Tension Mooring Research Articles

Dynamic Responses of the TLP System at Sea

In order to prove the tension mooring system is safe and useful, Japan Marine Science and Technology Center performed the TLP sea test by utilizing the prototype floating platform. The previous papers of this study discussed the design of the tension mooring system on basis of the results of theoretical considerations and model tests, and introduced the outline of the sea test.This paper discusses on the measuring system and data analysis in the TLP sea test and the dynamic response characteristics of the TLP.The tension measuring system which utilizes the combination of conventional chain-guide wheel and compressive load cell was proved to give the varying tensions about twice larger than the theoretically expected ones. The reason of this overestimation is supposed to be that under the static initial tensions, the varying tensions attenuate while going around the chain-guide wheel and then the load cell measures the larger load out of balance.This inference was proved to be true through the direct measurement of the loads of the chain at both sides of the wheel in the second sea test (1987).The significant values of the varying tensions are estimated and compared well with the theoretical values.The horizontal movement of the TLP is another topic in this study. The surge/sway acceleration of the TLP were measured by utilizing the pitch/roll inclination meters. The yawing motions were measured by the monitor TV camera installed on the deck of the TLP.

TLP sea test

Japan Marine Science and Technology Center carried out successfully the TLP sea test in 1986. The installation site is 41 m deep and 2. 6 km NNW offshore of Yura harbor. In the test the performance of the tension mooring system was evaluated in the real ocean environments. This paper discusses the experiences on the TLP sea test.The TLP consists of a platform, tension legs, a sinker and a monitoring system. The platform is a semisubmersible with 12 column-footings and the principal dimensions of the platform are length overall 34 m, breadth overall 24 m, height 13. 5 m, draft 5. 5 m and displacement 527 ton.The platform is tautly moored by the 4 tension legs which are at sea bottom connected to the sinker weighing 536 ton in water. Each tension leg is a chain filled and covered with rubber applied to absorb the shock loads and prevent the mooring line from local abrasion. The anchor connector is made robust and monitored by using underwater TV camera at any time. The total initial tension is 125 ton which is equal to 24% of the platform displacement. The maximum wave height experienced during the test period is 7. 3 m which is just under the critical condition not to hit the bottom face of the upper structure.This research and development was performed through the Special Coordination Fund for Promoting R & D of the Science and Technology Agency of the Japanese Government, as a part of the Research on the Utilization of Marine Space by Coastal and Offshore Structure.

Study on the Tension Mooring of the Floating Ocean Structure (4th Report)

Japan Marine Science and Technology Center plans the TLP sea test using a prototype floating offshore structure which has been designed by Ship Research Institute, Japan Ministry of Transport, for the joint at-sea experiments.The TLP sea tests will be excuted in this and next years. The test will be performed to evaluate the performance and reliability of the tension mooring system in the real ocean environment. The site is 40m deep and 3km offshore facing the Japan Sea. The environmental condition is favorable in summer, which allows us very easy installation work. It becomes, however, severe in winter. The prototype floating ocean structure is not originally designed for TLP. Moreover this test structure is made as small as the motions of the structure and the mooring line tensions can be measured in relatively severe sea conditions upon keeping enough stability.The air draft is only 5.5m. Consequently the great waves will crash the upper structure causing shock loads in the tension mooring line. By the 1/40 scale model test the critical wave heigh is proved to be 7.2m. Therefore, the test period during which the sea test can be excuted safely is decided from May till October. The stress check for yielding and buckling is proved that the working stresses under the critical environmental conditions are much less than the allowable stresses.

Study on the Tension Mooring of the Floating Ocean Structure (3rd Report)

Japan Marine Science and Technology Center plans to execute TLP sea test in this and next years. The site is 40 m deep and 3 km off Yura harbor, Yamagata prefecture. An outline of the TLP and the basic design results are described in the 2 nd report. In the tension mooring system, the rubber-chain which is a chain filled and covered with an elastomer, will be applied in order to absorb the shock loads, and prevent the mooring line from locally abrasing.This paper presents the fundamental test results of the rubber and the detail design and 1/3 model tests results of the rubber-chain. The tension elasticity of the rubber-chain is widely disposable with rubber hardness and the allowance ratio which is defined as the ratio of the rubber parts got between the chain links to the total rubber-chain length.In this design, the rubber hardness 50 and the allowance 26 mm are finally selected for the 10 m long and φ 50 grade IV chain.

Study on the Tension Mooring of the Floating Ocean Structures

The tension mooring of the floating ocean structures seems promising to expand the ocean space exploitation. This kind of mooring method becomes relatively inexpensive in deep ocean, and has many advantages such that it can suppress the motion of the floating structures and needs only minimum mooring space.This paper presents the results of the fundamental study on the tension mooring. A semisubmersible model with 4 footing columns is tautly moored by the rubber-chain cable in the regular waves. The results of the study are following.(1) The rubber-chain cable has a nonlinear load-elongation characteristics. The static characteristics before stress relaxation is applicable to the dynamic analysis.(2) The heaving and pitching motions of the floating structures are negligible, but the surging motion increases as the wave length does.(3) The frequency responses of the tension fluctuations of the cable do not depend on the material in case of the large initial tension. The axial rigidity of the rubber-chain cable is, however, much less than that of the wire cable so that in designing the rubber-chain tension mooring system, we must carefully set the natural frequency of the system sufficiently out of the wave spectrum.(4) The snap load easily occurs when the initial tension is small.(5) The analysis model is applicable to the design of the tension mooring system. It predicts, however, much larger snap load. The authors expect that the analysis model will be improved by including the nonlinear hydrodynamic forces.

Study on the Tension Mooring of the Floating Ocean Structure (2nd Report)

In order to increase the ocean space exploitation by using the floating structures, it ceems necessary to develop the tension mooring system. This system has many advantages such that it becomes relatively inexpensive with depth, suppresses the motions of the floating structure and needs only minimum mooring spaces.An at-sea experiment is planned to establish those fundamental technologies. The semi-submersible consists of twelve columns with footing, and its principal dimensions are 30 × 20 × 12.5 m. This is supposed as a proto-type of bigger scale floating platform.The structure will be moored mainly by the six catenary chain lines with anchors. Further mooring by the four tension legs will be tested. The tension leg is made of rubber and chain. This composite material has the higher breaking load of chain and also anti-impact and corrosion resistance of rubber.In this study the tension mooring system has been designed to match with the environmental conditions of test site. The mooring tension characteristics have been estimated both by the model tests and the theoretical calculations.

Motion, Fatigue, and the Reliability Characteristics of a Vertically Moored Platform

The vertically moored platform is a buoyant structure for deepwater drilling and production that uses tensioned vertical mooring risers integral with platform wells. The Dynamic response of the structure to wind, wave, and current is analyzed. A model for relating environmental conditions to riser stresses is outlined. Introduction As the search for hydrocarbons continues, offshore platforms are being installed in deeper and deeper water. platforms are being installed in deeper and deeper water. These platforms primarily have been of the fixed type, for water depths ranging from 10 to 1,000 ft (3 to 300 m). In deeper waters, the fabrication and installation of these platforms can become difficult and extremely costly. In addition, as the water depths increase, the first natural periods of vibration of these platforms increase from less than 1 second to 4 or 5 seconds and higher. Wave forces have significant energy at this level (Fig. 1); dynamic amplification also becomes significant. Thus, such structures may be subjected to a large number of cycles at a significant stress level, so that structural fatigue becomes a major design parameter. The vertically moored platform (VMP) concept Fig. 2) is a drilling and production platform specifically designed for deep water of 700 to 3,000 ft (200 to 900 m) or deeper. This platform can be fabricated and installed at a reasonable cost. A detailed engineering description of this concept has been presented.The VMP is a buoyant structure moored by vertical tubular members maintained in tension. The well casings are integral to the mooring risers, and the wellheads are on the deck. The basic motion is similar to that of an inverted pendulum; the structure is flexible horizontally and rigid vertically. While the VMP and semisubmersibles have similar horizontal motions, the latter have substantial vertical motions, making it difficult to tie in wells for production operations. VMP vertical motions are restrained. permitting conventional tie-in of wells at deck level.First, we describe VMP motions. Next, a computer model of VMP dynamic behavior is discussed, followed by a comparison of calculations with experimental data. We stress the selection of design environmental conditions for a VMP. An overview of the riser fatigue and reliability studies demonstrates the adequacy of the mooring system. VMP Motions VMP dynamic characteristics are inherently different from those of fixed offshore structures (Fig. 1). The motions of the VMP jacket (within 6 degrees of freedom) have differing characteristics, depending on the corresponding stiffness of the mooring system. The flexibilities are high for surge, sway, and yaw motions, with natural periods in the 60-second range, where wave energy is negligible. Surge, sway, and yaw motions were critically damped during model tests and computer simulations. No resonance can be expected for these motions. The mooring system resists the mean environmental forces (e.g.. averaged more than 30). while jacket inertia resists the shorter period environmental forces. Under the most severe environmental conditions, in water 1,000 ft (305 m) deep, a maximum horizontal placement or surge of 150 ft (46 m) is predicted. placement or surge of 150 ft (46 m) is predicted. JPT P. 267