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.
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 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.
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.
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 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.
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.
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
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
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.
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 xddot, yddot, and zddot 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;
MMSMSI = 10 x tan-1 (MMSmeasured)
MSI = MMSMSImax (t - 20 min, t) x R
Helideck MRU location
According to the HCA
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.