Significant wave height, vessel RAO and gangway AMC performance

Whitepaper

Significant wave height, vessel RAO and gangway AHC performance

Bj?rnar Huse Ulmatec Handing Systems

The?term significant wave?height (Hs) represents the average height of the 1/3 highest measured waves within a period of 20 minutes; it can also be provided for a 3 hour range. The highest sea state depends on the actual metocean area. Within a sea state described by a significant wave height, the actual maximum wave height could be a multiple of 2, 3 and more than 4 of the representative Hs. The maximum can be defined in several ways, for instance: “most probable largest wave in x number of waves”, “expected largest response” or “extreme response with 1% probability of exceedance”.

It must be emphasized that equally important as the wave height is the wave period. In terms of motion, the response of the vessel can be amplified several times. This means that the vessel response in a sea state with a low wave height and critical wave period can be higher compared to a sea state with a higher wave height but “better” wave period. In addition, a Hs – Tp combination (usually applied to describe a sea state) can occur in many ways and by many types of wave spectra. So the vessel response in a given Hs – Tp will vary dependent on the wave spectra that is utilized. Wave spectra is defined in statistical terms and can have a large influence on capability calculations.

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Various?hull designs?will respond differently to waves, but in general a larger hull has less heave and pitch motion. Roll motion is mainly influenced by the vessel’s natural period.?Seakeeping simulation software?is used by the ship designers to calculate the response of a given hull for various Hs and wave periods, headings and loading conditions. Most seakeeping software calculate a physical quantity called the “response amplitude operator”,?RAO. This describes the vessel’s motion characteristics in all degrees of freedom (surge, sway, heave, roll, pitch and yaw). The RAO assumes a relation between the wave spectra and vessel response, meaning that if the RAO and wave spectrum is known, the vessel response in a given sea state can be described for any fixed point on the vessel.

When RAOs are known, the motion?amplitude, velocities and accelerations?can be calculated for the end of a gangway over the ship side at a given height above deck. From this data it is possible to determine maximum velocity and acceleration based on wave period and ship heading. This is provided that the definition of the maximum is precisely defined.

See the document?DNV-CG-0130 Wave loads?for details and definitions from DNV.

A gangway designer can use this information to decide required speed, amplitude and acceleration of the gangway in active motion compensation and estimate the dynamic loads. The basic gangway is designed as general purpose equipment, not specially designed for a particular vessel. It is impossible to give a blanket statement of “this gangway will work in 3,5 mHs” on any vessel. The actual capability is dependent on the maximum speed, amplitude and acceleration of the gangway and the responses and motions of the hull.

Ulmatec uses the?ShipX?and Veres software from the Sintef research company, a partner of NTNU, Norwegian University of Science and Technology. Sintef is one of the leading research for ship design and simulation.

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With a set of RAOs for a hull and a defined wave spectrum, we can simulate the motions of the hull and the compensation required of the gangway tip. This capability plot will show the maximum Hs at different headings and gangway orientation, typically 90 degrees off to one side. With the rotating tower, the Ulmatec gangway can be turned to other headings which may increase workability. Simulations have shown a 30% increase for certain headings.

The control system will warn when any one of the operational limits for the system is exceeded.

An important factor is the distance from the gangway end to the ship centerline in the X-Y plane and distance from the gangway end to the axis of the motions eg. the axis the hull is pitching around.

Another factor is the distance from the origin of motion to the end of the gangway in the Z axis, the height. The greater the distance the more work the gangway has to do in order to stay stationary in 3d space, in contact with a wind turbine, for example.

The use of a gangway also exerts force on the structural components. Material fatigue is an important issue and needs to be simulated and calculated for the expected liftetime of the gangway. A typical use is estimated, hours per day in cantilever mode in a windfarm (for example), and the average and peak loads during the lifetime and the number of crane lifts or number of people crossing on the gangway in AMC mode. For a flotel gangway the number of emergency liftoffs and the load on the gangway during a liftoff is used. The FEM analysis is part of the DNV, ABS or other class approval of the gangway.

The loading condition (metacentre height, centre of gravity, actual weight distribution) for a SOV usually does not change much in operation and the motion characteristics are usually related to one loading condition.

Be aware of the fact that all simulations of ship motions are based on assumptions and will not be 100% accurate. In real life other motions may occur and safety factors should always be applied, depending on the consequence if the values are exceeded.