Hoist and boom wire dynamics during offshore heavy lifting

Abstract

HeeremaMarine Contractors (HMC) owns and operates several semi-submersible crane vessels (SSCV) used for offshore heavy lift operations. Throughout the engineering phase of a (dual crane) heavy lift, dynamic lift models are generated by combining bodies with their hydrodynamic properties, inertia and spring-dashpot elements. The models represent the mass-spring system of a heavy lift over the different lift stages during a topside installation. These stages characterize the free floating, load transfer, free hanging and set down
phases.

Hook load fluctuations are governed by the relative vertical motion of the load and the crane boom tip. At the load transfer phase this motion is governed by both the motion of the vessel and the barge, whereas for the free hanging phase it is mostly effected by the motion of the vessel. To achieve safe and successful projects, an accurate prediction of the load and motion responses is essential while advising offshore personnel about the lift to perform. At the moment an inconsistency exists between predicted load fluctuations and offshore
crane measurements. This is the main reason for this research.

Until now a simplified spring-damper system is used to incorporate the hoist wire reeving system of a crane.This simplification is not fully justified and three goals are set up to model this in a more detailed manner. Firstly, the driving parameters of the load-crane-vessel system are assessed. Secondly the dynamic behavior of the wire reeving system is captured in a numerical model in Simulink. Finally, the dynamic load fluctuations of the model are compared with offshore measurements to both validate the model and analyze the results.

The dynamical model of the load-crane-vessel system is solved in the time domain and it consists of two parts. The first part considers the sheave and wire system of the crane. An equation of motion is derived for each sheave where dry friction is taken into account and leads to a stick-slip effect. This friction originates from the sheave bearings and from the bending friction of the steel wire rope that runs over it. When a load is raised, the stress in each rope part increases from the winch to the dead end. With a lowering operation the effect is the opposite. Due to stick-slip this force difference remains in the crane wires after the operation. The friction factors of the sheaves in the numerical model are tuned with steps observed in offshore load measurements during crane operations.

The second part of the model imposes the measured vessel motions to calculate the motion of both crane boom tips and the topside. This is performed by applying their influences as external forces on the free bodies. With the relativemotion the response of the hoist wire forces is determined. A coupling of these two parts can be made by removing the element of the hoist wire in the imposed motion model and replacing it by a pair of two nonlinear forces determined fromthe sheave wire model. These forces have an opposite sign and
are equal in absolutemagnitude and phase.

The effect of friction on dynamic hook load fluctuations is also consideredwith inputs of different amplitudes and frequencies. The hook load fluctuations at the measuring sheave are lower than applied fluctuations in the model when friction is taken into account. The simulated force at the measuring sheave is better represented with higher load fluctuations as the stick condition is exceeded earlier.