A new method for analyzing frequency-dependent transmission line systems with nonlinear terminations is presented. The generalized scattering matrix formulation is used as the foundation for the time domain iteration scheme. Compared to the admittance matrix approach proposed in a previous paper, it has the advantage of shorter impulse response which leads to smaller computer memory requirement and faster computation time. Examples of a microstrip line loaded with nonlinear elements are given to illustrate the efficiency of this method.
The Smart?Leg system developed by ETPM ensures the shockless float-over installation of heavy fully-commissioned integrated decks on offshore jacket structures. ETPM successfully installed the Ekpe Gas Compression deck offshore Nigeria in June 1997, using, for the first time, the Smart?Leg float-over system. Two years before, ETPM undertook a program of numerical simulations and model test work as part of the development of the system. The numerical program was validated against the basin model test results. The design of the Ekpe deck, the cargo barge, and the Smart-Leg equipment was based on extensive numerical simulations performed with the Smart?Leg software. A detailed weather analysis and forecast were conducted during the Ekpe deck installation. Furthermore measurements of environment, barge motion and pressure and stroke of Smart?, Leg jacks were carried out to monitor the operation. A later study verified correlation between measured and predicted motions and forces. This paper presents the model basin test results, the calibration and computer work, and the field results from the Ekpe deck float-over installation. The results of the correlation between the predicted motions/forces and the as-measured values obtained during the float-over deck installation are presented, too. The main advantages of the method have been confirmed: smooth suppression of deck and jacket relative motion, absence of shock either horizontally or vertically, and great positioning accuracy. Introduction In June 1997, the 4,100 tonne Ekpe Gas Compression deck was successfully mated on the OY jacket using the Smart?Leg float-over system, as described in the companion paper reference [1]. The Smart?Leg float-over method is fully described in reference [2]. The main steps of the mating operation are listed below as well as the parameters that need to be evaluated in order to design structures and equipment and to define limit environmental conditions for the operation. Environmental conditions to be considered for offshore Nigeria operations include:–wind and current, although corresponding loads are much smaller than those due to waves–maximum tide of 2 meters–long swells with maximum incidence of ±20° relative to the jacket longitudinal axis–wind generated waves from any direction Analytical Work and Design Procedure Several computer programs have been used to check the feasibility of the mating operation. The main software is a new time domain simulation program developed by BMT [2] to describe the dynamic behavior in regular and irregular waves of three bodies: the barge, the deck and the jacket linked by the "Smart" components - i.e. the Smart?Fins, Smart?Fenders, Smart?Legs and Smart?Shoes - with their complex mechanical and structural characteristics. Other programs are MOSES commonly used for the simulation of marine operations and structural verifications, and FASTRUDL, a general purpose structural program. Vertical clearance between jacket and deck Legs. It is essential to be sure that there is no risk of shock between jacket and deck until the deck legs are immobilized. This was ensured by first designing the Smart?Leg system to leave enough clearance between the deck bottom and the jacket top, based on the numerical simulation results, and then by measuring, during the installation at the site, the heave motions prior to the beginning of the mating operation.
The Arctic offshore may hold the largest undiscovered oil deposits which could account for up to 25% of the world’s undiscovered hydrocarbons based on Gautier et al (2009). Access to the deepwater deposits in the Arctic Ocean presents a special challenge. In the past four decades only shallow water drilling campaigns have been executed in relatively mild ice environments and have accumulated valuable drilling experience. To drill an exploratory well at a deepwater Arctic location, a floating drilling platform is required. Floating platform design poses significant challenges given the harsh ice loading conditions and the demand on the hull and mooring system strengths. In most of the deep water Arctic regions, the winter season is characterized by the presence of first-year ice, multi-year ice, and in some areas ice islands and icebergs. Compared to the environmental loads due to waves, winds and currents, ice actions (both forces and moments) are considerably higher and are the governing loads for deepwater Arctic systems. The capability of a floater mooring system to withstand ice loads is limited as compared to gravity based structures. One of the solutions is a disconnectable system utilizing the ability to disconnect the floater from the mooring system and move off site when the ice loads are forecasted to approach the design limit. As of today, several disconnectable floating system concepts have been proposed, such as disconnectable FPSO, non ship-shaped circular FPSO, Arctic Spar and semi-rigid floater. These concepts are either intended for relatively mild Arctic ice conditions or require long durations for disconnection and re-connection. This paper presents an innovative disconnectable floating platform concept for deepwater Arctic, which can perform exploratory, development drilling and potentially year-round production in various deep water Arctic locations. This design, like many other similar concepts, by limiting the design ice loads to a pre-defined level, enables reasonable hull and mooring system configurations within existing technology limits for an environment where the environmental loading seems to approach infinity in practical terms, if unmanaged. In the event of an excessive ice feature approaching, the innovative platform can be quickly disconnected and towed away, and can then be quickly re-connected once the ice feature has passed.
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