Vessel functions converge to form "seismic data factory"

Rethinking the data gathering process

Victor Schmidt
Exploration Editor

The Geco Eagle displays the multiple tow points of its distinctive streamer-deck "wing."

Aft view of the vessel while under construction at Mjellem & Karlsen, Bergen, Norway. Shown are (left to right) the 1.5-meter gun deck wall for crew protection, deflector bay with wave-wall lowered, and rear slipway for source array deployment. [31,910 bytes]
Remote-controlled tow points lower streamers for attachment of telemetry equipment. Project Manager Arild Viddal is at the right. [37,332 bytes]

Schlumberger Geco-Prakla's new seismic vessel, the Geco Eagle, incorporates a proven ship-shape hull, a distinctive "wing" borrowed from aircraft carrier designs, and a rethinking of the marine seismic acquisition process. The product is what the firm calls a a "seismic data factory."

This "Bird" class vessel is an extension of the proven Gem class fleet and a response to the industry's continuing need for lower unit costs, through higher volume data sets. According to Arild Viddal, Marine Technical Manager and Product Manager for the newbuild, "One of our chief focus areas for the last nine years has been the handling and towing of in-sea equipment. We believe we should offer lower unit costs and proper turnaround for the client. We have to put more streamers in the water, and at the same time, do that in a safe and efficient manner." The Bird class is designed to tow streamers more efficiently.

The firm used quantified risk assessment to sort through the seismic acquisition work processes onboard the new vessel to discover ways to be more cost effective, improve acquisition, and operate safer. While an outside advisor helped the firm focus thinking, the experience of the firm's people and knowledge of details led to the new design. What resulted is a series of fully integrated material handling and work flow improvements that allow the Geco Eagle to achieve the following:

Carry up to 120 km of streamer, nearly doubling the highest current streamer capacity used on the Geco Triton
Tow up to 20 separate streamers
Deploy streamer spreads of over 1,400 meters, without the aid of a second vessel
Remain at sea for up to five years before making a port call.

Known hull design

The firm chose to stay with a ship-shape trawler design modified for seismic towing. Viddal commented: "We have invested in smart technology, which goes into the water. We don't want to invest in a vessel, as such, a towing platform. We prefer to invest in technology that goes into the water. That is really what the client is paying for." By using a well-known ship design, the firm was able to keep newbuild costs reasonable and focus on technology and workflow improvements.

Two rear screws power the hull while a forward azimuth thruster was installed to maintain forward movement in case of engine failure. This will keep the streamers in tension and prevent them from tangling. A bow thruster was added for turning flexibility in port. All the screws are fully variable electric systems powered by large diesel-electric generators.

Although not now required by current law, the firm made the vessel double-hulled with a 1.8-meter separation between the surfaces. This provides survivability in case of collision. The gun deck has a rear slipway for deployment of source arrays and retractable wave walls for easier deployment of deflectors. For the safety of the gun deck crew, 1.5-meter walls rise from the rear of the vessel and the slipway is gated when not in use.

Wing adaptation

The distinctive "wing" of the vessel is an adaptation of a design used by aircraft carriers to expand deck space. In the case of the Geco Eagle, the wing creates storage space for four streamer reels, two each in the outboard corners. The shape expands the useful midship space to stow sixteen reels in two banks of eight. This configuration improves vessel stability by placing the streamer mass near the vessel's center-of-gravity.

For standard rear-reel vessels, streamer deployment creates a fore-aft shift in the vessel's center-of-gravity. This new design creates mainly vertical displacement in the water, because of reel placement and the mass of streamer carried. It is easier to adjust for such movement and maintain the vessel's stability while deploying and towing streamers.

The added deck space increases the storage capacity of the vessel and the streamer handling space available to the crew. It keeps the deck free for material handling and improves crew safety.

New technologies

Viddal explained that just as the industry's needs led to the building of the Gem class fleet, technical advances now require a new towing platform to produce a new level of integration and performance. He said the in-house capacity to build equipment and optimize performance was utilized producing improved:

Towing leads: The towing lead, deflector, and streamer can be thought of as a kite in the water. The lead is the string, the deflector is the kite, and the streamer is the kite's tail. Any equipment in the water must be directly tied to the vessel by very strong cables. As the streamer spread increases in width, the tension and drag forces on the towing leads also increase. The firm has moved to a lightweight cable that is thinner than standard cables. This reduces drag forces creating a more efficient system.
Sources: The recent major advance found for the source arrays is in the source umbilical and distributed source controller. A fiber optic cable is used to control the guns and send signals back to the bridge. This results in a longer, lighter umbilical that can place the guns closer to or within the front of the streamer array to produce a near vertical offset, a goal and request of many geophysicists.
Deflectors: These devices carry very large forces and produce significant drag that the vessel's propulsion system must overcome. They usually require additional cables to hold against the lateral forces. Standard deflectors also "bite" as soon as they enter the water, creating a safety hazard for workers. Like flat kites in air, standard spread deflectors are very large flat "doors" that are dragged through water. Chain and cable systems manually set the deflector "attack angle." Like a highly maneuverable airfoil-shaped kite, the vessel uses the Mark II Monowingtrademark, an improved version of Mark I technology. The main improvement is in the extension of the waterfoil, making it longer to provide more outward lift. The device "flys" through the water, instead of being dragged.
Lift created by the foil is adjustable by a computer-driven actuator between the foil and the streamer. The actuators are controlled from the vessel. Two leads connect each deflector to the vessel creating less drag than the standard multi-cable configuration.
The new deflector is set for minimal attack angle during water entry. The angle is then gradually increased to create the full spread width. This prevents abrupt tensional stresses on the leads and provides more control over spread deployment. The engineer in charge of the spread can modify the attack angle while the vessel is moving to maintain the spread width, correct for skewing current forces, or shift the array to assure full bin coverage. "One pass coverage is the goal," Viddal explains. With fewer in-fill passes more data is collected and this keeps the vessel shooting (online) and maintaining high productivity.
Streamers: Like a kite's tail, the streamers provide a measure of stability for the deflector. They are the carriers for the hydrophone sensors and the array-positioning network "birds," attached by the crew to the outside of the streamers.
The firm's standard Nessie 3 streamer is 67 mm in diameter. The improved Nessie 4 system has been reduced to 54 mm in diameter. Streamer diameter is not that critical while towing. The drag caused by the front of the streamer is small, compared to other forces.
Streamer diameter becomes very important for vessel weight and space limits when stowed. The smaller diameter system allows the Geco Eagle to carry significantly more streamer, which allows longer offsets to be deployed. More streamers mean more flexibility of array options both in bin-density and array size.
Deployment system: An overheard conveyor system moves streamers from the central reels to the rear tow points while under tension. A new control system lowers the overhead tow points and streamers to a comfortable work height, about 1 meter, for the attachment of telemetry birds and in-sea positioning network components. Once attachments are done, the streamer is returned to the overhead position and spooled out.
This new system is ergonomically friendly. It prevents the crew from lifting and fixing birds to each streamer in an extended vertical position,. By lowering the working height, both crew safety and physical stability are improved. All streamer conveyors and tow points are controlled from a central operator console, an idea borrowed from highly automated drilling systems on modern semisubmersibles. The operator monitors the rear deck with video and is in constant communication with the bridge.

Five years at sea

The Geco Eagle is designed to stay at sea for five years, in order to increase the "online" time of the vessel and improve overall productivity. However, it creates the significant logistical challenge of feeding and servicing the vessel. Crew changes, provisioning, equipment replacement, and refueling all must be done without a port call.

Seismic streamer replacement and calibration, as well as battery replacement for birds are also key issues. These are normally done in port while provisioning between jobs. To solve these problems the marine architects added a starboard-side crane, a central opening from the top deck to the gun deck large enough for a 10-ft container, and a series of funnels between the top deck and streamer deck.

Crews will be changed by routine helicopter flights using the bow helipad. Containerized provisions and equipment will be sent by supply vessels and transferred by the crane. Containers will be stored on the upper deck and contents conveyed to the ship's interior by two elevators. Used equipment ready for onshore repair and anything to be returned for shore disposal will be placed in a container and returned to the supply vessel. The goal is to re-supply with a minimum number of lifts.

New seismic streamers will be delivered on a spool in 1-km increments. The funnels on the top deck are used to thread new streamers down to the streamer deck where they will be transferred to the proper reel.

Servicing

Streamer and bird servicing will be done both on the vessel and in the water. The service launch was upgraded to 32-ft length providing enough space for three 100-meter streamer sections. In-water repair or replacement of damaged sections can occur without disturbing the progress of the shoot. The launch can also replace batteries and conduct repairs on in-water equipment.

Streamer calibration is critical when the vessel is moving from one exploration region to another. Adjustments for spread configuration and hydrophone groups are normally done in port and take several days. The Geco Eagle will carry a set of pre-calibrated reference streamer sections or they can be delivered in a normal provisioning cycle.

While the vessel is relocating to a new exploration area, the crew can reset and calibrate the streamer configuration for the next job. This turns normal vessel downtime into productive time and increases the quantity of data collected in a year.

Crew training

Such a high-capacity system requires two available crews of 40 members to maintain the 24-hr, 7-day/week operation. It also requires that those crews be proficient and able to do their work with a minimum of downtime.

Schlumberger Geco-Prakla has invested in virtual reality training for the new streamer deck operation. Over $20 million in water-based assets will be at risk. In this light, the expense of crew training is an effective way to mitigate risks. There is classroom space aboard the vessel for crewmembers to upgrade skills and knowledge enroute.

The Geco Eagle is a response to industry's desire for a low cost, high capacity seismic vessel to produce large, high-quality 3D data sets. The firm has created a fully integrated, high-production system for seismic acquisition and onboard processing. Using the knowledge and experience of fleet crews, workflow processes have been enhanced and a new towing platform has been built around those processes.