UnderWater Magazine




Article reprint: Fall 1997
"Subsea Pipelines: From Survey to Start-Up New Technologies Conquer the Ocean Deep"
By Howie Doyle and Susan Troscinski

With onshore prospects for precious hydrocarbons facing inevitable depletion, an increasing number of the world's oil producers are looking to the ocean for sustenance. According to an April 1997 article in Offshore magazine, present known offshore reserves are estimated at an incredible 200 billion bbl, proving that we can harvest more from the ocean than just fish.

Opportunity of this magnitude is not without its difficulties. The ocean floor presents us with plunging ravines, yawning canyons and towering mountains, creating underwater seascapes that challenge even the most experienced producers. Yet significant progress has been made. Pipelaying capability in deep water has increased from roughly 500 feet in the 1970s to 5,400 feet today, with some experts predicting the plunge to 12,000 feet in the not too distant future.

Fortunately, recent advancements in pipeline technologies - from surveying to installation - are furthering the exploration and production of oil in areas deeper and more remote than ever before. This article represents the first in a two-part series on subsea pipelines. Our discussion will continue in the winter issue of UnderWater magazine with a comprehensive look at pipeline inspection and maintenance.

Surveying: A Prerequisite

Comprehensive, high-quality surveys are crucial to the planning and development of subsea pipelines, particularly at greater depths where both the complexity and the cost of the operation increases. While all pipeline projects are subject to certain minimum surveying standards (in the Gulf of Mexico these are stipulated by the Minerals Management Service, a division of the U.S. Department of the Interior), the project site and scope are also key factors in determining which surveying techniques to utilize.

Side-scan sonar techniques use a sophisticated frequency device housed in a remotely operated vehicle (ROV) or towfish to obtain seabed surface data which is then translated into a high-resolution topographical map.

The technique, which has both shallow and deepwater applications, continues to advance through the use of state-of-the-art digital data acquisition and interpretation software such as Elics' Delph-Sonar and Delph-Map systems. The Delph-Sonar offers realtime and offline processing of sonar data, logging of raw data, and interactive screen interpretation, explains Rich Dentzman, a geological oceanographer with Texas-based DWS International Inc., the exclusive North American representative for Elics' Delph seismic and sonar systems. Delph-Map is specifically designed to support the Delph-Sonar system by processing sonar images and vector drawing files to create a georeferenced visual database of images, vectorized sonar feature extractions, and spatially related map data.

Bathymetric sonar is a similar surveying technique producing a horizontal, three-dimensional view of the seabed, and is often cross referenced with side-scan technology in both shallow and deepwater projects. The ISIS 100 system, manufactured by the UK-based Submetrix, utilizes the combined technologies to provide realtime displays of high-density data using swath widths up to 15 times the water depth, says Dentzman, whose firm also represents Submetrix.

Use of sub-bottom sonar imaging techniques is critical to assessing the makeup of the seabed itself. Equipment such as Datasonics Chirp II sub-bottom profiler emits a lower-frequency signal capable of penetrating deep into the sea floor. Data collected aids personnel in identifying the ground composition (mud, sand, clay, etc.) and in determining the appropriate pipeline route.

Lastly, magnetometers are used to locate and track objects - including existing cables and pipelines - by their magnetic fields. (See related article on page 19.) Magnetometers may be pole mounted for shallow water application and either towed on or near the bottom or attached to an ROV for deepwater projects. According to Dentzman, field tests of four pipe-locating sensors in the Gulf of Mexico indicate that an ROV-housed magnetometer is capable of locating buried or exposed pipelines in water depths up to 900m (2,952 feet).

Clearly, the offshore survey market has seen steady advancement in recent years, driven by the quest for fossil fuels. According to Mary Kiernen, marketing director for UK-based CODA Technologies, the move to digital imaging techniques has provided survey contractors a critical edge. "Just a few years ago, most companies were printing sonar images to thermal paper which resulted in several difficulties," she says. "The move to digital images offers greater efficiency and effectiveness - personnel are now able to interpret, manipulate and store critial surveying images on screen.

"Recent global positioning techniques are also allowing us to position topographical images into correct global coordinates with amazing accuracy," she adds.

Dan Galligan, business development manager for C&C Technologies, agrees, but acknowledges the need for further advancement in this field. "Technology is racing just to catch up, particularly at greater depths," says Galligan, of the Lafayette, La., mapping and surveying services company. "Further progress is needed to enable us to routinely reach depths of 5,000 feet," he says.

So the challenge to both improve image quality and reduce survey operational costs in deeper water continues. While still in its developmental stages, autonomous underwater vehicle (AUV) technology is expected to offer the survey market improved efficiencies by providing an effective alternative to the conventional towfish and ROV.

The HUGIN system, developed in cooperation between Statoil, Norwegian Underwater Technology Centre (NUTEC), Kongsberg Simrad AS and the Norwegian Defence Research Etablishment (FFI), is an AUV equipped for collecting bathymetric seabed data. (A later version of the HUGIN system will introduce a multi-sensor system.) According to its manufacturers, the system offers operational effectiveness which is almost independent of water depth. And, by eliminating the umbilical system of traditional ROVs and towbodies, the AUV operates with greater freedom and positioning capability.

Mapping The Route

Due to recent advancements in surveying techniques, today's pipeline selection process is no longer left to guesswork. Instead, sophisticated images of seabed topography and profiles allow pipeline operators to thoroughly analyze a variety of irregularities which may compromise an optimum route.

Various features of the seabed topography, including sand waves, iceberg plow marks, pockmarks and boulders, impact route selection as does the seabed profile, or composition material (rock, sand, clay, etc.). Currents also warrant further evaluation when detected along the pipeline route.

According to Alan Evett, commercial manager with UK-based Underwater Excavation Limited, the decision of whether to alter the suggested route or to modify the existing seabed (via subsea excavation) remains a matter of cost. Route changes or pipeline design changes required to pass around or over these features are expensive. Nevertheless, they are often the economic solution due to the extreme high cost of modifying the seabed topography, he says.

Pipelaying: A New Generation of Lay Vessels and Methods

As the world's search for hydrocarbons continues at increasing depths, the necessity for more sophisticated pipelaying vessels and techniques has never been greater. In fact, the challenge for quicker, more cost-effective pipelaying methods remains a chief focus of pipeline service contractors such as Houma, La.-based Torch, Inc. "Torch has experienced a 31 percent increase in its pipeline laying capability in the first six months of 1997 over the same period last year," reports Jim McClaugherty, business development manager for Torch. "This is due to the upgrades we have made to our equipment," he says.

According to McClaugherty, the acquisition by underwater contractors of sophisticated vessels from the North Sea for use in the Gulf of Mexico - in addition to upgrades of existing equipment in the areas of thrust, stingers, towers, tension capacity, deck wench capabity, and jet sled capabilities - is responsible for notable progress in the pipelaying field. He also credits state-of-the-art dynamic positioning capabilities, which allow vessels to maintain their position precisely without anchoring, for increased pipelaying capacity in deeper waters. Adds Thomas Angel, president of Submar Inc. of Houma, La., "Dynamic positioning technology has significantly reduced the costs associated with laying pipelines."

Global Industries, Ltd. has shown a similar commitment to investing in deepwater technologies. In 1994-5 the company upgraded its barge Chickasaw with a dynamic positioning system, vertical departure angle stinger, and other equipment for deepwater operation. One of the world's few reel-lay vessels, the Chickasaw is capable of laying pipe up to 10 times faster than conventional methods at depths of up to 6,000 feet. Global Industries is currently engaged in a project to upgrade its 2,000-ton lift capacity, 445-foot derrick barge Hercules. Phase I of this project will add dynamic positioning, conventional pipelay equipment for pipe diameters up to 42 inches, an advanced 1,200-kip pipe tensioning system, and a 300-foot stinger capable of departure angles up to 90 degrees. Phase II will include a removable reel for laying FBE-coated pipe and insulated pipe-in-pipe systems, which is capable of carrying up to 10-miles of 18-inch pipe.

Earlier this year, Oceaneering purchased and upgraded the M/V Rambo in an effort to expand its GOM operations. Renamed the Ocean Project, the vessel's unusually wide beam and deckload capability make it particularly well suited for umbilical lay projects in deep water.

Other Gulf of Mexico pipelay contractors are engaged in similar vessel upgrades.

Pipelaying methods include the standard S-lay in which 40-foot lengths of flexible pipe are laid from the barge forming an S shape, and a more recent pipelaying method, the J-lay, which alllows for a more vertical drop and is, therefore, the method of choice for laying pipe in deeper waters.

Still, reel laying (also referred to as the spool method) represents the most significant advancement in the field. First introduced in 1970 and continually improved upon since then, this method allows lengths of pipe to be assembled either onshore or on the vessel, coiled onto a large reel and dropped from the barge using straighteners. The advantages of reel laying are significant; the preassembled pipe can be deployed from the vessel in less time and with fewer personnel than with traditional methods.

Connections: Weld vs. Mechanical

Whether installing a new pipeline, tying into an existing pipeline system, or repairing an existing structure, two primary connection methods are used - welding and flanges.

Whether installing a new pipeline, tying into an existing pipeline system, or repairing an existing structure, two primary connection methods are used - welding and mechanical bolt on hot tap tees.

The most popular connection is the mechanical bolt on fitting, reports Ed Buckley, North America sales manager with US- and UK-based International Piping Services Company (IPSCO). After installing a fitting, a hot tap is performed in which a pressure drilling machine is used to cut an opening into an operating pipeline so that a new "take off" can be created from the original pipe without any product leakage or loss. A similar technique known as a line stop, he reports, is a means of temporarily stopping the flow of an operating pipeline in an effort to isolate the section requiring repair, alteration or relocation.

Welding methods can be further classified as either wet welding, a technique performed within the water itself, or as a more sophisticated form of welding known as hyperbaric welding. This technology was introduced in an effort to overcome the difficulties of welding in a wet environment. Hyperbaric subsea welding is achieved by placing an open-bottomed steel box, or habitat, over the weld, from which the water has been displaced by gas pumped down from the diving support ship. Stolt Comex Seaway introduced the first autonomous hyperbaric welding system for subsea use, called THOR system (TIG hyperbaric orbital Robot), in 1986 and continues to use the same technology today for 80 to 90 percent of its welding operations, according to company president Patrice Chemin.

Weldless flange connectors provide an alternative to conventional methods and may prove more economical than welding in selected applications. Proof is Oceaneering's Smart Flange Plus Connectors, offering reliable and permanent pipeline tie-in and repair. According to Oceaneering Intervention Engineering's John Charalambides, product manager - pipeline repair systems, the system uses a conventional uniform metallic slip-and-cone mechanism and a large elastomeric element to grip and seal around the existing pipe end. The one-time stud bolt make-up operation, with the mating flange on the spool piece, automatically energizes the internal components of the Smart Flange Plus Connector and sets, locks and seals it onto the pipe permanently. More than 1,000 Smart Flange plus Connectors have been installed worldwide over the past 10 years without a single in-service failure, states Charalambides.

Withstanding The Test of Time

Although recent pipeline materials are far surpassing the track records of their predecessors with respect to longevity, all are susceptible to corrosion. This is due, in part, to the chemical composition of seawater and the pipeline itself, the pressure of the material pumped through it, and the natural wear and tear of the sub-bottom environment. While a certain level of corrosion is unavoidable, a variety of technologies are being utilized to slow the degenerative process, including pipeline coatings, cathodic protection methods and pipeline burial.

A wide selection of coatings is currently in use, including amastic, bitumastic, tape wraps, plastic and cementitious coating materials, reports Will Hux, vice president of Stroud Diving of Jacksonville, Fla. "We have seen significant improvement in all coatings in the past 10 years," he says.

Stroud explains that effective coating materials will resist marine and plant life, feature a high electrical resistivity ideally suited to complementary cathodic protection, have a virtually negligible water absorption factor, and be suited to operating conditions between -30 and 95 degrees C (up to 115 degrees C in subsea conditions under a concrete weight coating). They will also be suited to the concrete weight coating of subsea pipelines as their thermoplastic nature allows for differential movement of the steep pipe within the external concrete coating.

Cathodic protection, Stroud continues, uses aluminum, magnesium and other noble alloids to change the polarity of the metal pipe in an effort to resist electrochemical corrosion.

Finally, actual burial of the pipeline - using a high pressure water jet or trenching equipment to embed the line, or directional drilling techniques to physically drill the pipe underground - has been used with considerable success. Once installed within the trench, the pipeline is often covered with gravel or similar material for additional protection. Such sophisticated maneuvers are being aided by the use of state-of-the-art tracking systems such as the Innovatum Ultra System manufactured by Innovatum, Inc. of Houston, Texas. "Close proximity to the pipe could lead to collision or damage," explains Innovatum's General Manager Sam Marcello. "The Innovatum tracking unit offers superior detection capabilities - tracking pipe as small as eight inches in diameter and calculating vertical burial depths of up to 5m."

New Frontiers

Emerging technologies in the field of subsea pipeline surveying and installation are allowing today's oil and gas producers to tap reserves which only recently were thought too remote and too deep to conquer. Yet as H.O."Harvey" Mohr, president of Texas-based H.O. Mohr Research & Engineering , Inc., and Al Cooper, regional sales manager for North and South America for HydroTech Systems Inc., reported in their Spring 1997 article in UnderWater magazine, today's classification of deep water (approximately 1,000 feet), particularly in the Gulf of Mexico, will not hold long. This statement is based on the large number of leases made in the GOM during 1996 alone - many in depths of 6,000 to 9,000 feet.

As we challenge new frontiers in pursuit of valuable hydrocarbons, exploration and production technologies will continue to advance - providing a pipeline into the next millennium and beyond.


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