This invention relates to a pump apparatus and in particular to a pump apparatus configured to produce an output of water at high pressure in shallow water. The pump apparatus of the invention is particularly applicable to apparatus which uses jetting tools to cut subsea trenches. Such trenches are necessary for burying pipes or cable in the seabed, for example. 
     BACKGROUND 
     Trench cutting apparatus using high pressure water jets to form the trench are well known in the subsea environment. Until recently the primary use of such trench cutting apparatus has been in relatively deep water. However, there is increasing demand to employ trench cutting apparatus in shallower water conditions. The use of jetting tools in shallow water presents some problems. 
     To supply water at high pressure to the outlet(s) of a jetting tool, a pump is used to pump water from the environment around the jetting tool. Generally the pump is submerged and in relatively close proximity to the high pressure outlets. For example, the pump may conveniently be mounted to the body of an underwater trench cutting vehicle. For applications of this sort, it is preferred to use high specific speed pumps such as multi-stage axial pumps and mixed axial flow pumps since the size and weight of pumps of this type are generally lower than lower specific speed pumps such as centrifugal type pumps. 
     The high intake flows of high specific speed pumps have the effect of lowering the ambient fluid pressure in the intake side of the pump. In deep water conditions this is acceptable, but in shallow water conditions the reduction in ambient pressure can be sufficient to cause cavitation in the pump. As is well known, cavitation is likely to reduce the performance and service life of the pump. 
     One proposed solution to the problem of cavitation in shallower water is to install an auxiliary pump upstream of the primary pump, the auxiliary pump providing low pressure and high flow. Such an auxiliary pump can be mounted on the trenching vehicle or on a service vessel or on the shore, for example. However, this is inconvenient and adds complexity to the overall apparatus. It is also possible to use low specific speed pumps as the primary pump, but as noted above such pumps carry a weight penalty and are more expensive. 
     The present invention seeks to obviate or mitigate such problems and to provide a pump apparatus, in particular for a jetting tool of a trench cutting apparatus, which is suitable for use in shallower water and which reduces or avoids problems of cavitation. 
     BRIEF SUMMARY OF THE DISCLOSURE 
     In accordance with a first aspect of the present invention there is provided a pump apparatus comprising a primary pump having a relatively low pressure fluid input and a relatively high pressure fluid output and means, associated with said primary pump fluid input, operable where the ambient pressure is insufficient substantially to prevent cavitation in the primary pump, to locally increase the pressure at said primary pump fluid input. 
     In preferred embodiments said means operable to locally increase the pressure at said fluid input is a jet pump. 
     Preferably the jet pump has an input and an output, the input being in fluid communication with the high pressure output of the primary pump and the output being in fluid communication with the input of the primary pump. 
     Preferably the pump apparatus further comprises an isolator operable to isolate the jet pump from the high pressure outlet of the primary pump. 
     Preferably the primary pump comprises one or more controllable flow restricting valves upstream of the high pressure fluid output, said valves having a flow restricting condition and a non-flow restricting condition, said flow restricting condition operatively reducing the input flow to the primary pump and increasing the exhaust pressure at the primary pump fluid output, as compared to the non-flow restricting condition. 
     According to a second aspect of the present invention there is provided an underwater trenching apparatus for operation in shallow water, the apparatus comprising a trench-cutting high pressure jetting tool and a primary pump having a relatively high pressure fluid output in fluid communication with said jetting tool and a relatively low pressure fluid input, and means, associated with said primary pump fluid input, operable where the ambient pressure is insufficient to prevent cavitation in the primary pump, to locally increase the pressure at said primary pump fluid input. 
     In preferred embodiments of this second aspect, said means operable to locally increase the pressure at said fluid input is a jet pump. 
     Preferably in this second aspect the jet pump has an input and an output, the input being in fluid communication with the high pressure output of the primary pump and the output being in fluid communication with the input of the primary pump. 
     Preferably the underwater trenching apparatus further comprises an isolator operable to isolate the jet pump from the high pressure outlet of the primary pump, said isolator being operable where the ambient pressure is sufficient substantially to prevent cavitation in the primary pump. 
     Preferably in this second aspect the primary pump comprises one or more controllable flow restricting valves upstream of the high pressure fluid output, said valves having a flow restricting condition and a non-flow restricting condition, said flow restricting condition operatively reducing the input flow to the primary pump and increasing the exhaust pressure at the primary pump fluid output, as compared to the non-flow restricting condition. 
     According to a third aspect of the present invention there is provided a method of operating an underwater trenching apparatus for operation in shallow water, the apparatus comprising a trench-cutting high pressure jetting tool and a primary pump having a relatively high pressure fluid output in fluid communication with said jetting tool and a relatively low pressure fluid input, the method comprising, where the ambient pressure is insufficient substantially to prevent cavitation in the primary pump, locally increasing the pressure at said primary pump fluid input. 
     Preferably the step of locally increasing the pressure at said fluid input comprises using a jet pump in fluid communication with said fluid input. 
     Preferably the jet pump has an input and an output, the input being in fluid communication with the high pressure output of the primary pump and the output being in fluid communication with the input of the primary pump. 
     In preferred embodiments the method further comprises isolating the jet pump from the high pressure outlet of the primary pump where the ambient pressure is sufficient substantially to prevent cavitation in the primary pump. 
     Preferably the primary pump comprises one or more controllable flow restricting valves upstream of the high pressure fluid output, said valves having a flow restricting condition and a non-flow restricting condition, said flow restricting condition operatively reducing the input flow to the primary pump and increasing the exhaust pressure at the primary pump fluid output, as compared to the non-flow restricting condition, the method comprising, on initially starting the primary pump where the ambient pressure is insufficient substantially to prevent cavitation, causing the one or more flow restricting valves to adopt the flow restricting condition, and, when the pressure at the fluid input of the primary pump reaches a predetermined threshold value, causing the one or more flow restricting valves to adopt the non-flow restricting condition. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention are further described hereinafter by way of example only with reference to the accompanying drawing, in which: 
         FIG. 1  is a schematic illustration of a pump apparatus according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawing the pump apparatus  10  according to the invention comprises a primary pump  20  and a jet pump  50 . The primary pump  20  is a high specific speed pump such as a multi stage axial pump or a mixed axial flow pump. 
     In the illustrated embodiment, the primary pump  20  can be seen to comprise a primary pump motor  22 , a primary pump intake  24  and an exhaust manifold or output  26 . The primary pump outlet  26  communicates via lines  64  and  66  with a jetting tool  70 . Specifically, the primary pump  20  supplies water at high pressure to the jetting tool  70  via lines  64 ,  66 , the water being expelled from one or more nozzles of the jetting tool  70 , for cutting a trench in the seabed. 
     Jet pumps per se are known in the art and the construction of suitable jet pumps will be known to persons skilled in the art. The jet pump  50  in  FIG. 1  has a main water inlet via line  60  from an intake  52 . The intake  52  can usefully include a strainer or the like to prevent the take up of any debris which may be in the water. The output of the jet pump  50  is via a line  68  to the input  24  of the primary pump  20 . The motive fluid input of the jet pump  50  is via line  62  which is in fluid communication with the high pressure output  26  of the primary pump  20 . 
     An isolator valve  30  is included in line  62 , by means of which the jet pump  50  can be isolated from the output  26  of the primary pump. That is, in the isolating condition of the isolating valve  30  the flow of motive fluid from the output  26  of the primary pump to the jet pump  50  is stopped. 
     Line  64  from the output  26  of the primary pump terminates in fluid communication with one or more throttling valves (flow restricting valves)  40 . From the throttling valve or valves  40 , line  66  communicates with the jetting tool  70 . Thus the jetting tool  70  receives high pressure water from the output  26  of the primary pump  20 . 
     The one or more throttling valves  40  are controllable, such as by means of an actuator  42 , selectively to adopt a throttling (flow-restricting) condition or a non-throttling (non-flow-restricting) condition. 
     In operation the primary pump  20  is submerged. For example the primary pump  20  may be mounted on a main body of a trenching apparatus. In conventional deep water operation where there is little or no danger of cavitation in the primary pump  20 , the isolating valve  30  may be closed, so that the jet pump  50  is not in communication with the output  26  of the primary pump  20 . In this condition, the input  24  of the primary pump  20  receives fluid (i.e. water in which the primary pump  20  is submerged) at the ambient pressure of the surrounding water. It is noted that even when the jet pump  50  is not operational (because isolator valve  30  is closed) the jet pump still allows flow of water to the primary pump input  24 . In deep water operation, the pressure of the water in the immediate vicinity of the pump  20  is sufficiently high to avoid cavitation in the pump. Isolation of the jet pump  50  from the output  26  of the primary pump  50  in deep water conditions is advantageous in increasing the efficiency of the pump apparatus in such conditions. 
     However, in shallow water conditions, the ambient pressure of the water in the immediate vicinity of the primary pump  20  may not be sufficient to prevent cavitation in the pump. 
     The pump apparatus  10  of the invention is operated in shallow water conditions with the isolating valve  30  open. Thus the jet pump  50  communicates via line  62  with the high pressure output  26  of the primary pump  20 . That is, a portion of the output flow of output  26  is diverted via line  62  to the jet pump  50 . The high pressure water flow received via line  62  by jet pump  50  forms the motive fluid for the jet pump  50 . Thus, in this operational state, the local pressure of water received at the input  24  of the primary pump  20  is increased to the extent that the possibility of cavitation in the pump is reduced or eliminated. 
     The isolating valve  30  may be manually operated, or may be operated by an actuator  30 A. Where the pump apparatus  10  operates exclusively in shallow water environments, isolator valve  30  is not necessary. 
     In operational conditions, the pump apparatus  10  of the invention may move from shallow water conditions to deep water conditions, or vice versa, while in an operational state. In moving from shallow water conditions to deep water conditions, the valve  30  can be closed when the water in the vicinity of the primary pump  20  is sufficiently deep for there to be a sufficiently low risk of cavitation in the pump  20 . 
     Conversely in moving from deep water conditions to shallow water conditions the valve  30  can be opened when the water in the vicinity of the primary pump  20  is still sufficiently deep for there to be a sufficiently low risk of cavitation in the pump  20 , that is, before the water becomes so shallow that there is an appreciable risk of cavitation. 
     In the case either of a transition from deep water to shallow water or from shallow water to deep water it is not necessary interrupt the operation of the primary pump  20 . 
     When the pump apparatus  10  according to the invention is started (from rest) in deep water conditions, the isolating valve  30  can be set to the closed position before operation of the primary pump commences. 
     When the pump apparatus  10  according to the invention is started (from rest) in shallow water conditions, steps must be taken to prevent cavitation in the initial stages of operation, that is, before the jet pump  50  is effective in raising the input pressure to the primary pump input  24 . To this end, the output  26  of the primary pump  20  communicates via line  64  with one or more throttling valves  40  which can selectively adopt throttling and non-throttling conditions. For starting of the pump apparatus  10  in shallow water conditions, the one or more valves  40  are set to a throttling condition, for example by means of actuator  42 . Setting valve or valves  40  in the throttling condition has the effect of reducing the inlet flow to the primary pump  20  and increasing the exhaust pressure at the primary pump output  26 . The possibility of cavitation is thus avoided or at least significantly reduced. The jet pump  50  receives high pressure input via line  62  (since the isolator valve  30  is open) and thus increases the pressure at the primary pump input  24 , again reducing the possibility of cavitation. The throttling valve or valves  40  can then be opened and the pump apparatus  10  is in its fully operational state. 
     In some embodiments the pump apparatus  10  according to the invention includes a plurality of valves  40 . In such embodiments, the plurality of valves  40  may each independently be in communication with a plurality of nozzles on the jetting tool  70 . Each valve  40  may independently adopt a throttling and non-throttling condition as outlined above. Advantageously the use of a plurality of nozzles on the jetting tool  70  in combination with a plurality of valves  40  minimizes any reduction in pressure in the fluid expelled from the jetting tool  70 . Each nozzle on the jetting tool  70  may thus receive fluid from a separate valve  40  ensuring that any reduction in pressure occurs across the nozzle of the jetting tool  70  and not the valve  40 . The efficiency of the pump apparatus  10  in such embodiments can be optimised by each valve  40  adopting a fully open or fully closed position. 
     In some embodiments components of the pump apparatus  10  according to the invention can be operated at different speeds to further reduce the risk of cavitation. The primary pump  20  may therefore incorporate a variable speed drive system. Thus when the pump apparatus  10  according to the invention is started (from rest) in shallow water conditions, the primary pump  20  may be set to a reduced speed in order to reduce the risk of cavitation in the initial stages of operation. 
     Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of them mean “including but not limited to”, and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise. 
     Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. 
     The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed. 
     The reader&#39;s attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.
A pump apparatus comprises a primary pump having a relatively low pressure fluid input and a relatively high pressure fluid output and means, associated with said primary pump fluid input, operable where the ambient pressure is insufficient substantially to prevent cavitation in the primary pump, to locally increase the pressure at said primary pump fluid input.