Offshore Northern Europe Revolutionary Seismic Vessel Sees First Action In North Sea

Aug. 28, 1995
John Greenway PGS Exploration AS Oslo A few years ago not many oil companies saw the central role that 3D seismic surveying was destined to play in exploration and production (E&P) economics, though its proponents were aware of its many advantages. Today, no oil company commits to a production campaign without a 3D seismic survey of the prospect. Indeed, many will not commit to an exploration well without 3D seismic support.
John Greenway
PGS Exploration AS Oslo

A few years ago not many oil companies saw the central role that 3D seismic surveying was destined to play in exploration and production (E&P) economics, though its proponents were aware of its many advantages.

Today, no oil company commits to a production campaign without a 3D seismic survey of the prospect. Indeed, many will not commit to an exploration well without 3D seismic support.

In a recent E&P poll carried out by Salomon Bros., 75% of oil company respondents cited 3D seismic as a technology having most impact on their business. Fifty two percent cited horizontal drilling, while no other trend or technology was cited by more than 7% of respondents.

At present, 3D seismic technology is unique in providing quantitative information about structure, reservoir continuity, and fluid content away from the borehole.

The strengthening position of 3D seismic in E&P campaigns has prompted seismic service companies to invest in new technology and designs.

The latest and most dramatic development of recent years is the appearance of a completely new vessel concept for marine surveying. The first of these vessels was launched in May and is called the Ramform Explorer.

Ramform design

The Ramform design is characterized by its extremely wide maximum beam of 40 m in relation to its overall length of just 80 m. The maximum beam is at the transom, giving the vessel an unusual delta shape.

At first glance it seems the vessel would be equally at home in a Star Trek movie as in the North Sea. Only one vessel of this design has been built previously, the Marjata surveillance vessel operated by the Norwegian navy and launched in 1993.

A project was started in the same year to review the capability of the design to be adapted for seismic surveying. The design had potential to solve the limitations of traditional hull design and held the promise of yet further advantages for this type of use.

Marine 3D seismic

In marine seismic surveying a vessel tows an acoustic energy source and one or more streamer cables, which contain highly sensitive pressure sensors known as hydrophones.

Acoustic energy sources, typically based on the emission of high pressure air into the water, are fired at regular intervals while the vessel traverses the survey area. Acoustic waves emitted from the source travel through the water column into the subsurface, with a portion of the acoustic energy reflecting from geological boundary layers that the wavefield encounters along its propagation path.

Upcoming reflected energy is recorded by the sensors contained in the streamer cables. After extensive data processing, principally involving the analysis of travel paths and signal-to-noise enhancement, the data give a representation of subsurface geological features.

Over the last few years, as improved towing technology has developed, there has been a dramatic increase in the number of streamer cables towed by seismic vessels. This increase is related to demands for improved cost efficiency.

Multiple streamers

Increasing the number of streamer cables towed behind a seismic vessel increases the width of the data swath recorded during a traverse of the vessel over the survey area.

The marginal costs involved in adding streamer cables to existing vessels are relatively low compared to the cost of the vessel itself, so increasing the number of towed streamer cables results in substantially reduced acquisition costs on a per unit of data basis.

As towing technology has developed, seismic fleet operators have invested heavily in increased streamer capacity. The first five-streamer vessel appeared in 1993, the first six-streamer vessels in 1994, and 1995 has seen eight and 12-streamer vessels entering operations for the first time. Each step has seen consequent reductions in acquisition costs for the vessel operator and end-user.

Economics

The cost efficiency of a seismic operation is related to the width of the data swath recorded by the vessel during a traverse of the survey area. The width of the swath is related to the number of towed streamers but, equally, to the lateral spacing of the streamer cables.

The vast majority of marine seismic surveys are acquired with a cable spacing of 100 m, giving an interval of 25 m between lines of subsurface reflection points.

In general, a closer cable spacing, and therefore higher line density, is advantageous, improving the resolution of the final seismic images. Since reducing the cable spacing reduces the width of the data swath acquired on each traverse, however, specification of cable spacing involves a trade-off of resolution against cost.

Nonetheless, falling costs associated with high streamer cable counts are encouraging oil companies to reconsider this trade-off. In the North Sea in 1995, several commercial high-definition surveys are taking place, coinciding with the introduction of new or upgraded vessels towing eight or more streamers.

Conventional vessels

The cost-effectiveness of conventional multistreamer surveys and the economic feasibility of high definition surveying are encouraging the seismic service industry to look at ways of increasing streamer cable counts still further.

This puts strain on conventional hull designs. Indeed, basic seismic vessel design has evolved little over the last 20 years or more. Traditionally, seismic vessels have been based on supply boat or stern trawler designs, adapted somewhat to the specific handling requirements of seismic equipment.

A typical seismic vessel has an overall length of 65-90 m, a beam of 15-20 m, and a displacement of 3,000-4,000 metric tons.

This type of hull design, while adequate for low streamer cable counts, has serious limitations when the streamer count goes beyond six. These limitations relate to stability, internal volume, motion characteristics, and power, as follows:

  • Stability. A full-length seismic streamer cable is between 3,000 and 6,000 m in length. It has a diameter of several centimeters and is filled with a liquid agent which endows the cable with neutral buoyancy when submerged.

    Together with winching equipment and cable positioning equipment, each streamer cable system weighs more than 20 metric tons.

    The weight of a streamer cable system on a conventional seismic vessel is positioned above the water line. While not necessarily threatening the security of the ship, these added masses for which the vessel was not originally designed can have a serious impact on the trim stability and loadline conditions of the vessel.

    The high form stability of the Ramform hull eliminates the need for extensive ballasting and thereby increases the vessel's useful load-carrying capacity.

    Ramform Explorer is designed to have a minimum capacity of 12 streamer cables, comprising a combined weight of cable and drums of about 240 metric tons, with no need for counter-ballasting.

  • Internal volume. A full-size cable drum has a diameter of some 4 m and a width of some 3 m. Traditional vessels do not have space for more than six drums, arranged in two banks.

    Additionally, instrument rooms on conventional vessels are hard-pressed to find space for the volume of computer and data monitoring systems that are required for modern seismic surveys.

    The Ramform vessel is broad enough at the transom to accommodate 12 streamer drums side by side. The instrument room on the Ramform Explorer has more than 400 sq m of floor space. This is around three times the space found on traditional vessels.

  • Motion characteristics. A ship in a seaway pitches about a point approximately amidships. This means the amplitude of pitch is greatest at the bow and stern. The motion of the stern generates high accelerations which are transmitted to the trailing gear.

    Jerking motions in high seas require that trailing gear be retrieved when wave amplitude increases to 3 m or so. Deployment and retrieval for full-length streamer cables and source arrays for a six streamer vessel can take up to 24 hr.

    This time increases as the number of streamers increases. In areas such as the North Sea which suffer unstable weather patterns, fast deployment and retrieval are important to be able to make use of short windows between spells of inclement weather.

    This is the area where the Ramform design most spectacularly outperforms traditional vessels. Test figures have been based on weather data from the Statfjord area of the Norwegian North Sea collected over the last 10 years.

    As a measure of stability, the analysis suggested that for this period Ramform would have been stable enough to allow helicopter operations for 94% of the total time.

    Extreme stability is particularly significant for seismic operations in three ways. Firstly, the test results show that even in bad weather accelerations at the stern of the ship are modest.

    This implies that even during very poor weather, trailing equipment can be left in the water with little risk of damage. That in turn means that equipment need not be continually retrieved and deployed during spells of changeable weather, resulting in significant efficiency gains.

    Secondly, the results show that for Ramform Explorer the freeboard of the transom can be low, improving accessibility to the sea for the handling of equipment. Freeboard at the transom on the Ramform Explorer is just 0.8 m.

    Thirdly, vertical acceleration readings show that the vessel is an effective work platform for the crew. It is vertical acceleration which first affects human working capacity as weather deteriorates. The limit for manual work is usually set at 0.4 times the force of gravity (0.4 g), and the limits for effective intellectual work at 0.2 g. In tests, recorded values were all well under these limits.

  • Power. Hydrodynamic deflectors are attached to each side of the vessel to create the lateral forces required to attain a wide spread of streamer cables. The drag that the deflectors generate in the travel direction of the vessel, combined with the drag of the insea equipment, reaches tens of tons. Traditional seismic vessels generally need re-engining in order to cope with this.

The Ramform vessel possesses three propulsion units of equal output. The Ramform Explorer is powered by three azimuth thrusters, rotatable through 360, each with 4,000 hp at the shaft.

This gives a thrust at towing speed of about 70 metric tons, compared with conventional vessels where thrust is generally less than 50 metric tons.

Noise

Large engines normally fitted to conventional seismic vessel create structural noise and noise from propeller turbulence. This creates acoustic noise which can contaminate the seismic data.

The Ramform drive motors are ac electric motors, with current supplied by a large diesel generator in the bows of the vessel. Two of the thrusters are located at the stern, with the third at the bow. The drive and generator positioning, combined with shock mounting of all mechanical equipment, gives extremely low acoustic noise output.

Safety

While today's seismic vessels can be considered safe working platforms, there is considerable scope for enhancements in offshore safety through vessel design. This relates primarily to the back-deck work area environment, vessel motion characteristics, and seakeeping capabilities.

Ramform motion characteristics, as described, contribute to safe operation in terms of the effectiveness of the crew, reduced requirement for equipment handling in poor weather, and extended helicopter operations for medical evacuations.

Vessel security is improved by the possibility of building watertight chambers inside the hull. It has been calculated that the vessel would survive flooding as a result of damage which opened a complete hull side to the sea. There are no plans for a full-scale field test of this capability, however.

Ramform in action

Ramform Explorer began its operational life in U.K. North Sea Brent field, operated by Shell U.K. Exploration & Production, on May 29. The job is a high-definition survey, around 260 sq km in extent, centered around the field's Bravo platform.

The survey specifications call for an 18.75 m line spacing, use of a single source for high-fold recording, and 3,600 m streamers. The survey reads likes a seismic Guinness Book of Records, with its first-ever eight-streamer deployment, the most streamer cable ever deployed by a single vessel, and recording of more data samples per sq km than ever before.

The objective is to create a detailed image of the reservoir to help in squeezing out remaining oil from renovated installations and to extend the aging field's life as a gas producer.

Based on this first experience in Brent, Shell decided to retain the Ramform Explorer for a similar survey over the Tern/Eider complex to the northwest. As a testament to the efficiency of the Ramform design, following a spell of bad weather early in the Brent survey, the vessel was able to recommence work 24 hr ahead of conventional vessels in neighboring blocks.

There are a number of large fields in the North Sea at a similar stage of their life cycle as Brent, and it is expected that demand for high-definition surveying will increase sharply in the future, providing the ideal marketplace for the Ramform Explorer and a sister ship, due to be launched in early 1996.

The delta-shaped hull of Ramform Explorer increases stability of the vessel in high seas. This reduces the need for streamer retrieval in rough weather and increases vessel up-time.

The Ramform Explorer can hold down costs by surveying with a wide swath at a normal 100m cable spacing or increase resolution over a conventional swath with reduced cable spacing.

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The Author

John Greenway is in charge of market communications for PGS Exploration AS, Oslo. He has 16 years of experience in the seismic industry, including data processing, system sales, marketing, and management assignments in various locations around the world.

Greenway holds an honors degree in geology from Oxford University.

Copyright 1995 Oil & Gas Journal. All Rights Reserved.