Session: 01-06-01 CFD Modeling and Offshore Applications

Submission Number: 180944

Assessing Helideck Turbulence of an Offshore Platform Using Delayed Detached Eddy Simulation and the Lattice Boltzmann Method 

Operators of offshore platforms routinely employ helicopters to provide essential services such as the transport of crew and freight, equipment inspection, and in emergencies, evacuation and search and rescue missions. However, when the helicopter arrives at the offshore platform, the pilot is often faced with a difficult and dangerous landing, mainly due to strong winds. If the helicopter is unable conduct missions due to poor flying conditions, the operation of the offshore platform may be affected, leading to significant commercial penalties. 

The unsteady air flow generated by the wind moving over and around the platform structure creates a highly turbulent air flow known as the ‘airwake’. The airwake turbulence is regarded by pilots to be the principal safety hazard and source of the highest pilot workload during helicopter operations to offshore platforms. Prior to helicopter operations to new or modified offshore platforms, analysis of the air flow over the helideck is performed. Wind tunnel testing or Computational Fluid Dynamics (CFD) is typically used to establish the wind environment in which helicopters will be expected to operate; however, regulatory bodies do not stipulate the type of CFD turbulence modelling required and, therefore, analysis is often limited to low-cost time-averaged solutions. Airwakes generated by the complex superstructure of a typical offshore platform are inherently unsteady and characterised by both quasi-periodic, large-scale structures and chaotic small-scale turbulent features. Steady airwake modelling techniques are, therefore, not well suited to capturing such complex flow features.

The proposed paper will describe how the turbulence over the helideck of an offshore platform has been assessed for a specific wind direction using two time-accurate CFD methods: Delayed Detached Eddy Simulation (DDES) and the Lattice Boltzmann Method (LBM). The wind direction was chosen such that the helideck was positioned in the lee of the main derrick providing highly unsteady and complex flow over the helideck. DDES provides a hybrid approach to turbulence modelling, where Large Eddy Simulation (LES) is used to directly resolve the larger-scale turbulent structures, and URANS is applied to the wall boundary layers. Unlike traditional Navier-Stokes solvers, LBM simulates fluid dynamics on a mesoscopic scale based on classical gas kinetic equations. It’s main advantage is the use of a regular lattice which, due to the resulting algorithm, is inherently suited to massive parallel execution. LBM, therefore, has the potential to provide a rapid time-accurate assessment of the aerodynamic environment over the helideck of an offshore platform.

A CAD model of a generic semi-submersible offshore platform was used as the geometry for each CFD method. The unstructured mesh for DDES had an approximate cell count of 40 million and required a time of about 5 days on a High-Performance Computer. The unstructured mesh generated for CFD analysis included a number of regions of smaller cells in areas of specific interest, such as over the helideck, to ensure that the turbulence was accurately resolved. The LBM approach implemented in the open-source framework OpenLB employs cell-centred grid refinement in combination with a wall-modelled LES model. Based on the transparent GPU executability provided by OpenLB, a time to solution below five hours is achieved for the same minimal mesh size as the DDES method. Both CFD solutions generated a thirty second airwake from which unsteady statistics were used to assess the level of helideck turbulence at heights up to 30 m above the helideck.

The proposed paper will describe the methodology used by the DDES and LBM CFD models to generate the airwake of the offshore platform at one wind direction. The solutions of each model’s computed airwake will be presented and compared. An assessment of the helideck turbulence, in line with the current regulatory guidance, will also be conducted to demonstrate the feasibility of each CFD model for industrial airwake analysis.

Presenting Author: Neale Watson Flight Science and Technology, University of Liverpool

Presenting Author Biography: Dr Neale Watson is a Aerospace Engineering Lecturer conducting research on the operation and analysis of rotorcraft operating within challenging environments. NW utilises Computational Fluid Dynamics (CFD) to model the unsteady turbulent airflow over and around bluff body structures, such as ships, offshore platforms or hospitals to assess the effect on aircraft operations through the integration of the airflow within a piloted flight simulation environment. NW also performs experimental measurements in a large water channel to validate the CFD generated data.

Authors:

Neale Watson Flight Science and Technology, University of Liverpool
Adrian Kummerländer Lattice Boltzmann Research Group (LBRG), Karlsruhe Institute of Technology
Fedor Bukreev Lattice Boltzmann Research Group (LBRG), Karlsruhe Institute of Technology
Mathias Krause Lattice Boltzmann Research Group (LBRG), Karlsruhe Institute of Technology
Ieuan Owen Flight Science and Technology, University of Liverpool