Now challenges are posed on structural analysis of jack-ups when moving to deeper water, more harsh environment and application high tensile steel. Dynamic behaviour becomes more prevailing and fatigue due to wave loads may become governing. Furthermore, supply boat sizes tend to increase and they work in more severe weather conditions, increasing the risk for collisions with the jack-up logs.
The present paper describes simplified ways to cope with the effects above.
Basically the global structural system of a jack-up platform is very simple. Traditional platforms for shallow water and calm seas may be analysed by means of simple methods as outlined in Ref. 2.
In reality, however, the platform behaviour is quite complex mainly due to nonlinearity of wave loading and dynamic structural response. Due to the nonlinear response, the frequent occurring stresses are so small compared with the maximum stresses for most platforms that fatigue criteria have not been governing, and the platforms that fatigue criteria have not been governing, and the platforms have been designed to yield, buckling and overtuning platforms have been designed to yield, buckling and overtuning criteria only. The dynamic behaviour has normally been neglected. This can be justified as long as the effect is small because traditional analyses have normally been based on extreme regular wave analysis, which has shown to be conservative compared to more refined stochastic analysis Ref.12 and 13.
For platforms designed for deep water, harsh environment and with high tensile steel to keep down the weight there is a need for more accurate analyses as the built-in conservatism in the deterministic analysis may no longer fully compensate for dynamic amplification and because fatigue of legs and jack house may become a governing design criterion.
To superimpose dynamic amplification on top of the deterministic analysis, however, may be quite conservative and there is a definite need to take some advantage of the stochastic analysis when including dynamic behaviour. As an extension to the traditional simple analysis of jack-ups, the present paper discusses how dynamic behaviour and fatigue can be included in simplified ways. Furthermore, guidance for the range of wave periods which should be considered for deterministic wave analysis periods which should be considered for deterministic wave analysis is presented as well as a simplified approach for analysis of collision between a supply boat and a jack-up.
p. 7–17 p. 7–17