Vision 2009 - Rolls-Royce

12
VISION 1-2009. Rolls-Royce
The anatomy of the
UT 755 series
Since the first vessels went to sea in 1996 the UT 755 and its variants
has become an extremely popular platform supply vessel.
To date, vessels in service or on order
total some 160 with shipyard order
books for the latest version, the UT
755 CD, extending to 2011 deliveries.
The story actually begins in 1992/93.
UT-Design had built a strong
reputation for offshore vessels, on
the PSV side with the UT 705 and
UT 706, and the team saw a market
need for a new design to fill the
general needs of the offshore supply
market. The main design drivers
were efficiency, a good balance
between load and size, a simple
vessel to build, but with good hull
lines particularly for North Sea
conditions. The new vessel should
have a deadweight of about 3,000
tonnes and the ability to carry 1,500
tonnes of this on deck, yet have a
shallow enough draught to access
ports and bases worldwide.
Chine hull
Designer Sigmund Borgundvåg
recounts, “The vessel we came up
with was the UT 755; 67m long,
16m beam and about 6m draught.
A chine hull was chosen for ease
of construction, and to minimise
hull resistance the chine lines were
aligned with the water flow using
extensive tank testing. As well as
easing construction, the chines
helped with roll damping and also
allowed much larger bilge keels to
be installed without them extending
beyond the side or base lines. Large
passive stabilisation tanks provided
good roll reduction. Much thought
was given to tank layout in relation
to ship structure, and tanks for
800m³ of mud/brine were worked in,
together with ample tank space for
fuel, water and other liquids and a
very large cement bulk tank system.
The end result was a compact vessel
which, in practice, proved as capable
as far larger and more expensive to
run PSVs.”
The propulsion system was simple
but flexible for transit and DP
modes. Two Bergen K-series engines
provided 2 x 2,000kW driving 2.9m
diameter open water CP propellers
through reduction gears, one or two
tunnel thrusters were located aft
and either two tunnel units forward
or a tunnel thruster and a swing-up
azimuth thruster at the bow. In
combination with high lift rudders,
this gave excellent manoeuvrability
and propulsion redundancy.
Stretched version
Twenty-four of the original version
of the UT 755 were delivered
between 1996 and 2003. However,
in 1998 came the first example of
a stretched version, the UT 755 L,
which to date has proved by far the
most popular. The main change
was a 5 metre long extension in the
mid-body. The main design features
were retained, including the logical
and simple accommodation block,
but the extra length of the working
deck and hold provided a useful
bonus, and the extra length was
also beneficial for motions in the
North Sea wave spectrum. Although
the vessel was not maximised for
pipe carrying, four standard 12.5m
lengths of pipe could be carried on
the 51m by 13.5m working deck.
Below decks there was room for 20%
more mud/brine and cement. This
version proved to be an immediate
hit with the market, and sixty-five
vessels have so far gone into service
or are on order with both owners
and yards spread around the world.
The UT 755 L has become the
workhorse of the offshore industry,
having a good load capacity and
transport efficiency. It is popular
with charterers, and a favourite with
yards as a simple to build logical
design with low risk, especially since
the design is accompanied by all the
major equipment and systems from
Rolls-Royce.
Other variants have followed, to
meet particular market require ments.
For example, one short version has
been built, essentially the standard
UT 755 with a 5 metre section taken
out of the mid-body. This also met
a particular Petrobras ‘PSV 1500’
charter requirement.
The LN-version sports a large bow
bulb and hull lines optimised for
maintaining speed in big seas with
minimum fuel consumption. The LC
variant uses the same hull as the LN
and a revised superstructure. This
variant meets Clean Class rules.
Engine changes
Along the way, equipment
specifications have changed
somewhat to incorporate technical
advances. The biggest change is to
the engines. As the sturdy K-Series
reached the end of its production
life it was replaced by the newly
designed C-Series engines – compact
units 250mm bore by 330mm stroke
which meet emissions standards
without off-engine exhaust cleanup
equipment. LN and LC variants
have a simple electrical system to
reduce the zero pitch losses from the
propellers when at a platform, also
giving a higher propeller efficiency
in low speed mode. In this system
the pumps and motors accept
variable frequency from the shaft
generators in the 50Hz to 60Hz
range. This means that engine speed
and propeller revolutions can be
varied within limits while retaining
efficient electricity supply from shaft
generators.
This brings us to the latest variant,
which is fully diesel electric
and intended to meet future
requirements in terms of efficiency
and low emissions, without losing
the traditional virtues of the UT
755 concept. The UT 755 CD was
introduced in January 2008, and
since then Rolls-Royce has secured
orders for design and equipment for
a large number of vessels of this type.
Fast evolution
“The evolution of ship design is
currently running faster than ever,
and the market requires higher
environmental standards and
an improved safety level for the
crew,” says Svein Kleven, Design
Manager. “The extra length and
improvements in CFD techniques
allied to tank testing have enabled
the hull resistance to be reduced
by about 30 per cent compared to
the original UT 755 design, giving a
significantly better starting point for
demonstrating low fuel consumption
in transit conditions. Diesel electric
propulsion using azimuth thrusters
as the main propulsors also secures
a low fuel consumption in low speed
transit mode, DP mode and under
standby conditions. The Clean Design
standard gives a good protection
against oil pollution hazards and also
improved safety.”
“The UT 755 CD is also futureproofed.
For example, it meets the
MARPOL limitations for domestic
fuel oil storage in non-protected
fuel oil tanks and also covers future
legislative requirements such as
allowing space for water ballast
treatment systems.”
“Successful standardisation of PSV
designs requires a series of optional
multi-functionalities and optional
operational services to be taken
into account by the designers. Then
it is most important to integrate
these options with the minimum
of modifications to the generic
design solution. The way the design
is prepared defines the design’s
robustness.”
Freedom of choice
The UT 755 CD offers a series of
such options, including fire-fighting,
ROV services, standby and rescue
arrangements, oil recovery
arrangements, dynamic positioning
Class 1 or 2, PSV safer deck handling
systems, and offshore crane
arrangements with anti-heeling
systems.
A major improvement is the
flexibility in the choice of main
propulsion system. There are
always trade-offs: for example,
when evaluating azimuth thrusters
for main propulsion the optimum
system for transit conditions is
not necessarily the optimum for
DP/standby conditions. Azipull or
Contaz thrusters give the optimum
performance at transit speed, while
azimuth thrusters with nozzles
provide a higher performance
in DP mode. The final choice
depends on the vessel’s operational
profile, specifically the percentage
distribution of DP, transit and
standby modes.
“We have prepared similar inboard
arrangements and interfaces in the
UT 755 CD, enabling the customer
to choose between Azipull or
Contaz or nozzled azimuth thrusters,
depending on the expected area
of an operation and operational
profile,” says Svein Kleven.
The first UT 755 CD is due for
delivery in 2010 from Nam Cheong
Dockyard. A
13
VISION 1-2009. Rolls-Royce
B B
Many versions of the UT755 design. From left to right: the latest UT 755 now under construction; LC version Dina Mercur; LN showing
large deck area; Highland Rover (1st UT 755 L ) ; LN type Greatship Dhwan and UT755 Suffolk Supporter (1st UT 755).