Nonlinear Dynamic Analysis Of Coupled Axial And Lateral Motions Of Marine Risers
Authors: Bennett Bruce E. , Metcalf Michael F. 
Published: October 2008

ABSTRACT
Deep-ocean m1n1ng systems are calling on the technology of marine petroleum operations, and many principles of petroleum riser design apply to mining applications. The long, freely suspended riser configuration of petroleum applications can be developed during the emergency disconnect maneuver; this configuration could apply as well to a mining-type riser. Analysis of riser performance in this configuration in one application serves the other as well.
Long, freely-suspended riser columns can have dynamic stability problems under certain conditions. A mathematical model and solution technique are described that have been used to simulate in time the dynamic axial and lateral responses of such riser columns. When used in conjunction w1th .closed-for stability analysis, the dynam1c solutions provide valuable stability information of realistic configurations.
Stability analysis by time series solution is inefficient. Improved solution techniques appropriate for stability analysis should be developed if exhaustive stability investigations are to be done.

INTRODUCTION
Recently many concepts have been developed and designs have been carried out on ocean-going systems for the exploitation of mineral reserves on the ocean floor. Because many of the known commercial mineral .deposits are at great depths, ocean-g01ng m1neral collection systems capable of working in deep waters have received much attention. Accordingly, technological advancements have been required to cope with the rather novel problems that have been posed.
For many years the concept of the marine riser has been utilized in petroleum exploration and field development. The most common configurations are a riser pipe, or cluster of pipes, connecting a surface ship or work platform to a well-head on the ocean floor. water depths of about 1,000 feet have been considered deep. Recently, petroleum risers have been constructed for far deeper waters - 6,000 feet and more - and even greater depths are being considered.
Deep-ocean mining systems are using a riser pipe to connect a surface vessel to some sort of ocean-floor collector unit. Many of the engineering principles and design features developed for petroleum applications have been applied directly to these concepts, but because of the water depths involved (3 miles and more) and different operational requirements, many new problems have come to light. Nonetheless, many of the engineering principles of petroleum riser design apply to mineral collection systems, and accordingly, much of the technology is the same.
This paper describes an aspect of riser column design that, while couched in the realm of the deep-water petroleum application, could just as well apply to mining configurations. The situation of interest arises when a long riser is freely suspended from a surface vessel, the riser having been disconnected from the well-head. The specific problem is that of buckling near the riser top, and results from the complicated riser dynamics caused by vessel motion effects. A method of analysis is described that permits the engineer to investigate riser pipe buckling stability, and thus assists in designing a riser package that avoids altogether the buckling conditions.