Wind speed

The numeric mean of wind speed measurements recorded during the measurement period. Wind speed measurements are received from the measuring instrument at 2Hz or 4Hz according to client specifications. For moving installations, eg. ships, wind speed is corrected according to the vessel's course and speed.

Wind gust

The highest value of a 3-second rolling mean of recorded wind speed during the measurement period.

Speed corrected to MSL (or other reference height)

Measured wind speed, corrected for height of the installed wind speed sensor to wind speed at 10m above Mean Sea Level, or other point of reference such as airfield/helideck, according to a standard formula.

Wind speed units

It is usual for systems supporting marine and airborne vessels to report wind speed data in knots. Logging and display of data in other units can be configured on request. The units of displayed values are always shown adjacent to the data displayed.

Wind direction

The numeric mean of measured wind directions recorded during the preceding measurement interval. The reported value is a bearing (in degrees) with respect to north that identifies the origin of the wind driven airflow. HCA compliant helidecks must report wind direction relative to magnetic north. Other systems may report wind direction relative to true north. This will be indicated in the data display heading.

For mobile installations such as ships, wind direction is corrected for vessel course, speed and heading. For floating stationary systems, such as semi-subs and FPSOs, wind direction is corrected for vessel heading.

Selected wind sensor

For systems fitted with multiple wind sensors a diversity system is configured to maximise reported data quality for all angles of incident wind. This is a text display identifying the current sensor providing wind speed and direction measurements.

Dependent on the relative positioning of multiple wind sensors, the diversity system can be configured to present wind speed and direction data from the sensor with the highest measured speed, or it can be configured to present data from a particular sensor based on a look-up list of wind direction sectors.

Wind Severity Index (WSI)

As defined in Standard Helideck Monitoring Systems Rev 9: 1 April 2018

The 10 minute mean free stream wind speed, corrected to correspond to the height of the main rotor of a helicopter landed on the helideck. An average main rotor height of 4 metres above the helideck surface is to be assumed.

Relative Wind Direction (RWD)

As defined in Standard Helideck Monitoring Systems Rev 9: 1 April 2018

2 minute mean free stream wind direction relative to the longitudinal axis of a helicopter landed on the helideck.

Dry air temperature

The numeric mean of air temperature measurements recorded by a standard thermometer during the measurement period. Unless otherwise specified by the client, the MetOcean software generally receives temperature measurement data at 1Hz.

Wet air temperature

A parameter calculated from the measured Dry Air Temperature and measured Humidity parameters. It is the lowest temperature that can be reached by the evaporation of water only. It is the temperature felt by wet skin exposed to freely moving air. Wet Air Temperature is always less than or equal to the Dry Air Temperature.

Wind chill temperature

Wind chill, sometimes known as Wind chill factor, is a calculated value. It is the perceived air temperature of exposed skin due to the flow of air. This can be a useful parameter in identifying conditions that add risk to outdoor working.

Note that wind chill temperature is only defined for temperatures at or below 10 °C (50 °F) and wind speeds above 3 knots (5kmh).

Dewpoint

A parameter calculated from the measured dry air temperature and humidity parameters. Dewpoint temperature is the temperature at which moisture or dew will form on surfaces and in the air. Mist or fog is likely to form when the dewpoint is near the dry air temperature. Dewpoint will always be less than or equal to wet air temperature.

Humidity

Humidity is the measurement of the amount of water vapour in the air, measured as a percentage. As humidity approaches 100%, the air becomes fully saturated and water vapour condenses and can produce cloud, mist or fog.

Heat stress

A parameter calculated from dry air temperature and humidity parameters. Heat Stress is a unitless parameter that rates the human body's ability to control its internal temperature. For a particular Fugro Northstar™ system, thresholds between particular ranges of heat stress value can be configured in order to align with local colour indications of this parameter. e.g. black, red, amber, green, etc.

Ambient pressure

The numeric mean of barometric pressure measurements made at the elevation of the defined pressure sensor during the measurement period. Unless otherwise specified by the client, the Fugro MetOcean software receives pressure measurements at 1Hz.

QNH - Air pressure at MSL

A function of ambient pressure and the elevation of the pressure sensor. QNH is the calculated value of air pressure at mean sea level. The relationship between QNH and barometric pressure is illustrated in the diagram below.

The term QNH derives from the aviation Q-code and is sometimes described as "Near Horizon" or "No Height".

QFE - Air pressure at helideck

A function of ambient pressure and the difference in elevation between helideck and pressure sensor. QFE is the calculated value of air pressure at the elevation of the helideck. The relationship between QFE and barometric pressure is illustrated in the diagram below.

The term QFE derives from the aviation Q-code and is usually described as "Field Elevation".

Air pressure units

The SI unit for pressure measurement is the hectopascal hPa. 1hPa is equivalent to 1 millibar mbar. Other common units of atmospheric pressure are millimeters of mercury mmHg and pounds per square inch psi. A Fugro Northstar™ system reports pressure data in hPa unless specified otherwise by the client.

A Fugro Northstar™ system deployed to a vessel or platform with a variable draft will include functionality to allow the elevation of the pressure sensor and helideck location to be adjusted in relation to draft. Where draft information is provided to the Fugro system in real time this is accomplished automatically. Alternatively, the user can update the effective vessel or platform draft via a password protected data entry box within the system data display webpages.

Significant wave height H_s

This is usually a calculated (rather than directly measured) parameter, and is derived from a spectral analysis of the data and is defined as;

4 * SQR(M₀)

where M₀ is the zeroth moment

It can also be defined as 4 times the standard deviation of the sea surface variation (trough to crest) over the measurement period and is approximately equal to the average wave height of the ⅓ highest waves.

Maximum wave height H_max

This parameter is defined as the crest to succeeding trough measurement of the largest individual wave within the preceding measurement period.

Wave crest height H_crest

The height of the largest wave crest within the measurement period, measured from the mean of all sea levels measured within the preceding measurement period.

Significant wave period T_z

This is a calculated (rather than directly measured) parameter, and is derived from a spectral analysis of the data and is defined as SQR(M₀/ M₊₂). It is sometimes known as the mean wave period.

Peak wave period T_p

This is a calculated (rather than directly measured) parameter and is derived from a spectral analysis of the data and is defined as;

(M_-2 * M₊₁) / (M₀ * M₀)

An alternative definition of T_peak is that it is 1/F_p where F_p is the peak of the spectra. i.e. The frequency at which the maximum energy occurs

Wave direction

This depends on the sensor being used to acquire measurements, but is usually defined as the average direction of all incident waves. Angles are usually stated as waves coming from the stated bearing but this is not always the case. The data display should clarify this.

Calculating wave parameters

Unless specified otherwise by a client, a Fugro Northstar™ system with a downwards looking radar ranging sensor uses wave analysis calculations defined by the Shell Oil technical note TS211. A system configured to this standard processes 4096 measurements made at 2Hz (34min 8sec ensemble). Note: Calculations are repeated after each subsequent acquisition of 120 measurements (60 seconds).

The application of SWAP (Standard Wave Analysis Program) wave analysis is possible, according to client specifications. This method of wave analysis provides data fields as per those described above, but this data analysis method provides individual output parameters according to waves in each of several time-period categories. The SWAP method was derived from the original specification set by the Rijkswaterstaat Monitoring Network Infrastructure by the Oceanographic Company of the Netherlands (OCN). Fugro can provide further details if required.

northstar.support@fugro.com

Other wave analysis engines categorise waves in to two divisions according to their time period. Longer time period waves are referred to as swell waves and shorter time period waves are referred to as wind waves. Swell is normally caused by distant storms or other wind systems. Due to this swell is more stable in its direction and frequency than waves caused by the wind. The time period boundary between wind and swell waves is not fixed but varies, being determined from a combination of wave and wind data. This is discussed in greater depth in the WMO publication Guide to Wave Analysis and Forecasting.

When the measured significant wave height (H_s) is below approximately 0.3m, the Fugro Northstar™ system will show the word calm against wave parameters measured from downwards-looking radar units. It is not possible to calculate wave parameters with acceptable accuracy in such still conditions.

Measurement methods

Wave data within Fugro Northstar™ systems can be derived from a variety of measurement sources. It is most common to use a downwards looking radar ranging sensor to monitor the sea surface. This method provides accurate wave parameters, irrespective of weather conditions, but cannot provide wave direction measurements. The Fugro WeatherMonitor application can interface to directional wave measurement buoys to provide wave direction information if required.

Some Fugro Northstar™ systems provide wave direction information from an x-band scanning radar coupled with a wave analysis system, based on image processing techniques. Contact Fugro for further details of this measurement method if you are interested to know more.

northstar.support@fugro.com

Spectral wave data

When specified, Fugro Northstar™ systems with wave measurement functionality can provide wave data analysed within defined divisions in the wave-frequency domain. This is referred to as the provision of spectral wave data. Depending on client requirements spectral wave data may be plotted as a frequency vs energy X/Y plot based on the previous calculation period of data and can also be periodically provided as tabular numeric data within formatted text data files.

Air gap

This is the smallest distance between the lowest working level of an offshore installation, often referred to as the cellar deck, and the sea surface over the last wave measurement period. It is derived from the fixed dimensions of the installation and the parameter H_crest. For floating systems where draft information is provided to the Fugro Northstar™ system in real time the air gap calculations accommodate changing draft. For other floating systems the user can enter the existing vessel or platform draft through a password protected data entry box within the system data display webpages to ensure that the air gap is reported correctly.

Tidal level

This value is the mean height of the sea surface over the last wave measurement period with respect to the long term mean water level. Depending on client requirements systems may also present tidal predictions extracted from almanac data for the required location.

Extreme level

The highest individual sea surface height over the last wave measurement period with respect to the long term mean water level. This parameter differs from H_crest which is a measurement with respect to the mean water level identified within the wave measurement period.

Current speed

This parameter reports the average current speed at the depth indicated. Current data is usually presented as an average of measurements made in a discontinuous 10 minute window. This parameter is corrected for vessel course, speed, motion and heading in the case of moving or floating systems

Current direction

The average direction in which the measured current is flowing. Current direction is shown as towards the reported bearing and is presented with the associated depth below the water surface of the measured value. Current direction is usually presented as an average of measurements made in a discontinuous 10 minute window. This parameter is corrected for vessel course, speed, motion and heading in the case of mobile systems

Depth

The depth of water in which current speed and direction measurements are taken. Depth values reference the sea surface as the origin, increasing in value towards the sea bed.

Current profile

This data is usually presented as a plot of depth on the Y axis vs current speed and/or current direction along the X axis of an X/Y plot. This plot shows the variability of current speeds and/or directions throughout the monitored water column.

Current timeseries

This is a plot showing the variation of either measured current speed or current direction with time for a specific water depth.

Transducer health

It is usual to use acoustic doppler sensors for current measurements. Dependent on the specific system requirements such sensors may be seabed, buoy, or near-surface mounted. Antifoul precautions and appropriate periodic maintenance to control biofouling on the acoustic transducers is important in order to maintain the quality of reported data and prolong the life of the instrument.

Cloud bases (1, 2 and 3)

The height of each of the distinct cloud layer above long term mean sea level. Generally a ceilometer will report up to 3 cloud bases if detected. It is common to report cloud base height for aviation in feet. Where there is no cloud detected for a layer it will be reported as No Base.

Cloud measurement

Cloud base heights are measured using an eye-safe pulsed radar system. Such systems are configured to point nominally vertically, but may be adjusted to eliminate direct incident sunlight in near-equatorial applications. Different instruments identify cloud bases using different signal processing, but a rule of thumb is that a cloud base exists at an altitude where the horizontal visibility is 1000m or less. Measurements reported by an instrument relate to the sky directly above.

Vertical visibility

The ceilometer is able to make an estimate of vertical visibilty up to a pre-determined ceiling height based on the attentuation of the reflected signal from the laser.

Ceilometer CL31 uses a contrast threshold value which, through many tests, has been found to give vertical visibility values closest to those reported by ground-based human observers. A safety margin is obtained with regard to pilots looking down in the same conditions since the contrast objects, especially runway lights, are much more distinct on the ground.

Laser safety

The following is taken from the Vaisala CL31 user manual.

Vaisala Ceilometer CL31 is classified as a Class 1M laser device in accordance with International Standard IEC/EN 60 825-1. Complies with 21 CFR 1040.10 and 1040.11 except for the deviations pursuant to the Laser Notice No. 50, dated July 26, 2001. This means that when CL31 is installed in a field environment with instrument covers on and pointed vertically or near-vertically, it poses no established biological hazard to humans.

The device is equipped with the following label:

Ceilometer CL31 is intended for operation in an area restricted from public access, and to be pointed vertically or near-vertically. The following precautions must be followed during the service and maintenance of the instrument:

Warning

Never look directly into the Ceilometer Transmitter or Ceilometer Optics with magnifying optics (such as glasses, binoculars, and telescopes). Never remove the Ceilometer Transmitter from its normal position without first switching off both the line and the battery power.

When operating, avoid looking at the ceilometer unit from the beam direction.

Only trained personnel should perform maintenance functions. Access to the work area by unauthorized persons during service operations must be prevented.

Horizontal visibility

This parameter, sometimes referred to as MOR, is the distance over which the intensity of a light beam is attenuated to 5% of the original intensity. This measurement is usually presented in units of metres and is calculated by measuring the amount of infra-red scatter in a small volume of air at the sensor location and assuming this sample has similar properties to the surrounding air mass. Contaminants in the air at the sensor location or excessive reflections will intefere the inferred visibility and may affect the reported value.

Present weather

Reported present weather is a calculated category derived from precipitation, visibility and temperature measurements. These three independent measurements together provide sufficient data for an accurate evaluation of prevailing weather type. This parameter is presented as a category, either in US NWS or international WMO code formats.

Pitch

This measured parameter reports the rotation of an object around a left-right (port-starboard) lateral axis. It is usual for positive pitch to refer to the condition where the bow of a vessel or the forward face of a platform moves up.

Pitch is usually presented in units of degrees and the measured value applies equally at all locations around a rigid frame.

Roll

Rolling involves rotation of the vessel around the forward-backward longitudinal axis. The rolling period is defined as the time taken for a full rolling oscillation from the horizontal to the left, back to horizontal, then to the right and then back to horizontal. Positive roll is generally reported when the elevation of the starboard side of a vessel or installation decreases, i.e. port-side up.

Roll is usually presented in units of degrees and the measured value applies equally at all locations around a rigid frame.

Inclination

Inclination angle is the parameter used to report the maximum tilt of the vessel or system. It is calculated as a combination of both pitch and roll angles. The angle of maximum inclination within a defined time period is generally reported.

Surge

The backward and forward movement of the vessel. Generally positive values are forward and negative values are backward. This polarity of reporting surge may be different between MRU systems. Surge is generally reported in metres.

Sway

The port and starboard movement of the vessel. Generally positive values are to starboard and negative values are to port. This polarity of reporting sway may be different between MRU systems. Sway is generally reported in metres.

Heave

Heave is the linear vertical (up/down) motion of a vessel or platform. It is measured as a real-time displacement in metres from a calculated centre position. Note that the heave at one location on a rigid vessel or platform will not be the same as the heave at another location if there is any element of rotational (pitch or roll) motion.

Total heave

This parameter reports the total maximum vertical displacement within the specified continuous measurement window. It is the difference between the highest and lowest recorded elevations within the measurement duration.

Heave period

The heave period is defined as the time taken for the vessel or platform to execute one complete cycle of vertical movement (oscillation) from the upper and lower points of the vertical motion. It is calculated from identifying the zero crossing points of the heave measurements. The Fugro monitoring system software calculates heave period based on the time between successive motion zero-upcrossings identified in motion data at a 2Hz sampling rate. The quantity of motion data used within the calculation is either 1, 10 or 20 minutes and this duration is identified on the data displays.

For HCA compliant systems, the heave period data displayed on the main helideck monitoring page is based on a motion data sample of 20 minutes.

Heave rate (calculated)

This is a calculated parameter, the numeric mean of the ratio of heave amplitude to cycle time, when dividing the vertical motion of the measured system in to sequential oscillation cycles.

Heave rate (measured)

This is a measured parameter, reported directly by a motion sensing instrument. It is an instantaneous representation of the z-velocity of the motion sensor at the point of reporting.

Maximum Average Heave Rate (MAHR)

This is a calculated parameter, the ratio of maximum heave in the measurement period against half of the mean heave period in the measurement period and has units of ms^-2.

Heave compensation

This term relates to the use of heave measurements at the location of a downwards looking wave radar ranging unit, to compensate radar-to-water-surface measurements for vertical motion (heave) of the radar sensor unit.

Lever arm offset heave

As noted in the section heave above, the heave at one location on a rigid vessel or platform will not be the same as the heave at another location if there is any element of rotational (pitch or roll) motion.

Lever arm offset heave is the term used when a mathematical function is applied to the heave, pitch and roll measured at one point on a rigid body to determine the heave at another point on the same rigid body.

This calculation can be used, for example, to calculate the heave at the location of a radar ranging sensor from the motion measurements made at a motion sensor mounted to the vessel or system’s helideck. It is not acceptable to use this method to provide a corrected heave measurement for heave at helideck centre. See the section below regarding approved MRU siting for helideck monitoring systems.

The following parameters are as defined in
Standard Helideck Monitoring Systems Rev 9: 1 April 2018

Significant Heave Rate (SHR)

SHR is the average of the one-third highest values of instantaneous heave rate recorded during the previous 20 minute monitoring period. This can more conveniently be calculated by;

Significant Heave Rate m/sec = 2 x RMS of the instantaneous heave rate.

In order to ensure that this is accurately represented, the sampling rate of heave rate should be a made at a rate of 2Hz or greater.

Measure of Motion Severity (MMS):

MMS is the instantaneous value of the ratio of the total acceleration in the plane of the helideck divided by the component of the total acceleration normal to the helideck. It is calculated using the following formula;

where x_ddot, y_ddot, and z_ddot are the total accelerations of the helideck in each of the three axes.

Motion Severity Index (MSI)

MSI is the maximum value of MMS occurring during the previous 20 minutes. It represents the ratio of helideck accelerations in the horizontal and vertical axes as a unitless number. It is calculated using the following formulae;

MMS_MSI = 10 x tan^-1 (MMS_measured)

MSI = MMS_MSImax (t - 20 min, t) x R

Helideck MRU location

According to the HCA HMS documentation;

Measuring equipment sensors for helideck movement, wind and weather data shall be located in optimum positions in order to provide relevant information relating to the helideck.

In addition to this Enclosure L to Norwegian Oil and Gas Association Helideck Manual rev.01.05.2017 adds;

Helideck heave data shall be representative for the center of the helideck. It is recommended to locate the motion sensor within 4 meters from helideck center for new designs in order to meet a possible future requirement for measurement of MSI.

Rainfall

Measured as the amount of rainfall falling in a fixed time interval. It is measured in mm/min. This unit of (depth per unit time) is frequently referred to as rainfall but is more accurately described as rain intensity or rainfall rate. Data presented within the Fugro Northstar™ system is matched to client requirements, but often a 10 minute rolling sum of the rain intensity and a 1 hour rolling sum of rain intensity are presented and recorded.

Dependent on client requirements, rainfall can be measured by means of the disdrometer function within a present weather detection instrument or by means of a dedicated rain-gauge instrument. Measurements from a present weather instrument require scaling by a factor which can be determined by means of the rainfall collected within a WMO standard collection vessel. This scaling will usually be completed by a Fugro engineer at the point of commissioning of a deployed system.

Solar radiation

This is measured by an instrument called a pyranometer which measures the amount of solar energy incident on the sensor. Solar energy is measured in units W/m². Any inclination of a pyranometer's sensitive surface away from the direction of incident sunlight directly impacts the measurements. According to client requirements, the output from a pyranometer can also be used to determine poor lighting conditions of an outside work area, an accepted contributing factor to workplace accidents.

Fugro are able to provide a great many transport methods for importing and exporting data to and from your Fugro Northstar™ system. Below are just some common examples. If you would like any further information regarding the available interfaces or to specify a proprietary interface for your partner systems then please contact us to discuss this in more detail.

northstar.support@fugro.com

OPC

When this function is requested and implemented the Fugro Northstar™ system publishes data through an OPC server hosted as part of the system. The most recent measured parameters and associated tags (unique strings to identify each variable) are held by the OPC server which must be interrogated by an OPC client in order to provide data. Data transfer is instigated by the client.

A deployed Fugro Northstar™ system hosts the MetOcean parameters defined by the client against tag names provided by the client. It is usual at the point of FAT or SAT to validate the correct operation of the OPC server by using an OPC client application (such as OPC Explorer) to verify that the correct values are available.

The Fugro Northstar™ system OPC server implementation is compliant with GA version 2.05a of the OPC specification. A standard configuration will update the data held by the OPC server at a 1 minute interval.

Modbus

The Fugro Northstar™ system can output data in Modbus format either using TCP/IP or Serial Connectivity. The Modbus system is commonly configured as a Modbus slave but we are also able to provide a Modbus master solution. The Modbus interface complies with version V1.1b3 of the specifications produced by the Modbus Organisation and is able to support both 2-byte integer and 4-byte floating point point registers.

NMEA

When specified, the Fugro Northstar™ system can generate standard or vendor-specific NMEA0183 sentences populated with realtime data as required. The system can output NMEA sentences to any connected serial (COM) port or as a TCP/IP client and/or host running on a defined port(s) on the localhost.

NMEA sentences can be output at any desired frequency. Note that if the requirement for custom NMEA sentences exceeds the maximum length defined within the NMEA0183 specification the maximum output frequency can be reduced. The interface is also able to generate and append the NMEA sentence checksum for each generated sentence if required.

ASCII text file

The Fugro Northstar™ system can generate standard ASCII text files of comma separated values at programmable intervals. These files can be written to any local or remote folder location that is accessible from the host system. The values in the file can be any of the parameters currently available within the system RTDB.

File Transfer Protocol (FTP)

The Fugro Northstar™ system is able to both send a receive and data files, such as text in CSV format, images, reports, etc. via FTP and SFTP protocol at specified intervals or events. Some examples that are currently in use include;

• Uploading raw ADCP data to a server ashore for further analysis.
• Automatically downloading and displaying meteorological reports within the user interface.
• Transferring third party radar images for archiving and display.

Dry contact relay

A relay system can be integrated to control external devices or provide alarm indications to existing onboard ICSS installations. A variety of options exist for normally open or normally closed relays and the system can be supplied to control multiple relays in one unit. A high power unit can be used to enable hihg load AC and DC devices to be directly controlled.

It is also possible for the Fugro Northstar™ system to read in active (wet contact) and passive (dry contact) switched digital signals from external devices and then record and act upon these conditions.

Extensible Markup Language (XML)

The Fugro Northstar™ system can be configured to send data at regular intervals using the XML protocol. This is useful for clients who wish to mirror their local data displays on our secure site at https://fugroweather.com so that the live data can be viewed remotely. Controlled access to the displays is possible via a credential based login if desired.

METAR

A METAR is a standard meteorological message normally sent at hourly or 3 hourly intervals. It consists of a coded summary of the local meteorological conditions and is used for aircraft operations. The Fugro Northstar™ system can generate standard METAR messages in accordance with WMO FM15-XII specification (as ammended) in both international and North American formats.

MANMAR WMO FM13

These are similar in scope to the METAR messages described above. The Fugro Northstar™ system can generate a MANMAR output as specified by the Canadian Government. This is based on the WMO FM13-X specification.

Anchor tension monitoring

A number of deployed systems aquire anchor-chain tension measurements either from Fugro supplied instrumentation or from a client's third party systems. A Fugro Northstar™ system allows simultaneous monitoring and display of chain tensions along with relevant metocean parameters such as wind, wave and current data.

Excursion monitoring

For mobile installations it can be useful to graphically plot installation and/or asset position information on a chart against a target location. Fugro Northstar™ systems can present position data in this way alongside anchor tension measurements.

Squall Warning System (SWS)

The Fugro Northstar™ system can be interfaced with a SWS to both display and predict the path of incoming squalls, asses their possible impact on operations and raise a visual or audible alarm dependent on certain criteria such as size, intensity and predicted proximity to ongoing operations.