The Cranemaster calculation sheet

The calculation sheets provided by Ernst-B. Johansen A.S. contains data which is used for engineering and analysis of offshore lifts with Cranemaster.

Sheet 1 contains input and output data. Project specific inputs including operating water depth (I 8), air temperature when adjusting the gas pressure (I 10), water temperature at operating depth (I 11), oil volume (I 12) and surface load when extending (C 20) needs to be filled in. 
The sheet list values for stroke vs surface load&pressure and subsea load&pressure for three different gas states.

Adiabatic gas compression	Isoterm gas compression	Isoterm compression and adiabatic cycling
Adiabatic Gas Compression occurs in situations with rapid compression of gas with no heat dissipating to the environment. Mainly used for shock absorption and splash zone work.	Isotherm Gas Compression is gas compression with all heat dissipating to the environment. Normally used to make sure that the unit will not be extended to full stroke from static hanging load.	This assumes isotherm compression to the mid-stroke and with adiabatic retraction and extension +/- mid-stroke. Normally used for Passive Heave Compensation, Seabed Landings and  Resonance Reduction.

Figure 1 : Main gas states

When preparing the Cranemasters for different lifts there are two parameters that easily can be varied to adjust the characteristics of the unit: the Pretension Pressure and the Oil Reserve Volume.
The pretension pressure of the unit is adjusted by changing the weight needed to extend the piston rod at the surface (C20) - defining at what weight you want Cranemaster to start extending. The pretension pressure value ( I 13) will then change automatically (do not try to change I 13) - make sure the pressure do not extend max working pressure limit.

Increasing the reserve volume (at I 12) gives a higher spring constant. This will make the unit less sensitive to load variations - needing increased load per length of stroke.

Figure 2 : Sheet one of the Cranemaster calculation sheet

Sheet one of the calculations sheet for 250T 2500mm stroke Cranemaster. The unit is adjusted for installation at 1000 meters water depth. The structure to be installed has a submerged weight of 120T, which will make the unit work at about 1/3 of full stroke when approaching the seabed. The values for landing on the seabed is found under "isoterm compression and adiabatic cycling".

Figure 3 : Stroke vs Force graph

The stroke vs force diagram illustrate the characteristics of the unit. Increasing the oil volume will give a higher spring constant, hence show a steeper curve.

Sheet 2 gives input to be included in Orcaflex simulations. The relevant columns (gas state) listing Link length vs Load and the values for Speed vs Damping is the most important input to be copied and pasted into a spring link which represent the Cranemaster in Orcaflex.

Figure 4 : Sheet 2 of Cranemaster calculation sheet showing Orcaflex values