Constructibility Engineering for Large Offshore Structures
The very large and complex structures now being considered for deployment in deep water and in adverse environments such as the Arctic are increasingly demanding fuller consideration of the entire construction process, in order to reduce the time and cost of construction, to ensure optimum quality and long term behavior, and to integrate more effectively with the other systems involved in the total field development.
These considerations have led to the concept of “Constructibility” as an essential aspect of the entire planning, engineering, and construction process. This concept involves such considerations as to where and how to build, the procedures to be followed, the equipment and facilities required, material procurement and handling, quality control, personnel training and supervision, safety, scheduling, and cost control. While each of these elements has been individually considered in the past, Constructibility wraps these up into a coherent system.
The process starts during the initial feasibility stage where it may affect the decision as to the type of structure, the basic materials to be used, and the way in which the structure will be integrated into the overall field development. Consideration must be given to the fabrication site, the availability of land, and protected inshore harbor areas, the available water depths for exit to the sea, and the environmental, political and social conditions, both those affecting construction and those affected by the construction.
During construction, these huge structures pass through many stages, during which full consideration must be given to draft, freeboard, stability, damage control, structural loads and stresses, deformations and deflections, dynamic response, and access for further construction activity.
Various techniques of construction need to be evaluated. Typical schemes that have application in offshore structures include prefabrication, modular construction methods, steel-concrete construction in its several combinations, and temporary ballasting and grounding, etc. Special construction equipment such as floating gantries may be considered.
Because of the rigors to which the structure will be subjected in service, with consideration of fatigue, impact, low temperature, and corrosion, and recognizing the limitations and cost of in-service inspection and repair, the ability to control quality during construction assumes a major role.
Scheduling also becomes of more than normal importance because of the financial costs attached to any delay and the need to integrate so many complex and sophisticated systems.
Finally, Constructibility requires an evaluation of risks and the establishment of contingency plans to deal with such events as storms during installation, changed soil conditions, accidental flooding, strikes and other delays.
This paper will be illustrated with examples of offshore projects from the North Sea and Arctic. It will propose an initial methodology suitable for Constructibility Planning in Offshore Structures.
KeywordsBallast Water Mooring Line Offshore Structure Steel Jacket Concrete Shell
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