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Gravity-based offshore foundations generally consist of a bottom slab and one or more cylindrical shafts on top of it. The slab geometry can strongly affect the surrounding flow pattern, dynamic wave pressure distribution and further momentary soil liquefaction. In the present study, the effects of the slab geometry and the incoming wave angle to the foundations on the surrounding soil response are investigated. Gravity-based foundations with bottom slabs of cylindrical shape and hexagonal prismatic shape are considered. For the hexagonal foundation, two different incoming wave angles, i.e. propagating to the hexagon corner and the edge, are simulated. The waves are fully nonlinear steep non-breaking waves. The wave-structure interaction is validated against an existing experiment. Soil consolidation behaviour underneath the foundations is investigated. It is found that both the slab geometry and the incoming wave angle can affect the distributions of pore pressure and momentary liquefaction depth in the surrounding soil. [ABSTRACT FROM AUTHOR]

The additional snow and ice loads that a typical sixth generation semi-submersible in polar lows may experience are estimated and the effects on the intact stability of the semi-submersible are analysed. The additional snow loads could be very significant if the snow accrete on all the horizontal surfaces of the semisubmersible, and could have more impact on the initial stability of the semi-submersible when in the survival from operating condition. Ice loads from sea spray icing may not exceed 300 tons in any of the semi-submersible loading conditions, except when the icing event extends to the main deck of the semisubmersible. The resulting asymmetric ice loads will lead to increase heeling of the semi-submersible, with maximum heeling occurring when the icing event extends to the main deck. Generally, the additional snow and ice loads in polar lows are deemed to represent operation and safety challenges. [ABSTRACT FROM AUTHOR]

A steel riser has benefits over a flexible riser in terms of pressure rating and cost. A steel riser in lazy wave configuration (steel lazy wave riser (SLWR)) is often considered as a good alternative solution for harsh environments where large floater excursions take place. The SLWR configuration is achieved by introducing buoyancy modules into a steel catenary riser (SCR). The buoyancy modules act as a damper and isolate the floater motions from the critical touchdown area. Hence, the SLWR generally has better overall performance than an SCR configuration. This paper attempts to analyze the correlation between the geometric shapes of the SLWR configuration with its capability to absorb the dynamic loadings. For deepwater cases, the behavior of the bottom part of the riser is correlated with the velocity experiences at the riser's hang-off location. Hence, the riser's performance is analyzed by comparing the velocity at the riser's hang-off with the velocity at the sag, hog, and near touchdown. The geometric shape of an SLWR is represented by its arch height, which is the vertical distance between the lowest point at the sag and the highest point at the hog bend of a riser. The results show that there is a correlation between the arch height of an SLWR with the riser's strength and wave-induced fatigue performance. SLWR configurations with higher arch generally have greater capability to absorb the dynamic loadings, as indicated by the lower velocities along the riser, which leads to lower stress utilizations and lower fatigue damage. [ABSTRACT FROM AUTHOR]

ABSTRACT Onshore wind turbine technology is moving offshore, and the offshore wind industry tends to use larger turbines than those used over land. This calls for an improved understanding of the marine boundary layer. The standards used in the design of offshore wind turbines, particularly the rotor-nacelle assembly, are similar to those used for onshore wind turbines. As a result, simplifications regarding the marine boundary layer are made. Atmospheric stability considerations and wave effects, including the dynamic sea surface roughness, are two major factors affecting flow over sea versus land. Neutral stratification and a flat, smooth sea surface are routinely used as assumptions in wind energy calculations. Newly published literature in the field reveals that the assumption of a neutral stratification is not necessarily a conservative approach. Design tests based on neutral stratification give the lowest fatigue damage on the rotors. Turbulence, heat exchange and momentum transfer depend on the sea state, but this is usually ignored, and the sea surface is thought of as level and smooth. Field experiments and numerical simulations show that during swell conditions, the wind profile will no longer exhibit a logarithmic shape, and the surface drag relies on the sea state. Stratification and sea state are parameters that can be accounted for, and they should therefore be considered in design calculations, energy assessments and power output predictions. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]