Guidelines for Compression Modeling in Flexible Risers for Deepwater Applications
Authors: McCann D. , Smith F. , O&apos;Brien P. 
Published: October 2008

There is not currently in industry a consensus for modeling compression in flexible risers, particularly for deepwater pplications where compression is a key design driver. This paper gives guidelines on the particular type of behavior experienced by risers under compression and on the requirements for accurate modeling of this behavior. For example, the assessment of post buckling curvature requires a full understanding of the events occurring and it is only with tight guidelines on modeling this issue that accurate, consistent solutions can be delivered.

At present one of the critical design drivers for catenary risers, either flexible or metallic, is compression of the riser at the touchdown location and the associated buckling. An accurate assessment of this behavior is fundamental to the design process. The guidelines developed and described in this paper will help in galvanizing the understanding of this behavior and on how to accurately predict the response when using a global finite element modeling technique.

1.0 Introduction

During the 1990's and up to the present day, flexible risers have been installed in a number of deepwater developments offshore Brazil, in the Gulf of Mexico and elsewhere around the world. Where individual risers are selected as economic over a hybrid design, it is preferred to configure the flexible pipes as simple free-hanging catenaries. This configuration represents both the simplest and cheapest option in terms of installation and procurement in deepwater.

The catenary configuration couples the mean, low frequency offset and first order motion of the vessel to touchdown point response. Where vertical motion of the hangoff point is significant, compression forces can arise at the touchdown point that, when large, can cause global buckling of the pipe section significantly reducing the dynamic pipe minimum bend radius. Thus, if compressive response arises, global buckling of the pipe cross section must be verified.

In general, the catenary configuration will not experience compression unless the vertical motion of the hangoff is severe. This is not a problem that normally affects traditional Gulf of Mexico SPAR or TLP deepwater development vessels as they do not have significant heave motions.

A Floating Production Storage and Offloading Vessel (FPSO) has a considerably larger heave response. In addition, the vertical motion is a combination of both the heave and pitch. Therefore the hangoff motion is a function of distance from the center of motion. Figures 1 and 2 comprise Response Amplitude Operators (RAOs) of vertical motion and acceleration at the hangoff point of an FPSO respectively. The risers are assumed to hangoff at distances of 0m, 50m 100m, 150m and 200m horizontally along the vessel from the motion center. As the hangoff point moves away from the center of motion, pitch amplified heave motion increases considerably. Therefore, catenary risers attached to FPSO's, in particular those where the riser hangoff are located away for the vessel motion center, are more likely to experience of compressive loading.

The majority of the work detailed herein is performed on two generic flexible risers, namely a 6" production and 4-1/2" water injection riser.