Steel Lazy Wave Risers SLWRs Towing: A Cost Effective Alternative for Deepwater Gas Developments

Abstract

Deepwater gas fields usuallyrequire large diameter risers to provide high flowrate. Steel catenary risers (SCRs) are a feasible solutionwhere the requirement for large diametersrules out the use of flexible risers. To improve fatigue performance and platform payload, SCRsare sometimes fitted withpermanent buoyancy modules, forming a wave shape, and are called Steel Lazy Wave Risers (SLWRs). SLWRs are typically installed by large pipelay vessels using the J-lay method. Some challenges for SLWR installation are the high top tension requirement, as well as offshore welding of large diameter (and often CRA clad) lines on critical path. These factors contribute to the cost and risk of installation. Utilising a towed installation method canreduce these risks, and remove the need for a J-lay vessel. This paper describesthe methodology for towing a SLWR in asingle length using previously established methods for pipeline bundles. A case study based on a large deepwater gas field offshore Australia has been used to show the feasibility of the towed method for large diameter risers. Simulation of the tow was performed with OrcaFlex™ to confirmthat various outputs are within allowable limits of design stress and marine operability. The paper also lists advantages and disadvantages of the towed installation of a SLWR when compared to J-lay.