Research On The Concept And Global Performance Of Deepwater Very Large Floating Offshore Base

Although very large reserves of gas and oil hide under the deep water of South ChinaSea, no exploitation has been taken by now, which is because that long distance frommainland and the abominable weather make the supply of materials difficult and theworkers and facilities unsafe. Based on an overview of the research on Very LargeFloating Structures (VLFS), a new concept of Very Large Floating Offshore Base(VLFOB) is proposed for the application in deepwater oil and gas exploration andproduction in the South China Sea.After reasonable comparison of different schemes, an originate type of artificialharbor is adopted, which is a ringy type of VLFS with400m outside diameter and328m inside diameter respectively. This VLFOB concept is composed of eight similarcambered modules connected with each other. Each module looks like a Spar hull andmainly consists of one round pontoon in water, one deck box structure above the sealevel, and four columns connecting the pontoon with the deck box. The affiliatedwave-breakers are designed to reduce the wave height within the VLFOB. The tautmooring system including48chain-wire-chain lines is adopted to keep the VLFOB inplace with water depth2000m. The VLFOB is expected to be a storage andtranshipment station supporting the exploitation of the surrounding offshore oil andgas field, and also to be a safe harbor for workers and engineering vessels in theextreme sea state. In order to investigate the global performance of the innovationVLFOB, the following studies have been accomplished:1. In order to take the hybrid model test of VLFOB, a process is proposed to designthe truncated mooring system including diverse mooring lines. By this means,with considering800m as the water depth of model test, the truncated mooringsystem of VLFOB consisting of20truncated mooring lines has been designed,14of which have the same attribute and the other6of which have the other attribute. 2. A series of wave-breakers with different rectangular holes is designed and themodel test is carried out to choose the optimal couple of the wave-breakers, byevaluating the efficiency of wave breaking, motion performance, mooring loadsand connecting loads between two modules. In addition, some characters ofwave-breaker are researched, for example, the damping force.3. Frequency domain simulation is achieved successfully, the numerical results of theRAO of VLFOB’s motion response are compared with the model test results andgood agreements are observed. Furthermore, the effect of water depth on addedmass, damping coefficient, wave exciting force and motion response isinvestigated.4. The model test program of VLFOB is established, the model manufacture andsimulation and installation technique is developed. By the means of model test inSJTU deepwater basin, with taking1-year,100-year,1000-year and other specialsea states as the environment loads, the global performance of VLFOB isresearched comprehensively, including the motion performance, the mooringlines’ tension and conncecting forces between two modules. The feasibility of thechunnel for vessels passing in and out is made by evaluating the change of widthand the cables’ loads. The motion response and mooring loads of engineeringvessel is analysed to make sure whether the vessel could be moored at theVLFOB.5. The motion response of VLFOB is simulated numerically in time domain, and thecalculated results of that are compared with the model test results.The scheme of VLFOB is originated for the first time without any precedents in theworld, so many new problems need to be settled, for instance, the concept design, theglobal performance, the model test, the numerical simulation and so on. In general,VLFOB could adapt to the bad weather in South China Sea, but still have somedisadvantages, so some important advices are made to solve the key technique relatedto the design of VLFOB.