Subsea hyperbaric chambers are known for providing suitable locations and conditions for various underwater activities, particularly but not exclusively for subsea hyperbaric welding. Such welding is the process of welding at elevated pressures underwater.

Underwater hyperbaric welding involves the weld being performed in a chamber filled with a gas mixture and sealed around the structure being welded. The chamber is filled with a gas (commonly helium containing a percentage of oxygen). Although such welding may require large quantities of complex equipment and significant cost, nevertheless this method has the ability to produce welds of quality comparable to open-air welds, as well as performing the welding in a warm, dry and well- illuminated environment.

Further information about hyperbaric welding can be found in the book entitled "Underwater Repair Technology" by John H. Nixon, available from Waterhead Publishing, 2000. There is a hyperbaric complex, the National Hyperbaric Centre, in Aberdeen, Scotland, where procedures for hyperbaric welding are coded and tested.

During the welding, carbon monoxide (CO) is produced. This is a very toxic gas, and is generally converted by a catalyst into carbon dioxide (C02) in a continuous manner, generally by passing the atmosphere of the welding chamber through a 'regeneration canister' provided with several sections or layers. The first or top section is a catalyst such as platinum/palladium supported on tin oxide provided under the trade name Sofnocat 423 (available from Molecular Products Limited). Thereafter, there can be a non-porous amorphous carbon layer, followed by a carbon dioxide absorbent.

An object of the present invention is to provide an improved regeneration canister, as well as a system for using such a canister.

Thus, according to one aspect of the present invention, there is provided a subsea hyperbaric chamber regeneration canister comprising at least a catalyst, the catalyst comprising at least a heterogeneous catalyst system incorporating catalytically active gold on an inorganic support medium.

The use of such a catalyst system in a subsea hyperbaric chamber has been found to be much more efficient than a conventional catalyst regeneration canister using Sofnocat 423.

The regeneration canister of the present invention may have any suitable shape, size or design, generally being the same or similar to regeneration canisters currently used, such that the regeneration canister of the present invention can be a direct replacement for existing regeneration canisters.

The catalyst for use in the present invention comprises at least a heterogeneous catalyst system, and optionally one or more other components, such as those generally known in the formulation of catalysts and known to those skilled in the art, and not further discussed herein. The heterogeneous catalyst system for use in the present invention incorporates catalytically active gold. Such a material may be identified by one or more requisite characteristics including size, colour and/or electrical characteristics. Generally, if a gold sample has one or more of these requisite characteristics, and preferably two or more of these

characteristics, it will be deemed to be catalytically active for the practise of the present invention. Nanoscale size is generally required as the catalytic activity of gold to a large degree is a function of whether the gold has a thickness dimension in the nanoscale regime. Accordingly, preferred catalytically active gold may have a nanoscale size over a wide range, i.e. in particle or clustered dimensions in the range from about 0.5nm to about 50nm, preferably about 1 nm to about 10nm.

The inorganic support medium may be any suitable medium known in the art able to support catalytically active gold. Many materials are known including carbonaceous materials, siliceous materials, metal compounds such as metal oxides or sulphides, and combinations of these. Preferably, the inorganic support medium is a composite nanoporous support medium, more preferably derived from ingredients comprising a guest material and a carbonaceous host material. Guest materials include relatively fine materials (such as <100 micrometres), and host materials are usually larger.

Suitable guest materials include one or more metal oxides such as titiana and alumina, while suitable host materials include activated carbon and the like.

This guest/host composite structure provides higher total exterior surface area while retaining the desirable gas passing characteristics, i.e low pressure drop, or a coarser particle. A variety of methods may be used to construct the inorganic support medium, and for the application of the catalytically active gold onto the inorganic support medium. Such methods are well known in the art, and include a physical vapour disposition (PVD). PVD is well known in the art for the physical transfer of a first substance such as gold from a first substance-containing source onto a support. The possible and optimum conditions for PVD can be calculated by a person skilled in the art, examples of which are described in WO2006/074126, which is

incorporated herein in its entirety by way of reference.

The regeneration canister of the present invention preferably further comprises an amorphous carbon component. Such carbon components are well known in the art of regeneration canisters.

The regeneration canister of the present invention preferably further comprises a C02 adsorbent. Such adsorbents are also well known in the art of regeneration canisters. According to one embodiment of the present invention, there is provided a regeneration canister comprising a catalyst layer, and an amorphous layer and a C02 adsorbent layer.

The regeneration canister of the present invention may comprise one or more sections or layers, preferably a plurality of layers of components or materials through which the gas or gases pass. Each section or layer may be physically distinct or separate, or include one or more transition layers, boundaries or sections therein between. Separation of two or more layers may be achieved by any known separators, such as filters or baffles or baffle plates. The regeneration canister of the present invention may comprise one or more gaseous inlets, and one or more gaseous outlets, generally one of each. Where the canister comprises a catalyst layer, amorphous carbon component or layer, and a CO2 adsorbent layer, these are preferably in sequence from the outlet to the inlet: alternatively the canister comprises from its inlet; the CO2 adsorbent layer, the amorphous carbon component followed by the catalyst, prior to the gaseous outlet.

The proportion of the regeneration canister of the present invention occupied by the catalyst may be different or the same or similar to that presently occupied by current catalysts such as Sofnotcat. Similarly, the proportion or percentage of the regeneration canister occupied by an amorphous carbon component and/or a CO2 adsorbent maybe the same, similar or different to that currently provided in conventional regeneration canisters. According to one embodiment of the present invention, the canister is a subsea hyperbaric welding chamber regeneration canister.

According to a second aspect of the present invention, there is provided use of a catalyst comprising at least a heterogeneous catalyst system incorporating catalytically active gold on an inorganic support medium in a subsea hyperbaric chamber regeneration process.

The regeneration process preferably involves the catalysis of CO to CO2, and the absorbing of the so-formed CO2. Preferably, such a catalyst is as further defined herein above.

Preferably, the use of the catalyst is in a regeneration canister as defined herein above.

According to a third aspect of the present invention, there is provided a subsea hyperbaric chamber comprising a gas-regeneration system, which system includes a regeneration canister as defined herein before. The hyperbaric chamber may have any suitable size, shape and design, and may be intended for manned or unmanned underwater operation. The parameters and conditions of the atmosphere or habitat in a hyperbaric chamber are known in the art, and generally include an overpressure above at least 2 bar, generally at least 5-10 bar or above. The atmosphere is generally predominantly helium, with a proportion of oxygen. The gas mixture in the chamber is generally controlled on a regular basis for health and safety considerations and requirements, and the use, size and other parameters or characteristics of the hyperbaric chamber will determine the number of regeneration canisters required, depending upon their size and design, to maintain the required habitat in the chamber for health and safety considerations.

Preferably, the subsea hyperbaric chamber is intended for welding operations, being preferably a subsea hyperbaric welding chamber.

Embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings in which:

Figure 1 is a schematic side cross-sectional view of a subsea hyperbaric welding chamber according to one embodiment of the present invention, including a regeneration canister according to another embodiment of the present invention;

Figure 2 is a side cross-sectional view of the regeneration canister in Figure 1 ; and

Figure 3 is a graph of a comparison of the use of a regeneration canister according to the present invention and a prior art regeneration canister in a subsea hyperbaric welding chamber during a welding operation.

Referring to the drawings, Figure 1 shows a subsea hyperbaric chamber designed for subsea welding, thereby being a subsea hyperbaric welding chamber 2. The chamber 2 has an outer frame 4, an inner welding room 6, and base doors 8 in a manner known in the art. Access for divers is achieved through the doors 8, for welding a pipeline 10 in the dry habitat provided by the welding room 6.

During welding, carbon monoxide (CO) is produced. The chamber 2 includes a gas regeneration system, comprising a flexible duct 12, usually placed in or adjacent to the weld site inside or outside the chamber 2, and which draws gas or gases therethrough, and through a filter 14 and blower 16, and then into the inlet 18 of a regeneration canister 20. The flexible duct 12 sucks gas from the welding site or area. In addition, the

atmosphere in the chamber 2 is or could be sucked in from one or more other points in the chamber 2, and also pumped through the regeneration canister 20.

Figure 2 shows the regeneration canister 20 in more detail. Typically, the regeneration canister 20 is circular in cross-section, having a circular inlet 18 at one end or face, shown at the top end in Figures 1 and 2, followed by a number of layers or sections as described in more detail hereinbelow, and a corresponding outlet 22 generally being the other end or face of the regeneration catalyst. The regeneration canister 20 generally comprises a central core 24 able to support one of circular baffle plates 26 intermediate the inlet 18 and the outlet 22, and able to provide layers or sections to support and/or confine components within the regeneration canister 20.

The example of the regeneration canister 20 shown in Figure 2 has four sections. Starting from the inlet 18, the first two sections 28 comprise two C02 adsorbent layers. C02 adsorbents are well known in the art, and include various types of activated carbon, such as that provided under the trade name Sofnofil (available from Molecular Products Limited).

Thereafter, there is provided an amorphous carbon component layer, preferably a non-porous amorphous carbon such as charcoal 30, followed by a catalyst layer 32 and then the outlet 22.

The catalyst layer 32 comprises a catalyst comprising at least a

heterogeneous catalyst system incorporating catalytically active gold on an inorganic support medium.

The inorganic support medium is preferably anatase titanium dioxide in the proportion of 5-20% by weight of the catalyst, and 80-100% by weight of activated carbon. The catalytically active gold preferably makes up <1 % by weight of the catalyst system. One example of such material is provided under the trade name NanAucat (available from 3M), and uses the gold catalyst AUC-16-1.

The catalyst layer 32 is placed downstream of the other layers in order to avoid catalyst pollution. The catalyst layer 32 converts CO in the gas stream provided through the duct 12, etc. into CO2, which CO2 can then be adsorbed by the re-entry of such gas through the duct 12, etc. and by the CO2 absorbent layers 28. Thus, Figures 1 and 2 also show use of a catalyst in a catalyst layer 32 comprising at least a heterogeneous catalyst system incorporating catalytically active gold on an inorganic support medium in a subsea hyperbaric welding chamber 2 regeneration process. Figures 1 and 2 also show a subsea hyperbaric welding chamber 2 comprising a gas-regeneration system, which system includes a

regeneration canister 20 as defined hereinabove.

Comparison Example 1

Calculations of test canister dimension and amount of scrubber materials were based on a gas purification canister in a living chamber. A minimum of six air changes are required per hour during a hyperbaric welding operation. This corresponds to a gas flow of 84 m ³ /h for a 14m ³ living chamber, and 780 l/h (13 l/min) for the 130 litre test chamber used in this test. The test canister dimensions and the amount of CO catalyst, charcoal and CO2 adsorbent were scaled down correspondingly to obtain an equivalent gas residence time in the scrubber materials. A cylindrical vessel with an inner diameter of 30mm and 25cm high was used as canister for the catalysts and adsorbents. In order to avoid catalyst pollution, the catalyst was placed downstream to both the CO2 adsorbent and the charcoal. The adsorbents and catalyst were physically separated by using particle filters. The order of components in the canister was: 1 Sofnolime CO2 adsorbent (Molecular Products Limited),

- 2.0-3.0 mm, 100 ml

2 Activated charcoal, Norit RB3, 30ml

3 CO-catalyst, 20ml

A centrifugal fan was modified so that the canister outlet end was connected to the suction side of the fan. The arrows in Figure 2 show the gas flow direction through the canister. The flow rate was measured with a Velocicalc air velocity meter and adjusted to approximately 780 l/h before the chamber was pressurised. The fan speed was controlled with a potentiometer from outside the chamber.

The catalyst tests were performed in a chamber atmosphere with a CO level near the hyperbaric exposure level of 12 bar (see table 1 below). The level of CO2 was chosen to 1/3 of the exposure limit of 10 mbar, and the chamber pressure was chosen to be 1 1 bar.

The determined level of CO and CO2 was obtained by injecting 520 ml of a gas mixture containing 3000 ppm CO and 390 ml of pure CO2 via gas lines into the middle part of the chamber by use of a mass flow controller (gas flow 100ml/min) with an integrator.

Table 1. Chamber conditions for determination of catalyst efficiency.

Chamber pressure 1 1 bar (1.1 MPa)

Gas flow rate through canister 780 l/h

PCO 12 bar (1 .2 pa)

PCO ₂ 3.3 mbar (0.33 kPa)

PO ₂ 220 mbar (22 kPa) Temperature 30°C

Humidity 40-60% RH

Volume of Sofnolime 100 ml

Volume of activate charcoal 30 ml

Volume of NanAuCat 20 ml

Volume of Sofnocat 423 20 ml

Figure 3 shows a graph of the results of a comparison of the use of Sofnocat 423 as a conventional catalyst in a regeneration chamber, and the use of NanAuCat catalyst as a catalyst of the present invention, at the same amount and under the same conditions.

Figure 3 shows that with use of NanAuCat, there is clearly a dramatic reduction in the CO pressure within a matter of minutes, followed by a sustained period wherein the CO pressure is close to zero. In contrast, the use of the Sofnocat catalyst shows a much slower decline from higher initial levels, and a CO pressure still close to two after 150 mins, still indicating a significant presence of CO in the chamber atmosphere.

Such results show that the catalyst as defined by the present invention is much more efficient for use in a subsea hyperbaric welding chamber regeneration canister that conventional catalyst Sofnocat, leading increased health and safety for manned underwater welding operations in such a hyperbaric chamber. Such a canister could clearly also be used in other types or forms of subsea hyperbaric chambers requiring the same or similar atmospheric regeneration, and the same or similar regeneration canisters.

The increased efficiency also provides a cost saving in terms of a 'reused' gas requirement, where it is possible to reuse the chamber atmosphere for a longer period, avoiding or delaying for longer the need to pump in "new" gas while taking out "old" contaminated gas to reduce the CO level in the chamber atmosphere. When conducting offshore hyperbaric operations such as welding, this also improves the logistics concerning the amount of breathing gas that has to be brought by reducing same.

Various modifications and variations to the described embodiments of the invention will be apparent to those skilled in the art without departing from the scope of the invention as defined herein. Although the invention has been described in connection with specific preferred embodiments it should be understood that the invention as defined herein should not be unduly limited to such specific embodiments.