327 research outputs found

    Assessment of shallow water random wave-induced scour at the trunk section of breakwaters using deep water wind and wave conditions

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    This article provides a simple analytical method giving estimates of random wave-induced scour at the trunk section of vertical-wall and rubble-mound breakwaters in shallow water from deep water wind and wave conditions. Results are exemplified by using a Pierson-Moskowitz model wave spectrum for deep water wind waves with the mean wind speed at the 10 m elevation above the sea surface as the parameter. The significant value of the scour depth within a sea state of random waves is provided and an example typical for field conditions is given. The method should serve as a useful tool for assessing shallow water random wave-induced scour based on input from deep water wind and wave conditions.acceptedVersio

    Time scale for scour beneath pipelines due to long-crested and short-crested nonlinear random waves plus current

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    This article derives the time scale of pipeline scour caused by 2D (long-crested) and 3D (short-crested) nonlinear irregular waves and current for wave-dominant flow. The motivation is to provide a simple engineering tool suitable to use when assessing the time scale of equilibrium pipeline scour for these flow conditions. The method assumes the random wave process to be stationary and narrow banded adopting a distribution of the wave crest height representing 2D and 3D nonlinear irregular waves and a time scale formula for regular waves plus current. The presented results cover a range of random waves plus current flow conditions for which the method is valid. Results for typical field conditions are also presented. A possible application of the outcome of this study is that, e.g., consulting engineers can use it as part of assessing the on-bottom stability of seabed pipelines.publishedVersio

    Note on estimating bed shear stress caused by breaking random waves

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    This note presents a method of how the bed shear stress caused by breaking random waves on slopes can be estimated. This is obtained by adopting the Sumer et al. (2013) bed shear stress formula due to spilling and plunging breaking waves on hydraulically smooth slopes combined with the Myrhaug and Fouques (2012) joint distribution of surf similarity parameter and wave height for individual random waves in deep water. The conditional mean value of the maxima of mean bed shear stress during wave runup given wave height in deep water is provided including an example for spilling and plunging breaking random waves corresponding to typical field conditions. Another example compares the present results with one case from Thornton and Guza (1983) estimating the wave energy dissipation caused by bed shear stress beneath breaking random waves.publishedVersio

    Joint description of waves and currents applied in a simplified load case

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    In order to perform a more accurate analysis of marine structures, joint probability distributions of different metocean parameters have received an increasing interest during the last decade, facilitated by improved availability of reliable joint metocean data. There seems to be no general consensus with regard to the approach of estimating joint probability distributions of metocean parameters and a general overview of recent studies exploring different joint models for metocean parameters is presented. The main objective of this article is twofold: first to establish a joint distribution of significant wave height and current speed and then to assess the possible conservatism in the Norwegian design standard by applying this joint distribution in a simplified load case. Based on NORA10 wave data and simulated current data, a joint model for significant wave height and current speed at one location in the northern North Sea is presented. Since episodes of wind-generated inertial oscillations are governing the current conditions at this location, a joint conditional model with current speed conditional on significant wave height is suggested. A peak-over-threshold approach is selected. The significant wave height is found to be very well modelled by a 2-parameter Weibull distribution for significant wave height exceeding 8 m, while a log-normal distribution describes the current speed well. This model is used to Monte-Carlo simulate joint significant wave heights and current speeds for periods corresponding to the ultimate and accidental limit states (ULS and ALS), i.e. 100 and 10 000 years. The possible conservatism in the Norwegian design standard is assessed by a simplified case study. The results give a clear indication that the Norwegian design standard in not necessarily conservative, neither at ULS nor ALS level.acceptedVersio

    Simulated wind-generated inertial oscillations compared to current measurements in the northern North Sea

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    This is a pre-print of an article published in Ocean Dynamics. The final authenticated version is available online at: https://doi.org/10.1007/s10236-018-1150-zMeasured current speed data show that episodes of wind-generated inertial oscillations dominate the current conditions in parts of the northern North Sea. In order to acquire current data of sufficient duration for robust estimation of joint metocean design conditions, such as wind, waves and currents, a simple model for episodes of wind-generated inertial oscillations is adapted for the northern North Sea. The model is validated with and compared against measured current data at one location in the northern North Sea and found to reproduce the measured maximum current speed in each episode with considerably accuracy. The comparison is further improved when a small general background current is added to the simulated maximum current speeds. Extreme values of measured and simulated current speed are estimated and found to compare well. To assess the robustness of the model and also the sensitivity of current conditions from location to location, the validated model is applied at three other locations in the northern North Sea. In general, the simulated maximum current speeds are smaller than the measured, suggesting that wind-generated inertial oscillations are not as prominent at these locations and that other current conditions may be governing. Further analysis of the simulated current speed and joint distribution of wind, waves and currents for design of offshore structures will be presented in a separate paper.submittedVersio

    Note on Water Drag on Sea Ice for Different Surface Roughness

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    The approach in Myrhau

    Family-centred practices in the provision of interventions and services in primary health care: A survey of parents of preschool children with cerebral palsy

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    The aims of this study were to explore how parents of preschoolers with cerebral palsy (CP) experienced the level of family-centred services using the Measure of Processes of Care (MPOC- 20) within primary health care in Norway and to examine the relationships between these experiences and the provided everyday skills interventions and services. A survey was sent to 360 parents of preschool children with CP. The response rate was 34%. Of the MPOC scales respectful and supportive care and coordinated and comprehensive care received the highest ratings, and providing general information received the lowest. Our findings indicate lower levels of family centredness in primary health-care contexts than that reported in specialist health care. Significant positive associations were found between all the five MPOC-20 scales and the parents’ satisfaction with the amount of service coordination (p ¼ .000–.004). The high scores for respectful and supportive care and the low scores for general information indicate that the families experienced relational help giving practices to a larger extent and participatory practices to a lesser extent. To increase the participatory aspects of family-centred practice, further research needs to address facilitators and barriers of information sharing and ways of giving this information both in specialist and primary health care
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