54 |
Oilfield Technology
August
2015
of communications. In addition to the difficulty in finding an
acceptable and safe solution for the planning activities, a further
issue is the removal in flexibility of operations. If all of the non
interfering codes have been assigned, what happens when an
additional item needs to be positioned?
Multi-user challenges
Multi-user acoustic positioning is clearly one of the recognised
challenges associated with SIMOPS, and along with the obvious
interference risks of traditional acoustics systems, there are
other concessions to be made when carrying out multi-user
positioning.
The frequency of position measurement is important for moving
objects – obviously with slower updates in position the inaccuracy
and uncertainty increases. Conventional acoustic systems are
subject to a decrease in position update rate if more objects are to
be simultaneously positioned, which for DP purposes reduces their
credibility and for subsea positioning reduces the accuracy and
usability.
Battery life is also an important consideration – if the beacon is
interrogated more frequently the battery is therefore drained more
quickly and needs to be replaced, costing vessel time and money.
Multiple interrogators of a seabed array will proportionally decrease
the life of the battery, which will certainly have an impact on cost
and can also cause significant schedule variations.
These were key areas of focus during the development of the
NASNet® system, which aimed to provide a truly multi-user system
where multiple users could use a seabed array simultaneously with
no risk of interference and no degradation in the performance, or
battery life, of the system.
Overview
NASNet is essentially a long baseline (LBL) acoustic positioning
system but the most fundamental improvement over conventional
systems is in the method used to measure the ranges from which
positions are subsequently calculated. Pulses are continually
transmitted from NASNet stations at preset intervals, typically
every 4 - 5 sec. No interrogation of the station is required to prompt a
response, unlike conventional systems. This passive receive technique
allows multiple simultaneous users (no ‘frequency management’) and
provides high update rates for all positioned objects.
The benefits provided through this approach are significant with
respect to dynamic accuracy, latency and battery life.
Accuracy
The dynamic accuracy is largely linked to latency – the age of
the ranges used in the solution, and the age of the solution itself
(position, update rate). Use of one-way ranging removes the
majority of latency in the solution in two ways. Firstly, the range
itself is subject to no latency error when received by the positioned
object; because the time of reception is time stamped.
The second difference is there is no fixed cycle of interrogation
and receive, ranges can be entered into the position solution
immediately, minimising the latency between measurement of the
range and calculation of the position. NASNet uses each range as
soon as its received, replacing the previous range from the applicable
station, and therefore ensures a more up to date position solution.
The system recomputes when any individual range is received,
rather than waiting for a complete set of ranges. This has the effect of
increasing the frequency of position update significantly. The actual
frequency is dependent on both the transmission rate of the stations,
and the number of stations being used. An example of this is when
using five stations with a 5 sec. transmission interval would provide,
on average, 1 Hz position updates. The same scenario but using 10
stations would increase the update rate to 2 Hz. This update rate is not
only equal to GNSS, but better, meaning the NASNet acoustic solution
can be weighted as heavily as GNSS within a DP system.
Battery life
Although battery life does not directly affect the performance of
an acoustic system, practicality dictates that a longer battery life
is desirable for reliability and the logistics and time involved in
changing units or batteries in deepwater.
Most users will err on the side of caution where noise is
concerned, tending to set the transmission power to a high value
to ‘guarantee’ reception. One fairly elegant solution to this issue is
the continuous monitoring of signal-to-noise ratio at the receiver.
The volume of the transponder can then be automatically adjusted
Figure 1.
Subsea fieldwithNASNet® equipment.
