The paper describes the development and construction of a semi-submersible derrick barge with a load lifting capacity of 2000 tons. The design was developed to significantly increase the workability of crane ships, which is low for conventional types operating in northern North Sea conditions. This design is unique in as far as it allows the crane to be operated to its full extent (2000 sh tons at 35 meters, revolving) without the necessity to change the preselected ballast condition of the barge. The paper further describes the motion characteristics of the vessel, collected from model testing. On the basis of a vertical crane tip velocity of 0.7m/sec the workability is in excess of 325 days per year in the northern North Sea, as compared to 110 days for conventional crane vessels.
The new unit, of which the construction is completed in spring 1978, will generally be used in the North Sea area and other seas with similar characteristics. The application of the barge will lead to a considerably shorter and cost saving construction time of offshore oil and gas producing platforms. Where sizeable projects in the northern North Sea required approx 1 to 1 ½ year to complete by use of conventional crane ships, such construction time will now be reduced to approx half a year or less. The authors are the opinion that the development of the new construction unit will influence the industry?s choice of construction towards steel, piled platforms. The operator of the new barge, NOC, now accepts lumpsum contracts or work-rate-only contracts throughout the year.
Offshore oil exploration originated in the relatively calm waters of the Gulf of Mexico, the Arabian Gulf and areas such as Lake Maracaibo. In those relatively shallow waters flat bottom barges with crane capacities of say 300 tons were used in the construction of the production platforms. Increasing demand for oil and sometimes political reasons initiated exploration in other offshore areas as well. One of the principal areas in this respect was and still is the North Sea. Naturally the expertise and the fleet of established "Gulf" offshore contractors were used in the task of bringing this North Sea gas and oil onshore. Starting in the shallower waters of the southern part the flat bottom barges played an important role. However, moving further north to deeper and less protected waters, it was soon found that weather downtime during installation rose to unacceptable levels. The industry?s answer to such uneconomic situation resulted in the development of larger barges and bigger cranes. As oil and gas exploration moved into the middle and northern part of the North Sea it soon became apparent that the increased lift capacities which allowed quicker installation would not be the final answer to the problem now encountered in the more hostile waters. The next move was the development of ship-shape construction units, while crane capacities increased from 800 tons revolving to 3000 tons.