A Refined Method for Determining Wind Loads on Moored Vessels

Abstract: Analyzing wind loads on moored vessels is integral to the design process of marinas to ensure the structural integrity of dock infrastructure. Existing methods from Tobiasson and Kollmeyer and ASCE Manual 50 are effective, but each present limitations in their ability to provide a seamless and accurate approach. Key shortcomings include the subjective determination of effective area, particularly due to the lack of comprehensive vessel data, and the reliance of engineering judgement for selecting drag coefficients. Furthermore, controversy surrounds the determination of the appropriate wind duration to use, gusts, or sustained winds.
In response to these challenges, we propose a novel method for calculating wind loads on vessels and subsequently transferring these loads to piles, chains, or other supporting structures within marinas. This method utilizes an effective vessel area dependent on the orientation of the vessel to the wind, based on vessel specification data collected at a preliminary level, accounting for factors such as air draft, length overall, and beam. Next, a refined drag coefficient is derived based on the wind heading on the vessel and an existing representative data set. Finally, the magnitude and direction of the resulting unit pressure is calculated. The product of the effective area and the unit pressure yields the wind force on the vessel. This process accommodates variations in wind direction and speed, vessel size and orientation, and slip layout configuration.
This method combines aspects from existing methods while offering flexibility to address individual project requirements. Considerations for factors of safety and standard inland structures can be addressed using guidelines in ASCE-7. Different wind durations can be compared to select the value best tailored to project demands; this is often related to anticipated vessel size, as smaller vessels can typically be mobilized by shorter-duration winds. Additionally, this method accommodates the inclusion of shielding effects aligned with established industry practices. Ultimately, this process merges the structural and coastal design approaches to produce one universal method for analyzing and designing marina structures.

How does this project / topic contribute to the advancement of the industry and profession?: Our research contributes to the advancement of marina design by offering a comprehensive and efficient approach to wind load analysis. By addressing the limitations of current methods and incorporating vessel-specific data, our method enhances accuracy while still allowing for the necessary variability to meet the unique demands of each project.

Does the project / topic implement new and innovative techniques, materials, technologies, and delivery methods?: This method of calculating wind loads combines two traditionally used methods, from Tobiasson and ASCE Manual 50, to create a more thorough approach. It also incorporates methods from Gaythwaite to determine the resultant force as well as a unified drag coefficient, rather than relying solely on engineering judgement. The proposed method calculates a more accurate vessel area, depending on the boat and wind angles, and has an iterative process for calculating the unit pressure and drag coefficient.

What was the most challenging aspect of your project / topic and how did you handle it to ensure success?: The primary challenge we encountered was the absence of air draft dimensions in many vessel specifications. To overcome this limitation, we resorted to measuring air draft using photographs of vessel profiles, which, while resourceful, may not always provide the most accurate data. We acknowledge these challenges and are committed to ongoing efforts to address them, ensuring the continued refinement and reliability of our methodology. Additionally, as this method is refined, it will need to consider how to adjust shielding assumptions when there is a smaller boat on the windward side of a larger boat.

Who is the target audience for this paper?: The primary audience for this presentation includes small craft marina designers seeking to refine their design approach to ensure the adequacy of dock structures and anchorage. 'Adequacy' in this context refers to the structures’ ability to withstand various loading scenarios associated with vessels of varying sizes, without unnecessary overdesign. It is important that the designer understands the environmental site conditions and vessel characteristics for each site to determine the optimal marina layout. Additionally, structural engineers, coastal planners, and regulatory agencies involved in waterfront development may find value in the insights and methodologies presented herein.