Scheduling and cost estimation simulation for transport and installation of floating hybrid generator platform

Highlights

• We investigated the process of transportation and installation of floating wind turbine.

• We simulated the schedule and cost estimation as discrete event processes.

• We identified the optimum T&I plan in terms of the shortest duration or the lowest cost.

• Different T&I plans are preferred depending on the objective, minimizing time or cost.

Abstract

Concerns such as global warming, depletion of fossil fuels, and the dangers associated with the nuclear energy have led to the research and development of offshore wind and wave energy sources. However, the transportation and installation (T&I) planning for offshore energy platform is still being conducted in an unmanaged manner. In this paper, the T&I process of floating hybrid generator platforms is investigated. The schedule and cost estimation are simulated as a discrete event process, an alternative T&I process is evaluated, and the optimum T&I plan (with respect to the shortest duration or the lowest cost) is identified using various techniques.

Introduction

Transportation and installation (T&I) planning can affect the design of offshore structures [1]. However, the T&I processes are planned and executed in an unmanaged manner even today. In this study, we analyzed the T&I processes for floating hybrid generator platforms and developed a simulator for scheduling and cost estimation.

In this section, we briefly present the concept of the conventional wind turbine and wave generators and the floating hybrid generator platform considered in this research. We then describe the T&I processes and the related research. In section 2, we explain the suggested simulation framework, model, scenario editor, and result viewer. In section 3, the weather information implementation methodology is described. In Section 4, we describe the estimation of a simulation scenario and two T&I scenarios. Finally, in Section 5, we present the conclusion of this paper and the planned future research.

A wind turbine is a device that converts the wind energy into electric power. The power generating capacity of the present day offshore wind turbines has increased to more than 3.6 MW. The increased output of wind turbines makes their installation further out in the sea more feasible than in the past. Offshore wind turbine installations have some advantages over onshore installations, including the improved performance of lower altitude installations for a given wind speed [2]. However, the drawbacks of offshore installations include the extra cost of building and installing floating foundations, installing the wind turbine tower and blades, and installing transmission lines from the platform to the onshore power grid. These additional expenditures can increase the cost of an offshore wind turbine project by approximately 20% [3].

The foundations of offshore wind turbines can be designed as fixed or floating structures [4] depending upon the depth of water as shown in Fig. 1. A fixed foundation can be installed as a monopile platform with a single cylindrical geometry for a depth of water of up to 30 m and as jacket or tripod structures for a depth of water ranging from 30 m to 50 m. Floating structures have cost advantages over the fixed ones for installations at sites having a depth of water of 50 m or more. The structure of offshore wind turbine platforms can be the same as that of oil and gas production platforms, such as tension leg platform, semisubmersible platforms, and cylindrically shaped spar platforms.

The role of a wave generator is to convert the ocean surface wave energy into electric power. Fig. 2 shows six types of wave generators: point absorber, attenuator, oscillating wave surge converter, oscillating water column, overtopping device, and submerged differential pressure configurations [6]. The point absorber configuration is widely used. Fig. 3 shows various types of point absorbing wave generators.

The wind is a major source of energy for ocean waves; therefore, a suitable site for offshore wind turbines is also appropriate for wave generators. This has led to the development of combined wave–wind technologies including co-located systems, hybrid systems, and island systems, as shown in Fig. 4. In the co-located systems, independent wind turbines and wave generators are co-located and connected to the same grid. The hybrid systems can be installed on a bottom-fixed or floating platform. The island system is conceptually similar to the hybrid system, however, it is realized at a massive scale of an island. Co-located wind turbines and wave generators have increased accessibility for operations and maintenance [8].

The Korean Research Institute of Ship and Ocean Engineering developed the conceptual design of a 10 MW-class hybrid power generation platform [10]. The conceptual design is shown in Fig. 5 and the principal dimensions in Table 1.

The platform has a 150 m × 150 m semisubmersible substructure with four 2 MW-class wind turbines and 24,100 kW-class point absorbing type wave generators. The T&I of this platform have been addressed in detail in this research.

Two alternative methods are considered for the transportation of the semisubmersible platform: dry towing and wet towing.

The first method employs a self-propelled barge or barge with tugboats. This method is faster than the wet towing method. However, because of the size of the substructure, a heavy-lift vessel is needed for dry towing, resulting in a high cost of transportation. Furthermore, the beam length of the world's largest heavy-lift vessel is 79 m, which is 71 m narrower than the width of the hybrid platform.

Therefore, the second method of towing is considered for this research. For the wet towing process, three or four tugboats are used as shown in Fig. 6. In this study, it is assumed that the construction yard will be on Geoje Island, located off the southern coast of Korea. The platform will be transported to the west of Chagwi Island, near Jeju Island (Fig. 7). After the transportation of the semisubmersible platform to the site, eight mooring lines will be connected to the sea floor with suction anchors.

We researched on weather routing and offshore transport for this project. For weather routing, works by Hanssen [12], Sen [13], and Safetrans software package [14] are reviewed. For the simulation of offshore T&I, works by Walther [15], Walker [16] and Kaiser [17] are reviewed.

Weather routing includes the evaluation of the feasibility and routing of transportation considering the weather conditions. The first modern weather routing was attempted by Hanssen of the United States Hydrographic Office [12]. Sen developed the key components of weather routing: forecasting the sea conditions, estimating the ship behavior, and developing an appropriate and efficient track or path optimization algorithm [13]. For simulating the transportation of offshore structures, Safetrans is typically used. Safetrans was developed as a joint industry project that was funded by 31 companies, including 11 oil companies, six surveyors, and six transport companies [14]. Safetrans can be run in either vessel motion climate (VMC) mode or Monte Carlo simulator (MCS) mode. In the VMC mode, the ship motion caused by the wave action is calculated, and the result is used as the input for the MCS mode. In the MCS mode, severe cases are calculated by repeating the Monte Carlo simulations.

Table 2 presents a comparison of Hanssen's research in 1960, Sen's research in 2015, Safetrans software package developed in 2001, and our current research. The objective of the conventional weather routing is time or cost minimization. The objective of Safetrans is to test the feasibility of offshore transportation and to determine the worst case. On the other hand, our research focuses on the duration and cost of the T&I operation. In the related works, the prognostic wave charts, international towing tank conference (ITTC) spectrum, and an in-house database are used. In our research, the accurate sea-condition forecasting data, WAVEWATCH III, and ocean surface current analyses–real time (OSCAR^®) of the U.S. National Oceanic and Atmospheric Administration (NOAA) are used. For the ship behavior, the operation criteria are applied because the weather workability is important in T&I estimation. In the related works, performance curves or response amplitude operator (RAO) calculation is applied. The transport route of an offshore structure is regarded as fixed because the transport speed is slower than the naval or commercial ships, and procedure optimization is more important than route optimization in the offshore operation.

Research studies related to the scheduling and cost analysis for the T&I of offshore wind turbines are available. We studied the optimization of T&I operations of monopile offshore wind turbines because it is widely used for offshore wind turbines.

Walther calculated the operating time and cost using algorithms developed in Microsoft Excel^® and Visual Basic^® [15]. He used input variables such as the number of turbines, the distance to port, the transit speed of the vessel, its loading capacity, daily operating costs, and a weather window factor obtained from the statistical analysis. Walker used the marine economic risk management aid, Mojo MERMAID^® of Maritime^®, for the calculation of 300 wind turbines without foundation [16]. He considered four scenarios: a single blade installation with a self-propelled jack-up vessel, a single blade installation with a large self-propelled jack-up vessel, a bunny ear installation, and a rotor star installation. However, he did not consider the weather conditions.

Kaiser developed a mathematical cost model for the T&I of wind turbine substructures and applied his model to three wind farm construction cases. In his model, the total cost is the sum of the costs of the foundation and turbine installation, cable installation, substation installation, provision for scour protection, and mobilization.

Table 3 shows the comparison between the related works and our research.