Trans RINA, Vol 161, Part A4, Intl J Maritime Eng, Oct-Dec 2019
Forrest et al (2010) have undertaken CFD simulations using DES on unstructured grids for SFS2 and a Type 23 frigate and have shown a good agreement with experimental data including full scale data of Type 23 frigate. They have further shown that inclusion of atmospheric boundary layer in the CFD simulations improves the agreement with the trials data. Kaaria et al (2012) have undertaken experiments in the water tunnel using Airdyn (specially designed airwake dynamometer) and have measured the unsteady forces and moments on a 1:54 model scale helicopter based on Merlin AW-101 while working in a ship airwake. The causes of the observed loading patterns have been discussed with the results of unsteady CFD simulations. Tai & Carico (1995) and Tai (2001) report simulating the aerodynamic flow around the US Navy destroyer DD-963 using a RANSE code to determine the flow conditions on deck for landing helicopters.
2.1
HELO OPERATIONS ON OFFSHORE PLATFORMS
The Civil Aviation Authority of UK's Safety Regulation Group has compiled a comprehensive Guidance on standards to be achieved for Offshore Helicopter Landing Areas through CAP 437 (2010). The document gives guidance for assessment of standards for offshore helo landing areas located on fixed and mobile offshore installations, and vessels supporting offshore mineral exploitation. The first edition of the document was published in 1981, where-in the size and layout of the helicopter landing areas were determined as function of the overall tip to toe length of helicopter intended for operation. Over the years, and based on various research findings through agency funded projects, the criteria for safe helo landing have been refined.
The UK CAA’s guidance document mentioned above has been supported by several high value research publications over the last decade. The research paper CAA 99004 (Whitbread & Coleman, 2000) has reviewed literature available on the subject and brought out that following four principal objectives were set by the CAA in the original specification for the work:-
a) To establish the nature and extent of the environmental problems associated with operations of helicopters on helodecks on offshore platforms.
b) To review the "state of the art" in relation to the techniques and technology that could be deployed to mitigate the identified problems.
c) To plan a course of short, medium- and long-term actions to address the problems associated with the offshore helodeck environment.
d) To improve the quality and scope of the guidance material contained in CAP 437.
The principal sources of environmental hazard have been identified as vertical wind components, local ambient
temperature rise and turbulence. It has been further brought out that the greatest risk to helicopter operations is judged to be the point where the helicopter arrives over the helodeck and is required to hover prior to touchdown. The report brings out that the earlier editions of CAP 437, as a guidance document, had a number of limitations and difficulties in application, the most serious limitation being the lack of any quantitative limitation on an acceptable level of turbulence, since the problems most frequently reported by the pilots are those involving handling difficulties which are almost invariably associated with turbulence. Conclusions linking high workloads due to turbulence with the increase in risk of error of judgement by pilots have been indicated.
Having appreciated the limited knowledge base, the research set out by CAA in the year 2000 for helicopter operations on offshore platforms set certain aims and targets for research which can be considered very relevant to be taken as guidance in setting up the path for researchers in the field of warship-helodeck design. These are as follows:-
Basic objective - "establishing technologies and techniques which could potentially be used to either reduce the environmental hazards, improve their control, or mitigate their effects".
Ultimate aim - "To provide support for a strategy for improving the safety of helicopter operations by proposing short, medium- and long-term actions in which the roles and responsibilities of the platform designer, the platform operator, the helicopter operator and the regulators are clearly defined.
The approach identified four key topic areas as follows:- a) A review of practices and procedures currently adopted for offshore helicopter operations to ensure that the experience of the operators and, more particularly the pilots, was fully taken into account.
b) The sources and nature of different types of environmental hazard to be identified.
c) An assessment of the response of helicopters to various forms of environmental disturbance.
d) Finally, the content of CAP 437 needed to be examined to determine whether there was scope for change or additions to the current environmental criteria.
Based on the research results, the latest edition of CAP 437 adopts a "Turbulence Criteria" on helodeck defined as the standard deviation of the instantaneous vertical velocity and sets an upper limit for the same to 1.75 m/s. In the earlier edition, this value was set higher at 2.4 m/s and was reduced in the sixth edition to allow for flights in reduced cueing conditions (i.e. deterioration in visual appreciation of operating environment for pilots), for the less experienced pilot, and to better align the associated measure of pilot workload with operation experience. The
©2019: The Royal Institution of Naval Architects
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