Industrial activities at sea are not new but the development of sea going structures for the exploitation of natural resources in these locations is on a scale of technology unprecedented in mankind history. One recent example is the gigantic deep-draft caisson deployed by ExxonMobil over the Hoover Diana fields in the Gulf of Mexico, 260 km south of Galveston TX. Hoover was discovered in January 1997, 25 km east of Diana. The two fields contain estimated recoverable reserves exceeding 300 million oil-equivalent barrels.
When it was deployed in 1,570 meters of water, ExxonMobil 63,000-ton Hoover deep-draft caisson vessel set a world water-depth record for a drilling and production platform. (Internet reference 94)
Vertical pipelines called risers deliver oil and gas from wells in the Hoover reservoir up to the caisson vessel.
Two lines carry Hoover-Diana's oil and gas to shore through one of the longest pipeline systems in the Gulf of Mexico.
Subsea wells in the Diana field send oil and gas back to the Hoover caisson vessel for treatment before it goes by pipeline to shore.
The Hoover DDCV consists of a cylindrical hull, buoyed with air-filled compartments in the upper portion of the hull and ballasted with seawater and fixed ballasts in the bottom compartments. On top of the steel hull, the DDCV production facilities are designed to handle 100,000 barrels per day of crude, 325 million ft³/day of gas and produce up to 60,000 barrels/day of water. A three-level deck will house separation, dehydration and treatment facilities, as well as a drilling rig. Surface trees on the DDCV are similar to trees found on traditional platforms. All valves and flowlines are at the surface, located in the wellbay area of the DDCV. In this particular application, the trees have tensioned risers that connect the surface trees to the well at the seafloor. Surface trees allow for wellbore intervention, using conventional workover technology. (Internet reference 95)
The deep draft caisson vessel is moored via 12 anchor lines, which extend in a radial pattern from the hull. The mooring lines will be 2,100 m long and will connect to piles driven into the sea floor, approximately 2,000f m from the DDCV (at a lateral distance of about 1,500 m). The mooring lines are taut, but will have some flexibility and form a catenary between the hull and ocean floor. Each mooring line consists of chain in the top and bottom sections and spiral-strand wire rope in the middle section.
Such offshore environment has a considerable impact on the choice and use of materials offshore. Subsea pipelines are key both to field development and the transportation of oil and gas. It is also a key global industry. Since a broad range of conditions will usually be imposed on a material, the impact of seawater on its performance is determined by numerous variables such as condition of the material, system design, fabrication procedure, seawater temperature and flow regimes, biological activity, and presence of oxidizing compounds or other biocides. Additional factors to consider in choosing a material for a marine system are obviously its physical and mechanical properties, availability, cost, ease of fabrication and maintenance, anticipated design-life and previous design experience.
In subsea systems the effects of hydrogen embrittlement from the cathodic protection system also have to be taken into account. In the selection of materials for wellhead equipment the following considerations have had to be taken into account:
Composition of produced fluids in contact with valve body and internal parts - all wetted parts from downhole to the flowline require identification
Operating pressure ranges
Galvanic effects due to contact of dissimilar materials
Crevice corrosion resistance at seal and flange faces
Wear and galling resistance of moving parts
Temperature and chemical resistance for non-metallic materials
Cathodic protection (CP) on materials
Effectiveness of coatings on materials
Weldability for weld overlay
Material availability and cost
Compatibility of materials with injected fluids