THE REEL THING
The movement of the oil industry into the sea has, over the years,
required mariners to develop whole new areas of skill, and whole new
levels of nerve to carry out the unusual tasks which the industry requires of the support vessel.
Possible one of the most
unusual tasks is that of seismic survey, which used to consist of towing
two miles of cable behind a small ship which went bang every ten seconds.
Even earlier than that it is said that a surveyor would stand on the stern
of the small ship throwing explosive charges into the water, and it was
not until some-dropped one onto the deck instead of into the water that a
serious effort was made to find an alternative.
The process of throwing the
charge over the stern and picking up the resulting sound waves with one
geophone produced a load of lines on a chart which gave the geologists an
idea about the substrata. It was a small step to go from that activity to
having the ship moving through the water firing the charge at regular
intervals and picking up the resulting traces with a number of receivers -
geophones which gave a better picture of the line beneath the surface.
This is known as 2D seismic, and the explosion is created by compressed air guns .
The use of more than one
cable provides those viewing the resulting pictures of the substrata with
a "3D" view. And to achieve this modern seismic vessels tow
multiple cables, or "streamers" and arrays of air guns. Indeed
the required bollard pull means that a modern survey vessel should ideally
have something like 15,000 bhp available.
Finally there is 4D seismic
where the fourth dimension is time. 4D requires the ship to lay a cable on
the seabed and then for it, or possibly a second ship to fire the charges.
Because the cable is stationary the system can successfully detect changes
in the reservoir, hence such techniques are frequently used in mature
fields. This system is known as OBC (ocean bottom cable).
It might appear that these
ships sail randomly about the surface of the planet letting off small
explosions and collecting the results, but in fact the process is fairly well defined.
The initial task is to gain
general information about a prospect and this is often done on a
speculative basis by the operators of the ships when they are not on hire
to anybody. They will follow long lines, say 10 or 20 kilometres drawn a
kilometre apart in two directions. This work is usually carried out on
blocks which are to be licensed, and operators who are interested in
bidding for the blocks will buy the work to check on the value of the prospect.
Thereafter more detailed
work is required so that the geologists can advise the operators where to
drill the holes and finally further work may be required to update
existing information using more modern technology on areas which were
initially surveyed during the dark ages.
The marine problems
associated with these activities are many and not least is the fact that
all the cables are of high value and must be protected, event though they
may now extend six or eight kilometres astern of the ship. In addition the
geophones are sufficiently sensitive to pick up engine and propeller
noises from any vessels in the vicinity and any other seismic vessel working within a 50 mile radius.
The latter problem can be
particularly disturbing for the operators of such craft who usually get
paid for their work by the kilometre. Two vessels will bang away until the
noise - in seismic terms - becomes intolerable and then one will have to
call up the other and come to an arrangement to take it in turns, shooting lines in rotation.
Interference from some-one
shooting over the horizon is irritating, and, if it goes on, costly, and
so the two ships must co-operate to get their jobs done, or if possible
re-locate so that one is out of range of the other. If the re-location
really cannot be done then they have to take it in turns, and therefore
some precision is required to ensure that one ship is lining up to start a
line just as the other comes to the end of one. This timing and turning
used to be one of the skills developed by the marine staff on board the survey vessels.
The very activity of
navigating down a line and knowing where the ship is has changed
immeasurably over the years. In the 1970s positioning systems were quite
primitive. Satellite navigation was used or occasionally Decca Hifix, and
if greater accuracy was required Syledis or Artemis might be established
on board platforms or on the coast. Often the lines shot on spec would use
satnav, just to keep the cost down. The line would be drawn in the
computer and then presented on a screen on the bridge. Some-one, usually
the watchkeeper would then steer down the track on the basis of distance
off to port or starboard - plus to starboard, minus to port. Even with the
assistance of the autopilot this required some concentration and skill,
and no matter how well one was doing, a new satellite would come up over
the horizon, the system would acquire new data and suddenly the ship would
be 100 meters off the line.
Today DGPS is available to
all anywhere in the world and as a result the survey ships know where they
are more often. But even if the navigation is easier, the multiplicity of
cables makes the job a nightmare for the shipmaster. The cables are
positioned about 100 meters apart and are about six kilometres long, the
result is a ship effectively six hundred meters wide by several kilometres
long. It is no wonder that the industry used what are known as "chaseboats"
whose task is to guard the cable from approaching vessels.
It is amazing how difficult
it is to contact ships at sea. They appear to be moving robot like across
the surface of the planet without human intervention, or at least that is
how it seems to the survey ship watchkeeper. They vary in terms of danger
or irritation but all are of concern. Although damage to the cables by
merchant ships can usually be avoided by diving the cables the activity
still destroys the run and the ship must go through the laborious process
of returning to the start point and going though it all again. This is
some task when one considers that it may take a couple of hours to execute a 180 degree turn.
Diving the cable? At
intervals along the length of each cable small objects which look like
flying fish are attached. Their wings can be angled electronically and
collectively they can alter the depth at which the cable travels through
the water. This depth is in itself important to the survey team who
usually have an upper and lower limit as two of their parameters. Other
parameters include speed through the water and extraneous noise levels and
all are monitored by the oil company representatives - if the task is
being carried out on behalf of an oil company.
This means of altering the
depth of the cable in the sea is therefore used to save the cable from
damage by the hulls of large ships. Fishing boats with their gear down are
an entirely different prospect and need to be avoided, contacted or fended
off by the chase boats. Experienced survey ships crews get to know many
tricks and some of them relate to means of getting ships to alter course.
The simplest technique is to call the approaching vessel, give the survey
ship position and its position in relation to the other vessel get an
answer, tell the other ship what's going on and see it alter course.
Some-times it is possible to go through the first part of the process but
impossible to get a reply on the VHF. It may now be best to issue the
approaching vessel with an instruction as to what course to take up to
avoid the cable. This often works, indicating that many officers of the
watch are either afraid to answer he VHF or possibly not allowed to do so.
There is also the possibility, particularly in the case of fishing fleets,
that absolutely no-one is listening to the hailing channel. In this case
it is necessary to search the airwaves until a whole bunch of people
talking about fish can be heard - interrupt, issue instructions, carry on.
Chase boats have their own
tricks, rushing up to approaching merchant ships and firing flared on to
the bridge wing to or approaching in the dark with one side-light out -
giving the impression that they are on a collision course. Some-times non
of it works and lines have to be aborted, or even worse, cables get damaged.
Ideally the survey company
likes to keep the cables in the sea astern of the survey ship since
recovering them is time consuming and can result in damage, and so it
sometimes necessary to repair defects by using a small boat, and a number
of these were featured in the August 1999 edition of OSJ. Shipmasters are
generally loath to let their people drift off in an uncontrolled fashion
in a small boat but never-the-less this is what the captain of the survey
ship is required to do. He may slow down a bit while the boat is launched,
but once in the water it is on its own, or very nearly. The coxswains of
these little craft therefore must be level headed and skilled and the boat
itself must be sufficiently powerful to catch up with the mother craft
which is inexorably moving forward at four or five knots dragging its multiple streamers behind it.
Most survey ships also have
helidecks for crew changes and some refuel and store at sea, providing
even more interesting problems for the Master, because of course, if the
cables are going to stay out then the ship must be moving and it becomes
necessary for the two vessel to match speeds and for the refuelling vessel
to come alongside. Toisa, the featured ship operator in this issue had a
couple of their small platform ships on hire to a survey company, and
while they were no longer ideal for the North Sea they suited this task in
the calmer waters of the southern Atlantic.
The task of processing the
data has changed in line with the changes in the means of acquiring it,
and the development of both computer hardware and software has influenced
the activity. Early on, before the days of the PC, all the data was
collected and then offloaded at the base port for processing. Then with
the increasing power of computers it has become more common for the data
to be processed on board ship, or for at least part of it to be processed
on board, since the whole job still takes some months to complete. This
year Western Geophysical are using something called "SeismicStar"
which uses geostationary satellites and NASA ground stations to transfer
raw data, and therefore allows the whole job to be done on shore. It still
takes a long time but the clients can have a peek at the results and may
be able to change or reschedule tasks for the survey ship while it is still on location.
For those who appreciate
techno-speak Mr William K. Aylor of SpaceData International had this to
say about the system at a conference in May of this year. "Local
onshore processing is needed to deliver pre-stack depth migration of the
decimated, onboard processing data set, and complete reprocessing at the
main onshore processing centre is needed to deliver a full fold fully
tested and processed data volume."
Well you can't top that.
First published in the Offshore Support Journal in 2000
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