The effective prediction of wave-in-deck loads

Abstract

The present paper concerns the extreme wave loads acting on an offshore structure; specifically the wave-in-deck loading component that arises when the height of an incident wave crest exceeds the elevation of the topside structure. In this case wave inundation occurs, the resulting loads on the topside structure represent a significant part of the total wave load. A new model for the effective prediction of this important loading component is presented. This is based upon the conservation of momentum, is formulated in a Lagrangian frame of reference, can incorporate any incident wave form, and takes due account of the porosity (or openness) of the topside structure. Comparisons between the model predictions and wide-ranging laboratory observations are shown to be in good agreement; the latter based upon deterministic focused wave events that are known to be representative of the largest waves arising in realistic sea-states. In addition, comparisons are also made with independent cfd calculations. Taken together, the proposed model is shown to accommodate changes in the spectral shape, the spectral peak period, the incident crest elevation (and hence the level of inundation), the directional spread of the incident waves, and the porosity of the topside structure. Importantly, this agreement applies to both non-breaking and breaking waves, involves no empirical calibration, and can be achieved with limited computational resources. As such, the model is ideally suited to design/re-assessment calculations in which the reliability of any offshore structure must be based upon a rigorous assessment of the long-term distribution of the total wave loads, including any wave-in-deck loading component.

An experimental study of wave-in-deck loading and its dependence on the properties of the incident waves

“…In undertaking this process, a 50 th wave gauge, placed outside the plan area of the topside, remained in place to serve as an assurance of the repeatability of the incident wave conditions. Full verification of the wave generation and the repeatability of the measurements is provided in Masterton and Swan (2008), Latheef and Swan (2013) and Ma (2017).…”

“…This was checked using a static system calibration showing excellent agreement and linearity within the measurement range. Whilst the complete details of the system are provided in Ma (2017), the success of the design is clearly demonstrated in Table 1; the application of a load in any one dof having a negligible response in all other loading components.…”

“…This section presents an analysis of preliminary data appropriate to the quality/accuracy of the laboratory study. Its focus lies in the measured wid loading; the success of the wave generation having already been established in earlier work by Masterton and Swan (2008), Latheef and Swan (2013) and Ma (2017).…”

Structural Integrity of Fixed Offshore Platforms by Incorporating Wave-in-Deck

“…Wave-in-deck occurs when there is no deck clearance or air gap between the water level and the bottom steel of topside structure when it is hit by the waves [43,44]. To avoid wave-in-deck, all offshore platforms need to be adequately designed by providing an allowance for the air gap [31]. If the air gap is expected to reduce over time, the lowest deck may be designed at a higher elevation or the equipment seated on the deck need to be designed to cater for the wave-in-deck load.…”

“…Studies performed by [22,[27][28][29][30][31] concluded that the wave-in-deck load needed to be considered in the pushover analysis. This was due to the fact that a huge wave hitting the offshore platform led to a high wave-in-deck load that could eventually resulted in significant platform damage and collapse.…”