Trans RINA, Vol 161, Part A4, Intl J Maritime Eng, Oct-Dec 2019


As indicated in Figure 5, the simulation model utilises the wind tunnel measurements of turbulence as input to the FLIGHTLAB model in terms of the three axis components of wind speed and gradients of speed experienced at the rotor hub. In this regard, the document states the following:- "This is clearly a simplification of a complicated flow field that will, in reality, vary across the whole rotor disk, but again a degree of qualitative validation was obtained from the test pilots, who stated that the effect of the turbulence on the helicopter felt realistic." An important take-away from this validation exercise indicates that the airwake obtained on the helodeck on the rotor planes in wind tunnel or in CFD can be represented/replaced with single statistical quantities assumed to be acting on the rotor hub. Such quantities can be used in quantitative comparison of competing geometries studied through a parametric investigation spread over a large range of parameters using single representative values for complicated spatial variables such as turbulence.


Holdo (Holdo, 1997) reports modelling helicopter landing conditions onboard offshore structures. The paper presents the CFD modelling of wind interaction with offshore platform and its relevance to the location of the helicopter deck, the CFD modelling of gas turbine exhaust jets for a typical offshore production platform and some comparisons between CFD studies and wind tunnel model tests.


2.2


TURBULENCE INTENSITY AND FLUCTUATING VERTICAL VELOCITY


Silva et al (2010), in their investigation of helodeck of an FPSO, have mentioned the relationship between Turbulence Intensity as obtained in isotropic RANS model and the standard deviation of fluctuating vertical velocity. The CAP 437 guidelines for helodeck design recommends a maximum turbulence level based on the standard deviation of the vertical velocity (CAP 437, 2010). The numerical simulation software generally calculates the turbulence kinetic energy (k) or the turbulence intensity (I). The turbulence kinetic energy is defined by following equation:-


k = 1 2 (u′2̅̅̅̅ + v′2̅̅̅̅+w′2̅̅̅̅̅) (1)


Recalling that the turbulence is assumed as fully developed and isotropic in the RANS models, the velocity fluctuations can be assumed to be equal (Silva, et al, 2010). For the steady state regime, the term w′w′


̅̅̅̅̅̅̅ may be


interpreted as the variance of the vertical velocity w. Using these assumptions in Equation 1, the standard deviation of the vertical velocity Sw may be expressed in terms of the turbulence kinetic energy as follows:-


Sw = √w′2̅̅̅̅̅ = √2 3 k (2)


The turbulence intensity (I) is expressed in terms of turbulent kinetic energy (k) as follows:-


I = Uref


√2 3k


(3)


In view of the above, while analysing results of numerical modelling using RANS, the turbulence intensity (I) or the turbulent kinetic energy (k), hence, can be related to the Turbulence Criteria recommended in CAP 437 for offshore helodecks which places a limit on the standard deviation of the vertical velocity Sw.


2.3


HELODECK ENVIRONMENT FOR HELICOPTER PERFORMANCE


Extensive research undertaken by the British CAA for operation of helicopters on offshore platforms bring out some important understanding of the dynamics involved related to the operations aspect. This is an important feature for research in this field since it closes the loop by linking the behaviour of the helicopter, as experienced by the pilot, with the nature of the environmental disturbance, the origins of which can readily be traced back to the basic design of the ship superstructure.


Also, in order to have a better understanding of the helicopter handling qualities on Naval ships, the Research and Technology Organisation (RTO) of the North Atlantic Treaty Organisation (NATO) has published the AGARDograph 300 (2003) which documents the helicopter/ship qualification test procedures including the preparation, execution and data analysis of helicopter/ship flight testing that should be employed, combined with best safety practices to obtain maximum operational capability. Towards this pursuit, the document brings out the following:- "Basic helicopter flight limitations are usually determined in a land-based environment by the aircraft manufacturer and/or by the procuring agency. The land- based limitations are not valid in the shipboard environment due to the individual factors including ship air wake/turbulence, ship motion, confined landing areas and visual cue limitations and the combined effects of these factors. Future NATO operators and force commanders may require the maximum helicopter/ship operational capability that can be accomplished in any environmental condition."


Figure 6 shows the set-up of helicopter/ ship qualification programme as followed by Netherlands' National Aerospace Laboratory (NLR) at Amsterdam. Prior to embarking on Dynamic Interface Testing (SHOL trials), a candidate helicopter flight envelope is developed based on results of land-based hover trials of the particular helicopter and the ship airwake environment defined through wind tunnel tests. In many developing nations, SHOL trials are undertaken directly, without taking help of modern experimental and numerical tools.


©2019: The Royal Institution of Naval Architects


A-407


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