DESIGN CONSIDERATIONS FOR SINGLE BUOY MOORING STRUCTURE

Abstract

The increasing demand and limited reserves of oil and natural gas in the nearshore, and subsequent growth in technology have allowed exploration and production of offshore reserves further away from the sea shore into the deep sea. This precious liquid cargoes have been transported from one place to another by oil tankers or through pipelines. The sizes of oil tankers have increasd from 30,000 OWT to 700,000 DWT to facilitate the loading and unloading of oil bulk cargoes as quickly as possible. But economically viable large tankers with higher draughts pose problems in near shore terminals and are more suitable for the offshore terminals. In this dissertation, the single point mooring concept for offshore loading and unloading terminals have been discussed. The common characteristics of single point mooring is that the tanker is moored to the facility which bowlines to a single point only, allowing the vessel to rotate freely around the mooring point. A comparative studies have been done for different types of single buoy mooring (SBM) structures on the basis of operational and maintenance conditions to facilitate the option for selecting the suitable type of SBM-system. An exhaustive review of present available informations and a design considerations of SBM-structures have been discussed iii The main parameter for the design of SBM-system is the estimation of mooring forces. This forces can not be calcualted by a reliable design formula due to a large number of factors such as size of tankers, winds, currents, waves etc...and their inter relation with number of parameters. In this present study, the recently developed empirical formula (R.I.NA., 1990) based on design load factors have been used. The design load factors for different types of SBM-structures have been found out on the basis of informations taken from already designed and constructed SBM-structures for varying sea and environmental conditions. A computer program has been developed to facilitate the design of bow hawser mooring lines and chain anchor mooring lines based on energy principle. The program has also provision to design a submarine pipe line for transporting the crude taking into account the various design parameters such as length, fluid properties and their variation with temperature, inlet head of pump etc. A six leg chain anchor SBM-system has been designed to accommodate 30,000 DWT vessel at a 25m depth of water near Hazira, Gujarat. Design of submarine pipeline has been done based on the actual field data where fluid properties are changing with the variation of temperature. The program has been validated with the iv help of a data published in a research report. On the basis of present suty, it is concluded that the four leg catenary anchor leg mooring system is the most economical choice from the structural point of views but at the time of failure of one leg, the whole structure may be washed away. Hence six leg CALM-system is found to be the best option for economy as well as reliability point of view. It is also found that the m000ring forces depends upon the type of SBM-structures and the stiffness of bow hawser mooring lines. V