Chapter V

OCEAN ENGINEERING FOR SUBSEA OPERATIONS

The severity of the ocean environment dictates the need for a broad understanding of the various oceanic properties and processes that impact, to some degree, on the development of technology to facilitate manned and unmanned subsea operations. Consequently, such developments rely heavily on Government-supported fundamental research. Certain areas of fundamental research, however, are especially pertinent to undersea operations. For example, the National Science Foundation (NSF) is supporting research on the following: The effects of temperature on the geotechnical properties of deep-ocean sediments; the properties of deep-ocean sediments; the properties of buoyancy-induced flows; heat removal and thermal plumes issuing from objects in thermally stratified fluids; the turbulent nature of water below the airwater interface; the long-range problems in developing improved acousticimaging systems for use in the ocean.

The United States Navy (USN) has been involved in a number of geophysical investigations of the ocean bottom, studies of anomalies associated with the gravity field, investigations of sedimentation and bottom processes, and ocean bottom roughness as related to acoustic reverberation or reflection. The Office of Naval Research also supported a study to understand the phenomenon of cable strumming in the hydrospace environment and provided expertise and advice on acoustic and nonacoustic detection systems for clearance operations in the Suez Canal.

The U.S. Geological Survey (USGS) is coordinating a broad multidisciplinary research and engineering study of sediment instability and movement. The study involves several governmental agencies, universities, oil companies, and an electronics firm. It was initiated after earlier investigations had provided tangible evidence that major storms could trigger massive failures in sediments supporting offshore platforms and pipelines. During the initial study phase, USGS and university

scientists analyzed high-resolution seismic profiles of the Gulf of Mexico provided by oil and oil-service companies in order to identify, classify, and map a variety of features indicative of sediment instability and movement. The results of this work have provided the basis for continuing studies of sediments in the Gulf of Mexico and on the Atlantic Outer Continental Shelf.

Although a knowledge of environmental conditions is important to undersea operations, the success of these operations must ultimately depend on the development and improvement of materials, equipment, and instruments for undersea use as well as on the sound planning of the operations themselves. The development of undersea technology is supported largely by the USN. Coordination of the use of submersibles and habitats by the civil Federal agencies is the responsibility of the National Oceanic and Atmospheric Administration (NOAA).

NEW MATERIALS AND EQUIPMENT

As vehicles are designed for deeper and deeper ocean waters, the thickness and the corresponding weight of the pressure hull must be increased. This increase in weight results in a significant loss of buoyancy. If the vehicle is to carry out its assigned tasks, supplementary buoyancy must be provided. A number of materials and systems have been investigated to provide supplementary buoyancy, but, because of its ease of handling, safety, and fail-safe characteristics under pressure, syntactic foam was selected.

Syntactic foam is composed of hollow microspheres embedded in an epoxy matrix. The diameter of these microspheres is on the order of 10 to 150 microns. In order to produce blocks of minimum density, a blend of two distributions of microspheres is used. This blend of a component containing small microspheres with a component containing larger microspheres produces a binary mixture that optimizes the volume occupied by the microspheres in the finished product. Using binary mixtures, the volume of a syntactic foam block occupied by microspheres is characteristically 72 to 76 percent, depending on the fabrication pro

cedure.

The finished product is made by either the vacuum impregnation of a prepacked column of microspheres or by mixing the microspheres directly with the resin under vacuum conditions and then curing the resin component. The strength of the final product depends on the type of resin employed, the percentage of microspheres present, and the density of the microsphere component. The density of syntactic foam for use at depths of 20,000 feet has now been reduced from 42 to 34 pounds per cubic foot. This increase in buoyancy, because of the reduction in space required for foam, has resulted in significant savings in vehicle

size. A new, lower cost foam has also been developed, but research is continuing to further reduce foam costs.

A number of materials are being tested for use by the USN in submersible hulls and undersea structures. One of these materials, graphite-epoxy, because of its high-strength, low-weight properties, offers the potential for extending operating-depth capabilities without significant payload sacrifice. In a project to evaluate the capacity of graphite-epoxy structures to meet deep-sea requirements, especially built cylindrical models with a 312-inch-outside diameter were hydrostatically tested at the Naval Underwater Systems Center. These models satisfactorily met the test pressure requirements, and therefore, newer models with larger diameters are now being fabricated and tested. Although acrylic plastic spherical shell sectors have been widely used for manned submersible windows, they have not been found adequate for precision optical systems because they are subject to large displacement and deformation under high hydrostatic pressure. Experiments have, therefore, been undertaken to test shell sectors with 150° spherical angles, fabricated from optical glass, chemically surface-compressed glass, or transparent cermaic. The tests indicate that a submersible system equipped with a 150° spherical shell window flange assembly made of a chemically surface-compressed glass or ceramic can operate at any ocean depth.

A wide variety of concrete pressure-resistant structures can function successfully at 3,000-foot water depths. In an effort to extend these depths, concrete polymer is being studied as an underwater structural material. It has excellent properties for use in ocean environments because of its impermeability to water, high durability, and compressive strength on the order of 20,000 lb psi. The tests show an operational depth range for positively buoyant concrete polymer spheres of 4,000 feet. Moreover, even though the cost of polymer-strengthened concrete may be three times that of conventional concrete, the structural members need be only one-third to one-fifth as large as conventional concrete members, and the increase in durability reduces maintenance requirements, making the product cost-effective.

The problem of marine fouling on concrete underwater structures has also been investigated. A method has been developed for incorporating toxic chemicals into concrete to protect such structures from the approximately 2,000 species of marine foulants. Tests indicate that the antifouling concrete is sufficiently strong for construction in which a compressive strength of 3,500 lb psi is acceptable.

Several years ago the USN installed a new titanium alloy hull on the submersible Alvin. This hull extended Alvin's working depth from 6,000 to 12,000 feet. Data on the performance of this hull continue to be collected for use in the further application of titanium alloys to submersible

and submarine construction. One such application, now underway, is the development of a 20,000-foot-depth hull for the USN deep submersible vehicle Seacliff.

In addition to materials research, the USN ocean-engineering program focuses on the development of new equipment for undersea operation. A major effort of this type is the Remote Unmanned Work System (RUWS) project.

RUWS is an unmanned, cable-tethered work system designed to perform a variety of engineering and scientific tasks at ocean depths extending to 20,000 feet. When operational, RUWS will be capable of instrument and equipment inspection, recovery, repair, and emplantment, as well as data gathering functions for over 98 percent of the ocean floor. RUWS will be able to maneuver with 4° of freedom, operating in currents of up to 1 knot.

Designed to be air transportable, and operable from ships of opportunity, the RUWS will be used either alone or in conjunction with the operations of any USN deep-ocean vehicle. Components can be adapted to the basic RUWS configuration for all aspects of deep-ocean salvage. RUWS includes advanced technology for high-accuracy deep-ocean navigation and local-area bottom search. The navigation system will, for the first time, provide coordinated navigational information to the RUWS operators and the support ship's bridge. The bridge display will indicate the ship's safe maneuvering area, and other displays and recorders will provide precise bottom mapping data.

The RUWS design emphasizes the extension of man's senses to the seafloor site. The simulation of man's presence is accomplished primarily through the use of head-coupled television, force feedback from the RUWS manipulator, and the use of integrated displays and controls. RUWS is presently undergoing at-sea testing, evaluation and further development.

A recently initiated USN project is directed to the development of a closed cycle internal combustion engine. The goal of the project is to produce a closed system using propane fuel and a Wankel engine for operations requiring more horsepower than is practical with silver zinc batteries but less than the amounts generally obtained from nuclear/steam plants. A bench version of the closed internal combustion engine and a demonstration model have been built. This model is suitcase portable and demonstrates the basic operating principles of the fullscale system.

Experimental data to confirm the theoretical analyses were obtained in the USN's SEACON II project. The project represents a milestone in the USN effort to design and construct large and complex underwater cable structures. SEACON II is a three-legged, experimental cable structure installed off the coast of southern California. SEACON II was con