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

TO RETURN TO FEATURES INDEX CLICK HERE