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OFFSHORE SUPPORT VESSEL REVIEW
“W
hen designing controls for an active
heave winch you have to think
differently,” explains Scantrol's MD
Helge Hammersland.
Scantrol, although well established in the field of
more conventional winch controllers, has seen a
recent rise in interest in Active Heave Compensation
(AHC) systems “and you have to understand the
pressures the new technology puts on the whole
system in order to be able to integrate controls and
winches effectively”, he adds.
The rising interest is easy to understand: AHC
systems filter out the effect of wave movement,
allowing not just for more accurate operations and
readings but also fewer weather-related down times.
However, the dynamics of winch and wire in these
AHC applications, are, says Mr Hammersland, very
distinctive as they are constantly running from one
direction to another and what's more, “the changes
have to happen in real time as the paying out and
taking up needs to be very fast, accurate and smooth”.
RESPONSE TIMES
So, what exactly does 'fast' mean? “While a more
normal winch might be doing well if it's running from
zero to full speed in a few seconds, the response times
for an active heave winch can be down to a hundred
milliseconds,” he explains, adding: “The issue is that
you have to minimise the delays at every step
between the sensors and the winch reaction.”
‘
The AHC ‘toolbox’ does the job of outlining critical factors
Real time responses:
a very different game
Given any amount of
money, yes, you can Active heave requirements push winch parameters
easily do it - but the trick from one direction, while costs push from the other,
is to do it on a budget c writes Stevie Knight.
’
Helge Hammersland
Given any amount of money, yes, you can easily do
it - but the trick is to do it on a budget. Mr Hammersland
points out that profit margins being what they are, you
simply cannot over specify, or you'd price the
equipment out of any realistic market. “So, you have to
find a practical solution.”
However, the requirements aren't that easy to shape
as the areas of operation have varying characteristics says
Mr Hammersland: “If you are designing for operations in
the Far East you have to compensate for a wave motion
of around 5m, while in the North Sea you need to be
fully compensating for up to 6m or 7m heave.”
It's not simply the waves that need accounting for. Mr
Hammersland explains: “While Angola has a lesser wave
height and generally calmer seas requiring a slower
response from the winch, it also has much deeper water
and operations can be down to 2,500m. Even the weight
of the wire gives you much higher loads and if the winch
is carrying just a few tonnes, it puts a huge strain on the
system - especially if the ship is listing slightly.”
To help model the requirements Scantrol has
developed an 'AHC toolbox' for winch designers which
includes a number of interesting applications. The AHC
36 www.motorship.com
Analyzer, for example, does the job of outlining critical
factors and minimising the power investment. It
allows for simulation and testing at an early stage and
it can also be used to determine the performance of
an existing system. “With the collaboration of the
winch or crane designers it means getting things right
first time,” says Mr Hammersland.
WINCH DYNAMICS
The 'box' also includes the Dynatest tool; this records
the winch dynamics in a workshop run and allows
verification of the AHC characteristics using recorded or
simulated ship movement data.
Further, there's a tool that predicts performance
before the start of lifting operations based on
information from present vessel movement and
present or planned loads, plus performance and data
logging tools which show the percentage of winch
capacity being used to help guard against overloading.
Finally there is a distance monitoring and updating
application.
“As we supply to a lot of different crane and winch
manufacturers we already know what the systems will
do,” concludes Mr Hammersland. One of the points in
favour of a standardised approach is that “there's not
a lot of time spent doing prototype testing,” but he
does add that Scantrol leaves room for adaptability
and has recently been involved in developing a
flexible control system that can be used for different
application areas on the same vessel.
‘
If you are designing for operations in the Far East
you have to compensate for a wave motion of
around 5m, while in the North Sea you need to be
fully compensating for up to 6m or 7m heave c
’
February 2013