Total Pipelaying System Dynamics

ABSTRACT A method is presented here for predicting the dynamic responses of a total pipelaying system undergoing excitations from high sea states and vessel motions in deep or shallow water. The method is used to predict the interactions between all components in a pipelaying system such as the lay barge; rigid, flexible, or articulated stinger; pipe; soil; currents; and wave forces from regular or random seas. The method incorporates a combination of coupled rigid body and flexible finite element models in a three dimensional time domain. This technique accounts for nonlinearities such as hydrodynamic damping and large angle deflections in pipe and stinger as well as discontinuities such as pipe separation from supports on the vessel/stinger assembly and rotational stops between elements of an articulated stinger. The approach assembles all the essential components within a pipelaying system and evaluates the total system under various static and dynamic environments. The investigations can then lead to predicting limiting environments for specific pipelaying designs as well as establishing and optimizing designs for specificc operational and environmental conditions. The criteria used in evaluating pipelaying systems will consist of stress magnitudes in pipe and stinger, resonant conditions, dynamic deflections and excursions, tensioner requirements, vessel motions, and station keeping requirements. A sample case of a specific 2 dimensional system with a flexible stinger operating in deep water undergoing regular wave excitation was investigated with this technique and the results are presented herein. INTRODUCTION The approach to analyzing the statics and dynamics of a total pipelaying system is to first define all the individual elements of significance within the system and then incorporate them into a general framework or mathematical model. The basic philosophy in modeling most physical dynamic systems is to keep the model as simple as possible, but of sufficient complexity to adequately describe the motions and loads in the areas of interest to the investigator. For purposes of pipelaying we may start by defining the baseline requirements for which the pipelaying system must operate. This usually starts with a definition of maximum environment such as sea state and the wave spectrum which represents it. Then, a range of depths and pipe sizes are designated. A suitable platform or barge is picked with the proper propulsion features such as thrusters or mooring line winches. The tensioning subsystem is selected as well as the stinger subsystem. The control system logic must also be described in terms of what must be accomplished during a specific event. Such things as move-up, payout, hold, abandonment, and recovery procedures all pertain to specific control system functions which require some definition. Figure 1 shows a rudimentary interaction diagram of most of the essential elements necessary to model the pipelaying system. The total system is encompassed by the dashed line and the subsystem components by the solid ovals. Most of the interactive components have bi-directional arrows connecting them.