“Interactive Dry-docking Simulator in a 3D Environment”

José Miguel Rodrigues e Carlos Guedes Soares

Lisnave – Estaleiros Navais S.A., 2901-901 Setúbal

Centre of Marine Technology and Engineering – Instituto Superior Técnico, Av. Rovisco Pais

1049-001 Lisboa

jose.miguel.rodrigues@lisnave.pt, guedess@mar.ist.utl.pt

Abstract

A computer program is presented which consists of an interactive 3D real time simulation of dry-

docking of ships in a specific repair shipyard. The flooding and emptying of the dock, as well as

of the ballast tanks are controlled by the user. The changing physical values can be visualized

and there is the possibility to save all the data to a file for post-processing and analysis.

The main data consists on the loads applied to the ship’s primary structure, the loads supported

by the keel blocks and, consequently, the load applied to the dock’s structure, and the

hydrostatic values of the vessel at each instance.

The majority of the hydrostatic calculations come from interpolation of the values present in the

hydrostatic tables of the ships. The structural analysis is done considering the ship as a beam

with constant structural characteristics throughout its entire length. The loading calculations on

the keel blocks are based on semi-empirical formulae applied by the mentioned shipyard.

A few qualitative conclusions originating from the obtained results are shown and there are

suggestions concerning the future development of the program.

The shipyard mentioned is Lisnave – Estaleiros Navais, S.A. located at Mitrena – Setúbal.

1- Introduction

The overall objective of this paper has its origin in the intention to develop a computer program

capable of simulating the dry-docking operations of ships in a three-dimensional interactive

environment.

The starting point is the repair shipyard “Lisnave Estaleiros Navais S.A.” located at the Mitrena

peninsula in Setúbal.

Being the fundamental actors of these kind of operations, one of the goals was to accurately

model the various dry-docks, as well as the keel blocks, ships and respective ballast tanks.

Although at this first approach the mechanical equipments present (dock pumps, valves, etc.)

are not subject to detailed description, being on this case considered in terms of their average

operational characteristics, it should be noted that in future versions of the program these will

have to be implemented and calculations must be performed on a real-time basis so as to

comply with extreme situations.

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Nevertheless the detailed implementation of the equipments has to be limited to the

fundamental aspects of the operation and where there is an actual need for it, otherwise an

excessive intricate system will have an unnecessary cost on development and execution speed.

In conclusion it is intended for this program to be a prototype, a flexible, relatively simple

approach to the problem, which can be later refined and optimized according to some particular

aspect of the dry-docking operation, having nevertheless the ability to serve as a basic tool for

analyzing such operations even if only in a qualitative point of view.

2 – Calculations and other technical aspects in dry-docking of ships

2.1 – Ship maintenance and dry-dockings

Once it has been built, every ship, according to its classification society, port of register and the

owner, has its own maintenance plan. This plan is made of a series of scheduled visual

inspections, equipment tests, etc. as well as dry-dockings. Also the paints used in the hull

treatment and in ballast tanks have their own time limited warranty and the renew times usually

coincide with the dry-docking intervals delineated by the classification society. With the

exception of serious damages or other major problems that may arise while the ships are

operating, these are the only times that the ships dry-dock. Nevertheless the normal dry-docks

have to be carried out for the entire life of the ship.

2.2 – Types of dry-docks

There are mainly three types of dry-docks used in major shipyards, these are the graving, the

floating and the platform dry-dock.

The graving dry-dock consists on a deep well carved on the terrain, adjacent to the exterior

body of water. The bottom of the dock is several meters bellow the exterior waterline and has a

gate that makes the connection between the exterior and the inside of the dock. This gate is

serves to prevent the water from the exterior to flood the dock and is built to withstand the

pressure from the water, being therefore the most critical equipment of the dry-dock.

The floating dry-dock consists on a floating platform with several segregated ballast tanks that

enable the entire structure to immerge/emerge several metres and have a precise control on the

trim and list. It typically has a U-type configuration and it is designed to allow for ships to come

in the way of the central space while it is submerged. Then, while emptying the ballast tanks,

the dock emerges and when the operation is complete the dock’s floor is above the waterline

and the ship is completely supported on the blocks.

The platform consists on a basin similar to the graving dock but with its bottom level above the

exterior waterline. It therefore needs some adjacent basin based on the same concept as the

graving dock which is able to enclose the ship on a closed body of water and, through the use

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of pumps, elevate the waterline to the platform’s height, allowing the ship to enter inside and

then sit on the blocks.

2.3 – Docking processes

As Lisnave only has dry-docks of the platform and graving type, here it’s presented a brief

description of the operation sequence of the docking of ships on these types of dock alone.

Also, we will focus only on docking, as the un-docking is similar but with the inverse sequence.

In the graving dock (Fig. 1), once the berth arrangement has been completed, the dock is

flooded by gravity through the valves that communicate with the exterior water. Later, when the

tide change is reached, the gate is lowered. The ship can now (or on the next tide – depending

on the ship’s draft condition) enter the dock and be positioned in the way of the keel blocks.

Once the gate is closed, the water is pumped out and gradually the ship is seated on the blocks

until the dock is completely emptied. An important note is that there is the need to gradually de-

ballast the ship while the dock is emptied so as to prevent overloading on the blocks, on the

dock’s floor, and on the structure of the ship.

Figure 1: 1 - dock flooding by gravity,

2+3 - gate opening, 4 - pumping out water,

5 - seating on blocks

Figure 2: 1 – adjacent dock flooding by gravity,

2+3 - gate opening, 4 – pumping in water,

5 – going inside dock and seating on blocks

At Lisnave, the platform type dock (Fig. 2) that exists is adjacent to a graving dock, serving this

one as the intermediate step to raise the ship to the platform level. The sequence of the

operations is similar to the graving dock with the exception of the additional step where the ship

is transferred to the platform type dock and the fact that in the dock is emptied through valves

by gravity to the adjacent graving dock.

A somewhat particular case of dock exists at Lisnave, it is the case of the Hydrolift. However

this equipment operates on the same principle as the platforms, being here the special case of

having a common basin that serves three platform docks at the same time.

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2.4 – Variables and critical factors

There are several factors that are critical to the success of a dry-docking operation. The limited

allowable load on the keel blocks and on the dock’s floor and its structural beams, as well as on

the ship’s structure are to be taken in consideration. Also one must control the stability of the

ship (especially on ships having a significant hull dead rise such as tall ships) and obviously the

proper alignment of the vessel with the berth.

A particular critical factor that is commonly overlooked is the risk of having the ship with a

precarious contact with the keel blocks. That situation may give way to the tumbling of blocks

and more importantly lead to catastrophic consequences if the mooring ropes have already

been removed.

Finally the tide, as has been presented in section 2.3, serves as the time restriction factor.

2.5 – Berth arrangement and lightweight estimating

A risk free berth arrangement should always be based on the shipbuilding yard’s “Docking

Plan”, however, especially on older ships this element is not to be found and so the elements

considered as a minimum base that are required from the owner are:

- Lightweight longitudinal distribution

- General Arrangement

- Midship Section

- Capacity Plan

- Shell Expansion

When the lightweight is not given, there are semi-empirical tables that are used so as to

estimate these values. The determination of its longitudinal distribution then follows these

formulae:

mtL

sConsumableLWW

ER

ER

25.0(1)

mtL

BallastoCLWW

CZ

CZ

arg70.0(2)

mtL

BallastLWW

FP

FP

05.0(3)

Where LW is the Light Weight, Consumables is the sum of fuel, fresh water and other main

consumables, Cargo and Ballast are the sums of the cargo and Ballast weights respectively.

WER and LER, WCZ and LCZ, and WFP and LFP represent the weight and Length on each of the

Engine Room, Cargo and Fore Peak areas.

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The outline of the berth takes into consideration the location of the ship’s bulkheads and

carlings, where the blocks are to be positioned. Also there is a special attention to the

distribution of ballast and other masses on board so as to reinforce the ship’s support in the way

of these.

2.6 – Ship’s hydrostatics

In the process of docking, three distinct sequential conditions take place, the first is when the

vessel is floating free, the second is when it is in contact with one, and only one, block, and

finally when the ship is seating on the blocks throughout its length.

In the first condition the instantaneous hydrostatic values are the ones that arise from the

application of the general Naval Architecture hydrostatic theory. In the second condition, one

can consider the reaction force acting on the ship at the point of contact as a negative mass to

be added to the ship’s “free displacement” and then apply the usual equations.

0.100 ushipfree

CPdockCP dttF , with t in meters (4)

Where F is the reaction of the block, t is the draft and du represents the unitary displacement.

When the ship is completely seated on the blocks the hydrostatic calculations are carried out by

interpolation of the hydrostatic data table’s values only, dependent simply on the instantaneous

draft.

2.7 – Structural analysis

The ship is considered as a beam with constant structural characteristics throughout its length,

namely the Young’s Module and Moment of Inertia.

With the ship floating free, the longitudinal moment distribution and shear force is calculated

using the ship’s Bonjean Curves. The ship-beam is considered as a free-free beam with vertical

transverse loading coming from the sum of the positive longitudinal weight distribution and the

negative buoyancy distribution.

When the vessel is in contact with the first keel block, the beam is now a simply supported-free

beam with the addition of the block reaction on the ship’s hull at the point of contact.

However, when the ship is completely seated on the blocks, a hyperstatic system arises that,

due to the impossibility of downward forces acting on the hull by the blocks, makes the use of

the classical hyperstatic methods completely unrealistic.

Therefore, on this first approach, the method implemented is based on the semi-empiric

considerations used by Lisnave’s shipyard when producing the berth. This means that it is

considered the segregation of the ship in three distinct zones – Engine Room, Cargo Space,

Fore Peak Tank – with the loads applied on the keel blocks located at each of these areas

equal, given by the equations (1), (2) and (3), where these values are multiplied by each zone

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length, then divided by the number of longitudinal locations of blocks and finally divided by the

number of transversal blocks put at each of these longitudinal locations.

Figure 3 – Moment and Shear

Force on the free-free ship-beam

Figure 4 – Moment and Shear Force

on the supported-free ship-beam

3 – Code’s architecture

The code is written in C# and uses the XNA 2.0 – Game Studio Express. This revolutionary tool

enables a much easier development of 3D visual real-time applications when compared to

others, particularly when it comes to graphic content integration with the application’s logic.

The program’s code architecture is based on a real world entity to class correspondence as is

good practice when coding in an object oriented programming language.

Figure 5 – Hierarchical scheme and relations of classes

In Figure 5 it is represented the hierarchical scheme of the main classes as well as their

relations with each other. InputEventSystem and WindowSystem are frameworks that provide

the user interface controls, with UserInterface acting as the connection between these and the

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main program. Game1 is the main class of the program and there are the SceneManager,

MasterManager and Camera classes, which execute parallel with Game1 and provide scene,

real-time update and 3D manipulation control respectively.

4 – Program execution

The next figures show some selected images of the program running.

Figure 6 – Welcome screen Figure 7 – 3D environment

Figure 8 – Ballast control

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Figure 9 – Ship’s structural analysis

Figure 10 – Ship’s hydrostatic values Figure 11 – Block loading

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5 – Conclusions and future development

A prototype of a tool simulating dry-dockings was developed and the respective physical

concepts inherent to these operations have been successfully implemented.

The multi-disciplinary nature of such work implies a great deal of knowledge and time spent

regarding the creation of the various elements present, therefore it is clear that these kind of

projects can hardly be carried out by a single person having the need for a development team.

No actual precise tests have been made to the application as it still requires proper calibration

and validation, nevertheless the simulation of the docking of an actual ship being repaired at

Lisnave using the respective berth arrangement provided by the shipyard revealed a clear over

dimensioning in respect to the number of keel blocks being used even by using the same

conservative approach regarding the loading on the blocks as Lisnave does. If we take into

consideration that this simulation was carried out without discharging any ballast (the berth

provided had the restriction of the ship having no ballast whatsoever with the dry-dock

completely empty), it clearly suggest that there is a need for optimization regarding this subject.

It’s a fact that the application developed is presently in an initial prototype state, the

improvement regarding the modeling and visual effects of the various entities present should

dramatically transform the “professional feel” of the program.

On the calculations side, the possibility to comply with unorthodox draft and list conditions, as

well as with ships having major bottom shell damages would definitely be a plus.

However, the critical aspect that needs improvement, even without any expansion of the

program current functionality is the process of the calculation of the load applied to each block.

A solution must be implement that is able to consider the blocks with a deformable top surface

and that takes into account the actual, or at least a better approximation, to the real weight

distribution throughout the ship.

Finally one must say that the future of this work is dependent on the possible real world

practical applications that might be specified by third parties.

6 – References

1 – “Beginning XNA 2.0 Game Programming: From Novice to Professional”, A. S. Lobão,

Apress, 2008;

2 – “Texto de Apoio para Estruturas Navais I – Solicitações em águas tranquilas, Vol. 1”, Y.

Garbatov, 2001;

3 – “Mechanics of Materials”, F. P. Beer, E. R. Johnson Jr., McGraw-Hill, 2001;

4 – “Considerações Técnicas para Docar Navios – Formação de Docagem (Definição do Berço

para Assentamento”, D. Robalo, T. Carvalho e C. Rodrigues, Lisnave, 2008.