General

This chapter is written with mainly jacket structures in mind. However, the same principles can also be used for analysis of other types of bottom-fixed structures such as jack-ups and support structures for wind turbines. Fatigue analysis of jacket structures should be performed for:

1. • transportation of the jacket from the construction yard to the installation site;

2. • driving of piles;

3. • the installed condition.

As fatigue damage is accumulated over different phases, the calculated fatigue damage at each hot spot from each of the operations can be added together.

Transportation of the jacket on a barge to the installation site requires special considerations with respect to fatigue. The most significant hot spots during transportation may differ from those in the installed condition. However, transportation can involve significant fatigue damage, and in some cases, repairs have been needed before the jacket could be installed. Therefore, it is important to plan transportation and assess relevant environmental criteria for the transportation period and route; this includes assessment of available sheltered harbors on route should transportation take longer timer than predicted. In fatigue analysis of the transportation phase, the question arises regarding which Design Fatigue Factor (DFF) should be used. This should be considered in conjunction with the criteria used for transportation. If fatigue damage is expected to accumulate at the same hot spot during transportation as during the installed condition, it is reasonable to use a DFF that is sufficiently large so that the allowable calculated fatigue damage during transportation is low. This should be described in a design specification before the detailed design commences.

It is also important to remember that slender members in a jacket lying on a barge during transportation may be subjected to vortex-induced vibrations in wind. Vortex-induced vibrations in water need to be considered in design for the installed condition. However, the driving forces and damping are different in wind than in conditions with wave and current loading.

Fatigue analysis of the piles in jacket structures shows that a significant part of the fatigue damage can be accumulated at thickness transitions during pile driving. The accumulated fatigue damage governs the probability of fatigue failure, as explained in Chapter 12.