Monohull Dynamics During Heavy Lift

The dynamics of offshore heavy lift remains complex due to the interaction between the monohull construction vessel and the lifting object under the challenging environmental loads. This system becomes even more difficult to judge by common sense as the lifting weight offshore is usually above the motion reference point (the center of flotation of vessel), which makes the coupled system more unstable than the vessel without the lifting weight. This study proposes an analytical formula using the double pendulum, based on the Euler-Lagrangian equations, to explore the insight of the heavy lift dynamics. The effect of initial lifting condition is investigated to explain different possible outcomes for lifting similar weights. A range of vessel stiffness (GM), lifting heights and weights are considered in the parametric study for a better understanding of the coupled dynamic behaviour. While such analytical model is convenient for the understanding of the system mechanism, it is difficult to capture the effect from the wave loading. Therefore numerical models are used for this purpose. A comparison between the analytical model and the numerical model, performed in the frequency domain, evaluates the quality of ship motion response analysis performed by the numerical model. Results of these works will be useful in development of offshore crane curves for heavy lift offshore. Apart from the theoretical formulation, several real time ship instrumentation records including the ship motion and the crane hook load have been collected and investigated in this paper. The offshore instrumentation records provide a valuable benchmark to calibrate the numerical model or to accumulate “experience” for future projects. Challenges in such process are depicted and possible solutions are discussed. To make the comparison a fair “apple to apple”, the major difficulty arises from the “unknown” environment itself, where wave is not always measured and operation relies heavily on weather forecast and “experience”. There are also limitations in offshore measurement itself. Possibility of using the ship motion as the indicator is explored for decision making (to lift or not to lift). The paper develops a practical approach for obtaining reasonable numerical results in the engineering office and investigates scientifically sound interpretation of the ship motion time history onboard towards a safe offshore heavy lift.