Smart. Leg : A New Method for Heavy Deck Offshore Installation

Abstract A new shock less flotation method, called Smart-. Leg, developed by ETPM, is presented for the installation of a heavy deck on a jacket in open sea using a conventional cargo barge and specific recoverable devices, The details and advantages of the method are discussed. The paper also summarizes the dynamic analyses carried out to assess the efficiency of the specific devices; one is mounted on the barge and designed to stabilize it inside the jacket and another one is fitted inside the deck legs and designed to land the deck smoothly on the jacket. The advantages of the integrated deck concept for large offshore platforms have been widely publicized. However, so far, the installation of heavy decks in open sea using the floatation method has been limited either by the shocks to be absorbed during the mating operation or by the difficulty of maintaining the barge steady inside the jacket. With the new method and associated equipment, mating shocks are virtually eliminated and the barge is smoothly and steadily maintained within the jacket. The system allows, a deck weighing up to 20,000 tons to& installed in a 2m / 10s heading swell, The results of the numerical analyses performed provide crucial data on motions and forces for the design of the new flotation method. The prototype tests (scale 1/4) give precise indications about Smart-Leg parameters and the tank tests (scale 1/40) enable validation of the numerical analysis. Introduction Few methods permit the installation of heavy integrated decks offshore particularly in long swell seas where the ergo barge movements have high amplitudes. Smart-. Leg is the floatation method that can install the heaviest integrated deck in open sea using only a conventional cargo barge and specific reusable equipment. With Smart- Leg and the accompanying stabilization devicesSmart- Fin, and Smart-Fender shocks during mating and barge immobilization are eliminated, As a result, a deck weighing more than 20,000 t ean be installed in a 2rn/10s swell. The principle used by Smart-. Leg devices is that whatever the size and mass of a heaving, surging or swaying body, its kinetic energy is zero when its velocity is zero. Smart-Leg, Smart-Fin and Smart-Fender are based on a jacking device that um detect the outward zero speed of the ram and maintain that ram position. A non-return valve located between the high pressure chamber (ram side) and the low pressure accumulator of the jacking device seines both purposes, Another advantage of the Smart-Leg system is that the reverse operation (deck removal) is possible using the same principle and therefore without any shock. The principle of Smart-Leg has already been validated inshore during the installation of the main girders of the Prince Edward Island bridge. Each of the 5 girders already installed weighs more that 7000 t. The first Smart-Leg equipment will be ready for an offshore installation early 1997 and able to install a deck in the weight range 3500 t to 6000 t,