NTNU
Faculty of Engineering Science and Technology
Department of Marine Technology
TMR 4195
Due date: 30.03.11
EXERCISE 4 & 5
DESIGN OF OFFSHORE STRUCTURES
Distributed: 14.03.11
Sign: V.L.
The floating production platform shown in Fig. 1 is subject to a 100 year storm condition. In the following
a ULS control of the columns and the main girder at the deck south-side is to be carried out. Set the
characteristic vertical acceleration amplitude equal to 2.5 m/s2. Use a yield strength of 355 MPa and an Emodulus of 210 GPa for the deck structure and the columns.
Figure 1: Floating Production Platform
Problem 1
a)
The main girder can be modelled as a trusswork, with height of 12 m and 12 m spacing between
the vertical braces. The girder carries both functional and permanent loads which may be assumed
uniformly distributed with a characteristic total value of qF=0.3 MN/m.
Brace A in Fig. 1 is hot-finished and has a hollow square cross-section made of plates with
thickness 30 mm. What cross-sectional width/height will you recommend for this brace? See
DNV Classification Note 30.1 in the appendix. Use Excel, MATLAB, MathCad etc.
b) Each column has the following characteristic stress resultants at the lower ends,
Axial compressive force, N
Bending moment, M
Shear force, Q
Functional induced
50 MN
10 MNm
15 MN
Environmental induced
20 MN
90 MNm
10 MN
Table 1: Characteristic stress resultants at lower end of columns.
The columns are horizontally divided into compartments of height 3 m with atmospheric internal
pressure. Vertical L-stiffeners are welded onto the column shell with a spacing of 650 mm. Your
task is to examine three L-stiffeners in a buckling capacity check; L200x90x11.5x15,
L300x100x11.5x16 and L400x120x12.5x25. The aim of the buckling check is to find the optimal
scantlings with respect to material costs.
What stiffener scantlings will you recommend?
What thickness will you suggest for the column shell?
Assume for simplicity that the L-stiffeners have sufficient capacity to avoid torsional buckling.
When panel stiffener buckling is assessed the effective shell flange, se, can be set to 275 mm.
Stress analysis formulas and relevant failure modes are found in DNV-RP-C202. See also Fig. A
in appendix. It is recommended to use Excel in this problem.
c) The service temperature is set to 0 °C. What grade of steel material should be selected for brace A
and the column shell plating? Use DNV-OS-C101 in the appendix.
d) From a stochastic analysis it has been found that the largest wave-induced axial force amplitude
during 20 years is 1200 kN in brace A. The brace is connected to the chord by a gusset plate with
a favourable geometry. It can be assumed that the axial force cycles has zero up-crossing period of
6.3 s and follow a Weibull-distribution with a shape parameter h=1.1. The SN-curve is of type F1
and has no fatigue limit due to a corrosive environment. The brace is accessible for regular
inspection and repair in dry and clean conditions.
Is the fatigue life for the lower joint of brace A acceptable for a service life of 25 years? Use
DNV-OS-C101 and DNV-RP-C203 in the appendix.
e) To reduce the weight of the deck structure it is proposed to use a high strength steel with 50 %
increase of yield stress compared to problem a). Modify the plate thickness of brace A such that
utilization with respect to buckling remains unchanged. Is the fatigue life for the lower joint of
brace A acceptable now? Comment the result.
APPENDIX
Figure A: L-stiffener cross-sectional data
DNV Classification Note 30.1
The necessary pages of the code for problem 1a) is included in the four following pages.
DNV-RP-C202
The relevant pages of the code which is used in connection with problem 1b) is included in the six next
pages.
DNV-OS-C101
The pages necessary to answer problem 1c) is included in the three following pages.
DNV-RP-C203 and DNV-OS-C101
In the four following pages the required information to solve problem d) is found.