Hydromechanically Coupled Motions of a Crane Vessel and a Transport Barge

Abstract

ABSTRACT Present day offshore lift operations feature the lifting of substantial loads horn a transport barge by means of large capacity semi submersible crane vessels with stem mounted dual cranes. During such operations the transport barge is moored perpendicular to the stem of the crane vessel. The motional behaviour of the crane vessel and transport barge are affected by hydro-mechanical coupling effects arising either from the hydrodynamic interaction between the two nearby vessels, or from mechanical coupling via the cranes, hoisting tie and slings. In order to investigate the hydrodynamic interaction effects a two-body diffraction analysis has been performed for a crane vessel and a nearby transport barge. The coupled equations of motion have been solved to establish the absolute and relative importance of the hydro-mechanical coupling between the two vessels. Whilst the crane vessel responses are hardly affected by hydrodynamic interaction, the transport barge motions may be significantly altered. It is demonstrated, however, that the hydrodynamic interaction effects are an order of magnitude smaller than the mechanical coupling effects and may be ignored for practical purposes. INTRODUCTION Nowadays it is generally accepted that the application of large capacity crane vessels may result in significant cost savings in offshore installation work, e.g. by adopting integrated topsides or lift able jackets 1. Recent years have seen the advent of two giant Semi Submersible Crane Vessels (SSCV?S) with twin revolving cranes mounted at the stem, the SSCV "DB102" (2×6,000 t maximum capacity) and the SSCV "M7000" (2×7,000 t). Large scale offshore lift installations feature the lifting of loads of up to 10,000 t from a transport barge by means of an SSCV. The loaded transport barge is usually moored perpendicular to the stem of the SSCV at a separation distance of only a few meters. During such operations the crane vessel and transport barge are subjected to wave induced motions, while their motional behaviour is also influenced by mechanical coupling via the cranes, hoisting wires and slings. The resulting complex dynamic behaviour of the hydro-mechanically coupled crane vessel and transport barge has been the subject of several investigations 2 which have culminated, amongst others, in the development of the LIFSIM time simulation program 3. As part of this effort a study has been performed into the hydro-mechanically coupled motions of a crane vessel and a transport barge. Common engineering practice in computational lift dynamics is to ignore hydrodynamic interaction effects. It is rc3CO@Sd, however, that the hydrodynamic interaction effects between two nearby vessels can only be analysed properly by means of a complicated two-body diffraction analysis, a major and expensive task 4. However, evidence from model tests suggests that the hydrodynamic interaction effects are at least an order of magnitude smaller than the mechanical coupling effects via the cranes, hoisting wires and slings, indicating that, for most practical applications, the hydrodynamic interaction effects can be ignored.