Session: 04-01-05 Flexible Pipes and Umbilicals V

Submission Number: 156694

A Qualitative Comparison of Measured and Theoretical Metocean Data and its Impact on Flexible Riser Design 

Dynamic analyses of flexible risers and other marine structures are often based on standard metocean report data generated for a specific field. The main environmental parameters for analysis of flexible risers are waves and current. The metocean report contains wave, current, and wind data for 1 to 10,000 years return periods. Occasionally, additional data is available such as associated wave/current data or wave data from forecasts and surface buoy measurements. This represents more accurately the actual offshore conditions and may allow for a significant reduction in conservatism.

Conversely, current information is often available as surface velocity from which a profile shape is assumed. Alternatively, a few velocities throughout the water column can be used with linear interpolation applied between them to create a velocity profile. For analysis purposes, the current direction is usually defined relative to the wave propagation based on a combination of known conditions and most onerous scenarios in terms of reaction forces along the flexible riser. This leads to potentially unrealistically high conservatism in the analyses, with consequences for elements such as design, material choices, and installation, eventually leading to associated cost impacts and a non-optimal riser system design.

For many purposes, the metocean report provides sufficient data for acceptable and conservative design of marine structures and flexible risers. However, with the focus on environmental impact, as well as the known challenges of achieving feasible designs for flexible risers for existing platforms, the need to reduce unnecessary conservatism in global analysis is evident, and the driver for this work.

From October 2023 till July 2024 a current profiler was installed on the seabed at an oil field in the Eastern part of the Norwegian Sea, where the water depth is 360 m. Current velocity and direction were monitored up to 60 m above the seabed, providing a unique opportunity to investigate the impact of the difference between theoretical current profiles from the metocean report versus actual conditions at the water depths particularly important for riser motions due to distributed buoyancy modules and consequently large drag areas.

This paper includes the following:

- Detailed description of the measuring methodology, current profiler description, and graphic illustrations of representative scenarios. The measured and theoretical current profiles are compared based on the associated significant wave height, spectral peak period, and direction. This illustrates the difference between the theoretical and measured profiles, as well as providing a qualitative measure of the applied conservatism in analyses based directly on metocean report data.

- Global response analysis of a flexible riser performed in OrcaFlex. The analysis is based on a detailed model of the platform and a 12” flexible riser in a reverse pliant wave configuration, complete with discrete distributed buoyancy modules and hold down anchor system. Two parallel dynamic analyses are performed: Actual maximum measured current profile with associated measured wave height and period, as well as maximum measured wave height with associated measured current profile. The current profiler measures current velocity and direction up to 60 m above the seabed. Above this height, the velocity profile is derived based on information in the metocean report and from surface current velocity measurements. These analyses are based on directional environment and associated platform motions.

- Theoretical comparable (current and wave) from the metocean report, which may need to be scaled to some extent depending on the extreme conditions occurring during the measuring period. These analyses are based on collinear environments, which is the standard methodology for riser analyses where no information regarding co-dependence on extreme waves and current is available.

Characteristic loads and motions are extracted from the two analyses and compared, and the results demonstrate the direct impact of current data on flexible riser responses.

Presenting Author: Karina Bruun Mortensen 4Subsea

Presenting Author Biography: Managing & coordinating work and tasks within the flexible pipe discipline, responsible for experience transfer, quality focus, and compliance to company requirements.

Work experience includes dynamic modelling and analysis of flexible risers for the oil & gas industry, development of analysis methodologies to comply with applicable standards and Client requirements, development of analysis tools, management of analysis work, development & presentation of 2-4 days courses.

Educated at the Technical University of Denmark, DTU, and specialized in coastal and marine fluid mechanics with experience within modelling and small scale tests regarding sediment transport and marine structures.