Enhancing offshore service vessel concept design by involving seakeeping

Abstract

The growth in the offshore wind industry sees an increased demand for offshore service vessels (OSVs). These vessels often operate in harsh conditions, and their performance is heavily dependent on their seakeeping characteristics. Conventional ship design processes fail to effectively consider seakeeping early in the design process, leading to sub-optimal vessel design. A design framework has been developed in the software 'NAPA' to efficiently design high-performing OSV concept designs.

The developed framework is able to optimize a hull shape -specifically the main particulars and length of different hull sections- to maximize performance in certain objectives. These objectives are seakeeping, ship resistance, lightship weight, and station keeping power requirements. Regarding seakeeping, the attainable percentage operability is calculated for each iteration, although the Operability Robustness Index (ORI) is optimized. The ORI is a more robust seakeeping key performance indicator (KPI) than percentage operability, which is advantageous when facing concept design uncertainty. The framework maximizes ORI, thereby seakeeping performance, for a particular loading condition, motion sensitive criteria, and operational area. The ORI is evaluated based on the area of operation's scatter diagram and wave spectrum, governing motion limits, and iteration-specific RAOs. Designs are required to satisfy an initial stability constraint, to ensure feasibility.

The framework's output is a Pareto-frontier showing the trade-offs between different KPIs, and the corresponding variable combinations. Thereby, the naval architect can evaluate what design offers the best overall performance.

A 'feeder' OSV, designed to transport wind turbine components to wind installation vessels has been optimized to validate the framework. This OSV is currently being developed as a concept by C-Job naval architects. The ship has been optimized for maximum operability of an Ampelmann motion compensated platform. A single optimization run took three and a half hours to complete 300 iterations, thereby finding the Pareto-frontier. Comparing the Pareto-optimal solutions with the base vessel, the ORI can be increased up to 3.6%, the lightship weight decreased by 21.1% and the ship resistance decreased by 13.0%. The framework showed that smaller vessels can still attain good seakeeping performance, leading to a substantial reduction in lightship weight. The increase in seakeeping performance allows for the use of less expensive motion compensated equipment while maintaining high operability. The framework showed that there is a trade-off to be made with regard to seakeeping, and lightship weight, and ship resistance. The framework presents what variables and ship attributes cause these trade-offs. This information allows naval architects to determine the optimal design direction during concept design. Clients and naval architects can decide what trade-off in performance provides the ideal combination to achieve the ship's mission. Consequently, besides producing high-performance designs, the framework substantially increases early design knowledge. Thereby, the overall design process becomes more efficient.

The framework showed to be a valuable tool for OSV concept design. By the extensive incorporation of seakeeping early in the design process, naval architects can design high-performance OSVs efficiently. The produced designs maximize performance in any of the KPIs, ensuring vessels have a high operability, but do not weigh more, or have higher fuel costs than is needed.