It is also interesting to note that DAF values are typically lower for heavier decks for the same sea-states.

Installation contractors advertise engineering feats accomplished by successfully lifting the large topsides using heavy lift vessels (HLVs) offshore.

I believe they have every reason to be proud, as marine lifting operation requires complex engineering and planning and the complexity of operation increases further with the size and weight of the topside.

The article is intended for fellow Naval Architects, Engineers, Project Managers and anyone interested in learning about the field.

Irrespective of the size, the fundamental principles of lifting remain the same.

A offshore lift operation in its simplest form is equivalent to a combination of spring-mass system and pendulum, coupled together by a crane boom.

The lift vessel in the sea acts as a spring mass, and the lifted load behaves like a pendulum as described in Figure 1.

Besides, the problem involves forced oscillations where the vessel is oscillated by sea waves, and the motion of the lifted mass is affected by the motion of the crane boom tip.

Note that I have simplified the problem for better understanding and in actuality, the analysis is more complicated due to solving a multibody system with coupled degrees of freedom.

In Table-1, I have attempted to further simplify the problem by evaluating the degrees of freedom of each body individually.

As an engineer, it is imperative to understand the risks associated with the offshore operation and be well versed with the industry guidelines lessons learned for safe execution of the job.

Although every lift operation is different and may have unique operational constraints that need to be addressed, I have listed the main things that must be considered.

In simplest terms when a system is oscillated at or close to its natural frequency, it will exhibit the phenomenon of resonance, and the oscillations will become wild and in some cases undesirable.

Similarly, in a marine lifting operation, we have a hanging mass (just like the swing) which is forced to oscillate (in the horizontal plane) due to the crane tip motion which in turn is governed by the motion of the lifting vessel in sea waves.

In this scenario, large oscillations due to resonance may occur if the frequency of the crane tip motion gets close to the natural frequency of oscillation of hanging mass.

Therefore, it is crucial to analyze a marine lift operation for a range of wave periods representing the environmental conditions at the lift location.

As per London Offshore Consultant guidelines for marine lifting operations, a minimum clearance of 3m between lifted object and crane boom should be maintained at all times.

Another thing that needs to be kept in mind when dealing with resonance is that the natural frequency of simple pendulum changes with the suspended length of mass as per below equation.

The mass may be lifted off the material barge to required elevation and then lowered onto the jacket in case of deck lift or lowered to the seabed in case of subsea lift.

Therefore it is crucial to analyze the system at different operational stages to capture maximum motions due to resonance.

The lifted mass is connected to the crane boom by a set of slings and moves due to the motion of the crane boom.

Therefore, the maximum hook load is the sum of gravitation force and the force due to the maximum acceleration of the lifted mass as given by the equation below.

Note that DAF values are lower in the case of inshore (sheltered waters) due to reduced wave loads.

In addition, the motion response of a semi-submersible is lower compared to a monohull, and consequently, it has reduced DAFs.

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