Fatigue in Jacket Structures With Impaired Integrity
Present regulations for offshore structures on the Norwegian continental shelf have
a requirement for Accidental Limit State redundancy and Fatigue Limit State redundancy
in a damaged state. However, the requirement is far more defined for
the Accidental Limit State than for the Fatigue Limit State. An increased understanding
of factors governing fatigue redundancy would create a basis to form a
In literature, the term redundancy is defined in several ways. The different methods
can be divided into two major categories, namely deterministic and probabilistic
approaches. In general, redundancy may be defined as the absence of members
whose failure would lead to global collapse. Within both the deterministic and the
probabilistic framework, several redundancy factors are usually defined, and there
are resemblance between some of them. In probabilistic methods, the reliability
method is commonly applied through the First Order Reliability Method.
Fatigue damage is a primarily concern regarding the integrity for offshore structures.
A near constant subjection to cyclic loadings from wind, current and waves
initiates a cumulative damage process which leads to a certain fatigue life for the
members in the structure. The lifetime may be calculated using either a fracture
mechanics approach or a SN-curve approach. There exists several approaches to
calculate the stress levels to be used in the fatigue analyses, and the choice of
method is mainly based on whether or not the structure under consideration is
dynamically behaving or may be regarded as quasi-static. Also, there may be nonlinearities
that needs to be accounted for and naturally this will affect the choice
of analysis method. However, large uncertainties are associated with fatigue calculations
regardless of analysis approach. Thus, a probabilistic framework is highly
relevant in order to estimate the risk of failure due to fatigue.
A study on how impaired integrity affects the fatigue life has been performed for
two jackets; one highly redundant four-legged jacket and one less redundant threelegged
jacket. The main goal has been to investigate the fatigue redundancy of the
structures, in order to link up the risk of accelerated fatigue due to damage with
the risk of failure due to extreme environmental actions, which may eventually lead
to structural collapse. A stochastic fatigue analysis approach was chosen, and the
analyses was in agreement with the standards governing the Norwegian continental
shelf at the time of this thesis. Pushover analyses has been performed on the jackets
to give an insight in their redundancy, and a calculation of changes in the natural
periods under impaired integrity has also been done.
The four-legged jacket was proven to be highly redundant, and had small changes
in the natural period under impaired integrity. The three-legged jacket on the other
hand, had some damage cases with a rather large increase. Also, the redundancy
factor R4 was significantly lower for the three-legged jacket, thus confirming it to
be less redundant than the four-legged jacket. A large change in the natural period
will alter the dynamic response, thus the fatigue life is vastly connected to changes
in global stiffness.
Trying to explain the changes in fatigue life for the two jackets under impaired
integrity without using a deterministic approach, i.e. calculate the fatigue life for
the specific damage case, was proven to be very difficult. There seems to be no
easy way to isolate the severity of the fatigue life reduction since large changes
are occurring throughout almost the entire structure for several damage cases, as
well as large spread in the values them self. However, there was also found some
trends in the results. One of them, was that the closer a member is to the damaged
element, the larger is the expected reduction in fatigue life. Another trend, is that
a large fatigue accelerator factor is expected in almost every damage case, thus one
may expect large changes for most of the damage scenarios.
Another vastly occurring phenomenon were the location of the damaged members
who gave the lowest fatigue life in the structures. For the four-legged jacket, this
involved damage in the caisson supports. The three-legged jacket, however, had
the lowest fatigue lives occurring for damage cases in the top frame where there is
a lack of deterministic redundancy.
There seems to be no fatigue redundancy for the jackets, as there are large fatigue
accelerator factors occurring. There is also the very low calculated fatigue life in
the most extreme cases. However, there has been found a slight correlation between
a large reduction in fatigue life and a large initial fatigue life. What should also be
taken into account though, is both the risk related to the fatigue lives found, and
the accuracy of the values due to the linearised analysis.
Place, publisher, year, edition, pages
Institutt for marin teknikk , 2012. , 170 p.
ntnudaim:7156, MTMART Marin teknikk, Marin konstruksjonsteknikk
IdentifiersURN: urn:nbn:no:ntnu:diva-18530Local ID: ntnudaim:7156OAI: oai:DiVA.org:ntnu-18530DiVA: diva2:566015
Leira, Bernt Johan, ProfessorAmdahl, Professor JørgenHellevig, Nils-ChristianJia, Junbo