BACKGROUND OF THE INVENTION

The invention relates to a method for converting an underwater methane containing hydrate deposit into a marketable product.

Such a method is known from US patents 6,192,691 and 6,209,965, US patent application US2003/0136585 and International patent applications WO98/44078 and WO2005/088071.

In the known methods hydrate deposits are

generally excavated from the waterbottom and a slurry of hydrates, soil and water is then pumped through a riser to a floating platform at the water surface, where the hydrates are heated and induced to

dissociate in water and natural gas.

A disadvantage of the known methods is that large amounts of slurry need to be pumped to the platform, which requires a large diameter riser and a large amount of energy.

It is an object of the present invention to provide an improved method and system for converting an underwater hydrate deposit into a marketable product in a more efficient and cost effective manner than the known methods .

SUMMARY OF THE INVENTION

In accordance with the invention there is

provided a method of converting a methane containing hydrate deposit in the waterbottom into a marketable product, comprising:

- excavating a slurry of methane hydrate, sediment and water from the waterbottom; - pumping the slurry to an underwater processing station;

- pumping warm sufacial water having a higher temperature than the slurry from the vicinity of the water surface via a warm water injection riser into the underwater processing station, in which the warm surfacial water is mixed with the slurry, thereby inducing the dissociation of said methane hydrate into into water and methane gas;

- separating the methane gas from the slurry; and

- transferring the methane gas to a surface production platform in which the produced natural gas is processed to a marketable product, which is suitable to be transferred to an export facility.

It will be understood that the method according to the invention may be applied in any body of water at the earth surface, such as an ocean, sea, f ord, lake or river and that the waterbottom may be located more than a kilometer below the water surface, and that the terms seawater and seabed shall be

interpreted to refer to any body of water at the earth surface and the bottom of such body of water.

The surface production platform may float at the water surface and the underwater processing station may be suspended from the floating production

platform such that the underwater processing station is located at a selected height above the waterbottom and the slurry may be excavated by a mobile excavator from the waterbottom and pumped from the excavator to the underwater processing station through a flexible slurry transportation riser.

The mobile excavator may be provided with wheels, caterpillars and/or other means for moving the excavator in a desired lateral direction across the waterbottom and an arm on which a slurry cutter is mounted, which arm inserts the cutter to a desired depth into the hydrate deposit in the waterbottom and means for controlling the movement of the mobile excavator, arm and slurry cutter from a control room at the floating production platform.

The underwater processing station may be provided with electrical or other means for heating the slurry in addition to the heating by the flux of injected warm surfacial water and the underwater processing station, the warm seawater injection riser and a gas riser through which the methane gas is transferred from the underwater processing station may have outer surfaces which are at least partly covered by a thermal insulation layer.

The slurry may be separated in the underwater processing station into methane gas and a methane depleted tailings stream, which stream may be pumped from the underwater processing station to a tailings disposal site at the waterbottom though a tailings disposal pipe, which extends in a different lateral direction from the underwater processing station than the flexible slurry transportation riser.

The floating production platform may be

maintained at a predetermined location at the

watersurface above the hydrate deposit by a dynamic positioning system or be moored to the waterbottom by at least three catenary shaped mooring lines, which extend in different lateral directions from the platform to anchors in the waterbottom.

These and other features, embodiments and

advantages of the method and/or system according to the invention are described in the accompanying claims, abstract and the following detailed

description of non-limiting embodiments depicted in the accompanying drawings, in which description reference numerals are used which refer to

corresponding reference numerals that are depicted in the drawings .

BRIEF DESCRIPTION OF THE DRAWINGS

Fig.l is a schematic three-dimensional view of a hydrate excavation and conversion system according to the invention; and

Fig.2 is a flow-scheme of the conversion steps applied at the production platform and subsea hydrate conversion station.

DETAILED DESCRIPTION OF THE DEPICTED EMBODIMENTS

The method according to the present inventions enables the production of marketable fuel (methane gas) from hydrate deposits buried in shallow

sediments in deepwater offshore.

Fig.l shows that the method according to the invention thereto comprises dredging the seabed using a seabed excavator 1 of a type developed for dredging or deepsea mining of other commodities. This will produce a slurry of hydrate, water and sediment which enters the production facility from which the product is separated and transported to the surface as described below:

The seabed excavator (1) disrupts the hydrate deposit (2) and mixes the disrupted deposit (2) with sediment and ambient seawater to produce a slurry of methane hydrate, particulate sediment and seawater. This slurry passes into a flexible or articulated slurry transportation riser (3) . The slurry passes up the riser into a subsea processing station (4) . A detailed description of the subsea processing

facility is given below.

Warm surfacial seawater is passed from the water surface to the subsea processing station via a surfacial water injection riser (5) . The flow of seawater in the injection riser (5) is sufficiently fast, and the thermal conductivity of the riser walls is sufficiently low, that heat loss from the pumped seawater as it passes down the riser is low. Within the subsea processing station (4) the seawater is mixed with the hydrate slurry, such that the

temperature of the mixed slurry exceeds the

dissociation temperature for methane hydrate at the ambient pressure. This causes the methane hydrate to dissociate within the mixture, giving methane gas and fresh water as a by-product. The methane gas is separated from the mixture and passes into a gas production riser (6) where it flows up to a surface production platform (7) . The remaining mixture of seawater, freshwater and sediment passes into a tailings disposal pipe (8) where it is transferred horizontally an area of seabed suitable for tailings disposal ( 9 ) .

Fig.2 shows a flow diagram of the operations at the subsea processing station (4) and production platform (7) floating at the water surface.

Within the subsea processing station (4), a slurry of methane hydrate, sediment and seawater enters via the flexible/articulated riser (3) and passes through a grinder (10) which ensures that all of the methane hydrate crystals are smaller than the minimum size necessary to ensure full dissociation prior to leaving the subsea processing station (4) .

Next the slurry passes through a pump which provides the pressure difference necessary to draw the seawater/hydrate/sediment slurry up to the subsea processing station (4) from the excavator (1) via the flexible/articulated riser (3).

The slurry then enters a mixing/dissociation vessel (12) where it is thoroughly blended with warm seawater from the seawater riser (5) such that the resultant mixture has a temperature higher than the stable temperature for methane hydrate at the ambient pressure. This causes the methane hydrate to

dissociate into methane gas and fresh water, so that a gas/sediment /water slurry is formed. The volume and temperature of warm seawater introduced into the mixing/dissociation vessel (12) is sufficient to cause all of the methane hydrate present to

dissociate. The mixing/dissociation vessel (12) is sized so as to allow sufficient residence time in the vessel to allow all of the methane hydrate present to dissociate .

The resultant gas/sediment /water slurry then enters a separator (13) where the methane gas is separated from the sediment /water slurry. The methane gas is sent to the gas riser (6) and the

sediment /water slurry is sent to the tailings disposal pipe (8) .

The surface production platform (7), as well as providing an operating base for the operations carried out in the subsea excavation and processing system and a buoyant point from which to suspend the risers (5) and (6), also carries out the following processes .

Warm surfacial seawater is drawn in from a shallow depth in the open ocean. It passes through a filter (14) to remove solids. It than passes through a pump which provides both the suction necessary to draw in seawater from the ocean and the pressure necessary to drive the warm seawater down the seawater riser (5) .

Methane gas enters a dehydrator from the gas riser at high pressure (ambient pressure at the subsea processing station (4) less the head due to the weight of the gas itself in the riser) . The gas is expected to be saturated with water. It passes through a dehydrator where the water is removed, leaving pure methane gas .

This gas then passes to an export facility, such as an underwater natural gas transportation pipeline or a floating Liquid Natural Gas (LNG) or Gas To Liquid (GTL) production plant to which LNG or GTL transportation ships may be connected for product offloading .