Proceedings of the Thirty-second (2022) International Ocean and Polar Engineering Conference
Shanghai, China, June 5–10, 2022
Copyright © 2022 by the International Society of Offshore and Polar Engineers (ISOPE)
ISBN 978-1-880653-81-4; ISSN 1098-6189

www.isope.org

A procedure to evaluate the fatigue contribution to the collapse of ship structural components
Dario Bootea, Gianmarco Vergassolaa, Thomas Lindemannb, Patrick Kaendingba) Department of Electrical, Electronic, Telecommunication Engineering and Naval Architecture DITEN,
University of Genoa, Italyb) Chair of Ship Structures, University of Rostock, Germany

Most damages to structural components of ships are due to fatigue
phenomena. Particular care should then be devoted to fatigue
evaluation since the first phases of any ship project. All main
Classification Societies have in their Rules simplified procedures forfatigue evaluation, in order to make all designers able to perform
structure scantling complying with fatigue principles. Apart fromdesign necessities, there is another aspect for which it becomes
important to assess and evaluate fatigue phenomena; this happens when
it is necessary to ascertain the causes of an occurred accident. While fordesign purposes the aim is to calculate the fatigue life of a structuraldetail based on a probabilistic approach, in the case of an occurred
accident the driving philosophy is to accurately analyze the life historyof the structure and to calculate, by an inverse approach, the cumulative
damage of the component up to the event time. The aim of this paper is
to set up a procedure to analyze and verify the causes of the failure of aship structure component.

type occurred in the past to ships, mainly bulk carriers (Boote &Cecchini, 2008 and Boote & Cecchini, 2009), or offshore platforms in
the Gulf of Mexico and in the North Sea, it came out that a more
rigorous consideration of fatigue is mandatory since the first designphases of ships and offshore structures. In this perspective all main
Class Societies added to their Rules simplified procedures for fatigue
evaluation, in order to make all designers able to perform structure
scantling complying with fatigue principles.
There is also another perspective, other than design, for which it
becomes important to assess and evaluate fatigue phenomena, and thishappens when it is necessary to ascertain the causes of occurred
accidents. In this case the procedure to be followed goes, somehow, inthe opposite way with respect to the design phase. While for design
purposes the aim is to calculate the fatigue life of a structural detail
based on a probabilistic approach, in the case of an occurred accident
the driving philosophy is to accurately analyze the life history of thestructure and to calculate, by a deterministic approach, the cumulative
damage up to the event time.

KEY WORDS: ship structures; seakeeping; fatigue life

FATIGUE ASSESSMENT OVERVIEWINTRODUCTION

The fatigue collapse of a structure is caused by cyclic loads and thenumber of repetitions causing failure depends on the stress intensity.
When fatigue investigations are carried out in a research laboratory theexperiments can be carried out with constant stress values but, in realconditions, the fatigue failure is always the result of the damage
accumulation caused by loads of different kinds and entities. In the caseof ship structures cyclic loads are mainly represented by sea waves; theresponse of the hull girder to wave loads in terms of global and local
stress is the main aspect to be studied in order to predict and avoid
fatigue collapse of any structural components.
Depending on the assumed method to determine the stress distribution,
there are four approaches to evaluate the accumulated fatigue damages(Bai, 2003):• Spectral Fatigue Analysis (SFA): the total fatigue damage of astructural component is derived by combining the short-term damages
on all sea states of the wave scatter diagrams the ship pass through in
her operative life. In this way both sea states and their probabilities ofoccurrence are accounted for. SFA is a stochastic methodology based

ABSTRACTIn the operative life of a ship hull structures are exposed to a wide
variety of loads of both static and dynamic nature. The first category
includes gravity, hydrostatic and thermal loads, the second category
includes wave induced loads, impact, slamming and sloshing loads.
While at present static loads are well known in all their aspects andtheir influence on structure scantling can be easily assessed by direct
methods or Class Society Rules, dynamic loads are more difficult to betaken into account and sometimes they can generate very high,unexpected values. A valid approach to evaluate the capability of hull
structures to withstand extreme loads is represented by the ultimate
strength determination (Lindemann et alii, 2016).
Nevertheless most severe damages to structural components of ships
are due to fatigue phenomena. Even if fatigue reveals itself with small
cracks on local details, if they are not promptly detected, in addition toship ageing, they can lead to more serious damages up to the totalcollapse of the whole structure. After some catastrophic events of this3117