Deepwater gas fields require large diameter risers to provide high flowrate. Flexible Risers are limited to about 18", whereas Steel Catenary Risers (SCRs) can go beyond that, for example 20" or 24"OD. Because those SCRs are sensitive to fatigue, they are equipped with Buoyancy Modules (BM) to decouple them from surface dynamics: they form a Lazy Wave configuration and they are therefore called Steel Lazy Wave Risers (SLWR). To solve more stringent fatigue issue, decoupled riser system like SHRs are very efficient. The objective of this article is to present innovative alternatives to conventional development. This paper will present a novel decoupled riser system: the Tethered Catenary Riser (TCR) previously presented at the Offshore Technology Conference (Legras 2013; Legras & Neel 2014). The installation aspects have been further developed and are now only based on field proven and reliable installation procedures. The TCR can be considered as a robust solution for gas development projects, being the most cost efficient decoupled riser system. This paper will also present an alternative solution to install traditional Steel Lazy Wave Riser (SLWR) using a towing solution. By removing the need for a large installation vessel, a towing method can help debottleneck project planning and simplify offshore logistics.

Turret-Moored FPSOs are frequently used for deepwater developments worldwide, with consideration of disconnectable turrets for harsh environment applications. This trend makes the interaction between the FPSO hull, mooring system, and riser systems a vital design parameter for arctic conditions. This paper provides a review of the various riser systems that can be considered for turret-moored FPSOs. These include proven coupled and decoupled systems (flexibles, Steel Catenary Risers, Steel Lazy Wave Risers, and hybrid decoupled riser systems), and also new riser concepts (e.g. the TCR - Tethered Catenary Riser, or the TSLWR - Tethered Steel Lazy Wave Riser). These systems are described in terms of design and functionality. These riser systems are discussed with consideration of the particular challenges of disconnectable turret-moored FPSOs and specificities of arctic conditions.

Steel Lazy Wave Risers are one of the most efficient and compliant riser systems for deep-water projects. From a design perspective, designers have to deal with parameters such as the configuration itself, interaction with the FPSO, environmental loads, installation vessel capabilities and commercial issues like installation and procurement costs. This paper presents how to assess the need for Steel Lazy Wave Risers (as opposed to conventional Steel Catenary Risers) and how to develop a state of the art design by using an innovative methodology based on the efficiency of an optimisation through modern technology computation. The objective of the optimisation is to propose a riser configuration that allows the best compromise between all parameters, while remaining fit for purpose and cost-efficient. A methodology was developed using a Python script as an interface between Orcaflex software and the user-defined parameters to best respect the project specific weighted criteria. It will also highlight how the methodology can be applied for fast track conceptual studies, enabling the end user to account for optimised design at the start-up of a field development. All the constraints and objectives will be considered with possible challenges and solutions to be presented. The final design of a lazy waver riser is a compromise between all constraints. This paper provides comprehensive information relative to all the stages of steel lazy wave riser design, from the initial draft configuration to the most optimised and cost-efficient one. A few optimisation possibilities achieved thanks to this process will be developed and illustrated through examples. The concept can be applied to early stage engineering or during the course of a project.

Since Deepwater field developments started around year 2000, Deepwater Buoyancy has been dominated by syntactic foam and has not evolved significantly since then. Some Deepwater applications like towed Bundle, Steel Lazy Wave riser and Hybrid Riser towers require large amounts of it, both for temporary (installation aids) use and permanent use, becoming a significant part of the cost associated to those systems. In parallel, the present market conditions is pushing Oil companies to develop new technologies and to promote cost reduction initiatives. In this context, Subsea7 and Matrix, Composites & Engineering have developed a new concept of buoyancy, so called low cost buoyancy. This paper will present the Buoyancy concept, will provide insight on the qualification tests successfully passed, and present typical application where the low cost buoyancy is intended to be used.

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.