Seismic vessel footprint more relevant with 3-D

Accurate way of defining capacity for wide tows

Victor Schmidt
International Editor

In the early days of marine 3D seismic acquisition, the capacity of the world's fleet was considered in terms of numbers of vessels, the same way as it was for 2D seismic. As vessels were upgraded to tow multiple streamers and new ones launched, the relationship between vessels and capacity became less meaningful, so the number of streamers towed was also considered.

Now, with a wide diversity of 3D configurations, even the total number of streamers is no longer a reliable estimate of real capabilities. Vessels with a similar total capacity are likely to tow different numbers and lengths of streamers in different markets.

For example, a large vessel in the Gulf of Mexico might tow five or six streamers. Each would be typically 150-200 meters apart and 6,000-8,000 meters long. Towing long streamers is not simple, as it requires extremely reliable in-sea equipment and a very efficient operation, but the geophysical challenges of the region demand it.

The vessel, which tows six 6,000-meter streamers in the Gulf of Mexico, will have a total vessel capacity of 36,000 meters. The vessel also must achieve a total spread of 800 meters. Six streamers, 160 meters apart, will give a CMP bin width of 40 meters, almost standard in the Gulf of Mexico. For operations in the North Sea, streamer lengths are typically 3,000-4,500 meters, and the separation between each streamer is 50-100 meters. Given sufficient tow points, the same capacity vessel could tow up to 12 streamers.

Key factors

So, the capacity of a given vessel is defined by several factors, any one of which will define the operational limit of the vessel under different circumstances. These are:

Recording capacity
Number of streamer tow points.
Streamer storage capacity
Maximum streamer length
Spread width

For most purposes, recording capacity is rarely a limitation using the latest systems, and older instruments can be doubled up if needed. Streamer tow points are also less of a limitation. The practice in recent years has been to add some extra tow points in addition to the number of streamer reels, such that streamers can be "fed off" and split into two if required. This is useful in high-resolution 3D surveys. The three major limitations are total streamer storage capacity, maximum length of any one streamer and the towing spread.

Storage capacity

Total storage capacity is a major limitation. Of course, a vessel can never deploy more kilometers of streamer than it can store on board. With the advent of narrower streamers, more kilometers can be put on each reel and total storage increased.

Despite this volume reduction, the 30 km or more of streamer carried by a typical medium to high capacity vessel still represents several hundred tons of valuable equipment to be stored on board and deployed rapidly and safely.

Streamer length

Maximum streamer length is limited by the type of streamer used. The industry in general is very interested in having longer offsets. Using one vessel, this can only be achieved by deploying long streamers.

Most streamer systems on the market today are capable of operating up to 6,000 meters in length. Western was the first company to tow beyond this. Now, both Western and Geco-Prakla are operating large multiple 8,000-meter streamer spreads in the Gulf of Mexico.

Spread width

Total spread width is defined as the maximum achievable separation between the outer streamers. A wide separation (more than 500 meters) is difficult to do as it requires a great deal of force to pull the streamers out sideways. This is normally achieved with angled deflectors, of which there are various types available.

The deflectors used by most seismic contractors are Barovanes, which consist of a number of angled vanes suspended from a surface float. This demands significant towing force to achieve wide separations, and is limited to 600-800 meter spreads. Geco-Prakla developed hydrodynamically efficient single wing-shaped deflectors, called Monowings. These are claimed to offer higher lift and are also remotely controllable, unlike conventional deflectors.

To date, only Geco-Prakla has succeeded in towing wider than 800 meters with a single vessel. PGS has achieved wider separations using additional vessels attached to the edges of the spread to pull it out wide.

Vessel footprint

The most challenging configuration for a 3D seismic vessel is towing multiple long streamers with a wide spread. This can be quantified by a single factor, the "vessel footprint," which is defined as the maximum individual streamer length multiplied by the separation between the outer streamers.

If the 3D spread is thought of as a carpet, this is the size of that carpet as the vessel moves through the water. Clearly, the larger the area covered by the streamer carpet for each shotpoint, the better the quality (in terms of offset) and the more efficient the operation.

The vessel footprint serves as an accurate way of defining vessel capacity for wide-tow 3D exploration surveys. Of equal importance, is the capability of a vessel to shoot high-resolution 3D and 4D surveys. The demands on a vessel for these dense spreads are more dependent upon the experience and capabilities of the crew than upon brute force and wide spreads.

The ability to tow and change lines, while maintaining very accurate narrow streamer separations, is also important. In this case, the total number of deployable streamers is less critical. These surveys have generally been small, and have rarely warranted the use of a very high capacity seismic vessel.

Acknowledgements

The author wishes to thank Robin Walker, Marketing & Commercialization Manager-Marine and John Kingston, Marketing Services Manager of Schlumberger Geco-Prakla for their assistance in preparation of this article.