The vast majority of flexible pipe dynamic risers installed in deep water are for sweet service application, with no or a very limited level of H2S present in the produced fluid. For the Baobab development in 960m water depth, the 11" ID flexible production risers required to be fully sour service rated to safely accommodate potential future reservoir souring. The design of such riser structure considering the standard approach proved challenging and leading to significant installation constraints. The optimization of a one length sour service flexible pipe structure to satisfactorily accommodate all loadings associated with the deep water environment (high tension at the top, high hydrostatic collapse pressure and reverse end-cap effect loading at the bottom, high installation tension, etc ..) is indeed difficult and ultimately results in a compromise solution. Through detailed engineering work, the technically optimum and most cost effective solution was identified to be a two sections riser system. Each flexible riser structure can then be optimized for the design loading pertinent to the top or bottom area respectively. This approach leads to a significant weight reduction over a one section design, bringing the top tension and installation loading to a similar level as for a sweet service riser structure. Optimization of the riser configuration, using a Deep Lazy Wave where buoyancy modules are incorporated at the touch down point area, also proved very effective to maintain large bending radius at that location in extreme storm conditions. The installation considerations are shown to be an integral part of the riser system design optimization. In particular the use of partial flooding during the critical installation stages contributed in the optimization of the riser structure design and the reduction of the risk of armour lateral buckling. The paper demonstrates that innovative design and optimization of riser configuration and installation can enable the use of large diameter flexible pipe risers for sour service production application in deep water. Introduction Deep water developments increasingly require larger diameter production risers suitable for sour service application. For flexible pipe, compatibility with sour service production fluid means using lower yield strength carbon steel wires resistant to H2S induced corrosion. This makes the design of the riser structure much more challenging to accommodate the different loading associated with deep water application, particularly so for larger diameter. Figure 1: Typical flexible riser construction (available in full paper) The paper describes the design challenges associated with such risers and the solutions selected to overcome them for the Baobab development. The benefit of splitting the riser into two separate sections is demonstrated, each section being optimized for the pertinent loading that it needs to accommodate. The optimization of the riser system configuration along with the installation measures taken are also shown to be an integral part in achieving a technically suitable and cost effective solution. Baobab field description The Baobab field is owned and operated by Canadian National Resources Limited (CNR) and is located offshore Ivory Coast, in Block CI-40, approximately 65km South West of Abidjan, in water depth ranging from 915m to 1200m.
As part of the TOTAL E&P Angola (TEPA) PAZFLOR development, two 10” Integrated Production Bundle (IPB) flexible risers have been installed to connect the Pipe-In-Pipe Oligocene loop to the FPSO. While IPB risers have first been implemented on the nearby DALIA development, the PAZFLOR IPB had to accommodate much higher design and operating temperatures. From the on-set of the project, focus was put on improving the design of this second generation of IPB and ensuring that all the lessons learnt from DALIA were addressed from a design, manufacturing and installation point of view. The paper presents the main elements of the IPB design as well as the comprehensive set of specific qualification tests performed during the course of the project. Indeed new materials were qualified, a new end fitting design for the bundle was implemented and various mechanical and thermal full scale tests were performed. Both risers have now been successfully installed and commissioned further proving that IPB flexible risers are a technical and cost effective solution to address the flow assurance challenges of deep water production fluid recovery.
For the TOTAL E&P Angola (TEPA) PAZFLOR field development, the decision was taken at the tender stage to propose an innovative diverless Bend Stiffener Fixation system (BSF) to connect the flexible risers to the FPSO. Avoiding diving intervention was driven primarily in order to minimize weather downtime during this critical offshore installation phase. Technip decided to develop its own system to retain full control of the design process and ensure it fulfilled all the installation constraint requirements. The patented system consists of a 3 part clamp activated by a hydraulic motor controlled from topside. An innovative disconnection device is used to release the riser pulling head once the bend stiffener is clamped to the I-tube. The development of the BSF was handled by an integrated team as a “project within the project”. From the start a robust design gate review process was applied to rapidly freeze the design of the system main components and enable the initial order placement of the long lead items. This was critical to ensure on time delivery to the FPSO Contractor. Extensive full scale testing was performed to ensure good reliability of all the system components. This testing enabled further optimization of parts of the system, particularly the items mounted on the risers to provide the maximum operability. Finally the involvement of the installation team from the onset ensured that robust installation procedures and operations were developed. The hook-up operations for the 21 risers was conducted safely with no weather downtime and with a riser connection cycle faster than planned. This proves that implementation of innovative design solutions can be successfully managed from initial concept to final installation within a project environment, ultimately enabling faster hand over for first oil.
The PAZFLOR field is located in Block 17, deep offshore Angola, and is one of the largest offshore/subsea development operated by TOTAL E&P Angola (TEPA). As part of the SURF package scope of work, flow assurance analyses had to be performed to verify the global thermal integrity and operability of the subsea architecture. In particular, a cold spot management study was carried out to ensure that adequate insulation was implemented on all the SURF equipment to achieve the project specified thermal performances. The purpose of the cold spot management plan is to complete the general Flow Assurance study mainly using OLGA software by performing a detailed flow and thermal analyses, using dedicated CFD simulations, for each subsea singularities identified as potentially creating a local cold or hot spot. Hot spots could lead to an accelerated ageing of some of the equipment materials, whereas cold spot could lead to a quicker cooldown time than required creating a local risk of wax or hydrate plug formation. The Pazflor SURF cold spot management plan has combined detailed CFD modeling and full scale thermal testing to validate the equipment insulation design and check via a validated simulation the exact thermal behavior under subsea conditions. An extensive amount of subsea elements have been studied such as Rigid Pipe-in-Pipe (PIP) waterstops, flexible IPB riser, pipeline In Line Tee structure piping and valves, Field Joint Coating with anode pad, flexible end-fittings and thermal insulation covers amongst other things. This paper will detail the CFD simulations performed, present the comparison between the simulation results and the full scale thermal tests and draw conclusions on the benefit of such cold spot analyses for future projects.
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