Fixed offshore structures are commonly used on the continental shelf (in water depths of up to 200 m). Structures might comprise simple single pile structures in shallow water, a jack-up facility in intermediate depths, ranging to concrete gravity-based structures which ‘sit’ on the seabed, or steel jackets which are ‘pinned’ to the seabed by piles.
Apart from having sufficient strength to support the requisite deck loads, the main design considerations for offshore structures are based) shear and overturning moment. Other considerations might include structural fatigue and seabed scour. Of crucial importance is that the deck elevation is set sufficiently high to avoid wave impact at any time throughout the life of the structure.
Operational metocean information is also required for the planning of such activities as transfer of materials from supply vessels, and possibly, helicopter operations.
Establishment of ‘survival criteria’ will require:
- extreme winds, waves, currents and water levels
- wind and current vertical profile
- wind gust factor assessments
- wave spectrum characterisation
- individual wave height distribution
- near-seabed currents
Separate ‘installation criteria’ may be required, addressing noncylonic storm assessments (i.e. short return period assessments of storms which cannot be avoided by forward planning or forecasting, and potentially may need to be endured before installation is complete).
Installation criteria might also include ‘weather window’ analyses, which attempt to predict the proportion of time that suitable installation conditions might prevail.
‘Operational criteria’ are the complement of ‘weather window’ analysis, essentially attempting to predict the proportion of time that seastates and wind conditions might prove to be inoperable – principally for materials transfer and helicopter operations.