A Study on the Hull Form Optimization of Semi-submersible FPU Considering Seakeeping Capability and Structural Weight

Abstract

In this paper, the optimal hull form of the real semi-submersible FPU with the minimum vertical motion and the minimum structural weight is determined. As the oil and gas fields in deep water is developed widely these days, the demand for floating type offshore units is increasing. The floating units are usually encountered with severe seas so that the large vertical motion occurs which can induce the operational downtime. Since avoiding the downtime leads to the economic advantage, semi-submersible type units which have relatively good seakeeping capability are being preferred. The amplitude of wave-induced motion is closely related to the hull form. Therefore, it is necessary to determine the optimal hull form in order to maximize the advantage of floating type structures. In this paper, fully automated procedure for the optimization of semi-submersible FPU's hull form is introduced and optimization is performed based on real model. Commercial software DNV WADAM is used for motion analysis. In order to automatically generate two files which are needed for WADAM as input, three modules are developed. In panel generation module, hull structure is divided into ten parts and ten geometric parameters which correspond to the dimension of each part are defined. B-spline surface is used for geometric representation. Mesh elements are generate in equal interval according to the global mesh size. In mass estimation module, weight and vertical center of gravity of total structure are estimated. Mass element is divided into several part

topside, steel, outfit, ballast water and remaining. Estimation is based on geometric characteristics such as length, surface area and volume. In conditions setting module, conditions for analysis like angular frequency, wave heading angle, sea state are set. In addition, 53 points for air gap analysis are defined in this module. Simulated annealing algorithm is adopted for the optimization. Four design variables are used. Among ten geometric parameters, only three major dimensions are selected. Additional variable is defined for flexible control of the amount of ballast water. As constraints, GM, minimum air gap, freeboard, draught, total hull height are considered. The distance between columns remains constant to support topside structures. Two weighting factors which correspond to two objectives, minimizing heave response and minimizing structural weight, are applied. By alternating the value of the factors, four optimal solutions are found. As the preference for structural weight is increased, total hull height is shortened accordingly. But this leads to worse seakeeping capability. It is confirmed that total hull height is in inverse proportion to heave response.