Water depth

Semis are often used in water depths too deep for jack-ups. However, anehor handling and the length of the riser eventually impose a limit on the operating depth. Their stability makes them suitable vessels for hostile offshore environments. 

Steel piled jackets are the most common type of platform and are employed in a wide range of sea conditions, from the comparative calm of the South China Sea to the hostile Northern North Sea. Steel jackets are used in water depths of up to 150 metres and may support production facilities a further 50 metres above mean sea level. In deep water all the process and support facilities are normally supported on a single jacket, but in shallow seas it may be cheaper and safer to support drilling, production... 

Within the same geographical area (e.g. water depth, weather conditions, distance to shore, reservoir setting) this is a useful tool for comparing projects of different sizes. If the indicators vary significantly then the reasons should be sought. 

Ocean Basins. Ocean basins are primarily formed from oceanic basalts and maybe interspersed with continental remnants, ridges, seamounts, or volcanic islands rising from the depths. Average water depth is around 4000 m but the most significant mineralization is generally found at 5000 m for manganese nodules, 4000 m for biogenic oozes, and 3000 m for hydrothermal metalliferous sulfides. The area is poorly explored, however. 

Stabilization Ponds. Stabilization ponds have a water depth of 1—2 m and oxygen is suppHed by surface entrainment or by algae. The BOD loading must be low and the detention time is 5—25 days (218). 

Tank diameter Largely a function of clear water depth the maximum overflow rate in ni/h = 0.278 X clear water depth. Resulting underflow design settling velocity should be less than 1.0 i7i/h for 1% solids and 120 mL/g S T. 

Two primary settling basins are each 100 ft in diameter with an 8-ft side water depth. The tanks are equipped with single effluent weirs located on the peripheries. For a water flow of 10 mgd, calculate the overflow rate, gpd/ft, detention time, hr, and weir loading, gpd/ft. The overflow rate for a clarifier... 

Identify the equipment failures as a function of water depth... 

Enter failure probabilities (ones) in the system model to determine the plant damage states as a function of water depth... 

If the PSA were constructed in the ZIP format, these common cause effects would be incorporated by changing the event tree branching probabilities as a function of water depth a.s the flood fails the various systems. 

Jack-up rigs. For drilling in water depths of up to 400 ft (122 m), the legs are cranked downward until they reach the sea floor and then jacked up so that the platform is 15 to 30 ft (4.6 m to 9.1 m) above the surface. Workers often reach the rig by helicopter (left). 

Fixed rigs. Used in depths up to 1,500 ft (457 m), these rigs are imbedded in the ocean floor and tapered from bottom to top for stability. Compliant tower. Used in water depths from 1,500 to 3,000 ft (457 m to 915 m), these towers rely on mooring lines for stability. 

Despite the forces of wind, waves and ocean currents, at a water depth of. S,000 ft (1,526 in), a dynamic positioning system can reliably keep a drill ship within 50 ft (15.2 m) of the spot directly over the borehole. 

Offshore drilling vessels are classified as either bottom-supported or floating-type vessels. Water depth is generally the governing factor as to which type of vessel is employed. 

Bottom-supported vessels consist of drilling platforms, jackup rigs, and drilling barges. A drilling platform is not employed until reserves warranting field development is indicated. Platforms are currently being set in water depths up to 1,000 ft, and also serve as permanent production facilities. 

Drilling barges, commonly employed in inland waters and marshes, can be used where water depths do not exceed about 25 ft. These self-contained vessels provide the least expensive drilling vessel, but have limited applicability because of water depth limitations. 

A marine riser must be held in tension to prevent the riser from collapsing under its own weight. This can be accomplished by adding buoyant material to the riser pipe, or by mechanical tensioning devices (Figures 4-498, 4-499). Tensioning devices are usually required in water depths greater than 250 ft. 

Variation in water depth owing to backwater or tidal influences... 

Model is insensitive to expected range of changes in summertime water depth in the tidal reach. Predicted DO values differ from standard profile by an average of 1 percent saturation. 

Fig. 18-1 Benthic foraminiferal oxygen isotope record from 3477 m water depth in the eastern tropical Pacific ocean from Ocean Drilling Program site 677 (Shackleton et al, 1990). ratios are expressed in the S...

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