Steel Catenary Riser Touchdown Point Vertical Interaction Models
Authors: Bridge Christopher , Laver Katherine , Clukey Ed , Evans Trevor 
Published: October 2008

Steel catenary risers (SCR) are an enabling technology for deepwater environments. Tools to analyse and design SCRs are available which show that the point where the riser first touches the soil, termed the touchdown point (TDP), exhibits complex behaviour that has been the subject of a number of recent research programmes. The soil parameters used in SCR analysis can have a significant effect on riser response, especially the predicted fatigue life.

If soil parameters and analytical models are chosen too conservatively they can make the predicted fatigue life unrealistically low, conversely using non-conservative soil parameters and soft soil models results in fatigue lives that may be unrealistically high.

This paper describes state of the art vertical pipe/soil interaction models developed for use in SCR analysis. These model pipe movement vertically downwards (soil stiffness) and vertically upwards (soil suction). The models are based upon test data from the STRIDE and CARISIMA JIP's and information from existing papers. The models are currently being used in many Gulf of Mexico deepwater projects that involve SCRs.

Introduction

The seabed models used in SCR analysis can have a large effect on the predicted riser fatigue life. Case studies on generic SCRs conducted within the STRIDE JIP [1] show that the predicted fatigue damage is dependant on the value of soil stiffness used, Figure 1. High values of soil stiffness (around 10,000kN/m/m) produce fatigue damages similar to those calculated using a rigid seabed. If the soil stiffness is reduced to 1,000kN/m/m the fatigue damage reduces by around 30%, an increase in fatigue life of 43%. If the soil stiffness is further reduced to 100kN/m/m the fatigue damage is around 45% of the rigid seabed, and increase in the predicted fatigue life of 120%.

This shows that if the level of soil stiffness used in SCR analysis is too high then the predicted fatigue life may be too low, conversely if the soil stiffness is low then the analysis may not be conservative, or representative.

The STRIDE case studies also investigated the effect of soil suction, the soil resistance force to the pipe moving vertically upwards on SCR fatigue life. The studies show that soil suction has a small effect on fatigue damage, but a large effect on extreme stress when the riser is pulled away from the seabed. Events that cause this type of motion include slow drift caused by a failed mooring line or pulling the pipe away from the seabed during installation operations.

The soil stiffness and soil suction models presented in this paper are developed using STRIDE and CARISIMA test data in combination with information taken from published literature. The work was conducted during the STRIDE JIP, with additional work sponsored by BP.

Pipe / Soil Interaction

An example of the development of a pipe/soil interaction curve with an unloading/reloading cycle is presented in Figure 2. The right hand column of Figure 2 shows the relationship between the backbone curve (the maximum soil resistance force to pipe penetration at a given depth) and the pipe/soil interaction curve (the force/displacement relationship) of a pipe moving through the soil.