Buoyancy Spacing Implications for Fatigue Damage due to Vortex-Induced Vibrations on a Steel Lazy Wave Riser (SLWR)
Authors: Jhingran Vikas , Zhang Heping , Lie Halvor , Braaten Henning , Vandiver J. Kim 
Published: April 2012

As oil and gas exploration ventures into deeper waters, designing risers withsufficient extreme strength and fatigue life becomes a challenging task. Thistask is even more difficult in regions with strong currents at great depths. Among the handful of riser concepts that can be used in such conditions, theSteel Lazy Wave Riser (SLWR) concept has shown great promise.

The SLWR concept does come with its own share of challenges. When deep oceancurrents exist, obtaining sufficient fatigue life due to Vortex InducedVibrations (VIV) becomes a major design challenge. A critical area of concernfrom VIV fatigue damage is the buoyancy section on the SLWR riser. Shell OilCompany performed some state-of-the-art experiments in 2011 to study theeffects of buoyancy spacing on VIV response. The experiments were performed atthe Marintek Ocean Laboratory in Norway with a 38 m (~125 ft) long pipe whatwas 30 mm (1.18") in diameter. The buoyancy was simulated using 80 mm (3.15 ")diameter sections. This paper describes these experiments, discusses resultsand provides design insights on the effective sizing and placement of thebuoyancy modules in SLWRs to minimize the fatigue damage caused by VIV.

Introduction

As Oil and Gas exploration continues to advance into deeper waters and harsherenvironments, engineers are faced with the daunting task of developingpractical riser designs for these new develpments. In recent years, Steel LazyWave Catenary Risers (SLWR's) have provided engineers with riser solutions fora wide variety of challenging environment and field configurations. SWLR'scombine the robustness of Steel Catenary Risers (SCR's) with the fatiguechararteristics of flexible risers. As offshore oil exploration progresses intounchartered domains, we believe SLWR's will become the riser configuration ofchoice for many future projects.

There are however great challenges in designing a SLWR system, particularly ifthe site specific MetOcean data indicates strong currents at great depths. TheSLWR system, as shown in Figure 1, relies on a buoyant section in the riser toprovide flexibility and enhanced fatigue life, particularly in the touchdownarea. Typically, the buoyancy on the riser is staggered and not continuous tominimize time of installation. If however, the buoyant region of a SLWR issubject to strong currents, fatigue due to Vortex-Induced Vibration may becomeimportant even if the non-buoyant regions of the riser have VIV suppression onthem. There are two reasons for this. First, the variation in depth over thebuoyant region is typically small, making it possible for the entire region tobe exposed to a current of uniform magnitude with little directional variationin the current profile. Second, the buoyant region typically has very lowtension resulting in higher local curvature response due to VIV than otherregions of the SLWR. This results to higher fatigue damage in the buoyantregion compared to other areas of the SLWR for the same diaplacement amplitudeof VIV response.