Interaction between Deepwater Catenary Risers and a Soft Seabed: Large Scale Sea Trials
Authors: Willis N.R.T. , West P.T.J. 
Published: October 2008

The paper describes experiments completed in June 2000 as part of the STRIDE JIP - Steel Risers in Deepwater Environments1,2. The tests investigated the interaction between a soft clay seabed, typical of deepwater developments, and a steel catenary riser (SCR). ROV surveys of deepwater SCR's attached to floating production vessels have shown that they can cut deep trenches into the seabed. The following objectives were set:To assess the effect of seabed to SCR interface forces on local riser stresses for wave and slow drift vessel motionsTo assess the effect of different riser trenches on local riser stresses for wave and slow drift vessel motionsTo identify the key trenching mechanisms and rates.To use the test program findings to benchmark riser FEA tools, and to adjust modelling parameters to better simulate the real case as necessary.

To achieve this, the seabed end of a deepwater SCR was simulated using a 360-ft long 6-inch diameter steel pipe, hung as a catenary across the soft seabed of a tidal harbour. A particular harbour location was found that had seabed properties similar to those of a deepwater Gulf of Mexico seabed. The top end of the pipe string was then actuated with carefully controlled wave and vessel drift motions to simulate a spar platform in 3,300-ft water depth.

Introduction

Deepwater (&gt;1500-ft) oil and gas fields usually have seabeds of soft, sticky clay. A popular strategy for developing deepwater hydrocarbon reserves is the use of steel catenary risers to a floating production vessel, both for well production and processed fluid export purposes (Figure 1). This technology is still relatively young, and whilst there are a number of SCR's now installed most are just starting a field life of 25 years or more.

ROV surveys of installed catenary risers, steel and flexible, have shown deep trenches cut into the seabed beyond the touchdown point (TDP). Even after just a few months, trenches have been seen that are four to five pipe diameters deep and three to four pipe diameters wide, and with an amount of soil backfill in the trench.

Current practise for catenary riser design takes little account of such circumstances, with FEA programs typically assuming a flat, rigid, "non-interacting" seabed. The dynamic nature of deepwater SCR's connected to floating platforms suggests that such an approach may be un-conservative. Storm and current action on a deepwater production vessel can pull the riser upwards from its trench, or laterally against the trench wall. The suction effect of the soft seabed on the riser, coupled with trench wall interaction, could increase the local riser stresses by causing tighter riser curvatures or higher tensions than are predicted by conventional FEA.

As part of the STRIDE III JIP, 2H Offshore Engineering conducted a test programme to investigate the effects of a deepwater seabed on catenary riser response and wall stresses. The objective was to assess the importance of such seabed/riser interaction, and to produce FEA techniques to match the real response as necessary.