Development of Soil-Structure Interaction Models for Deepwater Environments

This one year project which kicked off in January 2004 with the support of the APPLIED (INDUSTRY) MARINE RTDI programme, will develop linear and nonlinear mathematical models for the interaction between slender structures, such as marine risers & pipelines, and clay soils that are typical of the on-bottom conditions in deepwater (>500m) environments in the Gulf of Mexico and offshore West of Africa and in harsh environments such as those experienced Offshore Ireland and West of Shetlands.

The background to this R&D initiative is related to the experience gained from recent Steel Catenary Riser (SCR) designs for deepwater applications. These applications have highlighted the criticality of accurate modelling of soil-structure interaction phenomena at seabed touchdown for fatigue life prediction. It is generally acknowledged that traditional linear spring models for soil-structure interaction cannot capture the full complexity of catenary riser-soil interaction. This is particularly true of clay soils, such as found in deepwater environments (>500m) from Offshore Ireland to the Gulf of Mexico.

In particular, it has been demonstrated that the spring models currently being used do not take account of changes in the load-deflection behaviour of the SCR resulting from vertical penetration into the seabed (embedment). This means that accurate prediction of touchdown point fatigue is not possible. As a result it is common practice to employ conservative margins of safety.

The objective of the proposed development is to develop nonlinear mathematical models for soil-structure interaction for clay soil conditions that are representative of Offshore Ireland, Gulf of Mexico (GoM) and West of Africa (WoA). The development and implementation of more realistic soil-structure interaction models into design tools will allow inherent over-conservatism in current design procedures to be removed thereby facilitating (i) the safe use of lower cost solutions and (ii) enabling the use of solutions that would otherwise be deemed unviable.

Furthermore this model will be implemented into the MCS Kenny nonlinear time domain program, Flexcom. In addition, linearised approximations to the soil-structure interaction models will be developed for use in the MCS Kenny linear frequency domain program, Freecom. Using these enhanced programs the soil-structure interaction models will be validated through comparison with model test and field data.

The models developed will be generic in nature and as a result will be applicable to any marine applications involving interaction between slender structures and a seabed, including hydrocarbon (SCR touchdown) and geological (soil-conductor casing).

The SOILSIM models are planned to be made available in Flexcom Version 8 and Freecom Version 6.

For further information on these R&D initiatives please contact Michael O'Sullivan at +353 91 781010