FEATURE
ENVIRONMENT bers are longitudinal, running the full length of the ship from bow to stern, complicates the joining of each frame across the entire length of the vessel. The typical length of a wave is about 300 feet and ships of this length or greater are subject to considerable bending stress. For such vessels, it is reasonable to longitudinally use framing or a combination of longitudinal or transverse framing but for vessels below the length of 300 feet, typical for most modern offshore vessels, longitudinal framing has no advantage over transverse framing but definitely shows a disadvantage in construction cost. The other factor decreasing construction time and cost, flanged plate framing, does not require the individual frames to be rolled in a complex, costly process, with many frames shaped differently, to achieve the faired hull form but the frames are cut into shape to achieve the faired characteristics of the hull at a much lower cost, which can be done efficiently at a lower skill level. Constructability offers improvements in all the metrics of shipbuilding: manhours, cost, and total build time. These improvements can clearly be recognized in the Bourbon Offshore Liberty vessel newbuild program, which is sustaining the delivery of a GPA designed vessel every two weeks. To determine the ideal cost/performance ratio customized to a vessel's operational profile, operators need to also consider an adequate propulsion system. Besides the simplified hull form, other applications can contribute to environmentally friendlier operations through efficiency and fuel savings, such as diesel-electric propulsion systems, which are becoming more prominent worldwide. While Engine Emissions regulations, as well as Clean Class notations, address and continuously improve the minimization of measured emissions output, these standards do not enforce limitations on the actual fuel consumption and therefore fuel efficiency of the vessel. Until recently, there was not sufficient motivation to consider overall fuel consumption and efficiency for PSV/OSV operations. The possibilities in improving fuel economy have grown through the use of diesel-electric propulsion configurations, facilitating improvements in fuel consumption and emissions by 30%. The requirements for Dynamic Positioning (DP) are detrimental to fuel economy due to operating at a reduced engine demand. Diesel-electric systems can considerably improve actual fuel consumption to mitigate this by providing greater flexibility in the use of power, especially during DP operations. Besides reduced fuel consumption, diesel-electric can further contribute to a vessel's profitability as it creates flexibility in lower deck cargo space arrangements, and if taken advantage of properly, can increase cargo capacities by up to 30% while offering the ability to carry a greater variety of products. Until certain amendments to further protect the environment and minimize the ecological impact of maritime operations become mandatory, a range of options is available to operators not requiring extensive financial investments but instead can even lead to construction and operational cost savings while taking the environment into consideration. Depending on a vessel's operational profile, the optimum hull might have to be a trade-off between production cost and operation cost while the ideal solution for fuel consumption has still not been widely accepted due to higher capital cost investments, but the industry seems to be embracing the idea of environmentally friendlier solutions.
forming: Cold forming involving rolls and presses or thermal forming using line heating with torches and lasers. Single-curvature plates, shaped only in one direction or with only a slight amount of backset, can be formed using rolls while the more complex doublecurvature plates need to be fabricated using large hydraulic presses. Such processes are very labor intensive and require high skills, demanding a significant portion of the total structural manhours. Furthermore, being a manual process requiring high skill, it is not repeatable and may suffer from inaccuracy. Not only the hull form itself, but transverse framing, as well as flanged plate framing, contribute to constructability and therefore to decreased construction time and cost. Transverse framing, with main frame members running from keel to deck and from side to side as deck frames, and featuring longitudinal frames spaced widely, simplifies the exact fitting process of frames within individual modules and later on in joining these modules. Longitudinal framing, in which a comparatively small number of large transverse frames is employed and the chief framing mem-
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Maritime Reporter & Engineering News