System and method for cutting of offshore structures

A system for cutting an underwater structure, the system comprising: a cutting head, said cutting head comprising: an abrasive water jetting nozzle; a cryogenic nozzle; said abrasive water jetting nozzle and cryogenic nozzle mounted in fixed spaced relation; said cutting head arranged to position tips of the nozzles proximate to a cutting surface, and arranged to form a cutting zone defined by the nozzle tips and cutting surface; wherein a water repelling shield is located about said cutting zone and arranged to hinder water entering said cutting zone.

FIELD OF THE INVENTION

The invention relates to offshore structures such as pylons, well heads and other steel structures located offshore including those located underwater. Specifically the invention relates to the means of cutting these structures above or below seabed.

BACKGROUND

In conducting maintenance or site reparation, it is necessary to cut through steel structures whilst underwater and, for offshore conditions, at significant depth. The cutting may be for the purpose of repairing the structure or for its removal. Further, in order to avoid a structure projecting from the sea bed, it may be necessary to cut below the level of the sea bed.

Several techniques have been used including hydraulic shears, being mechanical severing devices, particularly for relatively thin material such as less than 50 mm. More difficult and complex techniques include shaped explosive charges, laser cutting and chemical attack, however none of these provide an efficient means of cutting underwater structures for various reasons.

The two most popular methods include diamond wire cutting and abrasive water jetting. For offshore applications, both require a significant deck area to support the equipment infrastructure. Further, diamond wire cutting requires divers and remotely operated vehicle, whereas abrasive water jetting requires a considerable amount of time (six to eight hours) in order to effect the cut.

SUMMARY OF INVENTION

In a first aspect the invention provides a system for cutting an underwater structure, the system comprising: a cutting head, said cutting head comprising: an abrasive water jetting nozzle; a cryogenic nozzle; said abrasive water jetting nozzle and cryogenic nozzle mounted in fixed spaced relation; said cutting head arranged to position tips of the nozzles proximate to a cutting surface, and arranged to form a cutting zone defined by the nozzle tips and cutting surface; wherein a water repelling shield is located about said cutting zone and arranged to hinder water entering said cutting zone.

In a second aspect the invention provides a method for cutting an underwater structure, the method comprising the steps of: placing an abrasive water jetting nozzle and a cryogenic nozzle in fixed spaced relation to form a cutting head positioning said cutting head proximate to a cutting surface of said structure; forming a cutting zone defined by tips of the abrasive water jetting nozzle and a cryogenic nozzle and the cutting surface; locating a water repelling shield about said cutting zone and so; hindering water entering said cutting zone, and; cutting said cutting surface using the abrasive water jet emanating from said abrasive water jetting nozzle.

Thus, a combination of features assist to make abrasive water jetting a viable option. Firstly, using cryogenic assistance to the water jetting by reducing the metal temperature to below the ductile-brittle transition. By making the steel brittle, its ability to absorb energy is reduced making the water jet more effective, and so able to act faster. Further, by introducing an air envelope, the surrounding water is prevented from raising the temperature of the steel, through efficient heat transfer, before the water jet can act.

DETAILED DESCRIPTION

FIG.1shows an elevation view of the type of underwater structure5to which the present invention is applicable. In particular, a jacket10comprising a tubular steel structure maybe embedded in the seabed15. Embedment will inevitably create a soil plug25within the tubular structure. By way of example, the tubular steel structure10may have a 50 mm wall thickness and, in order, to return the site to its original condition such that no structure projects from the seabed15, a cut line20may be identified at, for instance, one to three meters below the seabed15. It will be appreciated that given the cut line20is below the seabed15, site preparation leading to the cutting process will depend upon the area around the jacket10that needs to be cleared in order to utilize the cutting system. Thus, a cutting system that is large and bulky, for instance, a hydraulic shear or diamond wire cutter will require additional preparation time extending the time for which the cutting process requires. In consideration of time taken to complete the task, given that conventional abrasive water jetting requires up to six to eight hours, there is a considerable amount of time that is required to provide support for the cutting process. If the entire project involves many such jackets10, then it is clear such an approach may not be economically viable.

The present invention therefore involves two steps. It is accepted that abrasive water jetting is a useful technique for cutting steel, however the duration required under normal circumstances is problematic. By cryogenically freezing the steel structure just prior to the water jetting, a significant benefit may be achieved leading to a faster cutting process of the cryogenically brittle steel.

However, cryogenically freezing the steel is difficult in an underwater application. The present invention therefore further includes an air envelope and habitat surrounding a cryogenic nozzle and nozzle of the water jetting system.

FIG.2shows a schematic view of a cutting arrangement30whereby a cryogenic nozzle35applies liquid nitrogen to the cutting zone37. Compressed air45is injected into the cutting zone which includes water jetting nozzle39. The cutting zone37provides arid conditions in which to cool and cut the steel33whilst underwater40. A thermocouple50may be provided within the cutting zone37so as to monitor temperature and ensure cryogenic conditions exist during the cutting process.

As discussed, an important consideration in cryogenically assisted water jetting is to ensure the material to be cut33remains brittle during cutting. At ambient temperatures, ductility of steel remains relatively constant. As the temperature of the metal is reduced, a ductile-brittle transition (DBT) is reached. As a non-limiting example, this transition may be in the range −90° C. to −130° C. It will be appreciated that the ductile-brittle transition for a material may be different for each material, and so appropriate testing of the material may be required.

Whilst above ground applications of cryogenic assisted water jetting can be carried out relatively simply, heat transfer under water is such that any time lag between the introduction of liquid nitrogen and subsequent water jetting may be sufficient to elevate the temperature of the cutting zone above the ductile-brittle transition temperature.

Accordingly, the introduction of an air envelope37, which in this case is provided by the introduction of compressed air45, removes water from the cutting zone37and thus limits heat gained through immersion in water, which is a more efficient heat transfer medium than air.

FIG.3shows a cutting head55for underwater cryogenic assisted water jetting. Here a nozzle60directs65liquid nitrogen70onto a cutting surface75. A bracket95is engaged with the cryogenic nozzle and a water jetting nozzle80directing90a stream of abrasive water jet85onto the cutting surface75. The bracket acts to ensure the synchronized movement of the nozzles60,75, in spaced relation, being 50 mm in this embodiment. The cutting head55is arranged to be moved100along the cutting surface75whereby the liquid nitrogen cryogenically cools the cutting surface75beyond the ductile-brittle transition so as to assist in the water cutting of the surface. It will be appreciated this movement may be linear for cutting flat plate, or rotational for cutting a cylindrical pipe or jacket. For a cylindrical pipe or jacket, the cutting head55may be directed outward for cutting an internal bore, or radially inward, for cutting from a peripheral circumference. A water repelling shield, in the form of a stream of compressed air81drives out water87from the cutting zone83so as to create arid conditions and thus avoid temperature increases in the cutting surface before the surface is able to be cut. Another form of water repelling shield may be used, including a physical barrier surrounding the cutting zone83. The barrier may be purpose built to fit the type of cutting surface. Alternatively, as will be shown with reference toFIGS.4A and4B, the structure being cut may provide part, or all, of the water repelling shield. Further still, the water repelling shield may be in the form of a de-watering pump, drawing water away from the cutting zone. It will be appreciated that various combinations of compressed air, physical barrier and pumping may for the water repelling shield. The diameter of the nozzle for cryogenic liquid may be between 5 mm to 50 mm. The separation distance, either linear or circumferential, range of distance vary between 0.5 cm to 20 cm. The orientation angle of the cryogenic liquid nozzle makes relative to the target material to cut vary between 90° to 45° to allow better cooling distribution and reduce heat loss. The distance is required to allow adequate cooling for the metal to reach its Ductile Brittle Transition before severance operation by the abrasive water jet. Cutting nozzle stand-off range between 3 mm to 5 mm from the target material. The AWJ slurry could be mix near or far from the nozzle. Subsequently, the diameter nozzle of AWJ is set between 1 mm to 3 mm.

FIGS.4A and4Bshow an alternative embodiment102. Here, a jacket110is located under water105and embedded in the seabed115. In this embodiment the cutting head125is positioned within the bore130of the jacket110. Further, the cutting head125is integrated with the soil plug removal tool150, which includes a de-watering pump for evacuating the water and soil. The cutting head125includes a first nozzle145for directing liquid nitrogen proximate to the water jetting nozzle140, which define a cutting zone144together with the cutting surface146. The distance between the water jet nozzle140and cryogenic nozzles145,155, and in particular the various nozzle tips147,149,157, may vary based upon in situ conditions and material to be cut. As a non-limiting example, the distance160between the water jet nozzle tip149and the first cryogenic nozzle147may be in the range 50 to 100 mm. In a further embodiment, a lip or retaining surface151may be coupled to the water jetting nozzle140, or arranged close thereto, such that the lip151is in close proximity to, or in contact with, the target material. The lip151is arranged to capture and retain a portion of the cryogenic fluid from the cryogenic nozzle145as excess cryogenic fluid falls from the nozzle or runs down the surface of the cutting surface. In this way, cryogenic fluid is retained for a longer period at the surface of the target material prior to full evaporation. It follows that the lip151, therefore, maintains cryogenic conditions at the target surface for a longer period. The lip151may be made from a highly ductile material.

In this embodiment, a second cryogenic nozzle155is located circumferentially about the cutting tool from the water jet nozzle140, the offset165between nozzle tips149,157being in the range 100 to 200 mm. Thus, it will be noted that in the position shown inFIG.4Aas the cutting head125is rotated142the first cryogenic nozzle145, having a nozzle tip147is positioned above the water jet nozzle140, having a nozzle tip149, and thus providing cryogenic conditions on a continuous basis proximate to the water jet nozzle140. The second nozzle155acts in a similar manner to that of the embodiment ofFIG.3whereby the second nozzle155is in the same horizontal plane as the water jet nozzle140and precedes the water jet nozzle as the cutting tool is rotated142. Thus, having two cryogenic nozzles operating proximate to the water jet, the second nozzle155reduces the temperature of the jacket wall112with the first nozzle145maintaining cryogenic conditions during the cutting process.

In the embodiment ofFIGS.4A and4B, the air envelope is provided by de-watering the bore130of the jacket110, and so the pump and the internal bore130form the water repelling shield for the cutting zone144. The de-watering pump will need to be sufficient to accommodate water ingress through the base of the jacket during the cutting process. Thus, in this further embodiment the cutting tool and soil plug removal device150may include a water pump135of greater capacity than may be standard for pumping water out of the borer130in order to maintain arid conditions. It follows that integration with the soil plug device150provides the air envelope required for underwater cryogenic assisted water jetting.

As a note, the ability to place the cutting head125inside the jacket110means no site preparation is required around the jacket, unlike several of the prior art systems. Whether the cut is to be made above or below the seabed120is irrelevant, subject to the depth of the soil plug. In this embodiment, this is also managed by including the soil plug removal device150.