[Feature] the deck, the wave front causes slamming loads at each new location. The magnitude of the slamming load is largest at the inflow side and reduces moderately as the wave reaches the other side, resulting in a relatively wide global force peak. The global vertical impact force has its maximum when the wave crest passes the front of the deck, at the minimum (negative) air-gap. The local impact force has its maximum at a slightly earlier stage. The inertia force acts downwards as the wave passes by, since the vertical fluid acceleration in the crest is negative. During the initial stage of the wave cycle, the inertia term is small due to the small wet area, and it acts in opposite direction of the slam and drag forces. When the whole underside of the deck structure is wet, the inertia term is at its maximum due to added mass force.
Wave-in-deck forces influence an offshore jacket structure not
only by their magnitude, which is significant compared to the wave load on the jacket itself, but also because they alter the load
distribution in a manner that introduces high forces into relatively weaker parts of the structure such as the deck legs immediately below the deck.
At this instant, which is important for global effects due to the large exposed area, the crest has passed the centre of the structure and the vertical velocity has changed to negative, i.e. acting downward in the same direction as the inertia force. The vertical force at water exit is dependent on the wetted length of the structure and to a lesser degree on the impact condition and the immersion. Slamming is not defined for water exit. When girders are present, the flow is disturbed, which in turn reduces the wetted length and the magnitude of the vertical downward force. When assessing the structural resistance, it is important to consider the transient nature of the wave-in deck loads. It should be noted that negative pressure force during water exit means that the normal pressure is lower than atmospheric pressure, resulting in a downward force. ‘Effects’ of Wave-in-deck The above discussion brings out the complex loads that the wave-in-deck brings to bear on the offshore structure. Extreme waves may be associated with a storm surge reducing the air-gap and this effect needs to be taken into account prior to analysis of wave-in-deck loading. Large surface elevations on account of a worst combination of tide, surge and wave height can accentuate the adverse impact on the jacket structure.
Traditionally fixed offshore platforms are not designed to withstand the large forces generated by wave-in-deck loads. In that scenario, if a wave strikes the deck, the deck legs, which are not sized to transfer shear forces of this magnitude from the deck into the jacket, may be excessively loaded. In addition, large up and downward acting vertical loads may be introduced in the structure, further reducing the deck legs’ capacity to carry transverse load. This may be true even to the jacket legs. Thus, failure modes other than those considered during design can be governing for platforms exposed to wave-in-deck loads. The response of a typical jacket analysed for normal loading as well as wave-in-deck loading, the latter leading to larger levels of inundation, is presented below graphically. The plot indicates the enhanced stress levels as well as deformations in the legs and braces on account of higher inundation bringing out effects of wave-in-deck loading. As the water depth increases and the deck load increases accordingly, a larger part of the total force has to be transferred from the deck through the braces in the upper bay and down into the lower part of the jacket structure. These braces are originally not intended to transfer large wave loads, and will therefore represent the ‘bottlenecks’ when the platform is exposed to large wave-in-deck loads.
(a) Depth 75 m / inundation 0.25 m
Static collapse modes for different water depths and corresponding inundation levels
Conclusion Wave-in-deck forces influence an offshore jacket structure not only by their magnitude, which is significant compared to the wave load on the jacket itself, but also because they alter the load distribution in a manner that introduces high forces into relatively weaker parts of the structure such as the deck legs immediately below the deck. Therefore, if a jacket structure is likely to face seabed subsidence then it is worth considering wave-in-deck loads during design, since, a reduction in air-gap in foreseeable future is a distinct possibility. Also if opportunity is available, such as when a requalification for life extension of a jacket is taken up, it would be desirable to undertake a wave-in-deck analysis. Dec 2013 - Jan 2014 |
Balakrishna nair.indd 21
(b) Depth 81 m / inundation 5.88 m
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