Predicting Behavior of Suspended Pipelines in the Sea
Authors: Dominguez Richard F. 
Published: October 2008

This paper describes an analytical procedure permitting the determination of the static and dynamic behavior of pipeline systems suspended below the sea surface. The procedures make it possible to evaluate behavior in either a two or three-dimensional manner. Examples applied to a ball jointed pipeline system are included.

Introduction

Pipelines are one of the most practical and economical means for transporting liquids and slurries in quantity. Many ocean engineering activities involve the use of pipelines in the sea in conjunction with offshore oil development; offshore tanker unloading; deep ocean mining operations; beach replenishment; harbor and channel maintenance dredging; and in ocean construction using hydraulically placed fills.

Most ocean pipelining experience to date has been in connection with pipelines placed on the ocean floor. In some cases, however, pipelines cannot economically be constructed in this manner because of excessive water depth, irregular bottom topography, unsuitable foundation conditions or due to applications of a short term nature necessitating ease in deployment and total recovery of the system. In other instances such as dredging and mining operations, the terminal points of the pipeline continually change. The placement of a fully or partially suspended pipeline below the ocean interface may offer the most practical solution.

However, a number of significant design and analysis difficulties arise due to the inherent flexibility of the system. This characteristic makes it responsive to the effects of waves and currents and can result in complex static and dynamic behavior of the pipeline.

System Description

The problem which initiated the author's interest and subsequent involvement in this subject dealt with design and analysis of submerged pipelines. These were fabricated of rigid sections joined by ball joints, as commonly used in conventional hydraulic pipeline dredge systems. Ball joint placement in the pipeline permits adjacent sections to rotate within the permissible deflection limits of the connection. An illustration of this type of submerged system construction is shown in Fig. 1. Fig. 2 shows a photo of a typical ball joint connection. Although the specific applications presented deal with this type of system, the approaches to be discussed are general and apply equally to other pipeline designs such as the type proposed for manganese nodule mining in deep water (fig. 3).

Steady-state analysis of a pipeline such as those of Fig. 1 and Fig. 3 are difficult since the systems are highly indeterminate and behave in a geometrically non-linear manner. The system's subjection to fluid dynamic loadings resulting from ocean currents or the motion of the pipeline produces a particular difficulty since the hydrodynamic force distribution is a function of the ultimate but initially unknown pipeline configuration. With the presence of such forces the problem generally becomes three dimensional in nature. The action of waves, vortex shedding (which produces fluctuating transverse forces) or other sources of unsteady loads makes it necessary to determine the dynamic characteristics of a proposed system to avoid those conditions which could result in adverse dynamic behavior.