Fugro GEOS: Glossary

Captain FPSO | Glossary

Introduction

This document provides a glossary of measurements and presented data relevant to Fugro GEOS Meteorological & Oceanographic (MetOcean) Monitoring Systems. The details and capabilities of the sensors of any particular system are separately documented. This is a generic document for all Fugro GEOS MetOcean systems: most systems will comprise a subset of the measured and calculated parameters described below. Please contact geos.support@fugro.com if you require any additional functionality for an existing deployed system.

November 2015

▲ ▼ Wind Speed and Direction

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 Speed Corrected to MSL:	Measured wind speed, corrected^{^[1]} for height of the installed wind speed sensor to wind speed at 10m above Mean Sea Level (MSL)
Wind Gust:	The highest value of a 3-second rolling mean of recorded wind speed during the measurement period.
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 true north that identifies the source of the wind driven airflow. For moving installations, e.g. ships, wind direction is corrected for vessel course, speed and heading. For floating stationary systems, 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 can be configured to present data from a particular sensor based on a look-up list of wind direction sectors.
Notes:	It is usual for Fugro GEOS Meteorological / oceanographic monitoring to present 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 within the web page data displays.
Wind Severity Index (WSI)	A parameter used within helideck monitoring systems - equal to a rolling 10-minute mean value of wind speed corrected^{^[1]} from the elevation of measurements to an elevation 3m above the helideck surface.

▲ ▼ Temperature and Humidity

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 Fugro GEOS WeatherMonitor MetOcean software receives temperature measurement data at 1Hz.
Wet Air Temperature:	A parameter calculated^{^[2]} from the measured Dry Air Temperature and measured Humidity parameters: 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 less than or equal to the Dry Air Temperature.
Dew Point:	A parameter calculated^{^[3]} from the measured Dry Air Temperature and Humidity parameters. Dew Point 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 Dew Point is near the Dry Air Temperature. Dew Point 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.
Wind Chill Temperature:	Wind Chill, sometimes known as Wind Chill Factor, is a calculated^{^[4]} value: the perceived air temperature of exposed skin due to the flow of cold air. This can be a useful parameter in identifying conditions that add risk to outdoor working. Note that Windchill Temperature is only defined for temperatures at or below 10°C/50°F and wind speeds above 3 mph/ 5kmh.
Heat stress:	A parameter calculated^{^[5]} 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 any Fugro GEOS MetOcean system, thresholds between particular ranges of heat stress value can be configured in order to align with local (Black/Red/Amber/Green) colour indications of this parameter.

▲ ▼ Barometric Pressure

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 GEOS WeatherMonitor MetOcean software receives pressure measurements at 1Hz.

Mean Sea Level Pressure (QNH):

A function^{^[6]} of Ambient Pressure and the elevation of the pressure sensor, QNH is a calculated value of air pressure at mean sea level (MSL). The relationship between QNH and barometric pressure is illustrated in figure 1. The term ‘QNH’ derives from the aviation Q-code for "Near Horizon" or "No Height”.

Helideck Height Pressure (QFE):

A function^{^[7]} of Ambient Pressure, helideck elevation and pressure sensor elevation, QFE is the calculated value of air pressure at the elevation of the helideck. The relationship between QFE and barometric pressure is illustrated in figure 1. The term ‘QFE’ derives from the Aviation Q-code for "Field Elevation".

Notes:

One Hectopascal [hPa] is equivalent to One Millibar [mb]. Other units of atmospheric pressure are Inches of Mercury (InHg). The Fugro GEOS WeatherMonitor system provides pressure data in hPa unless specified otherwise by a client.

A MetOcean system deployed to a vessel or platform with a variable draft will include the functionality for the elevation of the pressure sensor and helideck location to be adjusted according to draft. Where draft information is provided to the Fugro GEOS system in real time, this is accomplished automatically. Alternatively, the system user can enter the current vessel or platform draft through a password protected data entry box within the system data display webpages.

Diagram:

QFE

QFE correction factor

Sensor elevation

QNH correction factor

QNH

Relationship between elevation and barometric pressure values for a semisubmersible platform

▲ ▼ Waves

Significant Wave Height (H_s):

This is 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 1/3 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.

Diagram


H_max	H_s	H_crest
Wave height Parameters

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 Tpeak is that it is 1/Fp where Fp is the peak of the spectra i.e. the Frequency at which the maximum energy occurs

Diagram


T_z		T_p
Wave Time Periods

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 measured as waves coming from the stated bearing.

Notes:

Unless specified otherwise by a client, a Fugro GEOS WeatherMonitor 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). Note: Calculations are repeated after each subsequent acquisition of 120 measurements (60 seconds), not a true one-for-one fifo of values. 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 GEOS can provide further details if required (contact geos.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 ‘normal’ 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^{^[8]}. When the measured Significant Wave Height (H_s) is below approximately 0.3m, the Fugro GEOS HMS/Met 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 GEOS MetOcean monitoring 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 GEOS WeatherMonitor application can interface to directional wave measurement buoys to provide wave direction information. Some Fugro GEOS MetOcean systems provide wave direction information from an x-band scanning radar based wave analysis system, based on image processing techniques. Contact Fugro GEOS for further technical details of this measurement method if required.

Spectral wave data

When specified, Fugro GEOS MetOcean systems with wave measurement functionality can provide wave data analysed within defied divisions in the wave-frequency domain. This is referred to as the provision of spectral wave data. Depending on customer requirements, spectral wave data may be plotted as an Frequency/Energy X/Y plot based on the previous calculation period of data, or may be periodically provided as tabular numeric data within formatted text data files.

▲ ▼ Water Levels

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 GEOS system in real time, the air gap calculations accommodate changing draft. For other floating systems, the system user can enter the current vessel or platform draft through a password protected data entry box within the system data display webpages in order that air gap data is correct.
Tide 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 customer 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_crestwhich is a measurement with respect to the mean water level identified within the wave measurement period.

▲ ▼ Water Current

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 non-rolling 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’ 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 non-rolling 10 minute window. This parameter is corrected for vessel course, speed, motion and heading in the case of moving or floating 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. The initial configuration of the current meter element of a MetOcean monitoring system
Current Profile:	This data is usually presented as a plot of Depth (Y axis) versus Current Speed and/or Current Direction. This plot shows the variability of current speeds and/or directions throughout the monitored water column.
Current Time Series:	This is a plot showing the variation of either measured current speed or current direction with time for a specific water depth.
Notes	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 maintenance to elimiate biofouling from acoustic transducers is important in order to maintain the quality of reported data.

▲ ▼ Cloud

Cloud Base 1:	The height of the lowest cloud layer above long term mean water level. It is usual to present this parameter in the non-SI units of feet.
Cloud Base 2:	The height of the bottom of the second distinct cloud layer above long term mean water level. It is usual to present this parameter in the non-SI units of feet.
Cloud Base 3:	The height of the bottom of the third distinct cloud layer above long term mean water level. It is usual to present this parameter in the non-SI units of feet.
Notes	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.

▲ ▼ Visibility and Present Weather

Horizontal Visibility:

This parameter, sometimes referred to as Meteorological Optical Range, or 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 will lower the inferred visibility.

Present Weather

Reported Present Weather is a calculated category derived from precipitation, visibility and temperature measurements. These three independent measurements together provide sufficiently data for an accurate evaluation of prevailing weather type. This parameter is presented as a category, either in United States National Weather Service (NWS) or World Meteorological Organization (WMO) code formats.

▲ ▼ Motion

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 (rolling) measurement window – 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 GEOS WeatherMonitor 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 against any web page 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) to (half the mean heave period in the measurement period) and has units of ms^-2
Pitch:	This measured parameter reports the rotation of a floating object around a port-starboard 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 system.
Roll:	Rolling involves side-to-side movement of the vessel. 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. Roll is measured in degrees. Positive roll occurs when the elevation of the starboard side of a vessel or installation decreases, ie “port-up”
Surge:	The backward and forward movement of the vessel. Positive values are forward and negative values are backward. Surge is measured in metres.
Sway:	The Port and Starboard movement of the vessel. Positive values are to Port and negative values are to Starboard. Sway is measured in metres.
Inclination	Inclination angle is the parameter used to report the maximum tilt of the vessel or system. It is calculated from both pitch and roll angles. The angle of maximum inclination is not reported.
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.
Significant Heave Rate (SHR)	This parameter is specified within the scope of UK-North sea helideck monitoring systems as twice the RMS of heave rate measurements in the preceding 20 minute window. In order to ensure that this is accurately represented, the sampling rate of heave rate should be ≥2Hz
Motion Severity Index (MSI)	This parameter is specified within the scope of UK-North sea helideck monitoring systems. It represents the ratio of helideck accelerations in the horizontal and vertical axes as a unitless number, and is defined as follows: - where xdotdot, ydotdot, and zdotdot are the instantaneous accelerations of the helideck in each of the three axes.
Notes	The HCA document “*Standard Measuring Equipment for Helideck Monitoring System (HMS) and Weather DataRev 8c 2010.09.10* ” states “Due to accuracy requirements on the acceleration measurements the motion sensing equipment has to be mounted directly under the helideck centre. Alternatively the motion sensing equipment can be mounted in the accommodation below the helideck within a maximum distance of 4 meters from the helideck centre. If a longer distance is used the MSI calculation will be affected.”

▲ ▼ Rainfall and Solar Energy

Rainfall

Measured as the amount of rainfall falling in a fixed time interval. It is measured in mm min^-1. 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 GEOS WeatherMonitor application 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 GEOS engineer at the point of commissioning of a deployed WeatherMonitor system.

Solar Energy:

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 Wm^‑2. 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.

▲ ▼ Configurable system Outputs

OPC	When this function is requested and implemented, the Fugro GEOS WeatherMonitor system publishes data through an OPC server hosted as part of the WeatherMonitor 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 GEOS WeatherMonitor system, hosts the MetOcean parameters defined by the client against tag names provided by the client. It is usual at the point of Factory or Site acceptance testing, to validate the correct operation of the OPC server by using an OPC client application (OPC Explorer) to verify that the correct values are “available”. The Fugro GEOS WeatherMonitor OPC server implementation is compliant with version 2.05a of the OPC specification. A standard configuration will update the data held by the OPC server on a 1 minute interval.
Modbus	The Weather Monitor system can output data in MODBUS format either using TCP/IP (Preferred Output) or Serial Connectivity. The MODBUS system can either be a MODBUS MASTER (unusual but possible) OR (more normally) a MODBUS SLAVE system Our MODBUS system complies with version V1.1b3 of the specifications produced by the MODBUS Organisation Fugro GEOS can provide further details if required (contact geos.support@fugro.com)
NMEA	When specified, the Fugro GEOS WeatherMonitor application can generate standard or so-called “vendor-specific” NMEA0183 sentences populated with realtime data as specified. The WeatherMonitor application can output NMEA sentences to any connected serial (COM) port, or as a TCP/IP client running on a client defined port on the localhost. NMEA sentences can be output at any frequency between XXX and YYY. 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 Fugro GEOS WeatherMonitor application generates and appends the NMEA sentence checksum for each generated sentence.
Text file	The Weather Monitor System can generate standard ASCII TEXT files of comma separated values at prgrammable intervals. These files can be written to any location accessible from the Host PC. The Values in the file can be any of the parameters currently being measured. The Text file can be formatted in different ways and Fugro GEOS can provide further details if required (contact geos.support@fugro.com)
Text file (FTP)	Any of the TEXT files created by the Weather Monitor System can be sent via in built basic FTP system. Fugro GEOS can provide further details if required (contact geos.support@fugro.com)
Relay	An external relay system can be installed to control external devices or provide alarm signals. A variety of options exist for normally open or normally closed relays and the system can be supplied to control 4 or 8 relays in one unit. A high power unit can be used to enable AC voltages of 5 Amps at 250 volts to be switched. Fugro GEOS can provide further details if required (contact geos.support@fugro.com)
XML	Weather Monitor can be configured to send data at regular intervals using the XML protocol. This system is used for clients who wish to display data on our UK Hosted server FugroWeather. Fugro GEOS can providefurther details if required (contact geos.support@fugro.com)
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 GEOS Weather Monitor System can generate standard metar messages in accordance with WMO FM15-XII specification (as ammended). Fugro GEOS can provide further details if required (contact geos.support@fugro.com)
MANMAR FM13	The Weather Monitor System can generate a MANMAR Outputas specified by the Canadian Government. This is based on the WMO FM13-X specification. Fugro GEOS can provide further details if required (contact geos.support@fugro.com)

▲ ▼ Examples of customer-specific bespoke functionalities

Anchor tension monitoring	A number of deployed systems aquire anchor-chain tension measurements either from Fugro GEOS supplied instrumentation or from client's third party systems. A Fugro GEOS WeatherMonitor system allows simultaneous monitoring of chain tensions with relevant metocean parameters such as wind, wave and current data.
Excursion monitoring	For movable installations, it can be useful to graphically plot installation position information as an X/Y plot against a target location. Some Fugro GEOS systems present position data in this way alongside anchor tension measurements.

▲ ▼ Footnotes

[1]	Using the calculation – Measured Speed(1/ ((Wind Sensor Height/10)^0.1255))* following the agreement reached by the North Sea Meteorological Panel of the UK Met Office. Note that the exponent 0.1255 reflects surface roughness and atmospheric stability. The value is referred to as the Wind Shear Exponent, and is approximated to 0.13 for UK-CAA specified helideck monitoring systems.
[2]	Wet air temperature calculations – Magnus-Tetens formula: Barenbrug, A.W.T., Psychrometry and Psychrometric Charts, 3rd Edition, Cape Town, S.A.: Cape and Transvaal Printers Ltd., 1974
[3]	Dew Point calculations – Magnus-Tetens formula: Barenbrug, A.W.T., Psychrometry and Psychrometric Charts, 3rd Edition, Cape Town, S.A.: Cape and Transvaal Printers Ltd., 1974
[4]	Wind Chill calculations – As per definition by the US NOAA's NWS (http://www.nws.noaa.gov/om/windchill/)
[5]	The computation of the heat index is a refinement of a result obtained by multiple regression analysis carried out by Lans P. Rothfusz and described in the 1990 National Weather Service (NWS) Technical Attachment (SR 90-23)
[6]	Ambient pressure correction to mean sea level is calculated by the application of a scaling factor defined by Kempe's Equation, provided by Shell Oil's metocean department in 1985
[7]	Ambient pressure correction to field elevation is calculated by the application of a scaling factor defined by Kempe's Equation, provided by Shell Oil's metocean department in 1985
[8]	This is discussed in greater depth in the WMO “Guide to Wave Analysis and Forecasting”