Advancements in Methods for Quantifying Energy Dissipation in Unbonded Flexible Pipe
Authors: O&apos;Sullivan Enda , Witton Martyn , Winkle John 
Published: October 2008

The energy dissipation that occurs due to structural layer slippage in unbonded flexible pipe under dynamic loading conditions is typically represented under design conditions as a damping effect. For the numerical modelling of flexible pipe, the Rayleigh damping [Ref. 1] model is commonly adopted to characterize this damping. In recent years studies have focused on quantifying the contributors to flexible pipe damping levels. In this context, tradition modelling methods for damping are now considered somewhat crude, though in many cases conservative, and a greater emphasis is now being placed on modelling the contributors separately. In particular, advancements in the ability to distinguish between flexible pipe damping and non-linear hysteretic bending have, in some cases, lead to significant reductions in design conservatisms.

The design response of disconnectable riser systems, many of which operate in Australian waters, can be particularly sensitive to the refinement in methods for modelling dynamic non-linear bending response and energy dissipation, both at seabed touchdown and vessel interface regions under both extreme environmental and long term service life conditions. The sensitivity is further affected by pressurization levels across the pipe layer. Traditionally in riser design scenarios, the unpressurized (linear) equivalent bending stiffness combined with nominal levels of Rayleigh damping have been used for design. More recently, the bending response predicted from the depressurized bending stiffness has proven to be conservative and there has been an increasing demand to consider the larger hysteretic bending stiffness of the pressurized riser, which by definition removes uncertainty associated with damping and energy dissipation.

This paper describes the background to characterizing damping of unbonded flexible pipe, the application of hysteretic bending effects and their impact on design response and it outlines where efficiencies can be achieved in relation to practical design applications.

Introduction

Australia, the world's largest island and a continent in its own right has a Marine Exclusive Economic Zone of 8 million sq km (excluding Antarctica) and is larger than the land area of 7.6 million sq km. This region covers the tropical waters and cyclone prone areas in the North and the world's most ferocious Southern Ocean. These effects are seen in many of the hydrocarbon development areas to the North and West and around the Bass Strait in the South East. The North and West hydrocarbon basins (Northwest Shelf, Browse Basin and Timor Sea) are central to the cyclone paths and where the frequency averages 12 per year, as per Figures 1 and 2 [Ref. 2].

Disconnectable turret mooring systems have been used in Australian waters for 22 years, with the Jabiru facility commencing production in 1986. Since that time at least 13 additional disconnectable facilities have come on-stream in Australian waters.

Disconnectable FPSOs have migrated around the world as indicated in Figure 3, and have adapted to solve a variety of challenges including cyclonic conditions, iceberg activity and project economics; indeed a disconnectable system is soon to enter the Gulf of Mexico. The lessons learned over the past 22 years, with regard to the interaction and integrated design of the holistic buoy-mooring-riser systems, have pointed the industry towards to need to continuously refine riser design methods.