Steel catenary risers (SCR) have been a favored choice for recent ultra-deep waters field developments subjected to harsh environments and large floating production units (FPU) motions. The design of SCRs in such conditions is always a great challenge where the key issues are the fatigue near the hang-off and at the touch down point; clashing between lines, especially on fields with a large number of wells and high payloads on the production unit. This paper describes the solution provided by the Buoy Supporting Risers (BSR) concept which has recently been installed in the Santos basin offshore Brazil.Subsea7 detailed the original concept of BSR system, from the design to fabrication and installation. The BSR concept combines several benefits to overcome the challenges of installing SCRs on ultra-deep waters, such as: allowing decoupling between installation of risers/flowlines and the platform, reducing payload on the production unit, very small dynamics transmitted from production unit to the risers, resulting in significant less fatigue issues. Also, the BSR concept reduces the risks associated with clashing and interference due to the smaller dynamics when compared to traditional coupled solutions. Fabrication is also addressed to highlight main challenges associated to assembling and welding clad and lined pipelines.In this paper the key aspects of the design and qualification are presented. Due to sour and CO2 service requirements for the production and water injection risers, it was decided to use corrosion resistant alloy (CRA) mechanically lined pipe for the entire line, with the exception of the top and touch down sections, where metallurgical clad pipe was used. Considering its novelty, a significant amount of qualification and testing was demanded. Among the technological innovations part of the SCR package is the first successful application of a pressurized mechanically lined pipe by reel-lay method. The solution involved an extensive qualification program, including full scale tests on vessel. The methodology and rationale to allow the application of high strain levels during spooling mitigating potential risk of wrinkling within the internal liner, had to be addressed during the detailed design by extensive Finite Element Analysis and validation tests.The SCR itself, due to de-coupling of motions by the buoy, have negligible dynamic response from vessel motions, thereby behaving almost like a static system with robust fatigue performance. The only meaningful fatigue in SCRs is due to current induced VIV and is mitigated using strakes. There was negligible potential clashing with adjacent SCRs.This paper provides a summary of design and qualification work carried for SCRs lined pipes installed in the BSR system and a discussion regarding main outcomes.

Hydrocarbon exploration and production is moving into deep and ultra-deepwater to meet global energy demands. The industry is having, as a result, to face up to field developments with great challenges, including designing for a HP/HT product whilst also meeting sour service requirements. It has been a common practice in the past two decades to use pipelines with metallurgically bonded corrosion resistant alloys for such field developments. CRA lined (mechanically bonded) pipes are, however, a viable alternative. The viability of using CRA lined pipes depends largely upon the behaviour of the welds associated with the CRA liner under fatigue loading. Pressure and temperature loading that varies cyclically can be expected to result in plasticity induced fatigue in the CRA liner as a function of the local radial gap at the liner/weld overlay interface (the seal weld). This fatigue behaviour is important in the context of the different manufacturing processes adopted by linepipe manufacturers and the need for consistency in the local geometry of the seal weld overlay when pipe joints produced in large quantities. Conventional S N methods are not adequate to estimate the fatigue damage at seal weld. This paper describes an FE based analytical approach to estimate strain range and thus the fatigue damage at the seal weld for Low Cycle Fatigue (LCF). The study shows the fatigue damage variation as a function of the magnitude of the local radial gap between the liner and the backing steel, when the fatigue damage is caused by axial plastic strain ratcheting and elastic plastic stress strain hysteresis. The paper discusses the effects of ‘low cycle’ fatigue of HP/HT flowlines at the seal weld using an analytical approach. The paper also discusses the importance of the radial gap between the CRA layer and the backing steel in general and at the seal weld in particular as a key parameter to be considered at the manufacturing stage. Introduction - Why CRA Lined Pipes? An increasing share of world's oil & gas production is obtained from offshore areas in deep and ultra deepwater. Many deepwater hydrocarbon fields are required to be designed for sour service due to the presence of carbon dioxide and/or hydrogen sulphide either in the early or later part of design life. Development of such fields needs equipment with high reliability and low maintenance costs. The material selection philosophy forms an integral part of the development process. There are surveys indicating that 60% of all maintenance costs in oil & gas exploration and production are related to corrosion maintenance and hence providing cost effective solutions become imperative. The corrosive environments favour usage of corrosion resistant alloy materials in stainless steels (standard martensitic (13%Cr), austenitic (e.g.316L), duplex/super duplex (22%Cr, 25%Cr), nickel based alloy materials (e.g. Alloy 825, Alloy 625) and cupronickles). The cupronickel pipe is technically suitable for seawater, but less satisfactory when exposed to sulphide-containing oil. The austenitic steels are prone to pitting and crevice corrosion when exposed to chloride containing seawater. Thus the pipes made of solid CRA materials are technically not suitable for corrosion sensitive applications and they are also expensive to implement for a large application. The metallurgically bonded CRA clad pipes with C Mn backing steel to provide strength and corrosion resistance against seawater and incorporating 3mm thick thin layer of CRA clad material to provide the corrosion resistance against the presence of CO2/H2S offer a technical alternative to solid CRA pipes. Mechanically bonded lined pipes are the most cost effective alternatives to solid / clad CRA pipes for corrosion resistance applications by a factor of 3 to 4. Recent studies [3] have demonstrated that strength of CRA layer can be considered together with the strength of backing steel for pressure containment calculations and thus provide competitive advantage for lined pipes compared to clad pipes.

The High Pressure and High Temperature (HP–HT) subsea field developments are increasingly using Pipe-In-Pipe (PIP) systems for transportation of production fluids due to superior thermal conductivity performance compared to wet insulated single pipe system. PIP systems can provide the necessary thermal insulation with very low Overall Heat Transfer Coefficient (OHTC) in the order of 0.5 to 1.0 W/m2.K whether installed exposed on the seabed or trenched and buried. An additional inherent feature of PIP systems is that they offer increased protection against third party interaction such as fishing gear and dropped object impact. PIP system will provide a very long "no touch time" before any intervention (e.g. depresurisation) becomes necessary. This aspect can also be improved by using e.g. hot water circulating and electrical heating in order to overcome difficulties which may arise from prolonged shutdown periods. The mechanical design of a HP–HT PIP systems is more complex than conventional single pipe system and involves consideration of additional failure modes. This paper gives an overview of the PIP system design, with particular emphasis on in–service buckling and fatigue design. Design considerations for trawl gear impact on lateral buckling and operational aspects of clad and lined pipes for HP–HT and sour service applications are also discussed.

Accurate determination of residual ovality is an important parameter for a successful deployment of single pipeline and pipe-in-pipe in deep waters wherein the integrity of empty pipes during installation depends upon the collapse resistance under external hydrostatic pressure. The reel-lay process of installation during which pipeline undergoes multiple strain cycles due to spooling, reeling and straightening has a significant bearing on pipe ovalisation and hence accurate determination residual ovality at the end of straightening process is one of the key inputs. It is industry practice to use numerical finite element analysis techniques to predict residual ovality of pipelines as full scale testing is expensive and time consuming. In view of the importance of residual ovality on the pipeline integrity particularly for deepwater applications, an integrated approach of testing and finite element simulation have been used to identify the correct numerical model that predicts residual ovality accurately. This paper discusses the full scale tests performed which include material testing and bend tests performed to simulate spooling and straightening process and the pipeline deformations recorded using laser measurements at different cycles of bending process. The paper presents a brief summary of numerical finite element analyses performed to validate the test results and the effect of element types and material models used in the finite element analyses on the predictability of residual ovality. The material evolution models and their effect on the predictability of remaining ovality are discussed in the paper. Comparisons are made on the predictive residual ovality for reel lay process on single pipe and pipe-in-pipe. The effect of residual ovality on the pipeline integrity for the lateral buckling limit state under combined bending and external pressure are discussed in the paper.

Prevention and arresting buckle propagation under high external hydrostatic pressure is a necessity for the deepwater rigid pipelines during installation and operation. The local compromise in geometric integrity will propagate at a high velocity, flattening the pipeline until it encounters a physical barrier that arrests the buckle. Traditionally, external clamp-on type buckle arrestors are considered by the pipeline designers for rigid pipelines that are installed by reeling method. Integral Buckle Arrestors (IBAs) could provide a more reliable method of arresting the propagating buckle during installation and subsequent design life. However, design and installation of integral buckle arrestors for rigid reeled pipeline installation would be a challenge. IBA is fabricated from a thick walled pipe section with the same inner / outer diameter as that of pipeline and wall thickness transition to match the thickness of the pipe towards the ends. For the reeled pipelines, additional cost economies will be achieved by the use of IBAs which allow continuous installation by reel lay vessel without interruptions. This paper gives background information, design and installation considerations and practical issues in the fabrication of IBAs. Recent Subsea7 experiences in design and installation of IBAs will be also addressed in this paper. This paper does not cover the aspects to be taken into consideration, if the buckle arrestor is also serving dual purpose as a J-lay support collar.