Risers

Semi-submersible rigs are often referred to as semis , and are a floating type of rig. Like the jack-up, a semi is self contained. The structure is supported by large pontoons which are ballasted with water to provide the required stability and height. The rig is held in position by anchors and mooring lines or dynamically positioned by thrusters. A large diameter steel pipe ( riser ) is connected to the sea-bed and serves as a conduit for the drill string. The blowout preventer (BOP) is also located at the sea-bed ( sub sea stack ). 

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. 

As the first well is being drilled the template is connected to the host facility with flowlines, umbilicals and risers. A Chemical Injection Umbilical will also typically be laid to the template or subsea facility and connected to a distribution manifold. 

Since the crystal shape, or habit, can be determined by kinetic and other nonequilibrium effects, an actud crystal may have faces that differ from those of the Wulff construction. For example, if a (100) plane is a stable or singular plane but by processing one produces a plane at a small angle to this, describable as an (xOO) plane, where x is a large number, the surface may decompose into a set of (100) steps and (010) risers [39]. 

Because of the large difference in tp between successive molecular layers, each layer becomes complete at a relative pressure (plp°)g which is determined by the value of rp/kT for that layer, viz integral values). Each layer will therefore give rise to a step, such that the riser corresponds to the cooperative build-up of the layer and the tread to the transition between the layer and the next higher one. 

A further complication which not infrequently appears is the occurrence of a phase transition within the adsorbed film. Detailed investigation of a number of step-like isotherms by Rouquerol, Thorny and Duval, and by others has led to the discovery of a kink, or sub-step within the first riser, which has been interpreted in terms of a two-dimensional phase change in the first molecular layer. 

Fig. 7. Axial density profiles in the (—) bubbling, (------) turbulent, and (----) fast and ( ) riser circulating fluidization regimes. Typical gas velocities for...

Fresh butane mixed with recycled gas encounters freshly oxidized catalyst at the bottom of the transport-bed reactor and is oxidized to maleic anhydride and CO during its passage up the reactor. Catalyst densities (80 160 kg/m ) in the transport-bed reactor are substantially lower than the catalyst density in a typical fluidized-bed reactor (480 640 kg/m ) (109). The gas flow pattern in the riser is nearly plug flow which avoids the negative effect of backmixing on reaction selectivity. Reduced catalyst is separated from the reaction products by cyclones and is further stripped of products and reactants in a separate stripping vessel. The reduced catalyst is reoxidized in a separate fluidized-bed oxidizer where the exothermic heat of reaction is removed by steam cods. The rate of reoxidation of the VPO catalyst is slower than the rate of oxidation of butane, and consequently residence times are longer in the oxidizer than in the transport-bed reactor. 

Vented risers should be provided on most cooling towers to release only light hydrocarbon leakage from the cooling water before the spray header. No ignition or source of spark should be within 30 m of the vented riser. 

The modern HF alkylation processes are also differentiated primarily by the reactor system that is used. The Phillips process employs a gravity acid circulation system and a riser reactor (19). The UOP process uses a pumped acid circulation system and an exchanger reactor (20). 

Radial density gradients in FCC and other large-diameter pneumatic transfer risers reflect gas—soHd maldistributions and reduce product yields. Cold-flow units are used to measure the transverse catalyst profiles as functions of gas velocity, catalyst flux, and inlet design. Impacts of measured flow distributions have been evaluated using a simple four lump kinetic model and assuming dispersed catalyst clusters where all the reactions are assumed to occur coupled with a continuous gas phase. A 3 wt % conversion advantage is determined for injection feed around the riser circumference as compared with an axial injection design (28). 

In energy extraction, titanium alloys are being used in deep-water hydrocarbon and geothermal weUs for risers. Corrosion resistance, high strength, low modulus (flexible), and low density can result in risers one-fourth the weight and three times the flexibiHty of steel. 

Thus the ECCU always operates in complete heat balance at any desired hydrocarbon feed rate and reactor temperature this heat balance is achieved in units such as the one shown in Eigure 1 by varying the catalyst circulation rate. Catalyst flow is controlled by a sHde valve located in the catalyst transfer line from the regenerator to the reactor and in the catalyst return line from the reactor to the regenerator. In some older style units of the Exxon Model IV-type, where catalyst flow is controlled by pressure balance between the reactor and regenerator, the heat-balance control is more often achieved by changing the temperature of the hydrocarbon feed entering the riser. 

By the late 1980s six principal commercial CEBC technologies were available (42). In 1993 the largest CEBC ia operation is expected to be the Pyropower Corporation s 165 MWe reheat coal-fired unit, under constmction siace 1991 at the Poiat Aconi Station of Nova Scotia Power Corp. (43). Combustion and SO2 control ia this unit is to be carried out ia the water-cooled riser. The unit is expected to operate at 870°C to optimize sulfur capture. The cyclone separators are refractory-lined and are supported approximately 30 m above grade. 

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