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‘Wave-in-Deck Loading of Offshore Structures’
-Cdr Balakrishnan G Nair, (Indian Navy, Retd), Sr. Principal Structural Engineer, J Ray McDermott Engineering Services, Pvt Ltd, Chennai
Design approaches to offshore structures have evolved over the last century. Endeavor is made from time to time to effectively adapt these structures to their operating environment so that they are functionally available throughout their expected life. The understanding of the environment and the response of these structures to various loads, as well as analysis tools have undergone fine-tuning, even as the international norms concerning safety of these structures have become more demanding. During the last two decades or so, wave-in-deck loading on offshore structures has been acknowledged as an issue of concern to the offshore oil and gas industry. The article looks at theoretical and analytical aspects of wave-in-deck loading.

Offshore platforms are exposed to one of the most complex, hazardous environments during their life time compared to any other manmade structures. The table below broadly brings out some of the hazards faced by offshore structures. These are categorised into Insufficient Strength & Excessive Loading. While insufficient strength is the consequence of deficiencies in design, fabrication, installation or operation or on account of degradation, Excessive Loading is the outcome of environment, operation or accidental loads.

Wave-in-deck Loads
Among these loads, wave-in-deck loading has been acknowledged, during the last two decades or so, as an issue of concern to the offshore oil and gas industry. Seabed subsidence due to reservoir compaction is one of the reasons attributed for such structures getting exposed to wave-in-deck loading. According to Katrine van Raaij, the other prominent reason could be the occurrence of certain extreme environmental events, previously thought of as rare and hence not accounted for during the design in the absence of accurate data.

Typically, for North Sea structures failure due to extreme environmental conditions probably can only be associated with wave impact on topside (Dalane and Haver, 1995; Haver, 1995). A vertical distance between the extreme surface elevation (including tide and storm surge) and the underside of the lowest deck, termed air-gap, of 1.5 meters has been widely recognised as a minimum requirement for fixed offshore structures.

It is evident that the 1.5 meter requirement leads to an inconsistent level of reliability, due to different probability of air-gap extinction, among structures located in different areas of the world having different environmental conditions.

Requalification
Existing platforms, which reach the end of their design life may require extension of service life. The desired extension of service life may create a need for requalification of the structure. Other circumstances can also necessitate a requalification process on an earlier stage in the design life. These could be seabed subsidence as stated above, increased topside weight or operational loads, revised environmental criteria, reduced capacity due to damage, corrosion or deterioration, better knowledge of material behavior or new information on soil properties obtained during driving of piles.

A requalification process may also be needed as a consequence of structural damage caused by, for instance, extreme weather or boat impact. There is a real need for assessment of the wave-in-deck load effects on the structures under these cases.

Wave-in-deck Forces
In accordance with DNV-RP-C205, wave slamming may have both global and local effects. The impact of a massive bulk of water from a wave crest hitting the platform deck is a global load effect while wave slamming on a brace in the splash zone is a local load effect, the latter normally not influencing the global structural capacity.

The following is a physical interpretation of the wavein- deck interaction by DNV-RP-C205, as observed for a wave crest hitting head-on on a simple box-type deck structure on a fixed jacket platform. It illustrates the main contributions to the global force identifying local/ global structural impacts and the time instants for maximum & minimum wave-in-deck forces. The horizontal wave-in-deck force has contributions from slamming, drag and inertia. Slamming and drag contributions are quadratic in velocity and governed by the high wave particle velocity in the crest. Inertia contributions are proportional to fluid particle acceleration. The slamming contribution is of short duration and drops to zero shortly after the initial impact (See fig).

The fluid particles underneath the deck are accelerated in a jet-like flow when the wave crest hits the deck. The drag contribution remains reasonably steady as the wave passes the deck. The magnitude of the inertia contribution depends on the horizontal acceleration and the rate of change of the wetted vertical area.

As the horizontal acceleration is zero at the crest and increases at lower elevations, the inertia term contribution is dependent on the immersion of the structure.

The negative water exit forces are due to the low pressure at the frontal wall caused by the vertical downward fluid velocity. The magnitude is dependent on the crest velocity and the immersion of the structure.

The vertical upward force is critical for local structural details because the force acts over a small area, leading to high local pressures. It is dominated by slamming forces, which is proportional to the wetted length times the wave particle velocity squared. As the wave runs along the underside of 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.

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.

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.