ABSTRACT

Construction, installation and retrieval of offshore platforms are carried out in exposed areas and at still growing scales. Not only entire topsides can be lifted but also the installation of large jackets can now be carried out by vessels with dual cranes.

To take full advantage of the large cost-savings related to such operations, the feasibility, workability and risk level have to be evaluated accurately. To this end computer simulations of the dynamic behaviour of the total heavy lift system due to wind, waves, current and hoisting/de-ballasting are required.

A general purpose 18 degrees of freedom simulation model covering the mechanism of motion has been developed. The computer model is able to provide the motions of the crane vessel, load and transportation barge and the related tensions in hoisting wires, slings and mooring lines as well as the barge-load impact forces. Special attention is paid to the simulation of partly submerged jacket-type structures. The computer program has been validated with results of an extensive model test program.

INTRODUCTION

Large semi-submersible crane vessels (SSCV's) with lifting capacities of up to 14,000 tonnes are used nowadays for the installation of platforms in exposed areas. A considerable increase in load weights and volumes is expected because of the proven cost savings that result from heavy offshore lifts. (see ref. [1]).

By means of the dual crane option, it is not only possible to lift complete topsides, but also to perform installation of liftable jackets (Fig. 1). The conventional way of jacket platform installation is the transportation barge launch procedure. In general this launch causes an extreme loading on the structure. Furthermore buoyancy requirements result in large diameter tubulars and extra tanks. For upending ballast systems are needed. All launch related structural provisions increase the wave and current loading. It is therefore evident that the launch procedure has a strong impact on the design, engineering and construction of jacket-type platforms. A liftable jacket can be designed for loads exerted to the structure during its operational life time. Hence such jackets are outstanding from the point of view of design and construction work, use of steel and maintenance. It is generally accepted that liftable jackets are 30 to 40 percent more economic than conventionally launched jackets.

Lifting operations at exposed locations with rapidly increasing weights, volumes and operational complexity cannot be performed on the basis of experience only. Evaluation of the feasibility and workability of such operations requires a detailed analysis of the system's dynamic behaviour as induced by waves, wind and current in combination with the hoisting action and the (de)ballasting procedures.

For topside installation the following phases in the dynamic behaviour of the crane system may be distinguished:

  • crane vessel in mooring system

  • side-by-side mooring of crane vessel and barge

  • pre-hoist condition with pre-tensioned slings

  • hoisting/de-ballasting with load/barge impacts

  • pendulum condition with free hanging load

  • positioning and lowering on support structures

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