduct a sensitivity study on modal parameters for a range of changes. By conducting this on the same structure, this acts as a useful reference for those interested in the dynamic response of offshore wind jacket structures. Additionally, this paper addresses the issue of changes in mode parameters resulting from turbine yaw. This paper also considers the challenge of mode-swapping in semi-symmetric structures and proposes several approaches for addressing this. Damage typically results in a reduction of frequency and change in mode shapes, but in ways which can be distinguished from other structural changes, given the extent of this model. These findings are important considerations for modal-based damage detection of offshore wind support structures.
... Marine growth survey: The aim of this inspection is to estimate the coverage, thickness and type of marine growth colonisation on the monopile and sacrificial anodes and to compare its thickness against the one assumed in the design basis. Loading issues that could potentially arise from a significant deviation between the measurements and the design assumptions must be established [54]. Any marine growth formations on structural parts accessed by personnel, i.e., boat landings and access ladders, must be removed. ...
This paper investigates how the implementation of Structural Health Monitoring Systems (SHMS) in the support structure (SS) of offshore wind turbines (OWT) affects capital expenditure (CAPEX) and operational expenditure (OPEX) of offshore wind farms (WF). In order to determine the added value of Structural Health Monitoring (SHM), the balance between the reduction in OPEX and the increase in CAPEX is evaluated. In this paper, guidelines for SHM implementation in offshore WF are developed and applied to a baseline scenario. The application of these guidelines consist of a review of present regulations in the United Kingdom and Germany, the development of SHM strategy, where the first stage of the Statistical Pattern Recognition (SPR) paradigm is explored, failure modes that can be monitored are identified, and SHM technologies and sensor distributions within the turbines are described for a baseline scenario. Furthermore, an inspection strategy where the different structural inspections to be carried out above and below water is also developed, together with an inspection plan for the lifetime of the structures, for the aforementioned baseline scenario. Once the guidelines have been followed and the SHM and inspection strategies developed, a cost-benefit analysis is performed on the baseline case (10% instrumented assets) and three other scenarios with 20%, 30% and 50% of instrumented assets. Finally, a sensitivity analysis is conducted to evaluate the effects of SHM hardware cost and the time spent in completing the inspections on OPEX and CAPEX of the WF. The results show that SHM hardware cost increases CAPEX significantly, however this increase is much lower than the reduction in OPEX caused by SHM. The results also show that an increase in the percentage of instrumented assets will reduce OPEX and this reduction is considerably higher than the cost of SHM implementation.
Offshore structures have inherently uncertain structure characteristics, which lead to the uncertainty of barrier levels. Although the uncertainty of barrier levels may affect the uncertainty of structural failures, there is no guidance on how to conduct an analysis due to a lack of efficient methods, particularly related to the random nature of both barrier levels and seismic motions. This article contributes to the development of an efficient method to compute the probability density function (PDF) of the first passage probability of offshore structures with an uncertain barrier level under random seismic motion. By establishing a transformation relationship between the barrier level and the first passage probability through the curve fitting operation, an explicit PDF of the first passage probability is simplified to the product of the transformation relationship and the PDF of the barrier level. Once the transformation relationship is obtained, the proposed method can quickly calculate the PDF of the first passage probability of structures with an arbitrary PDF of the barrier level. In the numerical studies, a monopile offshore wind turbine subjected to two types of random seismic motions is studied. Three kinds of PDFs of the uncertain barrier level are chosen for each case. The high accuracy and efficiency of the proposed method are validated by comparing with Monte Carlo simulations. Numerical studies confirm that a small uncertainty of the barrier level leads to a very large uncertainty of the first passage probability. It indicates that the first passage probability computed by traditional deterministic methods might be far from the true failure probability. The quantitative calculation of the uncertainty of failure probability is recommended in design to improve the safety and economy of structures.
Parametric Finite Element Analysis (FEA) modelling is a powerful design tool often used for offshore wind. It is so effective because key design parameters (KDPs) can be modified directly within the python code, to assess their effect on the structure’s integrity, saving time and resources. A parametric FEA model of offshore wind turbine (OWT) support structures (consisting of monopile (MP), soil-structure interaction, transition piece (TP), grouted connection (GC) and tower) has been developed and validated. Furthermore, the different KDPs that impact on the design and scaling-up of OWT support structures were identified. The aim of the analyses is determining how different geometry variations will affect the structural integrity of the unit and if these could contribute to the turbine’s scale-up by either modifying the structure’s modal properties, improving its structural integrity, or reducing capital expenditure (CAPEX). To do so, three design cases, assessing different KDPs, have been developed and presented. Case A investigated how the TP’s and GC’s length influences the structural integrity. Case B evaluated the effect of size and number of stoppers in the TP, keeping a constant volume of steel; and Case C assessed the structure’s response to scour development. It is expected that this paper will provide useful information in the conceptual design and scale-up of OWT support structures, helping in the understanding of how KDPs can affect not only the structure’s health, but also its CAPEX.
Growth rates, condition indices and shell shapes of Mytilus galloprovincialis, an invasive alien mussel that has colonised the west coast of South Africa, were measured at a series of sites with different wave exposure regimes in 2 regions 500 km apart. Wave action at the sites was determined by maximum wave-force dynamometers. M galloprovincialis grew faster and had higher condition values on exposed than on sheltered shores, probably due to higher food availability at sites with greater water flow. Growth rates and condition values, however, declined at sites experiencing extreme wave action. This resulted in a polynomial relationship between the growth coefficient K, and wave force. Mussels grew slower in the southern region, where food concentrations are lower. The shells of M galloprovincialis tended to be lower and narrower at exposed sites, perhaps reducing the effect of hydrodynamic forces. Mussel shells were thickest on sheltered and extremely exposed shores. The results are discussed in the context of the impact that M galloprovincialis is likely to have on indigenous species. Because M galloprovincialis is scarce and slow-growing at sheltered sites, its competitive effects there are likely to be minimal, whereas on exposed sites its density, growth and condition are highest and its effects will peak, diminishing again at extremely exposed sites.
The rapid worldwide growth of offshore renewable energy production will provide marine organisms with new hard substrate for colonization, thus acting as artificial reefs. The artificial reef effect is important when constructing, for example, scour protections since it can generate an enhanced habitat. Specifically, artificial structures can create increased heterogeneity in the area important for species diversity and density. Offshore energy installations also have the positive side effect as they are a sanctuary area for trawled organisms. Higher survival of fish and bigger fish is an expected outcome that can contribute to a spillover to outer areas. One negative side effect is that invasive species can find new habitats in artificial reefs and thus influence the native habitats and their associated environment negatively. Different scour protections in offshore wind farms can create new habitats compensating for habitat loss by offshore energy installations. These created habitats differ from the lost habitat in species composition substantially. A positive reef effect is dependent on the nature and the location of the reef and the characteristics of the native populations. An increase in surface area of scour protections by using specially designed material can also support the reef effect and its productivity.
We study the mechanics of pull-off of a barnacle adhering to a thin elastic layer which is bonded to a rigid substrate. We address the case of barnacles having acorn shell geometry and hard, calcarious base plates. Pull-off is initiated by the propagation of an interface edge crack between the base plate and the layer. We compute the energy release rate of this crack as it grows along the interface using a finite element method. We also develop an approximate analytical model to interpret our numerical results and to give a closed-form expression for the energy release rate. Our result shows that the resistance of barnacles to interfacial failure arises from a crack-trapping mechanism.
During late spring of 2008 the first 6 windmills of the C-Power windmill park were built on the Thorntonbank, some 30 km off the Belgian coast. Within the coming years, more windmills will be implanted in various windmill parks in a designated area of the Belgian part of the North Sea (BPNS). With the construction of windmills, a new habitat of artificial hard substrate is being introduced in a region mostly characterized by sandy sediments. This will increase the habitat heterogeneity of the region and the effect of the introduction of these hard substrates the so-called reef effect is regarded as the most important change of the original marine environment caused by the construction of windmill farms. A monitoring programme was set up to sample the biofouling on the new hard substrates associated with the windmills. At the moment of sampling, only the sub- and intertidal parts of the turbine foundations, made of concrete, were available for colonisation as the scour protection was not yet fully deployed yet. Six semi-quantitative samples for epibiota were collected in the autumn of 2008. The subtidal samples were taken by scuba divers at four different depths all along the foundation of one of the windmills and a vertical video transect was made. Samples were taken by scraping the fouling organisms from a sampling surface area of 6.3 dm². The scraped material was collected in plastic bags that were sealed and transported to the laboratory for processing. After preservation of the sample, the organisms were indentified and an estimate of their density was made. After about 3½ months, the submersed part of the foundation was already totally and heavily colonised by epibionts and also the intertidal zone was almost completely covered. A clear depth zonation could be observed. A species list was compiled listing 49 species: 1 Protoctista, 4 algae and 44 invertebrates. The vegetation was restricted to the intertidal zone and rather sparsely developed. Only four species of mainly filamentous algae were present: Blidingia minima, Ulva intestinalis, U. Compressa and Bangia fuscopurpurea . A total of 44 invertebrate species was identified in the samples. However, only a few species were really abundant. The most numerous (> 1000 ind/m²) or abundant species were the giant midge Telmatogeton japonicus , the amphipod Jassa herdmani , the barnacle Balanus perforatus and the bryozoan Electra pilosa . All other species were far less abundant with the exception of Phtysica marina , the only caprellid present (100-1000 ind/m²). Taking into account the short (i.e. 4-6 months) period of time available for colonisation of the foundation, the number of 49 spp. is considered high compared to other hard substrata in the Belgian part of the North Sea (BPNS) and included several uncommon species for the Belgian fauna. Four non-indigenous species were found: the slipper limpet Crepidula fornicata, the New Zealand barnacle Elminius modestus , the giant barnacle Megabalanus coccopoma and the giant midge Telmatogeton japonicus . All four species, already known from the area, are opportunists and early colonisers after disturbance, taking advantage of man-made structures and disturbed conditions to settle.
Mytilid bivalves employ a set of threads (the byssus) to attach themselves to both hard and soft substrates. In this study, we measured the mechanical properties of byssal threads from two semi-infaunal mytilids (Geukensia demissa Dillwyn and Modiolus modiolus Linnaeus) and two epifaunal mytilids (Mytilus californianus Conrad and Mytilus edulis Linnaeus). We compared material properties with and without the assumption that changes of length and area during tensile testing are insignificant, demonstrating that previous researchers have overestimated extensibility values by 30% and may also have underestimated strength values. We detected significant differences in thread properties among tested mytilid species, contrary to previous findings. Threads from semi-infaunal species were significantly thinner than those from epifaunal species, perhaps to allow the production of a greater number of threads, which form a dense network within the substrate. Geukensia demissa threads were weaker than those of the other species, and had a significantly lower stiffness at failure. Modiolus modiolus threads were significantly stiffer than M. edulis threads but also significantly less extensible, suggesting a trade-off between stiffness and extensibility. The only thread property that did not show significant differences across species was toughness - even when byssal threads differ in strength or stiffness, they seem to absorb similar amounts of energy per unit volume prior to failure. This study reveals notable differences between the byssal thread properties of different mytilid bivalves and provides a reliable and thorough methodology for future comparative studies.
In light of the deteriorating state of coral reefs worldwide, the need to rehabilitate marine environments has greatly increased. Artificial reefs (ARs) have been suggested as a tool for reef conservation and rehabilitation. Although successions of AR communities have been thoroughly studied, current understanding of the interactions between artificial and natural reefs (NRs) is poor and a fundamental question still to be answered is that of whether AR communities can mimic adjacent NR communities. We suggest three alternative hypotheses: Neighboring ARs and NRs will (1) achieve a similar community structure given sufficient time; (2) be similar only if they possess similar structural features; (3) always differ, regardless of age or structural features. We examined these hypotheses by comparing the community structure on a 119-year old shipwreck to a neighboring NR. Fouling organisms, including stony and soft corals, sponges, tunicates, sea anemones and hydrozoans were recorded and measured along belt transects. The ahermatypic stony coral Tubastrea micrantha dominated vertical AR regions while the soft corals Nephthea sp. and Xenia sp. dominated both artificial and natural horizontal surfaces. Our results support the second hypothesis, indicating that even after a century an AR will mimic its adjacent NR communities only if it possesses structural features similar to those of the natural surroundings. However, if the two differ structurally, their communities will remain distinct.
Offshore wind has been identified as one of the emerging sustainable energy sources in the United Kingdom. Offshore wind turbine support structures are mainly fabricated of welded tubular members, similar to structures used for oil and gas applications, and are exposed to highly dynamic, harsh marine environments. However, their structural details and design requirements are significantly different due to the magnitude and frequency of operational and environmental loadings acting on the support structures. These conditions would significantly affect their structural dynamic response characteristics due to the magnitude of the applied load. This may therefore have some significant effects on the crack growth behaviour and the extent to which corrosion can be associated with damage to the support structures. However, the magnitude of the applied load might depend on turbine size, water depth, soil conditions and type of support structures. It is therefore essential to design wind turbine support structures against prescribed limit states to ensure economical and safe operation. This paper presents a review of corrosion fatigue in offshore structures as regards the effects of seawater, environment and mechanical loading. Existing literature which documents results from previous campaigns is presented, including works referring to oil and gas structures, highlighting the significant difference in the aspects of loading and use of modern fabrication processes, with a view to illustrating the requirements for an update to the existing corrosion fatigue database that will suit offshore wind structures׳ design requirements.
It is reputed in the Oil & Gas industry that marine growth is overestimated in the North Sea. Can we quantify this overestimation to better tackle the challenges marine growth incurs?
This paper provides insight into the intrinsic and diverse nature of marine growth. It documents the biofouling development and the factors that affect it globally. Focus is made on fixed steel platforms in the North Sea approaching the end of their economic lives. A methodology to quantify the weight of marine growth is proposed and illustrated. 28 years after its installation, the weight of marine growth accumulated on Valhall 2/4G jacket located in Southern North Sea is evaluated based on the proposed method. It is confirmed that the current regulations overestimate the weight of marine growth in the North Sea.
A thorough understanding of the interaction of waves and currents with offshore structures has now become a vital factor in the safe and economical design of various offshore technologies. There has been a significant increase in the research efforts to meet this need. Although considerable progress has been made in the offshore industry and in the understanding of the interaction of waves, currents, and wind with ocean structures, most of the available books concentrate only on practical applications without a grounding in the physics. This text integrates an understanding of the physics of ocean-structure interactions with numerous applications. This more complete understanding will allow the engineer and designer to solve problems heretofore not encountered, and to design new and innovative structures. The intent of this book is to serve the needs of future generations of engineers designing more sophisticated structures at ever increasing depths.
An adequate representation of the site-specific wind-wave joint distribution is essential for cost-efficient and reliable designs of offshore wind turbines. Therefore, the wind and wave climates are subjected to a correlation of wind and wave parameters for design purposes. These correlations are often based on a lumping of the directional wave climate and subsequent association of the lumped wave climate to the directional wind climate. Preservation of the hydrodynamic fatigue distribution from the full wave climate is an important aspect in the wind-wave correlation process which requires an adequate consideration of the dynamics from the offshore wind turbine. However, only a few wind-wave correlation methods exist for the fatigue design of offshore wind turbines and none of them take the dynamics of the full structure adequately into account. In this study a new wind-wave correlation method has been developed and introduced. The new method is based on the establishment of damage contour lines which are used to determine the sea-state parameters that ensure simultaneous compliance with damage equivalency criterions at different locations within the offshore wind turbine. This simultaneous damage equivalency throughout the structure together with the straightforward derivation of the corresponding damage equivalent sea-state parameters constitutes the novelty of the presented wind-wave correlation method.
The contribution of foundation damping to offshore wind turbines (OWTs) is not well known, though researchers have back-calculated foundation damping from “rotor-stop” tests after estimating aerodynamic, hydrodynamic, and structural damping with numerical models. Because design guidelines do not currently recommend methods for determining foundation damping, it is typically neglected. This paper investigates the significance of foundation damping on monopile-supported OWTs subjected to extreme storm loading using a linear elastic two-dimensional finite element model. The effect of foundation damping primarily on the first natural frequency of the OWT was considered as OWT behavior is dominated by the first mode under storm loading. A simplified foundation model based on the soil-pile mudline stiffness matrix was used to represent the monopile, hydrodynamic effects were modeled via added hydrodynamic mass, and 1.00% Rayleigh structural damping was assumed. Hysteretic energy loss in the foundation was converted into a viscous, rotational dashpot at the mudline to represent foundation damping. Using the logarithmic decrement method on a finite element free vibration time history, 0.17%-0.28% of critical damping was attributed to foundation damping. Stochastic time history analysis of extreme storm conditions indicated that mudline OWT foundation damping decreases the maximum and standard deviation of mudline moment by 7-9%.
The support structure damping of a 3.6 MW pitch controlled variable speed offshore wind turbine on a monopile foundation is estimated both in standstill conditions and in normal operation. The net substructure damping is identified from the parameters of an exponential curve fitted to the relative maxima of an impulse response caused by a boat impact. The result is used in the verification of the non aerodynamic damping in normal operation for low wind speeds. The auto-correlation function technique for damping estimation of a structure under ambient excitation was validated against the identified damping from the decaying time series. The Enhanced Frequency Domain Decomposition (EFDD) method was applied to the wind turbine response under ambient excitation, for estimation of the damping in normal operation. The aero-servo-hydro-elastic tool HAWC2 is validated with offshore foundation load measurements. The model was tuned to the damping values obtained from the boat impact to match the measured loads. Wind turbulence intensity and wave characteristics used in the simulations are based on site measurements. A flexible soil model is included in the analysis. The importance of the correctly simulated damping in the model is stressed for accurate load prediction. Differences in the identified damping between the model and the wind turbine are detailed and explained. Discrepancies between simulated and measured loads are discussed.
Between November 2000 and May 2002, epibenthos was sampled monthly with a 2-m beam trawl at three stations along a transect from the southern German Bight towards the northeastern part of the Dogger Bank (North Sea) in order to investigate the seasonal variability of the epibenthic communities. The stations were chosen to reflect a gradient in the hydrigraphic regime, organic matter supply, and fishing effort. The epibenthic community of the southern German Bight was characterized by high biomass and abundance, dominated by Asterias rubens and Ophiura albida. In contrast, at the northern stations in the Oyster Ground and at the Dogger Bank, epibenthic biomass and abundance were substantially lower and the dominant species were mainly crustaceans such as Corystes cassivelaunus, Liocarcinus holsatus, and Pagurus bernhardus. In terms of seasonal variability, mean abundance and biomass in the southern German Bight showed highest values in the summer months and lowest values in the winter months. A similar pattern, but less distinct, was observed in the Oyster Ground. But at the Dogger Bank the pattern was different, with highest abundance and biomass values in the winter months. The differences in spatial and temporal patterns are discussed in relation to differences in temperature, thermal stratification and fishing effort at the three study sites.
We predicted that both refuge dimension and growth form would influence settlement and short-term post-settlement success (less than or equal to 7 d) of sessile marine invertebrates that live attached to hard substrata in low energy environments. Individuals with unlimited attachment to the substrata should rapidly be protected by their growth form, thus decreasing their need to settle in refuges and limiting the length of time any locations on heterogeneous substrata act as refuges. Alternatively, organisms with limited attachment to the substrata should remain susceptible to the causes of mortality for a longer time, and as a result should settle in high quality refuges(sites that protect individuals from competitors, predators or physical disturbance events until either a size refuge or reproductive maturity is obtained). Results agreed with these predictions for 4 species of invertebrates examined on both the topographically complex surface of the solitary ascidian Styela plicata (hereafter Styela) and on settlement plates with uniformly spaced roughness elements that mimicked the heights of roughness elements (2.0 and 5.0 mm) found on Styela in Beaufort, North Carolina, USA. On all surfaces, the 2 species with limited attachment to the substrata, Balanus sp. (aclonal, solitary) and Bugula neritina (clonal, arborescent), settled almost exclusively in the location that provided individuals with the best refuge: the crevices formed where the bases of roughness elements intersect with the flat surfaces. Additionally, when roughness elements of various heights were present (Styela, range: 0.6 to 8.8 mm), intermediate size roughness elements (2.0 < x less than or equal to 5.0 mm) were picked over 72% of the time. Settlement locations and locations where survival were enhanced were less consistent for the 2 species with unlimited attachment to the substrata: a clonal, encrusting form (Schizoporella errata) and a clonal stolen-mat form (Tubularia crocea). Fewer individuals of these 2 species settled on roughness elements on Styela and when they did, they were not restricted to the bases of the roughness elements. On the plate surfaces, most settlement did occur in crevices, but both species grew away from this location within days and short-term survival was not consistently greater in this location. Additional trials were run on plates with pits of the same maximum dimensions as the tested roughness elements (2.0 and 5.0 mm depth) to see if crevices and pits provide refuges of equal quality for newly settled individuals. Only survival of Balanus sp. recruits was greatest in both crevices and pits. Evidence for active choice of settlement location comes from consistent results in trials in which some larvae settled in greater numbers on specific size roughness elements on Styela and in areas of high erosion. Overall, these results show that one must be very cautious when generalizing about refuge quality on heterogeneous surfaces, and to determine if a location is a spatial refuge, it is critical to consider: (1) the dimensions oi the larva, (2) the relative dimensions of the individual and potential refuge:location at any point in time from the moment settlement occurred, and (3) the growth form of the individual which is related to its need for protection from biotic and abiotic sources of mortality.
The mussel Mytilus californianus is the dominant competitor for space in the mid-intertidal zone of wave-swept rocky shores in the Pacific Northwest where it forms extensive tightly packed beds. The rate at which patches are formed in these beds, can play an important rôle in community ecology by controlling the establishment and persistence of fugitive species. Despite the biological importance of physical disturbance, the mechanism of patch initiation has not been adequately explained. Battering by logs can create patches, but is the predominant mechanism only on shores near active logging sites. In other areas, it has been speculated that the hydrodynamic forces associated with storm waves somehow cause patches to form. However, the forces acting along the direction of flow — drag and the acceleration reaction — are unlikely to initiate patch formation. Here, it is suggested that fluid-dynamic lift forces imposed on mussel beds by breaking waves are sufficient to dislodge individual mussels and trigger patch formation. Arguments are presented suggesting that the likelihood of dislodgment by lift is consistent with the observed rate of patch formation in the absence of log battering.
The United States Department of Energy (DOE) Wind Energy Research Program has begun a new effort to partner with U.S. industry to develop wind technology that will allow wind systems to compete in regions of low wind speed. The Class 4 and 5 sites targeted by this effort have annual average wind speeds of 5.8 m/s (13 mph), measured at 10 m (33 ft) height. Such sites are abundant in the United States and would increase the land area available for wind energy production twenty-fold. The new program is targeting a levelized cost of energy of 3 cents/kWh at these sites by 2010. A three-element approach has been initiated. These efforts are concept design, component development, and system development. This work builds on previous activities under the WindPACT program and the Next Generation Turbine program. If successful, DOE estimates that his new technology could result in 35 to 45 gigawatts of additional wind capacity being installed by 2020.
Methods to calculate extreme wind speeds are described and reviewed, including ‘classical’ methods based on the generalized extreme value (GEV) distribution and the generalized Pareto distribution (GPD), and approaches designed specifically to deal with short data sets. The emphasis is very much on the needs of users who seek an accurate method to derive extreme wind speeds but are not fully conversant with up-to-date developments in this complex subject area. First, ‘standard’ methods are reviewed: annual maxima, independent storms, r-largest extremes with the GEV distribution, and peak-over-threshold extremes with the GPD. Techniques for calculating the distribution parameters and quantiles are described. There follows a discussion of the factors which must be considered in order to fulfil the criterion that the data should be independent and identically distributed, and in order to minimize standard errors. It is commonplace in studies of extreme wind speeds that the time series available for analysis are very short. Finally, therefore, the paper deals with techniques applicable to data sets as short as two years, including simulation modelling and methods based on the parameters of the parent distribution. Copyright © 1999 Royal Meteorological Society
Fouling was investigated on Marathon Kinsale Field Alpha and Bravo platforms in the Celtic Sea between June 1978 and June 1981. In shallow depths, algae dominated, chiefly Polysiphonia brodiaei and Ulva lactuca. Mussels formed the dominant fouling organism between 6 and 20 m depth, below which were zones of Metridium senile and Alcyonium digitatum, serpulids and the deep water barnacle Balanus hameri. In September 1979 mussels exceeded 2500 m−2 with a modal length of 42 mm. By June 1981, modal length had increased to 67 mm at −4 m and 73 mm at −18 m (maximum size 97 mm). Populations on Bravo were similar. Comparison is made with growth rates on North Sea platforms. On Alpha, percentage cover m−2 in March 1980 was much greater at −4 m than at −18 m, but mean thickness was similar. At −18 m mussels were a heavier fouler on Bravo than Alpha. On Alpha mussel weight did not show a linear relationship with percentage cover.
Canopy-forming algae occur across of range of energy environments (i.e., wave sheltered to exposed coasts) where disturbances are frequent (i.e., gap formation) and benthic patterns largely reflect variation in post-disturbance processes. Disturbances vary in extent (area affected) and intensity (degree of damage), and this may affect recolonisation at local scales. On an open oceanic coast, we tested whether habitat structure (patches of canopy algae) differed between heavy and relatively lighter wave exposure (sheltered vs. exposed sides of islands), and whether wave exposure affected the response of prominent habitat-formers to varying disturbance regimes (different sizes of partial and complete canopy removal). Observations of naturally occurring patterns showed sheltered coasts to be characterised by small patches of fucoids, whereas exposed coasts were characterised by large patches of kelp. Canopy-gaps were larger at exposed than sheltered coasts, and mixed canopies constituted > 24% of the subtidal rocky habitat independently of wave exposure. Experimental disturbances showed the local density of kelps to affect recovery through greater recruitment to partial clearings (80% canopy removal). Fucalean algae, on the other hand, mainly recruited into complete clearings (100% removal), but when their recruits were abundant, they also recruited into partial clearings. The covers of filamentous, turf-forming algae increased in all clearings, and more so at exposed than sheltered sites. Extent of disturbance had no detectable effect on recolonisation by canopy-forming algae across the scales examined (i.e., 1.5 m, 3 m diameter loss of canopy). Recolonisation varied among islands kilometres apart, and correlations (r > 0.85) between cover of canopies and cover of their recruits in clearings at the scale of sites, suggested that differences in propagule supply could account for variation in patterns of recolonisation at scales of kilometres. There was no evidence to suggest that the effect of disturbance depended on wave exposure within the range of exposures tested in this study (i.e. open coasts). We recognise that wave exposure can be fundamental to habitat structure of subtidal rocky coasts, but we suggest that its influence may be mediated by the biological setting (e.g., canopy composition).
The effects of selected physical and biological factors on the early development of a subtidal invertebrate assemblage were examined at an offshore oil platform in the Santa Barbara Channel (California, USA). The effects of date, year, length, and depth of submersion were investigated by replacing sets of ceramic tiles with new tiles at frequencies of 2, 4, 6, 12, and 24 months at three depths (6, 12, and 18 m) over a period of 24 months. The effects of existing colonists and depth were explored in a second experiment by removing selected early colonists from ceramic tiles deployed at the same three depths over a period of 12 months. More than 40 invertebrate taxa from seven phyla colonized the tiles. Colonial tunicates and encrusting bryozoans appeared early in the successional sequence (∼2 months), in cover ranging from <5% to 80% and from <5% to 55%, respectively. Tubiculous amphipods, barnacles, and sponges could also appear early, but in low cover (<20%). Composition of the assemblage changed over time with barnacles, sponges, and mussels becoming the principal space holders on tiles submerged for 24 months. When potential competitors were removed monthly, variation in the cover of dominant taxa (i.e., bryozoans, amphipods, barnacles) was maintained to 12 months, suggesting that other factors, such as larval availability or post-settlement mortality, were responsible for these patterns. Development of this assemblage appeared to fit a pattern of early succession that was largely predictable in terms of the composition and sequence of occurrence of dominant taxa, but variable in rate of development, depending on date of submersion, year, and depth.
The microstructure and chemical composition of the calcite shell of the sea barnacle Tetraclita rufotincta (Pilsbry, 1916) were investigated using microscopic and analytical methods. The barnacle shell was separated mechanically into its three substructural units: outer, interior, and inner layers. The organic matrices of these structural parts were further separated into soluble and insoluble constituents and their characteristic functional groups were studied by FTIR. Investigation of the mechanical properties of the interior mass of the shell reveals remarkable viscoelastic behavior. In general, the mechanical behavior of the shell is a function of its geometry as well as of the material, of which it is constructed. In the case of T. rufotincta, as calcite is a brittle material, the elastic behavior of the shell is apparently related to its micro- and macroarchitecture. The latter enables the shell to fulfill its primary function which is to protect the organism from a hostile environment and enables its survival. Our detailed identification of the similarities and differences between the various structural components of the shell in regard to the composition and properties of the organic component will hopefully throw light on the role of organic matrices in biomineralization processes.
A 2-year study was conducted in Hong Kong to examine the effects of substratum, season and length of submersion on the development of a subtidal epibiotic community using four types of settlement panels (concrete, steel, wood and tyre). The season and length of submersion had a strong influence on the total biomass and on community structure while the type of substratum had very little impact on the total biomass or the structure of the epibiotic community. The season of submersion determined the species composition of the newly submerged surfaces. In the spring and summer, tubeworms were the most abundant. In the autumn and winter, barnacles and tunicates dominated. Community succession was not obvious in the first year of submersion as it was intermingled with strong seasonal settlement, growth and death of barnacles and tunicates. In the second year of submersion, green mussels and tunicates settled and grew to occupy most of the panel surfaces, forming an assemblage that was characteristic of climax communities in the local subtidal waters. The results suggest that the type of construction material has limited impact on the development of epibiotic communities on artificial reefs deployed in Hong Kong; the season of submersion may affect community structure in the early successional stage, but not the characteristics of the climax communities. This study indicates that the type of substratum should not be of concern when deploying artificial reefs in the subtidal waters in this region. The design of artificial reefs should focus more on other physical and economical aspects such as durability, flow dynamics, stability, cost, and effects on the ambient environment.
Investigations of major accidents show that technical, human, operational, as well as organisational factors influence the accident sequences. In spite of these facts, quantitative risk analyses of offshore oil and gas production platforms have focused on technical safety systems. This paper presents a method (called BORA-Release) for qualitative and quantitative risk analysis of the platform specific hydrocarbon release frequency. By using BORA-Release it is possible to analyse the effect of safety barriers introduced to prevent hydrocarbon releases, and how platform specific conditions of technical, human, operational, and organisational risk influencing factors influence the barrier performance. BORA-Release comprises the following main steps: (1) development of a basic risk model including release scenarios, (2) modelling the performance of safety barriers, (3) assignment of industry average probabilities/frequencies and risk quantification based on these probabilities/frequencies, (4) development of risk influence diagrams, (5) scoring of risk influencing factors, (6) weighting of risk influencing factors, (7) adjustment of industry average probabilities/frequencies, and (8) recalculation of the risk in order to determine the platform specific risk related to hydrocarbon release. The various steps in BORA-Release are presented and discussed. Part II of the paper presents results from a case study where BORA-Release is applied.
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