MEASURING DYNAMIC MOVEMENTS OF A VESSEL NAVIGATING IN SHALLOW WATER USING PRECISE POSITIONING

Introduction

In 2007 the Swedish Maritime Administration performed an investigation of a merchant vessel dynamic movements in Lake Mälaren, Sweden.

The primary objective was to examine the dynamic draft when passing shallow water and to determine “squat”, as well as movements and draft increase caused by vessel turning and other types of dynamic forces.

For the necessary three dimensional precise positioning Network-RTK was used.

Objectives

Primary objectives:Draft increase at varying speed and at different depthsDraft increase caused by list while turning at varying speed and at different depthsDraft change, forward and aft (trim), at varying speed and at different depthsUnder-keel clearance based on existing depth information

Additional objectives:Accuracy analysis of sensorsAccuracy analysis of the internal transformation of offset positionsCalibration method of the survey systemProvide information for the development of systems for dynamic draft in real time.

Squat - Definition

Sinkage – Unrestricted space around the vessel- A vessel underway induces a relative water flow around the vessel hull- Friction and vessel underwater body produces a non-uniform flow – irregular pressure- Lower water level in close vicinity of the vessel – apparent downward vertical force- Trim change due to the irregular pressure on the hull

Squat – Limited space around the vessel- Water flow around the hull increases to allow the same water mass to pass- Increased flow - negative pressure – downward vertical force – increased draft (Bernoulli's principle)- If the water flow can not increase the vessel speed will be reduced- Squat affect the vessel at a depth below keel of approximate 5-7 times the vessel draft

Vessel speed is the factor that has the largest influence on the size of trim change and squat

PIANC, Technical Committee II, report 30, june 1997 (PTC II-30)

The Vessel

IMO number: 9207039

Call Sign: LANH5

Ice class: 1A

Loa: 116 m

Lload (7.75 m): 108.9 m

Beam: 15.96 m

Summer draft: 7.75 m

DW: 6875 M/T

GRT: 5831

NRT: 1750

Speed: ~18 kn

Ex. Baltic Viking, build year 2000 in Korea. NIS flag.Summer draft 7.75 meter => slack i cargo tanks at a maximum draft of 6.80 meter.Maximum 6.80 meter draft in Lake Mälaren

M/T BW Helen, LPG tanker, liquefied ammonium, 15 year charter for Norsk Hydro between Poland and Köping in Lake Mälaren. Two cargo voyages ~4000 ton per week all year.

Survey system and sensors

- 3D Position: GNSS RTK, 3 Topcon Odyssey RS (L1/L2, GPS + GLONASS)

- Network RTK service: GSM modem, Swepos (LM)

- Attitudes: MRU, TSS DMS-05 (Roll/Pitch)

- Position and data collection: PC, software Navipac NT (Eiva a/s Denmark)

- Heading: Calculated between two GNSS antenna positions in the software

- Transformation of coordinates: In the position software

- Offset position calculation: In the position software

- Tide gauges, Tide scales

Note: LM (Lantmäteriet) is the Swedish Land Survey (Governmental institution)

GPS SB

GPS PS

GPS Aft

CL

Instrument room

Swedish network RTK service

Lake Mälaren

Calibration of the survey system

Most important:- Internal coordinate system on the vessel

- Aligning of the MRU internal axis to the vessel longitudinal an transversal axis

- Calibration of the MRU roll/pitch angles using draft readings on the vessel

- Measuring of baselines between the GNSS antennas

- Connect the GNSS antennas and MRU locations to the vessel internal coordinate system

- Levelling of antenna heights and connect to a height reference system using a Benchmark on the jetty

- Connect the antenna heights to the vessel internal coordinate system

The route Södertälje-Köping

- Locker in Södertälje, lake level ~0.8 meter above Baltic Sea level

- Distance ~68 nm, 6-7 hours transit

- Two bridges which has to be opened

- Depths from 7.6 meters to more than 50 meters

Position transformation

External positions:- Reference system, geographical latitude/longitude/ellipsoid height, SWEREF99 (from WGS84)

- Transformation into chart coordinates (m), Easting/Northing, SWEREF99 TM (same as UTM33)

- Height reference system, Swedish national reference system RH2000

- Transformation of ellipsoid heigt to local height using Swedish national geoid model SWEN05RH2000

- Additional height reduction to Lake Mälaren reference level (Chart Datum)

Internal calculations:- Establish offset positions on the vessel hull using construction drawing

- Internal offset calculations (transformation of position from GNSS antenna to offset points)

- Logging of positions in offset points, 1/second

MethodHeight relations, calculation of height and RTK-tide

MethodDynamic draft and squat calculations

MethodUnder-keel clearance

calculation

Calculation and visualization in IVS Fledermaus

Data loggingPosition drop outsRed positions are accepted RTK fix solutions

Number of accepted positions (%)

Squat curves

X-axis: Static draft (m)

Continuous line: Forward, Dotted line: Aft

Dynamic draft=“Djg”, Under-keel clearance=“Ukc”, Speed=“Fart”

Dynamic draftX-axis: Draft interval (m)

Y-axis: No (#)

Under-keel clearanceX-axis: Clearance interval (m)

Y-axis: No (#)

Variation in bottom topography

X-axis: Time (HH:MM:SS)

Left Y-axis: Draft w.r.t. C.D. (m)

Right Y-axis: Depth w.r.t. C.D. (m)

Y-axis: Speed (kn)

Left Y-axis: Roll (degrees)

Right Y-axis: Pitch (degrees)

Y-axis: Heading (degrees)

C.D. = Chart Datum

Draft increase: roll and speed change

Error budget

Offset 1: Horizontal

Offset 1: Vertical

Total horisontal error in offset 1: 0.49 meter

Total vertical error in offset 1: 0.21 meter

Conclusions

- The result fulfils the primary objects according to the project plan

- The methods used has been working satisfactory

- Network RTK services provides precise positioning over long distances, hence it is now

possible to perform extensive investigations of vessel dynamics

- Availability of the Network RTK is insufficient (80%), poor GSM-connection

- Network services must be provided with more reliable data transfer

- The dynamics movements of a vessel are not limited to squat

- Roll, speed change and meeting vessels must be included in Fairway design

Project Mälarsquat, Swedish Maritime Administration

Complete report are avaliable for downloading at:

http://www.sjofartsverket.se/upload/Listade-dokument/_Rapporter/C%202008-1%20Malarsquat.pdf (1 MB)

http://www.sjofartsverket.se/upload/Listade-dokument/_Rapporter/Rapport_Mälarsquat_BILAGOR.pdf (16 MB)

Report are only in Swedish at the moment and appendices are separatet from the report