A New, Highly Efficient Deep Water ROV Buoyancy System
Authors: Patton Edward M. , Rennick Timothy S. , Brambilla Mauro , Calzavara Adriano 
Published: October 2008

Typical syntactic foam buoyancy blocks for ROV's designed to be used in deep and ultradeep water are large and bulky devices. This is because the density of the syntactic foam to resist the resulting compressive load from water pressure must increase as water depth increases, requiring the use of more buoyancy foam for the same vehicle as the rated depth of that vehicle increases. The technology described in this paper, and demonstrated with both laboratory and operational tests, was developed in an effort to improve the buoyancy efficiency of deep water syntactic foam, or to reduce the density of the foam block for an ultra deep water application, without significantly increasing the cost of the fixed buoyancy system for a 3000 to 6000 meter water depth ROV.

Syntactic foam used as fixed buoyancy for subsea vehicles is a mixture of hollow glass microspheres and an epoxy resin, with the common addition of what are called macrospheres (3/8 to ½ inch diameter hollow spheres) to further lower the weight per cubic foot of the final foam block. Under contract to Saipem, S.p.A., Southwest Research Institute (SwRI) developed an inexpensive light weight hollow aluminum sphere that when cast into typical syntactic foams further reduces the weight of the foam, and allows for a reduction in the size of the buoyancy block on a deep or ultra deep water ROV. The aluminum sphere is forged as two hemispheres, and assembled in a slight vacuum with an epoxy or other suitable adhesive before being cast directly into a syntactic foam block. Using this technology with a 3000 meter water depth rated syntactic foam, SwRI has been able to achieve buoyancy block densities as low as .406 kg/m3 with an efficient packing of the spheres. Without this technology, densities of solid syntactic for a 3000 meter water depth application range from .5 to .6 kg/m3. In addition, the cost of the spheres in large quantities is roughly equivalent to the cost of buoyancy foam itself, when measured against syntactic foam on a cost per unit of buoyancy basis. In other words, the spheres are slightly higher in cost than an equivalent volume of foam, but their significantly lower density makes them cost competitive with a solid syntactic product.

Introduction

The history of the use of syntactic foam for buoyancy goes back to the mid-1960's with a search by the Navy for buoyancy materials and the advent of the commercial availability of hollow glass microspheres that could be cast into an epoxy resin and made into a suitable molded block. Since that time, there have been several enhancements to the technology of fixed buoyancy for deep submergence vehicles and equipment for the exploration and production of hydrocarbons from the sea floor. A large volume of syntactic foam is in use today in the offshore oil industry for fixed buoyancy collars on risers for either drilling or production of oil and natural gas. For a drilling riser, these buoyancy collars are typically attached in two reasonably symmetric halves to a jointed steel riser that extends from the surface of the ocean to the sea bed. In water depths greater than about a hundred meters, the weight of this riser column cannot be efficiently accommodated by a floating platform. Syntactic foam collars are used to counteract this weight. To date, risers for drilling in deep water have been used in water depths slightly in excess of 1500 meters. With the discovery of a very significant amount of oil in the deepest waters of the Gulf of Mexico, drilling contractors are quickly upgrading all of their drilling risers to a 3000 or 3500 meter depth capability.