NTNU
Faculty of Engineering Science and Technology
Department of Marine Technology
GRADED
PROJECT
TMR 4195 DESIGN OF OFFSHORE STRUCTURES
Distributed: 18.01.11
Sign: V.L. / S.H. / J.A.
Due date: 08.04.11
Quasi-static and dynamic analyses of a jacket structure
A jacket is installed at oilfield “Student” at a water depth of about 190m. A simplified finite element
model of the structure is available for the computer program, USFOS. The aim of the project work is
as follows:
1. Estimate metocean design conditions for the platform. One shall determine wave conditions
both for a design wave analysis and for a short term stochastic response analysis.
2. Calculate the 10-2 – and 10-4 - annual probability base shear and overturning moment. Both a
design wave analysis and a stochastic response wave analysis shall be performed.
3. Calculate the reserve strength ratio for a 10-2 – annual load profile. Do the same for the 10-4 –
annual probability load profile. The q-probability load profile is the load profile caused by the
q-probability design wave. Impact of current shall be investigated by using various levels of
current in combination with the extreme wave; 0m/s, 0.5m/s and 0.8m/s, constant through the
water column.
The student may adjust the subject for the project slightly in the direction of their major interest by
selecting to work with 1+2 in the project or 2 + 3. In case 2 + 3 is used, one shall take necessary
information regarding wave conditions from the Example Metocean Report being available on its
learning. If 1+2 is selected for the project, the metocean conditions are defined by the hindcast data
series available on its learning.
i)
If point 1 (if this is not included in project go to ii)) is selected, the 10-2 – and 10-4 –
annual exceedance probability significant wave height shall be estimated from the
hindcast data. Adopt the total sea value given in the actual file. Data are given every 6th
hour. Here it shall be assumed that the distribution function estimated from the hindcast
data are representative for the 3-hour significant wave height. The duration of the q-annual
probability sea state is assumed to be 3 hours. Based on the joint data for Hs and Tp one
shall estimate a conditional 90% band for the period given Hs = hs,q.
Estimate the q-annual probability wave height by performing a long term analysis. If one
decides to consider all wave heights, neglect the effect of varying zero-up-crossing
frequency. The corresponding period band for the associated can be estimated from the
corresponding bands of Tp by multiplying the interval limits by 0.9.
Proceed to point iii):
ii)
Find the quantities corresponding to those determined under point i) by using the Example
Metocean Report.
iii)
Perform an eigenvalue analysis of the structure. Give the 5 largest natural periods and the
corresponding mode shapes. Discuss whether or not a design wave analysis can be
adopted for the structure under consideration.
Investigate effect of deck weight and soil-structure stiffness by increasing and decreasing
both quantities (one at the time) by 30%.
iv)
Irrespective of conclusion regarding analysis given in iii) a design wave analysis shall be
performed. Estimate dynamics by a simplified approach considering the dynamic
amplification for a single degree of freedom system and calculate the q-probability values
for base shear and overturning moment accounting approximately for dynamics.
Regarding choice of coefficients reference is made to Norsok N-003, which is available on
its learning under folder: Regulations, Standards and Recommended Practises.
v)
Solve the equation of motion in the time domain for a selected sea q-probability sea state.
The selected sea state can be the lower end or the upper end of the confidence band for T p
given hs,q. Or it can be the mean. It is recommended that your group enter into an
agreement with two other groups so that all three groups can obtain results for all three sea
states.
In USFOS it is not possible within a reasonable computational time to solve the equation
of motion for a continuous 3-hour period. USFOS scan the simulated wave histories and
execute a time domain solution for the most severe groups. Groups must be specified by
the user. As the selected groups for a 3-hour period are simulated, the maximum base
shear and over tuning moment shall be identified. Both quasi-static and dynamic response
shall be simulated.
The procedure above is repeated say 20 times with a different random seed. That means
we end up with 20 observed values for 3-hour maximum base shear and 3-hour maximum
overturning moment.
Try to see if the maximum base shear and overturning moment always is occurring for the
largest 3-hour wave height (or maybe rather 3-hour maximum crest height).
vi)
Assume the 3-hour largest maximum to follow a Gumbel distribution and fit this model to
the available samples for quasi-static and dynamic 3-hour maxima.
Compare the result of the Stoke 5th order design wave analysis with the distribution
function for the 3-hour maximum quasi-static response. Which percentile does give the
same result as the Stoke 5th order wave?
Estimate the dynamic response corresponding to the same percentile. An equivalent
dynamic amplification is then obtained by the ration dynamic 3-hour maximum to quasistatic 3-hour maximum. Compare this equivalent dynamic amplification with the DAF
calculated from the single degree of freedom approach.
vii)
If a simplified metocean work is performed, a study of the reserved strength ratio shall be
included. (This is not expected to be included if full metocean work is performed.) The
reserved strength ratio is the ratio between the total load at structural collapse divided by
the 10-2 – annual probability load. The 10-2 – annual probability load is the load i\on the
structure experienced as the 10-2 – annual probability Stoke 5th order wave passes the
structure. By scaling this load profile until collapse, the RSR is found as the ratio. It is
somewhat inconsistent to scale the load without increasing the wave height. It is therefore
of some concern to also calculate the RSR for a 10-4 – annual probability profile.
In order to investigate effect of current 3 current speeds are to be considered; 0m/s, 0.5m/s
and 0.8m/s. Again it is recommended that agreements with other groups are made so that
each group does the analysis merely for one current speed.
It is not expected that all these points can be handled thoroughly during the project. It is therefore left
to the students to decide to which depth each point is solved. Work is expected to be shared between
the – up to three – members of the group. Doing that should ensure that a proper depth assessment
shall be possible.
The work shall be presented in a scientific report. The report is to be handed in by April 8, 2011.
Supervisors for the project will be: Prof. Jørgen Amdahl, Prof. II Sverre Haver and Vegard Longva