OTC | Offshore Technology COnference

Technical session: May 1, 14:00-16:30 | 32153 | Presentation: 15:20-15:38. Room 610

Performing numerical simulations of a scaled-down experimental setup of a floating offshore wind turbine allows for a two-way validation between the two approaches. Furthermore, additional insight in the physical behavior of the system can be obtained by tuning numerical results on experimental measurements, and exploring the results. Numerical simulations of a 10MW floating offshore wind turbine model in a wave basin are performed at model scale using computational fluid dynamics. The performance-scaled turbine is designed to match the Froude-scaled thrust force in a Froude-scaled wind field. Initial results show a discrepancy in calculated turbine thrust and torque results compared with experimental results. Two main challenges are identified: (1) selecting a numerical approach appropriate for the low-Reynolds number flow, and (2) accurately modelling the environmental conditions in the basin.

In this paper, a numerical sensitivity study is carried out by varying systematically the turbulence models and inflow conditions. A standard k-ε turbulence model is used, as well as a more extensive γ-Re_θ turbulence transition model. Different inflow conditions are set up to model the turbulent jet wind field in the wave basin. It is found that the k-ε turbulence model is unsuitable to match the model test results, while satisfying results are obtained using the γ-Re_θ model. Furthermore, it is seen that the turbulent jet inflow is represented well by both a vertical power law profile and a radial profile fitted to wind field measurements, while uniform and tabulated inflow conditions are a poor representation of the experimental conditions. It is concluded that effects from surface roughness, the Reynolds number, the inflow velocity and turbulence distribution must be included in the numerical evaluation.

Technical session: May 1, 14:00-16:30 | 32368 | Presentation: 15:40-15:58. Room 610

This paper discusses the application of a novel passive control technology (Rotating Tension Element Damper, RTED) in attenuating dynamics of a floating wind tension leg platform (TLP) to significantly reduce fatigue and strength loads which ultimately lower the cost of the system. Comprehensive pre-Front End Engineering Design (pre-FEED), including wave basin testing, was carried out to develop the TLP integrated with the RTED. The integrated application of the RTED to a 15 MW floating wind TLP is the first of its kind, and the assessment features model testing performed at 1:60 scale in the wave basin using operational and extreme seas. Results indicate that the RTED attenuates dynamics in all components of the system, over a broad range of frequencies. Findings also reveal complexities which can be associated with dynamic attributes of control system embodiments at sub-scale, thereby providing insights to avoid such effects in full-scale applications.

Technical session: May 3, 09:30:12:00 | 32156 | Presentation: 10:10-10:28. Room 610

Feeder vessels are one of the solutions being considered to facilitate Jones Act-compliant installation of offshore wind turbines on the US outer continental shelf (OCS). Feeder vessels transport wind turbine components from US ports to offshore locations and position next to an installation vessel, where the components are then lifted off using a crane on the installation vessel. The lifting operation is a highly critical and transient process. A successful lift depends on the lifting system’s capability, the feeder vessel’s performance, environmental conditions, operational processes and other factors.

In 2021, the National Offshore Wind Research and Development Consortium (NOWRDC) initiated Project #107, Comparative Operability of Floating Feeder Solutions, that was undertaken by MARIN, ABS and Saint James Marine. The project primarily entailed computer simulations of offshore wind turbine component lifts and the development of comparative operability assessment procedures for floating feeder solutions. Time domain simulations were used to model the lifting process and results were analyzed relating to wind turbine component accelerations, loads on the lifting system and the re-hit of wind turbine components with the feeder vessel. The study has demonstrated that the combination of time-domain analysis for lifting simulation and frequency-domain analysis for feeder vessel motions can be an effective approach to comparatively analyze a wide range of feeder solutions. Comparative weather downtime and seasonal utilization can also be effectively derived. This paper describes the development process of and provides recommendations for assessing the operability of offshore wind feeder vessels

Technical session: May 4, 09:30 - 12:00 | 32285 | Presentation: 10:10-10:28. Room 312

The Vortex Induced Motion (VIM) response of multi-column semi-submersible floaters may have a significant impact on the predicted fatigue life of mooring and riser systems. Over the last decades physical model testing using Froude-scaled floater geometries has been the main method to estimate the VIM response in the design stage. However, available field measurements seem to indicate that the VIM response when the floaters are installed, is typically lower than what is predicted in the model tests. Overly conservative design guidance for moorings and risers may be the result of only using standard model test information, with significant impact on costs.

Recent joint research projects, e.g. the VIM JIP led by MARIN and the RPSEA project led by Houston Offshore Engineering, investigated the effects of waves, current inflow conditions, external damping, mass ratio’s and Reynolds number using both Computational Fluid Dynamics (CFD) and model testing to explore the main reason for the observed reduction in VIM response in the field. According to these investigations external damping, which may come from mooring and risers being dragged through the water during the VIM of the platform, was identified as the main cause for the reduced VIM response.

In this paper a coupled approach between the CFD code ReFRESCO and the time-domain mooring code aNySIM-XMF is utilized to investigate the effect of the damping from the riser and mooring systems on the VIM response of a semi-submersible platform. The VIM excitation from the hydrodynamic loading on the platform is solved by the CFD code while the damping from mooring and risers is simulated using dynamic anchor line models within the mooring code. A representative deep-water mooring and riser system consisting of 14 anchor lines and two risers was built based on a recent wave basin and VIM model test campaign. CFD simulations with an equivalent linearized mooring system are first carried out to identify the VIM response without damping from mooring and risers. Coupled simulations are then carried out to identify the effect of damping from mooring and risers on the VIM response of the platform. Different mooring models are investigated as well as the effect of different current profiles.

The calculated results for the equivalent linearized mooring system are first blindly benchmarked against available model test data which gives confidence in the CFD results. Additional sensitivity studies on the influence of the time step size showed that the calculated VIM results lie within 5% from each other.

Comparing the co-simulation results using different mooring models, i.e. an equivalent linearized stiffness matrix, a quasi-static catenary model and a dynamic lumped mass model, it can be observed that with the dynamic lumped mass model the VIM response reduces by 35-60% depending on the reduced velocity compared to the other two mooring models. This indicates that the hydrodynamic effects, i.e. added mass and damping, from the mooring and risers significantly reduce the VIM response of the semi-submersible platform. By setting the CD value to zero in the lumped mass model, a very similar VIM response was found compared to when using the catenary mooring model indicating that the drag component is responsible for the reduction in VIM response.

The novelty of the presented co-simulation approach is in the ability to estimate the hydrodynamic effects, originating from mooring and risers, on the VIM response of the semi-submersible floater. As a consequence more realistic and less conservative predictions for VIM response can be obtained in the design stage of projects, which is important for the prediction of the associated fatigue life of mooring and riser systems.

Technical session: May 4, 09:30 - 12:00 | 32586 | Presentation: 10:30-10:48. Room 312

In 2017, Exmar has taken delivery of an FSRU which was intended to operate in Bangladesh to provide the country up to 600 MMSCFD of natural gas. The FSRU was purpose-built with a shallow draft to allow it to be moored in a river estuary. The project in Bangladesh never materialized and the FSRU ended up in lay-up in Singapore. With the energy crisis unfolding early 2022 several countries in Europe urgently started investigating the possibilities to bring FSRUs to Europe to ensure the security of natural gas supplies and in order to have a back-up for the decreasing natural gas flow from Russia. It was proposed to mobilize our FSRU from Singapore and eventually a 5-year contract with Gasunie was signed. This deal made it possible to operate the FSRU at the EemsEnergy Terminal in The Netherlands. For the FSRU to operate safely and efficiently at the new terminal, several modifications needed to be applied in order to adapt the FSRU to the new, colder environment. First and foremost, the regasification system needed to be provided with warm heating water. As per the original design, the FSRU regasification system needs seawater of at least 15 °C in order to avoid freezing in the LNG vaporizers. A closed loop heating water system was not foreseen for heating the water which is used to vaporize the LNG.

Several options have been investigated and together with the Client, it was decided to use heat from shore as input for vaporizing the LNG. The FSRU layout was also found incompatible with terminal layout and changes were required on the main piping system and on the mooring system. Within a couple of months time the mooring design was revisited, the main piping layout was modified to ensure the FSRU could fit the terminal and could stay safely moored in the new metocean environment. After the FSRU was mobilized from Singapore it went straight into the conversion yard in the Netherlands. Within a period of 2 months all changes were implemented and the FSRU could be deployed on time to supply LNG to The Netherlands. This project demonstrates that our FSRU concept is flexible and can be redeployed quickly to other sites when market conditions change. Thanks to our in-house engineering and operational capabilities, we could upgrade the FSRU design in a short time, matching the very demanding project timeline.