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1Design and Strength Assessment of Moonpool Region
Dr. Y. V. Satish KumarMarine Department
Infotech Enterprises Ltd.Hyderabad
Moonpool design should focus on elimination of hydrodynamic
disturbance to facilitate launching and retrieval of subsea
systems.
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2The Vessel
The vessel is originally a product carrier and is converted in
multi purpose offshore supply vessel with moonpool between frames
38~43.
The preliminary design for the moonpool region is developed ABS
rules.
Analysis of moonpool region is carried out similar to hold model
analysis. The size of the moonpool is 25' 25.
Main deck of the vessel with moonpool
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3Preliminary design
Issues of importance for preliminary design
Elimination of hydrodynamic disturbance by introducing openings
in
the non-water tight bulkheads and dampen the sloshing phenomenon
in
the moonpool region.
Identification of critical loading conditions
Development of scantlings for critical loading conditions
Preliminary design also deals with hull girder analysis
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4Finite element model
The rule based design is empirical in nature.
A first principle based finite element analysis is carried out
to obtain
accurate results and improve design with realistic loading
conditions
The finite element analysis is carried out with 3 hold model
concept with
the model extending from 32 to frame 50.
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5Loading conditions
The different wave conditions are considered and hogging wave is
found to
be critical loading condition
The loading components include Static loads ( Self weight)
Dynamic loads (Hydrodynamic loads) Machinery and equipment
loads
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6Hull girder loads
The hull girder loads such as bending moment and shear force are
applied
to FE model i.e. hold model
Verification of static equilibrium of the vessel under the
applied loads.
Sloshing loads are also considered in the moonpool region.
The motion accelerations for the dynamic loads are calculated
using ABS
rules.
Bending moment
Shear force
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7Preliminary design
Preliminary design showed that longitudinal strength is
insufficient with
introduction of moonpool.
A new girder is introduced to meet the longitudinal strength
requirements.
New girder
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8Hydrodynamic design
The hydrodynamic design involves placement of openings in the
non
watertight bulkheads at regular intervals and also baffles
between the
watertight and non watertight bulkheads at regular
intervals.
This provides damping of hydrodynamic disturbances and ensures
safe
launching and retrieval of the subsea systems.
Bulkheads in moonpool region Non watertight bulkheads
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9Structural design
The initial design is developed from the rule based design and
maximum
stress is found to be 37ksi which is above the permissible
limits.
The final design is developed with change of scantlings at all
critical
locations and the maximum stress is found to be 25ksi.
Initial design Final design
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10
Moonpool hatch cover design
The hatch cover of the moonpool is designed for weather
loads.
The designed stiffener scantlings and the shell thickness are
found to be adequate from strength point of view.
Moonpool hatch cover
vonMises stresses
Unity check ratios for stiffeners
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Stress concentration factor
The stress concentration factor at the moonpool corner is
calculated using local analysis.
Nominal stress = 1.33 ksi
Geometric stress =5.643ksi
SCF = 4.24
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Conclusions
The moonpool of size 25' 25' is designed for a multipurpose
supply vessel.
The preliminary design is done using rules of ABS for estimation
of longitudinal strength and development of scantlings.
The hogging wave condition is found to be critical loading
condition.
The openings in non watertight bulkheads and also baffles are
introduced to dampen the hydrodynamic disturbance.
The finite element analysis is carried out to improvise the
design. Several members in the moonpool region are strengthened to
withstand the design loads.
The moonpool hatch cover is designed to withstand the
environmental loads.