Ships – Mooring device – Having ship-mounted turret
Reexamination Certificate
2000-03-03
2002-01-29
Bagnell, David (Department: 3673)
Ships
Mooring device
Having ship-mounted turret
C114S230130, C166S356000, C166S363000, C441S004000
Reexamination Certificate
active
06341572
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to safety systems for preventing explosions in internal turret mooring systems where risers which are carrying hydrocarbons from subsea wells are connected to lines leading to process facilities. In particular the invention relates to an atmosphere control system for preventing explosions in such a mooring system.
2. Description of the Prior Art
In the past, ventilation has been the basis for preventing explosion due to leaks between risers and surface equipment of a turret mooring system. Ventilation systems have inherent difficulties in that explosion potential can remain unacceptably high under certain conditions.
Systems and methods based on the principle of filling an enclosure with inert gas are known in the art of safety systems for marine vessel cargo tanks and in land hydrocarbon storage tanks. Inert gas systems used on marine vessel cargo tanks are described in a book,
Inert Gas Systems,
International Maritime Organization (IMO), 1990. Guidelines are provided which apply to inert gas system on tankers, particularly to cargo tankers for hydrocarbons. The guidelines are based on current general practice used in the design and operation of inert gas systems using flue gas from the uptake from the ship's main or auxiliary boilers, and installed on crude oil tankers and combination carriers. The guidelines provide a method with an inert gas system where the protection against a tank explosion is achieved by introducing inert gas into the tank to keep the oxygen content low and reduce to safe proportions the hydrocarbon gas concentration of the tank atmosphere. It can be determined from flammability diagrams that as inert gas is added to hydrocarbon/air mixtures, the flammable range progressively decreases until the oxygen content reaches a level generally taken to be about 11% by volume, below which point no mixture can burn. There are three methods of replacement of gas in cargo tanks, namely: inerting, purging, and gas-freeing. The general policy of cargo tank atmosphere control is that tankers fitted with inert gas systems should have their cargo tanks kept in a nonflammable condition at all times. In line with that policy, tanks should be kept in the inert condition whenever they contain cargo residues or ballast. The oxygen contents should be kept at 8% or less by volume with a positive gas pressure in all the cargo tanks. The atmosphere within the tank should make the transition from the inert condition to the gas-free condition without passing through the flammable condition. In practice this means that before any tank is gas-freed, it should be purged with inert gas until the hydrocarbon content of the tank atmosphere is below the critical dilution line. When a ship is in a gas-free condition before arrival at a loading port, tanks should be inerted prior to loading.
A second inerting method and system is described in a publication,
NFPA
69:
Standard on Explosion Prevention Systems,
of the National Fire Protection Association (NFPA), 1997. The standard described in this publication applies to systems and equipment used for the prevention of explosions by the prevention or control of deflagrations (i.e., combustion with velocities less than the speed of sound).
The standard outlines the minimum requirements for installing systems for the prevention of explosions in enclosures that contain flammable concentrations of flammable gases, vapors, mists, dusts, or hybrid mixtures. Recognized techniques are grouped into two classes in the standard: one based on preventing combustion; the other based on preventing or limiting damage after combustion occurs.
One method of the standard for preventing combustion provides for oxidant concentration reduction which is a technique for maintaining the concentration of the oxidant (e.g. oxygen) in a closed space below the concentration required for ignition to occur. The technique for oxidant concentration reduction for deflagration prevention can be considered for application to any system where a mixture of oxidant and flammable material is confined to an enclosure within which the oxidant concentration can be controlled. The system is maintained at an oxidant concentration low enough to prevent a deflagration by using a purge gas (e.g., inert gas such as nitrogen). Flammability diagrams for specific flammable gases or vapors are used as a basis for determining the level of limiting oxidant concentrations (LOC).
U.S. Pat. No. 5,564,957 discloses an arrangement for dynamically positioning a vessel with thrusters and connecting a riser buoy in a lower receiving module at a submerged place at the bottom of the hull of the vessel. The buoy has an outer buoyant portion anchored to the sea bed by anchor legs. The outer portion of the buoy is locked to the vessel. An inner part of the buoy is rotatably mounted centrally of the outer part. A riser runs from the sea bed to the central part of the buoy which can be removably secured to a flow line of the vessel which leads to storage holds. A long vertical shaft runs from the vessel deck to the connection of the riser at the top of the central part of the buoy to the vessel flow line. Inert gas and ventilation are applied to the shaft from the inert gas and ventilation system of the vessel. Further the shaft at its upper end is provided with a shutter for closing the shaft. The shaft and the upper part of the receiving space can thereby be filled with inert gas (after removal of water) as a safety precaution prior to start of transfer of combustible or inflammable fluids.
Ventilation is also employed for atmosphere control in closed chambers for combustible concentration reduction by mixing and diluting combustible gas in air, followed by removal of the chamber atmosphere mixture via exhausting to the natural atmosphere on topsides of the vessel. This presupposes that combustible gas is present, as in the case of an accidental leak (i.e., upon confirmed detection of the presence of the combustible gas). Ventilating, either continuously or on demand (i.e., upon confirmed detection of gas), is intended to reduce the combustible gas concentration low enough (i.e., below the LEL of the gas) to prevent the formation of a flammable atmosphere. A disadvantage of ventilation for atmosphere control is that, unless the ventilation is designed to deliver a very high number of air changes per hour, even a moderate hydrocarbon release rate may be sufficient to overwhelm the ventilation system and result in a combustible gas concentration between the LEL and UEL (i.e., the flammable range), the atmosphere is potentially flammable, thereby increasing the probability of an explosion. Although for very large releases, the combustible gas concentration could pass through the flammable range quicker, thereby reducing the probability of an explosion, the problem of exhausting the gas after a leak has been controlled still presents a hazard, since ventilating with air would require the atmosphere to pass through the flammable range again. It is desirable therefore to ensure that, regardless of the characteristics of a gas leak within the closed chamber, the atmosphere within the closed chamber will not pass through the flammable range either during the leakage or during clearing of the leaked combustible from the closed chamber.
A disadvantage of employing continuous ventilation for atmosphere control within the QCDC room is that moist sea air is introduced into the atmosphere of the room, allowing for accelerated corrosion and subsequent degradation of critical equipment and instrumentation (e.g., ESD valves and actuators). The effects of corrosion and degradation are compounded in terms of increased risk by the increased potential for leaks from degradation over the life of the equipment. The necessity for more frequent maintenance and repair to control corrosion and degradation creates increased exposure of personnel to hazards as work is conducted within the QCDC room. Also, since more frequent maintenance and
Corder Richard M.
Howell Gordon B.
Jones David A.
Nandi Asis
Witten Lloyd D.
Andrews & Kurth - Mayor, Day, Caldwell & Keeton
Bagnell David
FMC Corporation
Jackson James L.
Lee Jong-Suk
LandOfFree
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