Proceedings of the 6th International Conference on the Application

of Physical Modelling in Coastal and Port Engineering and Science

(Coastlab16)

Ottawa, Canada, May 10-13, 2016

Copyright ©: Creative Commons CC BY-NC-ND 4.0

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ACCURATE SEAKEEPING MONITORING FOR FLOATING STRUCTURE DEPLOYMENT

DAVID BLANCO1, LUCÍA MENESES1, ÁLVARO ÁLVAREZ1, RAÚL GUANCHE1, IÑIGO J. LOSADA1,

MARÍA F. RODRIGUEZ DE SEGOVIA2, MANUEL RUIZ2, MIGUEL A. MARTÍN2, MARIA JOSÉ CONDE2,

FRANCISCO ESTEBAN2

1 Environmental Hydraulics Institute - IH Cantabria, University de Cantabria, Santander, Spain, blancod@unican.es,

menesesl@unican.es, alvaro.alvarez@unican.es, guancher@unican.es

2 Infrastructures Area, R+D Department, FCC Citizen Services, Madrid, Spain, mfrodriguezs@fcc.es, manueljaime.ruiz@fcc.es,

mamartinn@fcc.es, mconde@fcc.es, festeban@fcc.es

ABSTRACT

The seakeeping of floating bodies is not only related with environmental agents (waves, winds and currents), but also

with factors such as their interaction with mooring systems or even human decisions, the latter being difficult to manage and

often responsible for the success of the operation. State of the art technologies allow for accurate, real time monitoring and

tracking of floating bodies’ behavior, ensuring safety during all stages of the operations they are involved in.

For this reason, the scope of this study focuses on the development of a device that will lead to a greater knowledge of

the aspects that determine the behavior of floating bodies, in order to identify them with more clarity and improve operational

thresholds. In this case, the test structure is a floating caisson and the measurement instrument is a GNSS/INS based device.

KEWORDS: GNSS, INS, RTK, CAISSON, FLOATING STRUCTURE.

1 INTRODUCTION

This paper shows how a GNSS/INS based device attached to a floating structure can be used to learn about its behavior

and, hence, assist operators in minimizing human factors in an upcoming future. Moreover, unmanned and reliable

deployment is a big challenge that can be overcome with on-the-fly fusion of precise attitude determination and automation

techniques. This work has been developed within the DOVICAIM project (www.dovicaim.ihcantabria.com, 2015), a

collaboration between IH Cantabria and the construction company FCC CO. Since one of the main areas of expertise of FCC

CO is the use of reinforced concrete floating caissons in maritime works, the research concentrates on how the system and

instrumentation developed are applied to track and evaluate the behavior of these floating structures. A brief description of

the mentioned device and the tested caisson is included in the following chapters.

1.1 Measurement technology

Inertial Navigation Systems (INS) come in all shapes and sizes but one thing they have in common though is the use of

multiple inertial sensors, and some form of processing unit to keep track of the measurements coming from those sensors. An

INS comprises two-distinct parts; the first is the Inertial Measurement Unit (IMU). This is the collective name for the

accelerometers and gyros that provide acceleration and angular velocity measurements. The second part is the navigation

computer. The navigation computer takes measurements from the IMU and uses them to calculate the relative position,

orientation and velocity of the INS. By taking measurements along (and about) the x-, y- and z-axes, the navigation computer

can understand how it is moving and rotating. However, an INS generates measurements relative to their last known position

which means that: first, the absolute position is not known and, second, the inaccuracies are accumulative and hence the

position error increases with time. Figure 1 shows a simplified example of how an INS estimates its position and attitude.

(OxTS - Oxford Technical Solutions, January 2016).

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Figure 1. Simplified 2D example of how an INS works.

This trouble can be fixed if a Global Navigation Satellite System (GNSS) receiver provides the absolute position to the

INS. Moreover, if, instead of one GNSS receiver, two receivers are used then it is possible to measure true heading as well.

For INS, true heading can be a difficult quantity to measure accurately, especially in dynamic conditions. The earth’s magnetic

field can rarely be relied on; gyro-compasses are expensive and can be upset by high dynamic motion. In addition, nowadays

GNSS receivers have improved a lot their performance and availability thanks to the multi-constellation networks (GPS,

GLONASS, GALILEO, BEIDOU), Satellite Based Augmentation System (SBAS) and ground stations corrections (DGPS,

RTK). The above features added by GNSS technology are depicted in Figure 2. (ADVANCED NAVIGATION, January

2016).

Figure 2. True heading with two GNSS receivers (left); Multi-constellation GNSS RTK receiver (right)

From the paragraphs above can be easily deduced that high performance current navigation systems rely on both INS

and GNSS technologies in order to ensure accurate, stable and reliable data.

1.2 The floating structure

The floating caissons are large structures that by their lightened cross section - multicellular - can float once completed.

That gives them great versatility in construction (by sliding concrete), floating transport and final positioning. Floating

caissons are precast concrete box-like structures that can reach more than 10,000 m3 of concrete.

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Typical infrastructures that use this type of caissons are the docks and other berthing structures, the vertical breakwaters,

etc. This floating structure is a widely used type in maritime works of Spanish ports. See Figure 3 as an example.

Figure 3. Floating caissons construction (Açu, Brazil). FCC CO.

2 THE MEASUREMENT INSTRUMENT

The developed device has been enclosed in a compact and robust cabinet which comprises the following elements (Figure

4 shows a simplified schematic of how they are interconnected):

1. Miniature GNSS/INS & AHRS system for attitude and position determination;

2. Tri-axial anemometer for wind load estimation;

3. Low power industrial computer for data logging and monitoring;

4. UPS (Uninterruptible Power Supply) for autonomous operation.

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Figure 4. Main components of the instrumentation device.

2.1 GNSS/INS & AHRS system

This device is the core unit of the whole system. It is a miniature GNSS/INS & AHRS system that provides accurate

position, velocity, acceleration and orientation under the most demanding conditions. It combines temperature calibrated

accelerometers, gyroscopes, magnetometers and a pressure sensor with a dual antenna RTK GNSS receiver. These are coupled

in a sophisticated fusion algorithm to deliver accurate and reliable navigation and orientation. Table 1 shows its main

specifications in terms of accuracy. It can output data at a rate up to 1KHz (configurable) and the GNSS receiver supports all

of the current and future satellite navigation systems, including GPS, GLONASS, GALILEO and BeiDou. It also supports

the Omnistar service for hassle free high accuracy positioning apart from all today available SBAS systems. Resilience against

corrupted GNSS signals is assured through the Receiver Autonomous Integrity Monitoring (RAIM) technology.

(ADVANCED NAVIGATION, 16th April 2015).

Table 1. Main specifications of the GNSS/INS & AHRS unit.

Feature Accuracy

Horizontal position 0.008m

Vertical position 0.015m

Velocity 0.007m/s

Roll & Pitch 0.15º

Heading 0.1º

The unit also comes with a powerful communication interface, including two RS232 ports with software configurable

data packets and two General Purpose Input Output (GPIO) pins with configurable functions to ensure compatibility with

external sensors or systems.

Finally, it supports a wide operating voltage (9 to 36V) and requires less than 3W as nominal power consumption. All

assembled in a robust and small enclosure.

2.2 Tri-axial anemometer

Wind loads has an important impact on the behavior of floating structures so it is important to know the wind magnitude

and direction in order to correlate it with the data coming from the navigation system. For that purpose, an ultrasonic

anemometer has been included as part of the instrumentation. It has been attached to the instrumentation cabinet trying to

keep the size of the whole system and the installation requirements as small as possible, but at the same time setting the wind

measurement point at a distance quite above the cabinet to avoid turbulences from the surroundings objects. The main

12V Battery

+12V

GND

Coaxial

RS-232

GNSS/INS AHRS - Navigation system

Navigation system main unit

GNSS Antennas

Instrumentation cabinet

GNSS antennas mounting arm

DC-DC Regulator

Triaxial anemometer

Low power industrial computer

Charger - PS230V AC input

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specifications of the anemometer are shown in Table 2. (Gill Instruments Limited, 7th June 2011).

Table 2. Main specifications of the ultrasonic anemometer.

Feature Value

Wind speed range 0 – 45 m/s

Wind speed resolution 0.001 m/s

Wind speed accuracy < 1.5% RMS

Direction range 0 – 359.9º

Direction resolution 0.1º

Direction accuracy 2º

Power requirement 9 -30 V 0.6W

Data rate Up to 20 Hz

Interface RS232/422/485

Ultrasonic technology for wind measurement has as an advantage the fact that there are no moving parts which means

there is no need to worry about maintenance and recalibrations as usually happens with cup anemometers. On the other hand,

ultrasonic anemometers often have difficulties resolving wind speed and direction when heavy rain is present.

2.3 Industrial computer

The computer has two primary tasks:

Data logging;

Communications management.

The data delivered by the navigation system and the anemometer through the RS-232 serial ports is properly formatted,

compressed and stored in the computer’s Solid State Disk (SSD).

The communication system provides two alternatives depending on the scenario. The first one, and the most common

nowadays, is based on the 3G/4G cellular network which has excellent radio coverage, good data transfer rates and affordable

price for reasonable amounts of data. The second one, as a backup or whenever is possible to use, is based on Wi-Fi which

allows the integration of the system in existing Wi-Fi networks or to create a point to point ad-hoc Wi-Fi link between two

devices (laptop and computer for instance) at zero cost. The Wi-Fi option is very useful when a Wi-Fi network already exists

in the floating structure to monitor, or for remote access and/or data download from the surroundings.

On one hand, it must be ensured that the computer has enough processing power to handle the data logging and manage

the communications or other tasks needed in the system. On the other hand, having in mind that must be an autonomous

system and that it has a high impact in the power consumption budget, a trade-off between power and performance is

mandatory. Finally, reliability is another issue to deal with when computers are deployed on the field.

In this case, the computer is equipped with a fanless Intel Atom N2600 1.6GHz dual core processor embedded in a

rugged-design box and demands less that 10W of power. Its main specifications are shown in Table 3.

Table 3. Main specifications of the industrial computer

Feature Value

CPU Atom N2600 1.6GHz

Memory (RAM) 4GB DDR3-1333

Storage 128GB 2.5”SDD

Operating System Windows 7 Pro

Cellular card 6-band HSPA

Wi-Fi card USB dongle

USB 6 x USB 2.0

Serial 3 x RS-232 / 1 x RS-232/422/285

Ethernet 2 x LAN 1Gbps

GPIO 10 x 5V

Power requirement 12V@3A

2.4 UPS

The power supply has to guarantee that the instrumentation system works without interruption during all the

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measurement campaign. In this case, since the duration of the deployment is known, it is enough to dimension properly the

capacity of the battery. In other scenarios, it may be needed to add a solar panel to enable real autonomous operation. The

UPS is comprised of three main components:

12V battery, as main power source;

Charger – power supply, for external power source conditioning;

DC-DC regulator, as isolator and to keep always the same voltage on the devices.

2.5 Enclosure

All the elements introduced in the previous sections have been assembled in a small cabinet equipped with a front door

to facilitate maintenance tasks and system operation. Figure 5.

Figure 5. Outside and inside views of the cabinet

3 THE APPLICATION CASE

The application case is based on the tracking of a floating caisson. The caissons are manufactured in Santa Cruz de Tenerife

and then transported to Granadilla within the Canary Islands (Spain).

3.1 Installation

The first task to address is to install the measurement instrument (the cabinet) on the caisson. For that purpose, a custom

structure was manufactured to fix the cabinet on one of the walls of the caisson. The installation sequence is as shown in

Figure 6:

1. Installation of the cabinet on its mount support.

2. Lifting of the cabinet from the dock over the caisson;

3. Fixing to the caisson wall;

4. Installation of the anemometer.

5. Opening the cabinet’s door and powering up.

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Figure 6. Instrumentation cabinet installation sequence

Once the sequence above is finished, the cabinet connects to the cellular network by default and remote access is possible to

activate the data logging, configure the RTK station, etc.

3.1 Axis definition

The cabinet is installed so that the X axis points the prow of the caisson (the direction of movement during the transport).

Figure 7.

Figure 7. Caisson’s axis definition.

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3.2 Measurement results

This chapter shows some of the main parameters monitored during the transport of the caisson. They are defined as follows

(positive direction indicated by the arrow in each axis of Figure 7):

Roll. Angle around the X axis.

Pitch. Angle around the Y axis.

Heading and wind direction. Angle around the Z axis.

The route followed by the caisson towed by a tugboat (see Figure 8) is presented in Figure 9 together with the distribution

of Speed Over Ground (SOG).

Figure 8. Caisson towed by a tugboat leaving the dock.

Figure 9. Route followed by the caisson (left) and SOG (right).

The roll and pitch data is presented in Figure 10. The graphs show the sample distribution during the transport. As it can be

seen, the standard deviation for the roll and pitch are around 0.12 and 0.28 degrees respectively, while the maximum values

are within ±0.4º for roll and ±1º for pitch. Below these graphs, in Figure 11, the power spectrum shows how the roll has a

19.2s oscillation period while the pitch is around 11.2s.

Finally, the apparent wind speed along all the route is presented in Figure 12. The wind data has been analysed in periods of

10 minutes, the average speed, burst speed and direction for each of those periods are shown in the figure.

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Figure 10. Roll (left) and pitch (right) distribution during the transport.

Figure 11. Roll (left) and pitch (right) power spectrum as a result of the transport.

Figure 12. Apparent wind speed

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4 CONCLUSIONS

This work shows how accurate seakeeping monitoring of floating structures is possible thanks to the combination of GNSS

and INS technologies. State of the art devices using this technology at reasonable costs foster its use during the deployment

phase and provides invaluable information that may be used for the improvement of procedures or even automation techniques

implementation for futures unmanned operation.

Obviously, the data collected by the GNSS/INS device must be correlated with external agents such as wind, waves, currents,

mooring lines forces, etc., to fully understand the behaviour of the floating structure and, hence, be able to address

improvements.

ACKNOWLEDGEMENT

DOVICAIM project and, hence, this work is supported by the “Ministerio de Economía y Competividad” of Spain within the

call “Convoctoria RETOS 2014, RTC-2014-3077-4”. The authors gratefully acknowledge the Spanish ministry funding

received to carry out this research.

Thank you as well to all FCC CO team since they have facilitated the required resources and actively collaborated during the

tests in order to come this measurement campaign true.

REFERENCES

ADVANCED NAVIGATION, Copyright © 2015. POSITIONING EVERYWHERE, [Online]. Available:

http://www.advancednavigation.com.au/product/spatial-dual [January, 2016].

ADVANCED NAVIGATION, 16th April 2015, Spatial Dual Reference Manual, Version 2.2.

DOVICAIM project, 2015. “DOVICAIM: Metodología para el Diseño y Optimización del ciclo de Vida de CAjones en Infraestructuras

Marítimas”, [Online]. Available: http://dovicaim.ihcantabria.com [October, 2015].

Gill Instruments Limited, 7th June 2011, WindMaster User Manual, 1561-PS-0001, Issue 07.

OxTS - Oxford Technical Solutions, Copyright © 2014. OXTS Inertial+GPS, [Online]. Available: http://www.oxts.com/technical-notes/

[January, 2016].