Offshore discharge study

OFFSHORE DISCHARGE STUDY

FINAL REPORT

Document number: Final report 1.0

Students:

Robin van Hattum 0816549 Maarten Nijenhuis 0806925

Study:

Civil engineering

Specialization: hydraulic engineering

Date:

June 2013

Cosco Shipping Europe B.V. Strevelsweg 700, App. 608/609 3083 AS Rotterdam, the Netherlands

Tel.: +31-10-240 4777 www.coscoht.com

Rev. Description Date Prepared Reviewed

C Final 12-06-2013 MN RH MB LG B Concept 28-05-2013 MN RH MB LG A First issue 21-05-2013 MN RH MB LG

OFFSHORE DISCHARGE STUDY

FINAL REPORT

Document number: Final report 1.0

Students:

Robin van Hattum 0816549 rvanhattum11@gmail.com Tel.: +31-6 52 17 72 39 Maarten Nijenhuis 0806925 maartennijenhuis8@gmail.com Tel.: +31-6 11 08 75 09 Study: Civil engineering

Specialization: hydraulic engineering

Company:

Cosco Heavy Transport

Strevelsweg 700, App. 608/609 3083 AS Rotterdam, the Netherlands Tel.: +31-10 240 4777 www.coscoht.com Mentor: Marc Beerendonk mbeerendonk@coscoht.com Tel.: +31-6 15 87 72 92 University Rotterdam University G.J. de Jonghweg 4 - 6 3015 GG Rotterdam Tel.: +31-10 79 44 801 First reader

Leo van Gelder l.a.van.gelder@hr.nl Tel.: +31-6 50 43 57 69 Second reader William Kuppen w.j.j.m.kuppen@hr.nl Tel.: +31-10 79 44 911 Date: June 2013

SAMENVATTING

Voor het transport van zware en grote ladingen kunnen half afzinkbare schepen worden gebruikt. Door het afzinken van het schip kan de drijvende lading worden gelost.

Tegenwoordig wordt de losoperatie alleen uitgevoerd op een beschutte locatie. Dit wordt gedaan, omdat het gedrag van het schip en de lading in de deining onbekend is. Daardoor is een offshore losoperatie tot op heden nog nooit uitgevoerd.

Deze studie onderzoekt of het mogelijk is om tijdens een offshore losoperatie de krachten die door deining ontstaan te reduceren. Om dit te bepalen zijn ontwerpcriteria opgesteld die resulteren in een ontwerp ‘lading’ en een ontwerp ‘schip’. Ook is een onderzoek gedaan naar de karakteristieken van de deining. Voor deze studie is een maximale deininghoogte van 2.00m beschouwd.

Tijdens de losoperatie komt de lading los van het schip. Dit resulteert in een kleine ruimte tussen de lading en het schip. Het gedrag van het water in deze ruimte is zeer complex. Het gedrag kan worden benaderd met behulp van de multi-domein theorie. Echter is deze theorie een benadering van de werkelijkheid, waardoor het leidt tot onnauwkeurige resultaten. Daarom zal het gedrag van het water in deze ruimte buiten beschouwing worden gelaten.

Over de bewegingen van een drijvende lading boven een schip is weinig bekend. Om inzicht te krijgen in deze bewegingen is een modeltest uitgevoerd. De modeltest is gebaseerd op visuele observaties en is alleen gebruikt om een beter inzicht te krijgen in deze bewegingen. In de modeltest zijn de lading en het schip afzonderlijk en in combinatie getest. De resultaten van de modeltest geven een reductie van bewegingen weer. Omdat de modeltest is

gebaseerd op visuele resultaten is het niet mogelijk om een reductiecoëfficiënt toe te passen.

Met de resultaten van de modeltest is een krachtsberekening gemaakt. Hierin zijn statische en dynamische krachten vastgesteld. De statische krachten zijn gelijk aan het totale gewicht van de lading. De dynamische krachten zijn de krachten die ontstaan tijdens het lossen door de tegengestelde beweging van de lading en het schip. Deze bewegingen zijn in de berekening in tegenfase beschouwd en zijn daarom conservatief. Voor de berekening is een deining met een periode van 12.5[s] en een hoogte van 2.00m gebruikt. De optredende statische krachten zijn 189,245kN en de dynamische krachten 42,901kN.

De benodigde vrije ruimte tussen de lading en het schip voor een veilige offshore losoperatie is vastgesteld op 2.00m. Dit is gebaseerd op de theoretische achtergrond en twee

verschillende modeltesten.

Om vast te stellen of er een oplossing bestaat voor het reduceren van de krachten die door de deining ontstaan, zijn verschillende potentiële oplossingen beschreven. Door middel van een Multi Criteria Analyse (MCA) zijn de oplossingen getest op hun potentie. In de MCA zijn de volgende criteria gebruikt: risico’s, uitvoerbaarheid, milieu, duurzaamheid en kosten. Op basis van hun belangrijkheid zijn de criteria gebonden aan een vermenigvuldigingsfactor. De resultaten van de MCA zijn besproken met COSCO Heavy Transport. Tijdens de

vergaderingen zijn drie oplossingen geselecteerd die meer in detail zijn uitgewerkt. Dit zijn het ‘jetsysteem’, de ‘hydrauliek’ en de ‘cones’. Dit wijkt af van de resultaten die zijn verkregen door de MCA. De ’drijvende golfbreker’ en de ‘wave dragon’ worden niet verder onderzocht vanwege de verwachte moeilijkheden voor de uitvoerbaarheid.

Een onderzoeksvraag voor de drie potentiële oplossingen is vastgesteld om de haalbaarheid van de oplossingen vast te stellen. Wanneer de conclusie negatief is, zal de oplossing is niet verder worden onderzocht.

Het jetsysteem wordt geïntegreerd in het dek van het schip. Tijdens de offshore losoperatie, wordt de ontstane ruimte gevuld met water dat onder hoge druk staat. Dit zorgt voor een opwaartse druk op de onderkant van de lading. Het jetsysteem is getest op het benodigd vermogen. Om het benodigde vermogen te berekenen, moeten de dynamische krachten gelijk of lager zijn dan de opgewekte druk van de jets.

Voor de berekening is gebruik gemaakt van twee type jets. Dit zijn de ‘Jet Thruster’ (klein) en de ‘Rolls Royce jets’ (groter). Uit berekeningen blijkt dat beide jets verreweg het huidige beschikbare vermogen in combinatie met een externe krachtsbron overschrijden.

De cones worden geïnstalleerd op het dek. Tijdens transport van de lading zullen de cones volledig zijn ingedrukt. Het heeft de voorkeur dat de ingedrukte hoogte van de cone gelijk is aan de hoogte van de houten cribbing. Zodat de krachten verspreid worden over de

draagconstructie. Tijdens het lossen moeten de cones uitzetten en altijd boven de cribbing uitkomen.

Twee verschillende cones zijn in dit rapport gebruikt. Dit zijn de cell fender (KCEF) en de Super Cone fender (SCN). Gebleken is dat de optredende krachten tijdens transport groter zijn dan de dynamische krachten. Hierdoor zijn beide cones volledig ingedrukt tijdens

transport en veerkrachtig tijdens een offshore losoperatie. De SCN heeft het voordeel met een hoge deflectie van 72,5% en is daardoor leidend in dit rapport. Door de hoge deflectie kan de SCN gebruikt worden in combinatie met de houten cribbing. Om de dynamische krachten op te vangen zijn minimaal 76 SCN’s nodig.

Echter overschrijdt de optredende dekbelasting de toegestane dekbelasting. Hierdoor zullen aanpassingen aan het dek gemaakt moeten worden of zal een ander type cone gekozen moeten worden. Indien aanpassingen aan het dek gemaakt kunnen worden, is het mogelijk om de cones te gebruiken in combinatie met cribbing op het dek.

Float-over operaties hebben bewezen dat het mogelijk is om een topside op een jacket te plaatsen met behulp van hydrauliek. Voor een offshore losoperatie zal de hydrauliek worden geïntegreerd in het hoofddek. Bovenop de hydrauliek zal een stalen dek worden bevestigd. Door dit stalen dek is de hydrauliek niet ladingafhankelijk.

Het hydraulische systeem creëert een acceptabele vrije ruimte. Wanneer het schip start met afzinken houdt de hydrauliek de lading boven zijn drijfpunt. Het schip zinkt af tot de vrije ruimte 3.00m is. Op dat moment wordt het heave compensatiesysteem uitgeschakeld en de hydrauliek zo snel mogelijk ingetrokken. Een verlies van gewicht zal ontstaan door het intrekken van het hydrauliek. Hierdoor zal de vrije ruimte kleiner worden. Omdat een vrije ruimte van 3.00m is aangenomen, resulteert dit in een vrije ruimte van 2.00m. Bij deze ruimte is veiligheid gegarandeerd.

Het voordeel van het hydraulische systeem is dat het een botsing voorkomt. Hierdoor is het een gecontroleerde operatie en zijn de risico’s gelimiteerd. Hydrauliek kan dubbelwerkend worden uitgevoerd. Hierdoor is het mogelijk om het stalen dek sneller te kunnen intrekken.

Hydrauliek kan 20% van zijn verticale draagcapaciteit opnemen in horizontale richting. Een oplossing voor het opvangen van deze horizontale kracht, kan worden gevonden in een casing om de buitenste hydrauliek. Door deze casing zullen de horizontale krachten worden afgedragen naar de sterke basis van de hydrauliek.

Uit het rapport kan worden geconcludeerd dat een hydraulisch systeem een botsing voorkomt. Hierdoor is de operatie gecontroleerd en zijn de risico’s gelimiteerd. De cones zijn echter gebaseerd op het opvangen van een botsing. Door deze botsing kan ongecontroleerd gedrag ontstaan. Tijdens een offshore losoperatie is het gewenst om de risico’s te limiteren. Daarom is het niet aannemelijk dat een oplossing gebaseerd op een botsing gewild is. Om deze reden heeft het hydraulisch systeem de meeste potentie voor COSCO Heavy Transport.

De eindconclusie is dat de kans op het slagen van een offshore losoperatie het grootst lijkt met een hydraulisch systeem. Echter zal meer onderzoek gedaan moeten worden naar de bewegingen van het schip en de lading. Dit is noodzakelijk in verband met het onbekende gedrag tussen de lading en schip tijdens een offshore losoperatie. Tevens zal een

gedetailleerde studie naar de exacte afmetingen en eventuele toepasbaarheid van het hydraulisch systeem moeten worden uitgevoerd.

ABSTRACT

For the transportation of heavy, oversized cargo, semi-submersible vessels can be used. Submerging the vessel will allow discharging the floating cargo. Nowadays discharge

operations are executed in a sheltered environment. Because of the unknown behaviour of the vessel and the cargo in more severe wave conditions, a discharge operation has never been performed in an offshore environment.

This study investigates whether there is a solution to reduce swell forces or impact loads during an offshore discharge. To do so, principles have been determined. This resulted in a design cargo and a design vessel. Swell waves have also been studied. For this study, a maximum swell wave of 2.00m height has been considered.

During a semi-submersible discharge operation, the cargo gets separated from the vessel. This results in a small gap between the cargo and the vessel. The behaviour of the water in this small gap is highly complex. This behaviour can be estimated with the use of the multi domain theory. However this theory is still an approximation of the reality and leads to a highly questionable result. Therefore the behaviour of water within this small gap will be neglected in this report.

Little is known about the heave motions of a cargo floating above the vessel. To determine these motions, a model test is performed. This model test is based on visual observations, and is only used to get a better understanding of the motions. The result of this model test shows a reduction in heave motions when both floating bodies are positioned above each other, compared to independently floating bodies. Since the model test is based on visual results, it is not possible to add a reduction coefficient.

A force calculation is made using the results of the model test. Static forces and dynamic forces are determined. The static forces relate to the total weight of the cargo. The dynamic forces relate to the occurring forces during the discharge, due to the opposing heave motion of the cargo and the vessel. In this anti phasing is considered and is therefore conservative. For the calculation a swell wave with a period of 12.5[s] and a wave height of 2.00m is considered. The static forces are 189,245kN and the dynamic forces are 42,901kN.

The clearance depth is determined at 2.00m. Clearance depth is the distance between the cargo and the vessel that is required for a safe discharge. This is based on a theoretical background and two different model tests.

To determine whether it is possible to reduce swell forces or impact loads during an offshore discharge, several potential solutions are described. These solutions are evaluated on their potential by the use of a Multi Criteria Analysis (MCA). In this MCA the following criteria are used: risks, practicability, environment, durability and costs. These criteria are bound to a multiplier depending on their importance. The results of the MCA are discussed with COSCO Heavy Transport. During this discussion, three solutions were selected that are studied in more detail. These solutions are; the jet system, the use of hydraulics and the use of cones. This deviates from the results that are shown in the MCA. Due to the expected difficulties in practicability the solutions “floating breakwater” and the “wave dragon” have not been further considered.

To determine the feasibility of the three selected solutions, a research question was defined. This means that when the conclusion is negative the solution will not be feasible.

The jet system will be integrated in the deck of the vessel. During an offshore discharge it will fill the expanding gap with compressed water. This generates a lifting pressure on the bottom of the cargo. The jet system will be tested on the required amount of power. To determine the required power, the dynamic forces need to be lower or equal to the generated pressure of the jets.

Calculations have been done for two types of jets. These are the jet thruster (small) and the Rolls Royce jet (greater). Calculations show that both types of jets are exceeding the present power installed in the vessel in combination with an external power source by far. Therefore the jets lose their potential as a solution for the offshore discharge operation.

The cones will be installed on the deck. During the transportation of the cargo, the cones should be fully compressed. It is preferred that the height of the cone whilst being compressed has the same height as the wooden cribbing. This is preferred because of the required

spreading of forces onto the support area. During the discharge itself the cones should expand. At this point the cones should always be above the cribbing.

In this report two different cones are used. These are the cell fender (KCEF) and the Super Cone fender (SCN). As can be concluded in this report, occurring forces during transportation are higher than the dynamic forces. Therefore both of the cones can be fully compressed during transportation and will be resilient during the offshore discharge. The SCN has the advantage of a deflection of 72.5% and is therefore decisive in this report. Due to this

deflection the SCN can be used in combination with wooden cribbing. Optimally 76 SCN’s are required to absorb the dynamic forces.

However, the occurring deck pressure is exceeding the allowable deck pressure. Therefore adjustments to the deck or a different type of cones will be required. If it is possible to adjust the deck, it will be possible to distribute the cones in combination with cribbing on the deck.

Float-over operations have shown that it is possible to place a topside on a jacket with the use of hydraulics. In an offshore discharge operation the hydraulics will be integrated in the main deck. On top of the hydraulics a small steel deck will be attached. Because of the small deck, the hydraulic system is not cargo dependent.

The hydraulic system helps to create an acceptable clearance depth. When the vessel starts ballasting, the hydraulics keeps the cargo above his buoyancy point. The vessel starts

ballasting till the clearance is 3.00m. At that moment, the heave compensation will be powered off and the hydraulics will be retracted as fast as possible. Due to the retraction, a loss of weight will occur on the vessel. This results in a decrease of clearance depth. Because of the 3.00m clearance, it results in a safe clearance of 2.00m whereby safety is guaranteed.

The advantage of the hydraulic system is that it prevents a collision. Therefore the operation is controlled, and the risks are limited. Hydraulics can be carried out double acting. Therefore it is possible to retract the steel deck faster.

Hydraulics can absorb 20% of its vertical bearing capacity in horizontal direction. To counter the horizontal forces, a solution can be found in a casing around the outer segment of the hydraulic. Due to this casing, the horizontal forces will be directly transferred to the base segment of the hydraulic.

The hydraulic system prevents a collision. Therefore the operation is controlled and the risks are limited. The cones are designed to absorb an impact. Due to this impact uncontrolled behaviour can occur. During a discharge, risks should be reduced to a minimum. It is not likely that a solution based on an impact is wanted. Therefore the hydraulic system seems the solution with most potential for COSCO Heavy Transport.

Based on the conclusions that are found in this report, it seems possible to perform an offshore discharge operation. The hydraulic system seems to have most potential to perform this operation. Because of the unknown behaviour of the cargo and the vessel, a more detailed study to the motions of the vessel and the cargo is advised. Also more detailed studies have to be done to the exact dimensions and applicability of the hydraulics.

PREFACE

For our final assignment of the study Civil Engineering followed at the Rotterdam University a thesis has to be made. This thesis is made by Robin van Hattum and Maarten Nijenhuis. Our wish was to find a subject in the offshore branch. After some research, a subject was found for the company COSCO Heavy Transport. After several meetings, both parties got enthusiastic and the research subject was formed. The research subject is to find a complete new solution for a present day phenomenon. This causes different kind of challenges like new theories of other study directions, unknown theoretical elaborations and to challenge problems in a ‘’out-of-the-box view’’.

Using this thesis we want to prove that we have sufficient capabilities to perform as a civil engineer at starting level.

In this we want to thank COSCO Heavy Transport and especially Marc Beerendonk for the pleasant cooperation. Also we want to thank Leo van Gelder and William Kuppen for the pleasant accompaniment from the Rotterdam University. Knowledge and comments from the persons above has helped us to prepare a better thesis.

Rotterdam, 12th of June 2013

CONTENTS 1 INTRODUCTION ... 9 1.1 Maritime guidelines ... 9 1.2 References ... 9 1.3 Appendix list ... 10 1.4 Symbol list ... 10 1.5 Abbreviation list ... 10 1.6 Background... 11 1.7 Project assignment ... 12 1.8 Thesis outline... 13

2 DEFINITIONS AND PRINCIPLES ... 14

2.1 Definitions ... 14 2.2 Principles ... 17 3 MODEL TEST ... 19 3.1 Introduction ... 19 3.2 Execution: ... 19 3.3 Test setup: ... 20

3.4 Results model test: ... 21

3.5 Conclusion model test: ... 23

4 FORCES AND CLEARANCE DEPTH ... 24

4.1 Forces: ... 24

4.2 Determining the required clearance depth: ... 27

5 SOLUTIONS ... 29

6 MULTI CRITERIA ANALYSIS ... 30

6.1 Description MCA ... 30

6.2 Results MCA ... 31

6.3 Conclusion ... 31

6.4 Solutions with insufficient potential: ... 32

6.5 Solutions with sufficient potential:... 32

7 SOLUTION JETS ... 34

7.1 Introduction jet system ... 34

7.2 Amount of jets/power needed: ... 35

7.3 Conclusion: ... 35

8 SOLUTION CONES ... 36

8.1 Introduction rubber cones ... 36

8.2 Research question 1: Feasibility ... 36

8.3 Research question 2: Suitable cones ... 36

8.4 Required amount of cones ... 38

8.5 Conclusion research question 3 ... 39

9 SOLUTION HYDRAULICS ... 40

9.1 Introduction hydraulics ... 40

9.2 Calculation sinking speed steel deck: ... 41

9.3 Discharge operation with hydraulic steps. ... 42

9.4 Horizontal forces ... 46

10 CONCLUSION AND RECOMMENDATION ... 48

10.1 Conclusion: ... 48

1 INTRODUCTION

For the transportation of heavy, oversized cargo, semi-submersible vessels can be used. Submerging the vessel will allow discharging the floating cargo. Nowadays discharge

operations are executed in sheltered environment. Because of the unknown behaviour of the vessel and the cargo in more severe wave conditions, a discharge operation has never been performed in an offshore environment. This thesis investigates the possibilities to perform an offshore discharge operation.

Figure 1. Semi-submersible vessel with cargo1 1.1 Maritime guidelines

This report is based on the present state of the art maritime calculation methods. Unless stated otherwise, in general all the calculations are in accordance with the Noble Denton General Guidelines for Marine Transportations.

All units used in this report as well as other engineering documents prepared by Cosco Shipping Europe B.V. will be in the SI metric system.

1.2 References

In this report reference is made to the following documents:

Document Title

[1] Offshore Hydrodynamics; J.M.J. Journée and W.W. Massie 2001

[2]

An efficient method for hydrodynamic analyses of a floating vertical sided structure in shallow water; Y. Drobyshevski 2010

Hydrodynamic coefficients of a floating, truncated vertical cylinder in shallow water; Y. Drobyshevski 2010

Hydrodynamic coefficients of a floating, truncated rectangular floating structure in shallow water; Y. Drobyshevski 2010

[3] An accurate and efficient hydrodynamic analysis method for offshore discharge operations; J.B. de Jonge 2008

[4] Hydrodynamic behaviour during Offshore Loading and Discharge; Onno A.J. Peters, Rene Huijsmans and Michel Seij 2012

[5] Theoretical manual of “SEA WAY FOR WINDOWS” made by J.M.J Journee and L.J.M Adegeest, TUD report 1370, revision 1412 ,2003

Table 1. Reference documents

1

1.3 Appendix list

In this report information obtained from the following documents are used. The documents can be found in the appendix:

Appendix Document Title Rev.

[A] Document of definitions 2.0 [B] Document of principles 2.0 [C] Model Test 4.0 [D] Forces and clearance depth 2.0 [E] Document of solutions 2.0

[F] MCA 2.0

[G] Solution jets 2.0 [H] Solution cones 3.0 [I] Solution hydraulics 3.0

Table 2. Input documentation 1.4 Symbol list

In this report the following symbols are used:

Symbol Description A Area cm Centimetre E Energy F Force g Gravitational acceleration h Water depth H Wave Height

Hsig Significant wave height

hp Horsepower kg Kilograms kN KiloNewton kW Kilowatt m Metre mm Millimetre m/s Metre/second MT Metric Ton Nm Newtonmetre s Second t Wave period v Velocity ρ Density

Table 3. Symbol list 1.5 Abbreviation list

In this report the following abbreviations are used:

Abbreviation Description

ABS American Bureau of Shipping

COSCO China Ocean Shipping (Group) Company COSCOL COSCOL

HT Heavy Transport

KCEF Kemenangan Cell Fender MCA Multi Criteria Analysis

RAO Response Amplitude Operator SCN Supercone fender

Semi-sub Semi-submersible

SI International system of units TU Technical University

1.6 Background

1.6.1 Company

NMA Maritime and Offshore Contractors was founded as a service company for the shipping industry 35-years ago. Different kind of services were offered such as; marketing, technical support, chartering and the purchase and selling of vessels.

NMA is specialised in heavy transport. The company has nowadays completed more than 250 offshore heavy lift projects. During the projects fast, safe transport and installation are playing a central role. The markets NMA works for are oil and gas related, dredging industry, offshore engineering and construction.

The company has 3 offices around the world. The head office is located in Rotterdam, the Netherlands. The 2 supporting offices are located in China and in the USA. Recently NMA offshore has founded a joint venture with COSCO.

There has always been a good connection between NMA and COSCO. In 1983 NMA sold two semi-submersible dock ships to COSCO. In 1990 the relationship continued with chartering vessels for COSCO. This led to marketing vessels worldwide. Since the end of the year 2012 NMA Maritime and Offshore Contractors is named COSCO Heavy Transport.

COSCO Shipping Company Limited (COSCOL) focuses his core business on transport for special loads. Such as oversized, extremely heavy and loads that cannot be transported by containers.

1.6.2 Contractors

The contractors of the project are two students of the Rotterdam University. Both students study Civil Engineering specialized in hydraulic engineering.

1.6.3 Stakeholders

COSCO Heavy Transport

With this thesis COSCO hopes to get more information about the possibilities of an offshore discharge operation. If there are potential solutions, COSCO wants to know if the potential solutions are feasible. If the solutions seem feasible, COSCO can start a more detailed study to this potential solution. If the study is performed positive it may lead to the execution of an offshore discharge. This will result in an additional service that COSCO can offer to their clients.

Rotterdam University

The Rotterdam University hopes that the students perform on a significant level. When the students deliver a proper research answering the main question, the students will have sufficient capabilities to perform as engineer at starting level. For a proper research, the students have to practice their capabilities learned in the previous years.

1.7 Project assignment

1.7.1 Motivation

During an offshore discharge for heavy loads, the swell waves cause a major problem. This problem is caused by the vessel and cargo starting to behave independently the moment the cargo separates from the submerged vessel. The small initial gap may result in high impact loads on both vessel and cargo. This could cause severe damage to the load and the vessel. The costs for repairs may be significant. Therefore COSCO Heavy Transport has asked to put this matter into research.

The research question that is asked:

‘’Is it possible to reduce or neutralize the swell forces or the impact loads?’’

1.7.2 Main question

Based on the motivation a main question is formed. This question is more specific, and is subject to boundaries. The project is tuned to this main question.

The main question that is formed:

‘’Is there a solution to reduce the swell forces or the impact loads during an offshore discharge with a “ semi-submersible heavy lift ship” in a pre-defined coastal area?’’

1.7.3 Goal final report

The intended result for the project is a report in which multiple solutions will be described. These solutions will be tested by the use of a multi criteria analysis. The three solutions with the most potential will be studied in more detail. At the end of this report conclusions and recommendations are given.

1.8 Thesis outline

In chapter two the definitions and principles are described. These definitions and principles are commonly used throughout the document. In this the definition of an offshore discharge is given. Also the definitions of swell and the theoretical elaborations of the occurring motions of the cargo and the vessel, during the discharge operation, are described in this chapter. In the principles the design criteria are given, like the type of platform, vessel, and location that are used for the project.

In the third chapter, the model test is discussed. This model test is performed to get a better understanding on the motions of the vessel and the cargo, and the relations between them. In this chapter the execution and the conclusions are given.

After the conclusions from the model test, the forces that may occur during an impact between the vessel and the cargo are calculated in chapter four. Also substantiation is given for the required clearance depth.

Chapter five is an overview of the different solution directions and solutions that have been looked into. In this the solution directions are a guideline for the solutions.

In chapter six, the solutions will be tested with the use of a multi criteria analysis (MCA). In this chapter the criteria and the motivation of the MCA are given. For the results of the MCA, a conclusion is made.

In the seventh, eighth and ninth chapter, the three most potential solutions will be elaborated. These solutions will be independently studied in more detail. If the solution seems promising, an execution plan is made.

In the tenth, and final chapter, the conclusion of the research question is given. After this conclusion a recommendation from the students to the company is made. This

recommendation will describe critical points of the different solutions and the possibilities that occur after this study.

2 DEFINITIONS AND PRINCIPLES 2.1 Definitions

The definitions that are described in this chapter give a better understanding of the terms that are used in the main question. A more detailed description of these definitions can be found in document of definitions [A].

2.1.1 Definition process discharge operation

Preparations

Before the vessel can be loaded, several documents have to be made. This contains vessel and cargo information, engineering, and a loading and unloading plan. This is all included in a transport manual.

Cribbing

Before the cargo can be loaded on the vessel, cribbing has to be placed on the deck. Cribbing is made of wooden beams (often 300*300*3000mm) and is placed to prevent damage to the deck of the vessel, and to distribute the cargo load over the deck structure. During the engineering a cribbing plan will be made. An example of a cribbing plan is shown in Figure 2.

Loading the cargo on the vessel

The ballast tanks are used to submerge the vessel.

Depending on the submerge level of the cargo the ballast tanks will be filled.

Positioning the cargo on the vessel

Once the vessel is at its required draft, the cargo will be carefully positioned above the vessel by means of tugger lines. This system is the most accurate method.

Tugger lines system.

First the vessel is anchored. The cargo gets positioned by tugboats. When the cargo is next to the vessel, small boats will transport the tuggerlines from the vessel and those will be attached to the cargo. The tuggerlines will be crossly attached (for optimal control) to the pre-defined locations. As soon as the tuggerlines are attached, the winches will start pulling the cargo towards the vessel. When the cargo is positioned above the vessel, the ballast tanks will be emptied. A schematic view of this operation is shown in Figure 3.

Figure 3. Tugger system2

2

Transport Manual XYK COSCO Shipping co.

Seafastening

As soon as the vessels main deck is dry, the fastenings can be brought into position, in accordance to the sea fastening plan. Sea fastening is needed to secure the cargo during transport. Welding of the sea fastenings shall not commence until the vessel is de-ballasted to the transportation draft. If the deck is dry, the sea fastenings will be welded on the deck of the vessel. Once the cargo is sea fastened it is ready for transport. Figure 4 shows the sea fastening for a jack-up rig.

Discharge operation

Before the discharge commences the sea fastening will be removed by means of cutting. When the sea fastening is loose they will be removed from the deck.

The vessel will ballast to discharge draft. Once at the discharge draft the cargo will be attached to the tug boat and will be carefully towed away from the vessel. This operation requires a good understanding between the captain of the tug boat and the winch operator. As soon as the cargo is off the deck, the tuggerlines will be disconnected from the cargo. Tug boats will transport the cargo to its final destination. The process is shown schematically in Figure 5.

Figure 5. Discharge operation4 Offshore discharge:

At the moment offshore discharge is not used. The risks involved with this operation are too high in relation to the costs.

During an offshore discharge for heavy loads, the swell waves cause a major problem. This problem is caused by the vessel and cargo starting to behave independently the moment the cargo separates from the submerged vessel. The small initial gap may result in high impact loads on both vessel and cargo. This could cause severe damage to the load and the vessel.

3 _{Transport ENSCO 88 – COSCO Heavy Transport} 4

Transport Manual XYK COSCO Shipping co.

2.1.2 Definition swell wave, and the influence of swell on the vessel

Swell is a series of surface gravity waves that is not generated by local winds. These waves are created by storms on the ocean. During the storm, wind waves are generated. When these propagate in direction of the deep ocean, the waves will group together. Swell waves often have a long wavelength (100-200m). During severe storms the wavelength can increase to 700m. The wave period is often well above 10 seconds. The kinetic energy of swell is much higher than short waves.

Swell waves cause motions on the vessel. These motions are split up in three translations in the ship centre in the direction of the x-, y-, and z-axis. Translating in a surge (x), sway (y), and heave (z) motion. The other motions are rotations around the gravity point: pitch (θ): rotation around the y-axis, Roll (φ): rotation around the X-axis, and Yaw (ψ): rotation around the Z-axis. The motions work on the vessel as shown in Figure 6.

2.1.3 Theoretic background

During a discharge in an offshore environment, the behaviour of the cargo is unknown. Various studies have been performed to investigate this problem. During the discharge, the initial gap between the cargo and the deck of the vessel will change the behaviour of the cargo.

Since the single linear mass-spring system theory [1] is based on a single object in deep water, it is not representing the phenomenon happening with the gap between the cargo and the vessel. To get a more realistic view of this phenomenon the multi domain potential theory is used.

[2] Note that this theory cannot accurately describe the phenomenon of the gap. This is because the theory is based on a 3 dimensional flow, however the fluid flow within the gap is considered 2 dimensional. The second problem is that the velocity of the fluid can be very high inside the gap. This happens in particular between the gap and open fluid domain. As

researched by J.B. de Jonge [3].

The basis of the multi domain method [4] is the standard linear potential theory. The derivate of the motion is used as a gradient to describe the velocity of the fluid around a body within a controlled volume. When the velocity is known, the pressure can be calculated. When the pressure is known, the forces can be determined. Finally the motions of the cargo can be determined.

5

Offshore Hydrodynamics; J.M.J. Journée and W.W. Massie 2001

2.2 Principles

For the offshore discharge study several definitions needs to be defined. In this chapter the cargo, vessel and location that will be used in the resulting study will be described. For the more detailed considerations is referred to the document of principles [B].

2.2.1 Design platform

For the discharge study, a design platform is needed. The dimensions of the design platform will be based on an existing platform. This is done to make the study more realistic.

First a decision is made on the type of cargo that the vessel should be transporting. The types of cargo that are being compared are a semi-sub and a jack-up rig. For the study, it is preferred to select a jack-up rig, because of the less complicated behaviour of the cargo when it gets released. This complication comes forth out of two floaters of a semi-sub. The water between these floaters can cause

uncontrolled behaviour of the semi-sub.

The jack-up rig that is chosen for the study is the ENSCO 120 (Figure 7).This cargo is selected because its dimensions are representative for a modern jack-up rig. This will result in a more realistic study. Average dimensions are preferred as it is no study that researches extremes. The study researches whether it is realistic for the future to perform an offshore discharge operation.

2.2.2 Design vessel

For the offshore discharge study, a vessel type is defined. The vessel type is determined during a discussion with COSCO HT. As a design vessel the X-class Xiang Yun Kou (Figure 8) is used. The specifications for the X-class are found in the document of principles [B].

The X-class has the biggest potential because the vessels have a larger draught then the other vessels in the COSCOL fleet. Due to this draught it is not always possible to discharge in a sheltered environment. If the study shows that it may be possible to discharge in an offshore environment, this vessel has the possibility to be used in this kind of situation.

Figure 8. Xiang Yun Kou7

6 http://www.enscoplc.com/files/Documents/Brochures/ENSCO120121Brochure_Web.pdf 7 http://www.shipspotting.com/gallery/photo.php?lid=1262382 Figure 7. ENSCO 1206

2.2.3 Weather conditions

Normal discharge operation:

To get a better understanding which conditions are acceptable in a normal discharge operation, the guidelines for a normal operation have been checked. These criteria are not strict criteria. The vessel master and the project superintendent will decide on commencement of the discharging operation. Here for the actual weather conditions, the weather forecast, and the relative motions of the vessel and the cargo will be taken in consideration.

Favourable weather conditions and preferably daylight are required for the discharging operation. The guidelines as shown in Table 5 are in general the limiting weather criteria8:

Maximum wind speed 15.0 knots Maximum wave height 0.50 meter Maximum swell 0.30 meter Maximum swell/wave period 5-7 seconds Maximum current 1.0 knots

Table 5: Design weather conditions Offshore discharge operation:

During an offshore discharge operation the weather conditions will be more severe. This is caused by swell waves. A study of these swell waves is done for the coast of Brazil and West Africa. The frequency and the wave height are studied. A maximum swell wave of 1.00-2.00m is used. This is based on the allowed maximum swell height used for a float over operation. A float over operation is generally executed in an offshore location.

The results of this study are shown in Table 6 and Table 7.

In the period June to August 58.0% of the waves are lower than 2.00m for the location Brazil. Hence offshore discharging may be possible. The workability in Brazil is at its best in the period March to May, with a possibility of 70.8%.

The applicability at the coast of West Africa is averagely better than the coast of Brazil. In the period of June to August a workability of 67.2% is reached. The period December to February shows a workability of 99.7%.

It can be concluded that a wave height of 2.00m is a realistic wave height for an offshore discharge operation. The whole report on the weather condition diagrams can be found in the document of principles [B].

Season Hsig 0-1m Hsig 1-2m Total % observations

Mar-May 24.3 46.5 70.8

Jun-Aug 14.9 43.1 58.0

Sep-Nov 16.4 44.6 61.0

Dec-Feb 23.2 42.5 65.7

Table 6. Seasonal probabilities of wave heights in Brazil Season Hsig 0-1m Hsig 1-2m Total % observations

Mar-May 36.5 49.5 86.0

Jun-Aug 16.2 51.0 67.2

Sep-Nov 24.3 54.0 78.3

Dec-Feb 77.3 22.4 99.7

Table 7. Seasonal probabilities of wave heights in West Africa

8

3 MODEL TEST

3.1 Introduction

During a semi-submersible discharge operation, the cargo gets separated from the vessel. When the cargo and the vessel are getting separated a small gap will result. This small gap is a major problem when the operation is performed in an offshore environment operation. Since the cargo is still floating above the vessel, its behaviour is hard to determine. Studies are still researching this phenomenon.

For the more detailed conditions, arrangements, results and conclusion is referred to the document model test [C]. An overview of the testing setup is shown in Figure 10.

The results are based on findings during the model test. These results have been recorded and can be found on the CD delivered with the report.

3.2 Execution:

Before the model test can start two scale models are made; one model to simulate the cargo and one to simulate the vessel (Figure 9). The model test is performed in a flume tank executed with a wave generator. The flume tank should have two points were the objects can be fixed to avoid horizontal movements during the tests. Three different tests have been done:

1. Cargo only 2. Vessel only

3. Combination of both in which the cargo floats above the vessel

These three tests will be executed in six different circumstances. These situations will vary in wave length and in wave height.

3.3 Test setup:

3.3.1 Test 1 ‘cargo only’:

In the first test, the cargo will be placed and strained in the flume tank as shown in Figure 11. The draft of the cargo is 26mm. The cargo will be tested to the different wave heights. This will later be used as comparison with motions in test 3.

3.3.2 Test 2 ‘vessel only’:

In the second test, the vessel will be placed and strained in the flume tank. The head of water above main deck of the vessel is 30mm. The vessel will be tested to the different wave heights. This will later be used as comparison with the motions in test 3, as shown in Figure 12.

Figure 11. Set up test 1 Figure 12. Set up test 2

3.3.3 Test 3 ‘cargo above the vessel’:

In the third test the cargo will be placed above the vessel. This test will also be tested on the different wave heights. The gap height is 4.00mm (Figure 13) which correspond with 2.00m in reality. Figure 9 and Figure 10 shows the testing models in the flume tank.

3.4 Results model test:

This section shows the end result of the practicum. A more detailed study can be found in the model test report [C].

To determine if there is a difference in motions whenever the objects are separated or together the results gained by the model testare compared. In this the average results of test 3 have been subtracted by the average results found in test 1 and 2, depending on the object. The results are shown in Table 8 and Table 9.

In the tables the following should be taken into account:

A positive figure means a decrease in vertical motion and will be coloured green. A figure which remains approximately equal will be coloured orange.

A negative figure means an increase in vertical motion and will be coloured red.

*This figure shows the “cargo only” vertical motion as percentage of the wave height. **This figure shows the “cargo combined” vertical motion as percentage of the wave height.

Table 8. Differences in test results cargo

Differences in test results vessel

Test number Up (cm) Down (cm) *Wave 1 (%) **Wave 2 (%) Total displacement (cm) Displacement percentage (%) 1 -0.4 0.5 85.0 80.0 0.1 94.1 2 0.4 0.0 46.3 30.9 0.4 66.7 3 0.0 -0.4 60.0 86.9 -0.4 145.0 4 0.3 -0.5 82.0 88.5 -0.1 107.9 5 0.1 0.2 65.6 53.9 0.3 82.2 6 -0.3 0.8 81.9 56.4 0.6 68.9

*This figure shows the “vessel only” vertical motion as percentage of the wave height. **This figure shows the “vessel combined” vertical motion as percentage of the wave height.

Table 9. Differences in test results vessel

Differences in test results cargo

Test number Up (cm) Down (cm) *Wave 1 (%) **Wave 2 (%) Total displacement (cm) Displacement percentage (%) 1 -0.5 -0.3 68.3 100.0 -0.8 146.3 2 0.6 0.0 72.0 48.0 0.6 66.7 3 0.1 0.4 100.8 84.4 0.3 83.8 4 0.2 0.8 149.0 97.2 1.0 65.2 5 0.0 -0.1 64.1 68.5 -0.1 106.8 6 0.0 0.8 101.9 69.1 0.8 67.9

3.4.1 Findings:

Generally the motions of the vessel should always be lower or equal to the motions of the cargo. This comes forth out of the length and the width of the objects. Since the vessel is longer than the cargo, it will have a lower natural frequency. Therefore it will be put slower into motion. This corresponds with the results of the model test.

In Test 3, 4 and 6 for the cargo some figures are shown red. This is done since these figures have crossed the 100% in vertical motions as a percentage of the wave height. Test 3 and 6 both show a percentage slightly higher than the wave height. With extremely long waves this can happen. Software such as Octopus shows an increased vertical motion exceeding 100% when waves are getting extreme. Test 3 and 6 both present the longest wave that has been tested. Therefore the results obtained are still accurate and useable for comparing.

The results shown in test 4 are considered to be false. It is expected that this test has not been run properly. During the 3 testing phases the wave height accidently changed. This explains the extreme deviations. This test result will therefore not be used for the rest of the model test.

Highly increased motions:

There are two tests which have a high increase of motion. These are test 1 for the cargo and test 3 for the vessel.

Looking at test 1 the combined motions seem to have the same height as the wave height. It seems that in this case the combination follows the period of the wave to the full 100%. This is remarkable since the waves in test 1 have the shortest period of tested waves. Computer models like “Octopus” show that motions increase to 100% whenever the wave period

becomes longer. As such, the motions deviate of the theory and the experience in the field. In this case the cargo does not seem to be influenced by the vessel. This can be concluded looking at the test results of the vessel. In this, the vessel seems to move equally.

With test 3 an increase in motion seems to occur as well. Although the motions have the same percentage of increase, it is less extreme than the motions seen in test 1. The vessel does not take over the full 100% of the wave. Instead, a vertical motion has been noticed of 87% of the wave height. Apart from this deviation, the combined wave does not show exceptional results compared with other tests. It is remarkable however that the results of the cargo in test 3 seem to act directly in the opposite way. In this, a reduction of motion takes place by 17%.

No change in motions:

There are three tests which show almost no change in behaviour. These are test 5 for the cargo and test 1 and 4 for the vessel. The two compared tests differ 1mm from each other. Due to this small difference and taken the inaccuracy of the observation method in account, these results will be taken as “no change in motion”. This means that for these tests, the sole object and the combination of the two objects have no influence on each other.

Highly decreased motions:

There are seven tests which show a high decrease in motion. These are test 2, 3, 4 and 6 for the cargo, and test 2, 5 and 6 for the vessel. These tests have a deviation of at least 3mm and have an average deviation of 5mm. This average is comparable with an average reduction in motion of 21%. It should also be noted that 66% of the observations lead to a decrease in motion.

3.5 Conclusion model test:

In general the motions of cargo and the vessel seem to reduce. The motions of the cargo show a clear reduction for 66% of the test results, when the “equal results” are taken in account this percentage goes up to 83%. 17% seems to turn out in an increase of motion. In which one of the test results shows a doubtful figure. The other test seems to show an

increase. However the gained results are still in line with the other results in this category. It is clear that in the flume tank a reduction in motion occurs for either the cargo or the vessel. In this the cargo is most influenced.

The model test is performed in a flume tank in which the conditions are optimal. It is likely that a similar effect will occur in an offshore environment. Further testing of the model is advised to obtain a more accurate view on the behaviour between cargo and vessel. It is also

recommended to look into the reduction of the motions. For this report the results are sufficient for its purpose.

4 FORCES AND CLEARANCE DEPTH

4.1 Forces:

To determine the occurring forces two different phases are looked upon. This is the static force that occurs due to the weight of the cargo, and the dynamic force that occurs during the discharge phase. To simplify the calculation it is assumed that the cargo is supported directly on the deck of the vessel.

Dimensions for the cargo and the vessel are as described in the document of principles [B].

4.1.1 Static forces:

The static forces relate to the total weight of the cargo. The weight of the ENSCO 120 is 19,291MT. The support area on the vessel has an effective surface of 2,454m2. A schematic view of the contact area is shown in Figure 14. This results in a total deck pressure of 7.86MT/m2

.

Figure 14. Contact area of the ENSCO 120 on the Xiang Yun Kou

4.1.2 Dynamic forces during discharge operation:

The dynamic forces occur when the cargo is floating. These dynamic forces occur due to Archimedes law in combination with the swell waves. This makes the dynamic force

dependent on the characteristics of the swell wave, and the behaviour in waves of the cargo related to the vessel. The motions of the cargo and the vessel reduce when they are

combined as seen in the model test. The results however are not sufficient to define a

reduction coefficient to the occurring motions. Therefore the motions will be considered as two separated bodies in the water.

The computer program Octopus [5] is a simulation program that simulates the behaviour of a rigid body in water. This behaviour is expressed in a Response Amplitude Operator (RAO). For both the cargo and the vessel three different RAO’s have been calculated. These are the heave, heave velocities and the accelerations. The motions, velocities and accelerations are expressed in wave period [s].Figure 15shows an example of a RAO. The appendix of the document forces and clearance depth [D]contains the used RAO’s calculated with Octopus.

With these results the occurring forces can be determined. The occurring forces are directly related to the amount of kinetic energy.

To determine the kinetic energy the velocity of the cargo and the vessel are looked into. The velocity is assumed normative because of its quadratic behaviour in the formula (…4.1). Since the velocity is normative the most extreme value is required to find the wave with the most kinetic energy. To find the maximum velocity, phase difference is neglected. By neglecting the phase difference the cargo and the vessel can be seen as moving in opposite direction. This results in a higher dynamic force. Phase difference will be used and more clearly defined in chapter 4.2.

To determine which wave leads to the highest velocity, different wave periods are looked upon. These are the wave periods, 12.5[s], 15.0[s], 17.5[s] and 20.0[s]. The occurring velocity per swell wave is shown in Table 10. In Table 10 the wave height is set at 2.00m. As long as the wave height is a constant figure the most negative wave period can be found.

Now that the differences in velocity are known the resulting kinetic energy can be calculated. The values per swell wave are shown in Table 10.

The formula for kinetic energy is:

(…4.1) In which: E: Kinetic energy [Nm] m: Mass platform [kg] v: Velocity [m/s] T: Wave period [s] H: Wave height [m]

T (s)

H (m)

V

(m/s)

V

(m/s)

E

(Nm)

E

(Nm)

12.5

2.0

0.65

0.90

22983087

7812855

15.0

2.0

0.64

0.78

22281355

5868322

17.5

2.0

0.55

0.70

16455347

4726295

20.0

2.0

0.50

0.62

13599460

3707730

Table 10. Maximum energy values for different swell waves

With the results as shown in Table 10 it is clear that the occurring forces will be highest with a swell wave of 12.5 seconds. If the amount of kinetic energy is known it is possible to calculate the resulting force. To accomplish this, the heave motion of the cargo and the vessel in a swell wave of 12.5[s] are required. The document of forces and clearance depth includes the RAO’s used for the calculations.

To calculate the occurring force the following formula is used: (…4.2) In which: F: Occurring force [kN] E: Kinetic energy [Nm] H: Motion due heave object [m]

T

(s)

H

(m)

E

(Nm)

E

(Nm)

Heave

vessel

(m)

Heave

cargo

(m)

F

(kN)

F

(kN)

F

(kN)

12.5

1.0

5745772

1953214

0.30

0.85

19153

2298

21450

12.5

1.5

12927987

4394731

0.45

1.28

28729

3447

32176

12.5

2.0

22983087

7812855

0.60

1.70

38305

4596

42901

12.5

2.5

35911074

12207586

0.75

2.13

47881

5745

53626

Table 11. Occurring forces (swell wave T=12.5s)

For the wave heights 1.00m, 1.50m, 2.00m and 2.50m, the occurring forces are calculated. The document of principles [B] states that a wave height of 2.50m is not realistic. This wave height is merely added as an indication.

Since 2.50m is not realistic, a wave height of 2.00m is used for further calculations. The scatter diagrams that are presented in the document of principles show that a wave height of 1.00-2.00m provides suitable workability for the investigated areas.

The resulting force that occurs from a swell wave with a period 12.5[s] and a wave height of 2.00m is 42,901kN. This is corresponding with 4,373MT. The vessel has an effective surface of 2,454m2. This results in a total deck pressure of 1.78MT/m2. Note that this is used as an indication of the occurring deck pressure.

Note:

Calculations made to determine the forces do not represent the exact occurring forces during an offshore discharge. If the cargo and the vessel move in the opposite direction the water in the gap will damp the motions. This damping leads to a lower force in reality. However the document of definitions states that the behaviour of damping within this gap is complex. Since damping can be considered positive it can be neglected for the discharge study.

4.2 Determining the required clearance depth:

During a discharge operation the vessel will start ballasting. Due to the ballast the vessel will gain a deeper draft. When sufficient draft is created the cargo will create buoyancy. This results in a gap between the cargo and the vessel. This gap will slowly increase depending on the ballasting speed of the vessel. When sufficient clearance between the cargo and the vessel is made, the cargo can be towed away by tug boats. When the discharge operation occurs in a sheltered environment the motions due to waves are minimal. This is different during an offshore discharge.

Note:

In this document a swell wave with a period of 12.5[s] and a wave height of 2.00m is considered.

4.2.1 Relative motions of the vessel and the cargo:

During an offshore discharge the vessel and the cargo will be affected by the swell waves. This results in a heave motion. In section 4.2.2 it is stated that the motions of the vessel and the cargo are considered to be directly opposing each other (anti phasing). If the heave motions of the cargo and the vessel are in opposite directions it will result in a total heave motion of 2.30m (Table 11). During an offshore discharge 2.30m clearance is required to avoid an impact between the cargo and the vessel.

In reality this is not the case. An object that is floating in water and that is affected by waves tends to follow the motion of the wave. Depending on the area and the shape of the object, phase difference will occur. Due to the phase difference, the motion of the object will occur with a delay. Model test performed by the TU-Delft [4] shows that a cylinder, triangle and a square shape have similar trends in motion.

4.2.2 Response Amplitude Operator:

When phasing is taken into account the results can be found in Figure 16. The motions shown in Figure 16 correspond with a wave period of 12.5[s] and a wave height of 2.00m.

The RAO for the phasing differences and the associated heave motions of the cargo and the vessel are found in forces and clearance depth [D]. The RAO’s are calculated with the use of the program Octopus.

The maximum opposing distance occurs when the wave has passed 8.0[s]. This shows that the minimum theoretical clearance that is required to avoid an impact is 0.85m. It should be noted that Octopus makes use of the damping coefficient that corresponds with deep water.

Figure 16. Heave motion due phasing

-1.5 -1 -0.5 0 0.5 1 1.5 0 2 4 6 8 10 12 14 16 18 20 Wave vessel cargo 0.85

4.2.3 Model test TU Delft [4]:

Model testing performed by the TU Delft shows that a clearance of 2.00m is sufficient to avoid an impact between the cargo and the vessel. The waves used in this model test are similar to the theoretic waves used in the discharge study. The waves used by the TU Delft correspond with wave heights of 1.00m or 2.00m and a period varying between 9.9[s] and 11.0[s]. During 45 tests only 5 impacts have been measured with a clearance depth of 1.00m. These tests lasted for 3 hours and have been monitored. During the tests with a clearance depth of 2.00m no impacts have been measured.

4.2.4 Model test University of Rotterdam [C]:

The results gained by the program Octopus and the model test performed by the TU-Delft are in line with the results found in the model test performed at the University of Rotterdam. Based on visual observation no impacts have been made with a clearance depth of 2.00m. Impacts that are registered in the model test happened due to the occurring reflection of the wave at the end of the flume tank.

4.2.5 Conclusion:

With these three observations it is safe to assume that a clearing depth of 2.00m is sufficient for an offshore discharge. It should be noted that this is only for a wave with a period of 12.5[s] and a wave height of 2.00m. It is likely that the clearing depth differs for different waves. However the findings are probably a worst case scenario. This can be concluded from Table 10 and Table 11.

5 SOLUTIONS

Now that the motions and the forces during an offshore discharge are determined, solutions can be looked into. Table 12 shows the list of solutions that are used in this report. The solutions are based on different methods. A description of these methods is given in the table. A more detailed description of these solutions is found in the document of solutions [E].

To determine which solution has most potential for the offshore discharge study, a multi criteria analysis has been made. This will be described in chapter 6.

Solution number Solution name Description of the method 1 2

Compressible deck layer

Airbag Rubber deck

These solutions are based on a compressible deck layer to absorb occurring impact forces.

3 4 5

Anti earthquake systems

Viscous dampers Coil springs Rubber cones

The solutions are based on existing solutions within the engineering of anti earthquake mechanics.

6

Anchors

Suction buckets Solutions that are based on pulling the vessel down by the use of anchors. Creating a higher draft in a shorter time.

7

Lifting system

Hydraulics combined with heave compensators

Solutions that lift the cargo above the vessel. When the clearance depth is large enough the lifting system can be retrieved.

8

Capacity ballast tanks

New valve system Solutions that increase the speed of the ballasting. This results in achieving the required clearance depth faster.

9 10

Gap filling

Jet system Liner

Solutions that are based on temporary filling the occurring gap.

11 12 13 14 External solution Wave dragon

Reduction of wave height by oil Anti waves

Floating breakwater

These solutions are not directly related to the vessel or the cargo, but focuses on reducing the external factors.

6 MULTI CRITERIA ANALYSIS

In chapter five a list of possible solutions is given for an offshore discharge. To decide which solutions have the highest potential to succeed, a Multi Criteria Analysis (MCA) is made. To avoid analyzing all solutions in detail, the MCA is used to separate the ideas with most potential from the ones that have no potential. After the test with the MCA, a selection of solutions is made according to their rating. These solutions will be discussed with

professionals. After this discussion, the solutions with the most potential will be studied in more detail.

6.1 Description MCA

The evaluation of the solutions will be based on five main criteria. These are costs, practicability, durability, environment and risks. Each criterion is divided in sub categories. This is done to get a more comprehensive analysis.

When all sub categories are graded, the grades will be added to each other and divided by the amount of sub categories. This will result in an average score for the main criteria. The main criteria are bound to a multiplier. The value of the multiplier depends on the importance of the criteria. This multiplier varies between 1 and 4.

6.1.1 Risks

The most important criterion is risks. The multiplier given to the criterion risks is 4. This is based on the strict rules in offshore conditions and to maintain a good reputation for the company. Beside the strict regulations, the costs that have to be made when a project fails will be significant. Therefore high risks are to be avoided. The sub criterion of risks is failing chance and reliability. In this criterion the reliability of the solution will be valued. In this, the failure probability will be based upon references. When no references are available, the value will be discussed with professionals. Only the frequency of failing is looked upon.

6.1.2 Practicability

The multiplier given to the criterion practicability is 3. When a solution is not practical it will never be used. The sub criteria of practicability are technical feasibility, execution limitations and delay. Technical feasible is measured in the amount of technical effort. This means that when a lot of work has to be done, this criterion will be valued lower. If the solution can only operated for in a certain kind of wave or when adjustments need to be made to the solution because of the limitations the criterion execution limitations will be valued lower. The delay is valued on the time needed to execute the whole installation process compared to other solutions.

6.1.3 Environment

A multiplier factor of 3 is given to this criterion. The clients of COSCO Heavy Transport, which are mostly gas and oil winning companies, give a high value to the environment. This is based on the strict rules in offshore conditions and to maintain a good reputation.

6.1.4 Durability

The multiplier given to the criterion durability is 2. This is done since the durability is of less importance than risks and the environment. The sub criteria are maintenance costs and reusability. Maintenance costs are the costs to maintain the solution. Reusability is the period of time until maintenance is needed.

6.1.5 Costs

The multiplier given to the criterion costs is 2. When a solution is low on risks and is not harming the environment, the profit are higher than the costs. Therefore costs are of less importance in the MCA. Costs are divided in construction-, transport -, installation - and staff costs. These criteria are based on the amount of expected costs for construction,

6.2 Results MCA

The maximum score of the MCA is 140 points. The MCA will be tested in a matrix. This matrix and the filled-in matrixes can be found in the document MCA [F].The end scores of each solution are shown in Table 13.

Note:

Results are based on assumptions. The scores gained are to be considered as an indication of the potential of a solution.

Table 13: MCA scores per solution

As can be concluded from the results shown in Table 13 , the hydraulic solution scored highest with 100 points. This corresponds to 70% of the total possible score. The floating breakwater scores second best with 95 points and the wave dragon follows with 87 points. On the fourth place, the cones solution is found. The fifth place is shared by the new valve system and the use of jets.

Solutions rated lower than 75 are considered too low in potential to include them in detailed offshore discharge study.

The results gained from the MCA will be discussed with professionals. This discussion will determine which solutions will be more deeply defined in the offshore discharge study.

6.3 Conclusion

During the discussion with COSCO Heavy Transport, all the solutions have been discussed. The conclusions made during the discussion are in line with the results gained in the MCA. These conclusions consist out of solutions with no potential and solutions with a potential for the final solution.

The solutions with potential will be deeper defined in the different chapters according to the potential solution. Since it is not known whether the solutions are technical feasible, a feasibility check will be made. This check will look into the most critical link of a solution.

0 20 40 60 80 100 120 To tal sco re Name solution

MCA results

6.4 Solutions with insufficient potential:

6.4.1 Wave dragon/Floating breakwater:

The wave dragon and floating breakwater may reduce the swell strength if the solutions are dimensioned big enough. It is likely that the dimensions of these solutions will be too big for easy handling. Due to the dimensions an additional transportation is required. This transport takes a lot of time, which results in additional costs. Due to this additional transportation another offshore discharge is required. Because of these reasons, the wave dragon and the floating breakwater will not be taken in account during this offshore discharge study.

6.4.2 Other solutions:

The document of the MCA [F] describes why the other solutions have insufficient potential.

6.5 Solutions with sufficient potential:

6.5.1 Jets:

The jets have potential for a final solution. The jets can be operated by compressed air or water. The most potential is seen in a jet system using water, since air has almost no constructive value. Due to air the water in the gap will be pushed away and will result in a smaller clearance. This may result in an impact between the cargo and the vessel.

The principle of the jet system is that the compressed water lifts, and stabilizes the cargo during an offshore discharge. To do so a lot of energy is required. If the amount of energy is exceeding the present power of the vessel, an extra external power station will be required. If the required energy is exceeding the capacity of the vessel and a power station by far, the solution will lose its potential.

This results in the following research question:

Is the amount of power required to run a jet system during an offshore discharge exceeding the present power of the vessel in combination with a power station?

A more detailed study of this critical issue will be given in chapter 7.

6.5.2 Rubber Cones:

The rubber cones show potential for the final solution. During the transportation of the cargo the cones should be fully compressed. Whenever the cones are not fully compressed, it is possible that the cargo starts bouncing. If the cones are fully compressed, it is preferred that the cone has the same height as the cribbing. This is preferred because of the required spreading of support area onto the deck during transportation.

During the discharge itself the cones should expand. At this point the cones should always be above the cribbing. This means that the occurring forces should be lower than the

transportation forces.

This results in the following research question:

Are the occurring dynamic forces during an offshore discharge lower than the occurring static forces during a transport?

6.5.3 Hydraulic:

Hydraulics has potential for a final solution. It is a proven solution for float-over operations. Therefore some of its properties are already known. It is likely that during an offshore discharge the hydraulics will have to be pre-installed into the deck of the vessel. The

hydraulics cannot be placed onto the main deck due to the amount of space that is required to reach a safe clearance depth.

Since the hydraulics have to be pre-installed, it is likely that the grid used for the hydraulics is not in line with the bearing points in the cargo. In order to achieve maximum flexibility a deck may be needed. This deck will spread the forces of the hydraulics equally over the cargo. The use of a deck brings a technical challenge. If the clearance depth during the discharge is reached, the hydraulics will be retracted. This should happen within seconds to avoid the chance of an impact. This chance may occur since the cargo starts floating.

This results in the following research question:

How much time is required to pull in a pre designed deck under water, by using the hydraulic lifting system?