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.

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

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

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.

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

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

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

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

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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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.