Dynamic Design and Analysis Methodology for Deepwater Bottom-Founded Structures

Abstract

ABSTRACT This paper describes random and regular wave responses of compliant and stiff bottom-founded structures in a water depth range of 900 to 1500ft for the Gulf of Mexico (GOM). Design force levels based on conventional design wave force models are shown to be generally lower than for state-of-the-art models. Attempts to design dynamically sensitive structures using single waves is shown to be inappropriate. A practical dynamic design approach is presented and applied to the design of a 1030ft GOM Jacket. INTRODUCTION Shallow water structures in which the dynamic response is considered small (say less than 109!. of the statically applied loads), have typically been designed by use of a single, regular design wave with prescribed period and height. The applied hydrodynamic forces are calculated using Morison's equation (Morison et al, 1950). For GOM applications the procedure is defined in API RP 2A in terms of actual values of the parameters to be combined for the purposes of design. Given the successful record of the thousands of structures installed in the GOM this procedure in its eatery can be considered as calibrated by experience, As the need for structures in deeper water in the GOM has increased, new challenges have been presented to the designers. Structures in water beyond lOOOft have significant dynamic responses to design waves. However, the single regular wave design approach is susceptible to "quirks" when the structural period coincides with harmonics in the applied wave load. Therefore a rigorous approach to the dynamics in design is prudent. Considerable progress has also been made in understanding wave hydrodynamics and the associated wave forces. As a result it is recognized that the regular wave formulations and the force coefficients such as those recommended in the RP 2A are not true representations of the physical reality of the offshore environment (eg. Rodenbush and Forristall, 1986; Rodenbush, 1986). To obtain a more rigorous approach to the dynamic design of deep water bottom-founded structures, a series of simulations and studies were undertaken. Both compliant towers and fixed platforms were addressed. A detailed design procedure was established for the latter. DESCRIPTION OF THE MODELS The compliant structure was represented by a 1250ft WD four- Leg Tower. A full 3-D representation was used for stiffness and hydrodynamic calculations; the conductors were represented by two groups of 12 each for a total of 24 wells. A payload of 6800 tons was assumed. The piled foundation was linear zed at the expected maximum load levels (Secant stiffness at pile head). The resulting periods in the two main orthogonal directions were 31.3 and 26.8 sec. (Fig.1). Three different models were used for the fixed-base structure since this investigation formed part of the conceptual and preliminary engineering for an actual design. Model 1 was represented by a 4-leg 1050ft WD configuration with 24 wells and 6800 ton payload. Conductors and foundation were represented as for the compliant tower. The chosen model had periods in the range of interest of 5.2 and 4.7 seconds.