A Comparison of Spar Riser Response Measurements with Results from Application of New Analysis Tools
Authors: Lang Donogh , Kavanagh W.K. , Lane Michael , Stear James , Griffin Paul , Thompson Hugh 
Published: October 2008

The design of dry tree spar risers with top tension provided only by passive buoyancy differs in significant respects from the design of other top tensioned risers, such as TLP risers. Critical differences are the provision of tension using buoyancy cans, the position and type of can (integral or nonintegral) and the interfaces or contact areas between the risers and hull guide supports. Predicting riser response to environmental loads and vessel motions requires the accurate modeling of the riser configuration, lateral hull guides and moonpool hydrodynamics.

This paper presents a comparison between recent full-scale offshore measurements of risers on the Chevron Genesis spar platform with response predictions made using an enhanced riser analysis software tool (DeepRiser) capable of modeling the mechanics of spar riser behavior. Comparisons made between the measurements and the results from the analysis software show reasonable agreement, and highlight major areas of response sensitivity. Important lessons from this work relate to the significance of hull motion on the riser response and the sensitivity of response to the width of the gap between air cans and hull guides.

Introduction

The use of spar platforms like Neptune and Genesis as facilities for deep water hydrocarbon production poses special challenges for the design of riser systems. The Genesis classic spar hull form and riser system, located in 2,600ft water depth in Green Canyon 205, are illustrated in Figures 1 and 2. The riser moonpool and seabed layout is presented in Figure 3.

Unlike tension-leg platform risers, the spar production and export risers used to date have been tensioned only by passive buoyancy. The buoyancy support system has consisted either of a group of air cans which sleeve the riser, with the riser being suspended from the top of the sleeved section, as with those on Neptune, or of air cans integral with the wall of the riser, such as those on Genesis.

While the moonpool of the spar affords a protected environment for the risers, the structure and geometry are such that contact between the risers and the hull are an essential consideration in the riser design. Early spar designs like eptune and Genesis have hull guides at several elevations to provide lateral support to the risers. These hull guides have clearances of one inch or more on the diameter, initially considered necessary to prevent the binding of the risers in the guides as the spar displaces horizontally and pitches. As the platform moves relative to the risers, these gaps will open and close, resulting in a highly non-linear problem of riser response. The intermittent contact problem is further complicated by the friction between the riser and the guides. This intermittent contact challenges the validity of the use of tools that assume the riser to be either in contact with hull guides or laterally unsupported. Recent experience with such designs has emphasized the need for design tools that accurately model this intermittent contact behavior.

Recent software advances have produced an analysis tool capable of modeling the complex hull-riser interaction.