Multi-Objective Collaborative Flexibility Optimization Design of the Strengthening Layer of a Flexible Riser

Marine flexible risers are widely used in ocean oil and gas extraction, and need to withstand environment loads (wave and current) and the large offset of the floater. Therefore, the flexible riser is subjected to tension, bending and torsion loads, which are mainly resisted by the key strengthening layer. Small bending stiffness of a cross section of the strengthening layer with larger tension and torsion stiffness are required to be compliant with the ocean environment. The traditional design of the key strengthening layer is partially rigid with larger cross-sectional stiffnesses. Therefore, the innovative configurations of the strengthening layer are imperative to make sure that the flexible riser is reliable and safe during the installation and operation. The strengthening layer of the flexible riser is treated as the cylindrical shell composed of periodic unit-cell beam structures, which is a hypothetical model. The optimization design is conducted through the novel implementation of the asymptotic homogenization (NIAH) method. The multi-objective collaborative flexibility optimization formulation of cylindrical shell structure is proposed, considering the ratio of cross-sectional tensile torsion stiffness to bending stiffness of the strengthening layer as the objectives. The optimal configuration results, the helically wound structures, are obtained, which are the alternative strengthening components of flexible risers. Finally, the optimal structures are compared with the commonly used marine flexible riser, which gives a great verification of the methodology feasibility, and explains why the strengthening layer is designed as the type of helically wound structure.

A Real Time Fatigue Monitoring Platform for Flexible Risers

Fatigue life of flexible risers is a critical design factor in offshore riser system design. To estimate the fatigue damage, global and local analyses are performed inhouse with inputs from operators. Metocean data, riser properties, vessel or platform RAO data are typical inputs for the fatigue analysis. In addition to the conservatism inherited in the product design methodology and API safety factor 10 for fatigue estimation, the input data may also contain some additional conservatism compared to actual operational conditions as these were estimated/projected from limited time-span of observations or predicted purely from numerical modelling. In combination, these inheriated conservatism permits the feasibility of service life extension of the flexible riser systems. Many platforms or vessels already have real time monitoring of motions in multiple degrees of freedom and wave/current data. Furthermore, pipe internal operating conditions like pressure and temperature are usually recorded onboard. Compared to the typical input data/assumptions made in predictive analysis, these records are more precisely and directly related to the actual fatigue damage accumulation. With real time data, a more accurate estimation on fatigue performance of riser can be achieved, which could enable the service life extension without compromising the design safety factors. A special software tool has been developed to calculate the real time fatigue damage of flexible tensile wires. The software is based on product design tools which are calibrated and independently validated with over 30 years installed operational experience. Developed in Python, the software utilizes an OrcaFlex API and a local analysis algorithm in the background. The software can be customized towards different platforms and pipe systems, as well as input data types. Depending on clients’ needs, multiple fatigue hot spots like top end fitting, bend stiffener region or touch down zone can be monitored at the same time. Other parameters like riser real time extreme response or statistical results can also be checked in the software. This paper summarizes the development of the pipe monitoring system. It is believed that with real time inputs, the software can better assist clients to monitor the pipe fatigue performance and other related riser responses.

FPU Mooring Footprint Reduction in Buzios Field: Key Driver to its Successful Subsea Layout

Buzios field development has the potential to implement several production systems due to large reservoir volumes. Considering the oil specification, the drive to use standard solutions already in place in Pre-salt area, associated with the high production indexes of the wells, Petrobras decided to tie back all production wells in satellite configuration. These facts, together with geological hazards in the area, lead to a potentially congested seabed scenario. Hence, FPU positioning has been challenging and demanding innovative engineering solutions to optimize FPU mooring as to overcome these challenges and enable FPU positioning close to wells. This optimization gave birth to new issues, such as risk of clashing between mooring lines and lazy-wave flexible risers. Integrated riser and mooring lines dynamic analysis, together with subsea layout assessment were performed to ensure technical and economic feasibility. Furthermore, due to the Buzios reservoir, well design requirements and subsea layout specificities, all FPU were located on the edge of the reservoir and flexible risers were tied back mainly from only one board of each FPU. Hence, enhancing clearance between bow and stern mooring clusters and the optimization of the risers’ configuration were of paramount importance for enabling most of the risers’ connections on the desirable board. FPU mooring optimization led to up to 30% of mooring lines’ radius reduction (horizontal projection), and an average of up to 500m per flowline reduction, saving CAPEX, OPEX and increasing the return on investment.

Development of a robot for in-service radiography inspection of subsea flexible risers

The extreme operational environmental conditions and aging conditions of subsea structures pose a risk to their structural integrity and is critical to their safety. Non-destructive testing is essential to identify defects developing within the structure, allowing repair in a timely manner to mitigate against failures that cause damage to the environment and pose a hazard to human operators. However, in order to be cost effective, inspections must be carried out without taking the risers out of service. This poses significant safety risks if undertaken manually. This paper presents the development of an automated inspection system for flexible risers that are used to connect wellheads on the seafloor to the offshore production and storage facility. Due to the complex structure of risers, radiography is considered as the best technique to inspect multiple layers of the risers. However, radiography inspection in turn requires a robotic system for in-situ inspection with higher payload capacity, precise movement of source and detector which is able to withstand an extreme operational environment. The deployment of a radiography inspection system has been achieved by developing a customized subsea robotic system called RiserSure that can precisely provide the scanning motion of a gamma ray source and digital detector moving in alignment. The prototype has been tested on a flexible riser during shallow water sea trials with the system placed around a riser by a remotely operated vehicle. The results from the trials show that the internal inner and outer tensile armour layer and defects in the riser can be successfully imaged in real operational conditions.