MARITIME

EYE ON DESIGN
CFD to Assess SSP’s
Crew Boat Docking Tunnel
BY JOOP HELDER
MARIN combined CFD with fast-time
simulations and intensive testing on
its bridge simulator to explore the full
potential of the new SSP HUB.
To save on costly helicopter crew
transport, SSP Offshore took a
good look at its SSP Plus design
and decided to add a large crew boat
docking tunnel, running straight through
the center of the big round floater. The
tunnel allows fast sailing crew boats to
dock inside the HUB, thereby cutting out
helicopter transfers. MARIN was put to
work testing the HUB design to make
sure a safe crew boat entry into the unit
was possible under extreme operational
conditions. A multi-stage campaign was
required, ranging from CFD calculations
to determine the complex interaction of
the HUB’s exceptional design with the
environment, to intensive simulation exercises
on the bridge simulator with experts
from SSP, Austal and MARIN.
To be located offshore Brazil, the SSP
HUB features an omni-directional shape
and the ability to rotate, thereby redirecting
its tunnel position depending on the
environmental conditions. If beneficial,
the tunnel can be closed by weather tight
doors on the wave-ward side of the unit.
Safe Crew Bboat Entry
One of the key questions SSP needed
answering was whether the HUB was
able to accommodate safe crew boat
entry in the harsh environmental conditions
of the Santos Basin. For this, the
unique design of the shallow docking
tunnel running through the floater could
be a determining factor. Waves typically
change their height, length and direction
when travelling over a sudden transition
from deep to shallow water, and could
thereby hamper the crew boat pilot during
entry into the tunnel. Current and
wind travelling around the HUB, and
possibly through the tunnel, could further
restrict the approach and docking
operation. By using a combination of
CFD tools, MARIN set out to study the
complex wave, wind and current flow
phenomena that occur around the entrance
of the HUB tunnel. As these flows
are driven by non-linear effects, conventional
linear potential flow tools are
simply not capable of capturing the flow
details with enough accuracy.
Environmental Conditions
Wind and current velocities inside and
outside the HUB were computed using
ReFRESCO, for both a completely open
tunnel and a tunnel that was closed on
the side affected by the environment.
For a range of environmental conditions
flow fields could be visualized in detail
and showed recirculation, shielding and
intensification zones. The results will
allow SSP to anticipate environmental
directions, for which the CFD predicted
strong cross flows at the entrance of the
tunnel, by using the rotating capabilities
of HUB.
To get a better understanding of the
wave patterns inside the docking tunnel,
unsteady wave simulations were
performed using ComFLOW. Special
attention was paid to possible resonant
wave modes inside the tunnel. The results
of the CFD provided SSP with critical
wave amplification factors inside the
tunnel, together with the environmental
conditions for which these amplifications
occur. With this knowledge, SSP
can set criteria for redirecting the HUB,
using the weather tight tunnel doors and
possibly other mitigation means to avoid
large water motions inside the tunnel.
Simulation Study
To provide SSP with the answers it
is looking for, the knowledge obtained
from the CFD simulations were only
half of the equation. The capabilities of
the Austal Trimaran crew boat and captain
also had to be taken into account. To
that end, a realistic and fast-time simulation
model of the HUB, environment
and crew boat was developed and put
to the test on MARIN’s bridge simulator.
To achieve realistic simulation exercises,
the environmental conditions on
the simulator were tuned using the CFD
simulations and a detailed model of the
interior of the tunnel was made including
moveable fenders that guide the vessel
into the tunnel. An accurate maneuvering
model of the trimaran was created
in cooperation with Austal. With the use
of the CFD results the HUB’s rotational
capabilities and weather tight doors were
exploited to the maximum to create the
optimum entry condition for any given
environment.
In the end, a week of extensive simulations
found that the SSP HUB was indeed
able to accommodate a safe entry in
the harshest environments – a conclusion
made possible by a unique combination
of CFD analysis, real-time simulations
and comprehensive simulator exercise.
The Author
Joop Helder is Project Manager of the
Offshore Department of MARIN. MARIN
offers simulation, model testing, fullscale
measurements and training programs,
to the shipbuilding and offshore
industry and governments.
e. j.helder@marin.nl
w. www.marin.nl
14 Maritime Reporter & Engineering News • SEPTEMBER 2015