Touch Down Zone Curvature Measurements on Insulated Steel Catenary Risers in Ultra Deep Water (Development, Implementation and Results)

Abstract

1.0 Abstract Bending strain is the main contributor to fatigue damage in the touch-down zone (TDZ) of a Steel Catenary Riser (SCR). Measurement of the curvature in the SCR at the TDZ due to bending is essential to the validation of numerical models for SCR response and fatigue estimation. Long term SCR integrity monitoring systems may also require tools for the measurement of bending strain in the TDZ. In deep water, production SCRs typically have thick anti-corrosion and insulation layers that make the inference of the strain in the underlying pipe challenging. This paper describes the development of a strain measurement system and unique attachment scheme that can be literally built into the insulation and anti-corrosion layers on ultra deep water SCRs. Results of qualification tests will be presented that prove that the " insulation strain measurement system?? produces a very accurate estimate of the strain in the underlying pipe steel. In addition, test results are presented that demonstrate that the attachment scheme does not threaten the integrity of the insulation and corrosion protection layers. Finally full scale bending strain measurement results will be presented from the TDZ of the Total AKPO Gas Export SCR in 1200 meters of water that illustrate the quality of the data, the excellent resolution and reasonableness of the results compared to numerical model predictions. 2.0 Introduction and Functional Requirements of the AKPO Riser Monitoring System Total's Akpo field lies offshore Nigeria within OML130, some 200 km south of Port Harcourt, in water depths ranging from 1250 m to 1480 m, and is operated by TOTAL UPSTREAM NIGERIA LIMITED. The spread-moored 2 million barrel AKPO FPSO receives oil and gas through insulated SCRs, produces the fluids, and offloads the product via 2 Off-Loading Lines (OOLs) to the OLT which is a Single Point Mooring (SPM) buoy, and subsequently to a shuttle tanker. The current field production plan includes 22 producing wells, 20 water injection wells, and 2 gas injection wells, arranged in clusters around subsea manifolds. The subsea manifolds are connected to the FPSO through looped and insulated flowlines and SCRs. The overall project schematic is shown in Figure 1. The SCR system is composed of:8 off 10?? ID production loops with Flex Joint (FJ) at FPSO side1 off 8?? ID gas injection line with FJ at FPSO side4 off 10?? ID water injection line with FJ at FPSO side1 off 16?? OD gas export line with FJ at FPSO side The OOL system is composed of:2 off 16?? ID OOL with FJ at FPSO side and FJ at SPM buoy side.