ABSTRACT:

A deepwater lifting operation using crane vessel is investigated by a time-domain analysis program for floating crane vessel systems. In the analysis, a coupled analysis of floating vessel and lifted object dynamics connected via crane wire are simulated with and without heave compensator. For deepwater installation cases, amplified vertical motion of lifted object is anticipated even under mild sea state due to axial resonance of wire because of elastic property of the wire. The vessel motion of 6 degree-of-freedom is solved in time-domain including memory effect, while 3-d.o.f. motion is considered for lifted structures. The wire is modeled as linear spring and heave compensator is modeled as combination of spring and damper in axial direction. A series of numerical simulations conducted for a wide range of wave periods, and discussions are made for making effective operational range of heave compensator and effectiveness of active heave compensator.

INTRODUCTION

A typical deepwater installation operation consists of four main phases: lift off from deck of a transport barge, lowering through the wave zone, deepwater lowering/lift operation, landing on seabed and retrieval (DNV, 2011). During the deepwater lifting operation, heave compensation system may be used to control the vertical motion of the lifted object and reduce the dynamic loads in the hoisting system. There are three types of heave compensators that are used in deepwater lifts: Passive, Active and combined heave compensator. A passive heave compensator is a kind of spring-damper system which shift heave natural frequency of hoisting system and reduce dynamic loads. Passive heave compensator is also designed to reduce impacts on offshore cranes by reducing the dynamic force in the hoisting wire. An active heave compensator uses controlled winches, hydraulic pistons and reference signal. Clauss et al. (2000) presents a comparative study of the operation capabilities of floating cranes.

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