Heavy Lift Monitoring And Prediction In The Construction Of North Sea Platforms

Abstract

ABSTRACT The role of a crane ship in the construction of production platforms in the northern North Sea is described with special emphasis on a newly developed on-board computerized vessel performance monitoring and prediction system. The planning stages of the operation are discussed with reference to analytical studies performed in order to assess operational constraints such as module motions and velocities and their effects, ship motion parameters, relative motion, etc. and to compare the performance of different crane ships for the specific job. An onboard system capable of providing the crane ship operating personnel with an on-line access to the data along with real time monitoring of key operational parameters is described. A description of the system is given illustrating its modes of operation, i.e., automatic and trend display in the monitoring mode and a response simulation in the prediction mode. Examples of heavy lifts performed are illustrated, giving full account of the system application during the operation. The merits of the quantification of responses and their simulation prior to actual operation is illustrated and discussed. INTRODUCTION Construction offshore has traditionally been achieved through the use of floating cranes of various sizes. As long as the work was limited to coastal waters, rivers and other sheltered locations, the response of the vessel to environmental effects was of minor importance and selection of equipment was based primarily on crane size, availability, cost and other factors such as support services, hotel accommodations, mobility, etc. Most cranes were installed on flat bottom barges of various sizes with the crane capacity rarely in excess of 500 tons. The extension of offshore construction into deep sea sites subjected to adverse sea conditions has ruled out the practical use of these extremely motion-sensitive derrick barges and has lead to the introduction of ship shaped hulls. The gradual increase in the need for larger crane capacity and the search for oil in deeper, more hostile waters has lead to the demand for heavy lift crane ships of substantia1 displacement, as large as 50–60,000 tons. The need for these converted tankers and specially designed heavy lift ships, which currently constitute the backbone of the offshore construction fleet in the northern North Sea became apparent in the mid-seventies The extreme sensitivity of the heavy lift operation to various ship responses excited by the sea and the fact that such vessels bear one of the highest daily rates charged for any offshore type vessel suggests that the evolution of the crane ship has not yet reached its climax. The use of semi submersibles as a floating platform, the installation of more than one crane on each vessel and the introduction of sophisticated on-board guidance systems are expected to dominate the offshore construction industry in the years to come, particularly in areas subjected to adverse weather conditions such as the North Sea. Simultaneously, other approaches to offshore construction problems are being investigated. Integrated deck float-out and installation is just another possible means of solving the monumental task of constructing production platforms in deep water.