Heavy lift operations.
Report 99.3.TT.5143, Transport Engineering and Logistics.
Offshore heavy lift operation means the transportation of heavy units in the
open sea. These units, weight 10.000 ton or more, are installed with the help
of floating cranes on oil-rigs: or in case of decommissioning, removed from
the oil-rig. These modules are often transported to the destination by
transport barge. The job of the floating crane is to hoist up these modules
from the barge to the oil platform or vice versa. Usually three types of
craneships are brought into action: a barge, a conventional craneship or a
The workability of floating offshore cranes for the
installation / decommission of offshore structures depends, beside
on the weight of the module, also on wave climate and motion behaviour of the
crane ships. The three crane vessels are discussed and compared with each other.
In general pitch and roll are important motions, while the heave of the crane
ship is the dominant limiting motion during the lifting. The complicated motion
behaviour of crane vessels (especially the SSCV) depends also on the hoisting
wire length and the suspended load weight. These motions are discussed in
detail. At SSCVs coupled resonance motions of heave, pitch, roll and hanging
load are of special influence. As pitch, roll and load oscillations are coupled,
a decrease of horizontal load motions reduces the vessel pitch, roll and,
consequently, the vertical load motions.
This report presents a linear mathematical model with eight degrees of freedom.
The model describes the complicated motion behaviour of crane vessels with
suspended load and demonstrated that the structural shape of the crane vessels
determines its motion behaviour. Also because heavy liftings operations do not
take place in rough weather, semisubmersibles are superior as compared to the
other two crane vessels. SSCVs are characterised by minimal motions in moderate
sea states, whereas in long waves weakly damped resonance oscillations are
excited. At crane barges and crane ships resonance effects are less significant
as the potential damping is high. However, their motions in moderate sea states
are larger. But this fact is rather a consequence of their sheer size than of
their shape. Simulations show that crane ships and SSCVs with the same
displacement experience comparable motion behaviour. Big offshore companies,
like Heerema, are studying the possibilities of a crane ship with two large
hoisting cranes and a 300.000 ton plus displacement.
The motions of crane vessels depends strongly on the sea states. The motion
behaviour of the different types crane vessels is not the same in an
identical golf pattern. To find out how the crane vessels behave at open sea
a lot of tests and simulations have to be run. One have to describe the sea
states properly to get the most reliable results. Using the sine curve as the
staring point to define the regular waves. By changing the amplitude, wave
length and phase various regular waves are created. The energy quality of a
wave pattern in any frequency can be found in the energy spectrum. If the
wave length is next to the length of the ship there will be big motions
and the ship goes on oscillating. One does not need any technical insight of
understand that the sea state will not cause any harm when the wave length is
very long (in steady weather).
An irregular wave can be seen as the superposition of series of these
sinusoidal waves with various amplitude, wave length and phase angle. These
irregular waves are as like as the real sea state, so it can be used as the
input of the computer program, with the vessel motion characteristics as
In this report two examples about active heave compensation systems are
discussed. These systems have a maximum hook up capacity of 200 ton. These
systems contain series of sheaves and one hydraulic cylinder or more including
the accessory control units. The task of active compensation system is to
change the hook height rapidly (following the waves actually) to hook up or
lift off the load save and without damage from/on the supply boat.
The reason why these systems are not being applied in the heavy lift operations
is the energy demand that is needed, is simply not available. To hoist a module
up to 1 metre, the main pulley need to shortens much more than 1 metre due the
cables has been pulled through series of guiding sheaves. To change,
continuously and rapidly, the height of a load weight over 10.000 ton the
energy request that is needed only generating station can provide.
Reports on Transport Engineering and Logistics (in Dutch)
, TU Delft