without consideration of the creeping behaviour and the rupture strength of the material [ 14 ]. b . the nominal diving depth NDD [ msw ] defines the maximum diving depth for unrestricted diving operations in sea water . The nominal diving pressure NDP [ bar ] is calculated by multiplying the NDD value by 0,101 [ bar / msw ], if not agreed otherwise for special operations or different diving conditions [ 14 ]. c . the test diving depth TDD [ msw ] corresponds to the external test diving pressure TDP [ bar ], which the pressure hull and the equipment will be exposed to under testing conditions after completion and after main repairs and modifications , respectively [ 14 ]. d . other external loads that may impact the design such as wind loads , flow resistance , wash of sea , and ice accretion are explained in the document .
3.2 Vehicle Hydraulic Power Systems
A common subsea trenching machine or ROV has a similar basic hydraulic power unit as a work class ROV . The system is shown in a simple representation in Figure 3.2.1 . The common systems are always powered by an electrical motor and a hydraulic pump . The power of the electrical motor and the hydraulic pump is determined by considering the force that may be encountered or taken by the vehicle such as vehicle drag in seawater , thruster or propeller thrust , crawler track system , and tool forces that need to be served by the auxiliary valve packs .
In a common system , a soft start is introduced to provide a smooth system start-up . It can be an electrical soft start or a mechanical soft start . The function of a soft start is to avoid any spike in the electrical supply of the vehicle . A mechanical soft start is normally introduced at the outlet of the mechanical pump in the hydraulic system . A bypass valve is installed on the hydraulic reservoir
Figure 3.2.1 : Main hydraulic flow diagram .
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