The oil tankers that operate on the Trans-Alaska Pipeline Service (TAPS) route have
exhibited a large number of structural fatigue cracks. These cracks can be attributed to
the increase in use of high strength steel in tanker construction and to the harsh operating
environment in the Gulf of Alaska. In response to the TAPS fatigue problem, this project
examines the topic of preliminary design for fatigue resistance. The TAPS tankers have
previously been the target of several studies on the subject of fatigue cracking. Most of
these studies have concentrated on reducing the costs and risks involved with operating
the current tanker fleet. Preliminary design, however, is oriented at reducing the fatigue
risk in future tanker designs. To that end, the design method outlined within concentrates
on the level of analysis that is appropriate for preliminary design.
The design method consists of four steps: the specification of a wave environment,
generation of a hydrodynamic model and subsequent wave loads, evaluation of cyclic
stresses and an assessment of fatigue damage. A series of example calculations that is
typical of preliminary design has been performed for one of the TAPS tanker classes.
These calculations employed Buckley's climatic
wave spectra, a 3-dimensional panel based
hydrodynamics package by Lin and a Miner's rule fatigue assessment based on the S-N
curves of the British Welding Institute.
The example calculations yield two important results. First, relatively inexpensive
methods can yie1d important and accurate fatigue results; for a side shell longitudinal at
the water line the example calculations predict a fatigue life of approximately 3 operating
years. This corresponds quite well to the published inspection data and obviously
represents insufficient fatigue life. Second, local panel pressures can have a significant
contribution to, and even dominate, total fatigue damage in the side shell. This contrasts
with conventional fatigue studies of ship hulls which focus on global loads; i.e., hull girder
bending.
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