to the most efficient levels via commands from the topside unit.
This approach does have limitations though, most notably in
multi-user applications. What may work for one receiver may be
completely unacceptable to another, due to noise levels, sensitivity
etc. Having separate controlling units with their own automatic
agenda would be a disaster for the less efficient receiver.
Multiple users are also a challenge to batteries, with most
systems being required to respond to additional interrogations
if more users are involved, and therefore using a proportionally
greater amount of power from the battery pack.
With regards to update rates, ping stacking can improve the rate
of position update but at a cost to the battery life. Ping stacking is
effectively increasing the interrogation and subsequent response
rate which obviously reduces the life of the battery.
Updaterate
The increased update rate of NASNet over conventional systems
is not at the cost of battery life, as each station is typically
only transmitting every 5 sec. A major gain over conventional
approaches is in the multi-user environment when multiple users
are using the same array. Even the latest generation conventional
systems require additional interrogations and response for multiple
users, and these impact drastically on battery life. By making use
of the one way transmission technique multiple users can position
from the same array with no increase in demand on the NASNet
stations, as each pulse can be received and used by an unlimited
number of receivers.
UnexpectedSIMOPS, learningfromMacondo
Unexpected or unplanned SIMOPS can encompass a scale of
situations from the requirement to accurately position, for example,
two ROVs instead of one from the same vessel, through to needing
to position multiple vessels unexpectedly.
It may not be the case that SIMOPS were not planned, rather
that the scale of SIMOPS were not anticipated.
Beyond question the most significant, and high profile,
SIMOPS challenge to date arose from the Macondo incident.
Multiple vessels, ROVs and subsea items working in extreme close
proximity provided ‘worst case scenario’ challenges from various
standpoints, including subsea positioning.
As the industry revisits its crisis management procedures and
assesses again the worst case scenarios, it is time to examine the options
for improving acoustic positioning in extreme SIMOPS situations.
The first step in such a situation is generally to reduce the number
of objects to be positioned, by identifying critical and non-critical
positioning requirements. In extreme circumstances the ‘critical’
point may be different from less extreme situations. For example
some vessels may not ‘have’ to use acoustics into their DP system,
and some ROVs may not ‘have’ to be acoustically positioned.
So by restricting the number of objects being acoustically
positioned the situation can be managed to fit the limitations of
the acoustic positioning systems. By deployment of a number of
different sets of subsea transponders it may be possible for different
objects to be positioned relative to different transponder arrays.
Conclusion
The use of a single NASNet broadcast system would provide
significant benefits to any positioning SIMOPS situation, from a
number of perspectives including safety, operation efficiency and
management overhead.
SIMOPS may be unexpected, but they should never be
unplanned.
Keep up to date
with us to hear the
latest in upstream
oil and gas news
Stay informed
