10072_001

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Offshore workspaceWP2 Task 7b Alternative installation

methods

Dutch Offshore Wind Energy Converter project

Dowec-072/00-P

Dowec 072 rev. 01

Name: Signature: Date:

Written by: A. Vos Ballast Nedam 14-05-2002

version Date No ofpages

0 14-05-2002 26 First issue (PRELIMINARY)

1 15-12-2003 26+1 For publication

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DOWEC-F1W1-xxx-yy-nnn/vv-C

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Contents

1 INTRODUCTION.............................................................................................................................3

2 STARTING POINTS AND CONDITIONS.......................................................................................4

2.1

PROPOSED LOCATION.................................................................................................................4 2.2 NUMBER OF WIND TURBINES AND PLANNING ................................................................................4

2.3 D ATA OF COMPONENTS ..............................................................................................................5

3 INSTALLATION..............................................................................................................................5

3.1 OPTIONS FOR INSTALLATION .......................................................................................................5 3.2 INSTALLATION METHOD – FOUNDATION MONO PILE .......................................................................6

3.2.1 Foundation Mono Pile by tilting Pontoon ..........................................................................6 3.2.2 Foundation Mono Pile by Svanen.....................................................................................9

3.3 INSTALLATION METHOD - OWEC ................................................................................................9 3.3.1 Preassembled OWEC’s ....................................................................................................9 3.3.2 Assembly on location by other ships ..............................................................................13 3.3.3 Assembly on location by Self elevating Platforms .........................................................13

3.4 OFFSHORE ASSEMBLY LOCATION ..............................................................................................13 3.4.1 Installation process .........................................................................................................13 3.4.2 General requirements .....................................................................................................14 3.4.3 Concepts.........................................................................................................................18 3.4.4 Concept I.........................................................................................................................18 3.4.5 Concept II........................................................................................................................21 3.4.6 Cost.................................................................................................................................25

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1 Introduction

In WP1 task 12 “installation of structure”, a work method is described for building a wind farm at thelocation of site III. Site III is located about 20 km offshore. For installing OWEC’s relatively near shore itproved to be possible to completely built up the superstructure onshore and transport it by the Svanento the site of the wind farm. Building a wind farm conform this installation method at he location of site

VII, implicates that the sailing time of the Svanen will be that much that it will be difficult to install thedesired amount of OWEC’s in the given time span.

This document describes the use of an offshore workspace for assembly, and accommodation ofworking crew and eventually O&M crew. It also looks at the possibilities of sheltering the installationvessel Svanen during periods of unworkable sea conditions.

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2 Starting points and conditions

2.1 Proposed location

The location of the wind park is 60 km offshore. For this report the DOWEC Site VII site is used. It is

located in the North Sea west of Hoek van Holland (see map). Although the actual sailing distance ofthe site is 55 km for the purpose of the study it is assumed that the sailing distance is 60 km.

Figure 2.1-1: Location site VII approx. 60 km offshore of Hoek van Holland

2.2 Number of wind turbines and planning

This report is based on a total number of 80 turbines with a capacity of 6 MW each. The total numberof turbines has to be installed in one year. The workability is based on DOWEC document DOWEC-F1W1-WB-01-047/00-C (Wind and Wave conditions).

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2.3 Data of components

Sizes and weights

6 MW

hub height m 95

rotor diameter m 130

mast length m 85

water depth m 35

scour depth m 0

monopile diameter m 6

monopile top m 10

monopile length m 75

monopile wall thickness mm 60

monopile toe depth m -65

transition piece top m 10

transition piece bottom m 0

transition piece length m 10

transition piece diameter m 6,42

transition piece wall thickness mm 60

nacelle weight ton 203

rotor weight ton 91

blades weight ton 111

mast weight ton 316

Total weight OWEC ton 721

transition piece weight ton 94

monopile weight ton 659

3 Installation

3.1 Options for installation

The installation of 80 OWEC’s of 6 MW at a distance of 60 km results in a combination of problems.

The main problem areas are how to install the big number of turbines in the required period of time.In the baseline case the Svanen was used to do the piling and the installation of the wind turbine of2.75 MW. In this package the capacity of the Svanen in not sufficient to be able to install the wholepark within the required time of on year (read one season).

To solve the problem we need to;

1. Double the equipment as used in WP1 (baseline).2. Break down the activities in smaller packages in order to spread the number of activities over more

workflows.

Ad 1. Double the equipment as used in WP1.

The Sailing time for the 60 km location will be 60 km / 1.6 m/miles / 4 knots = 9.5 hours. This is fromthe Maasvlakte Harbour.This results in a total cycle time of the Svanen of 55.5 hours. (Att. Fair weather planning)

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Task Name Duration

Placing activities Svanen 55,5 hrs

place anchors Svanen 5 hrs

Activities by Svanen at on-shore yard 17 hrs

Pick up foundation pile from shore 2 hrs

Reposition Svanen 1 hr

Pick up Transition piece from shore 1 hr

Pick up J-tubes from shore 0,5 hrs

Pick up windturbine from shore 3 hrs

Sail to construction location 9,5 hrs

Activities Svanen off-shore 38,5 hrs

Placing windturbine 12 hrs

Position Svanen and fix anchors 4 hrs

Final positioning Saven 1 hr

Place windturbine 4 hrs

Cleaning and coating touch-up 1 hr

Inspection by Consultant/Engineer 1 hr

Loosen anchors 2 hrs

Placing foundation pile 12 hrs

Relocate Svanen to nex t pos it ion (500 m) 2 hrs

Pick-up foundation pile and place in template 2 hrs

Final positioning and drive pile 4 hrs

Install temporary works 1 hr

Prepare and pick up transition piece 1 hr

Place top structure and adjust 2 hrs

Grouting preparation 1 hr

Grouting annulus 2 hrs

Placing J-tube 14,5 hrs

Pick-up J-tube assembly 1 hr

Place J-tube assembly 2 hrs

Cleaning and coating touch-up 2 hrs

Inspection by Consultant/Engineer 1 hr

Loosen anchors 2 hrs

Sail back to harbour 9,5 hrs

4 6 8 10 12 14 16 18 20 22 0 2 4 6 8 10 12 14 16 18 20 22 0 2 4 6 8 10 12 14 16

Thu 07 Jun Fri 08 Jun

Task

Split

Progress

Milestone

Summary

Project Summary

External Tasks

External Milestone

Deadline

DOWEC

Cycle time planning excl. down time

14-05-02

Page 1

Project: Dowec Svanen Fair Weather

Date: 14-05-02

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Looking at the additional data from the DOWEC team it was clear that an availability of uninterruptedperiods of 56 hours was 60 % during the spring and summer period.

This results in a total project cycle time of 55,5 hours/ 0,6 * 80 turbines = 7400 hours. Taking aninefficiency factor of 20% the total number of hours needed is 8880 hoursThe available hours in the same period are 184 days = 4416 hours.By doubling the equipment the available production hours are 8832, which is just sufficient to finish theproject in time.In this case the available work space on shore also has to be extended to at least 5 (1.67 timesbaseline area) temporary erection positions so that the onshore activities will match the production ofthe off shore activities.

Ad 2. Break down the activities in smaller packages in order to spread the number of activities

over more workflowsLooking at the components of the total OWEC the following parts can be identified;

• Foundation pile

• Transition piece

• Mast

• Nacelle

• Rotor• Blades

A logical split can be laid at the top of the transition piece whereby the foundation installation procedureis separated from the mast and turbine.This results in two separate workflows with each having its own equipment.

The separate workflows can be divided as various options;

1. Foundation 1. mono pile

2. tripod steel

3. tripod concrete

4. triple pile steel

5. segmented mono pile

2. Turbine 1. pre assembled

2. assembly on location For this report we choose to look further into the solution whereby the foundation consists of a monopile. We will also look at the methods for installing the OWEC pre-assembled as well as assembly onlocation.

3.2 Installation method – foundation mono pile

3.2.1 Foundation Mono Pile by tilting Pontoon

For this method the foundation activities are going to be handled by a separate foundation/ tiltingpontoon while the Svanen is doing the placing of the pre-assembled turbine. The piles can betransported to the floating tilting pontoon by means of a tugboat while floating in the water or they canbe transported on a floating barge.

At the piling location the piles are loaded into the tilting frame on the piling pontoon. The pontoon isfixed to anchors by means of which it can manoeuvre itself into the proper position for piling and it cantilt the piles in a vertical position. It is assumed that the pile has a length of 75 meters while the waterdepth is 30 meters. This results in the pivoting point to be appr. 7,5 meters above water level. If this

pivoting point can be altered in height the pile can be lowered into the seabed in a vertical position. Thebottom of the pile will sink into the bottom and its location will therefore be fixed. While the pile isresting on the bottom a crane on the tilting pontoon can place the piling hammer on the pile and drivingcan start. (Fig 3.2.1.1)

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Depending on the sensitivity of the tilting pontoon for waves and wind the maximum wave height andwind speed for this operation can be determined. For the moment we assume a workability of 60 %(being equal to the base case).

The whole proces will take 108 hours per cycle for 4 piles. This results in a total time of 20 * 108 hours/ 0,6 = 3600 hours. Taking an inefficiency factor of 20 % the total time is 4320 hours which is within theset limits. (planning dpfw 60km piling by pontoon)

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3.2.2 Foundation Mono Pile by Svanen

The mono pile cannot be split into smaller components, this in contradiction to the OWEC’s.From this it is sensible to review whether the Svanen can pile the mono pile and the OWEC can beassembled by smaller equipment. For this purpose the Svanen will load several piles in the harbour

and sail to the required location. At the construction location the pile will be brought under the mainhook of the Svanen and the hammer can be placed on the pile. Subsequently the pile can be tiltedwhile a secondary hoist assists the hammer. The hoisting height of the Svanen is 74 meters abovedeck level. The monopile is 75 meters long without the Hammer. The hammer length is 15 meterswhich means that the total length of pile and hammer is 90 meters. This means that part of themonopile has to be in the water during the tilting operation. The cycle times for this operation will be inthe order of the planning used for piling with the pontoon.

3.3 Installation method - OWEC

3.3.1 Preassembled OWEC’s

There are several ways of installing the preassembled OWEC;

• By Svanen one by one

• Pontoon for shipment

• More OWEC’s per trip

The activities of the Svanen will be limited to the placing of the pre-assembled OWEC. From theschedule it can be determined that the cycle time for this operation only is 80 turbines * 34 hours / 0,6= 4533 hours. By adding again the inefficiency factor of 20 % the total cycle time comes to 5440hours.This again is too long compared to the available 4416 hours per season. The conclusion is that thisworkmethod is not fit for its purpose. Main reason is that the extended sailing time takes up too much

working time.

If a pontoon is being used to do the transport of the OWEC the Svanen can stay offshore to install theOWEC’s. The erection of the OWEC has to be done on shore however the complete OWEC then hasto be moved from its erection position to the transport barge. For this an “industrial” sliding system hasto be used which is capable of loading an OWEC on a pontoon every day. (Fig. 3.3.1.1)

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ID Task Name Duration

1

2 Piling by Tilting Pontoon 108 hrs

3 place anchors pilling pontoon 5 hrs

4 A ct iv it ie s b y P il in g P on to on a t o n-s ho re y ar d 2 2,5 h rs

5 Load mono piles 4x 5 hrs

6 Pick up Transition piece from shore 4x 4 hrs

7 Pick up J-tubes from shore 4x 4 hrs

8 Sail to construction location 9,5 hrs

9 Activities Piling Pontoon off-shore 85,5 hrs

10 Placing foundation pile A 14 hrs

11 Positioning Piling Pontoon and tilting pile 4 hrs

12 Final positioning and drive pile 4 hrs

13 Install temporary works 1 hr

14 Prepare and pick up transition piece 1 hr

15 Place transition piece and adjust 2 hrs

16 Grouting preparation 1 hr

17 Grouting annulus 2 hrs

18 Placing J-tube 5 hrs

19 Pick-up J-tube assembly 1 hr

20 Place J-tube assembly 2 hrs

21 Cleaning and coating touch-up 2 hrs

22 Inspection by Consultant/Engineer 1 hr

23 Loosen anchors 2 hrs

24

25 Placing foundation pile B 14 hrs














39

40 Placing foundation pile C 14 hrs














54

55 Placing foundation pile D 14 hrs








63 Placing J-tube 14,5 hrs






69 Sail back to harbour 9,5 hrs

18 0 6 12 18 0 6 12 18 0 6 12 18 0 6 12 18 0 6 12 18 0

Wed 06 Jun Thu 07 Jun Fri 08 Jun Sat 09 Jun Sun 10 Jun Mon 11 Jun

Task Split Progress Milestone Summary Project Summary External Tasks External Milestone Deadline

DOWEC

Planning excl. down time

Page1

Project: DPFW 60 km monopile by pon

Date: 14-05-02

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The maximum beam of the pontoon is limited to the distance between the legs of the Svanen, which is22 m. For the reference OWEC this results in the pontoon being 22x67x4 meters.

An alternative in order to omit the sliding of the OWEC from the shore position to the transport barge isto erect the OWEC on the barge. Instead of the sliding installation a heavier crane is now needed inorder to be able to assemble the different components. If this is done it is sensible to consider using agantry like crane so that the OWEC’s can be assembled in an industrial like assembly line. Where thetransport pontoon is used as a building platform.

The third option to install the preassembled OWEC’s is to transport several OWEC’s in per trip. Thisovercome the problem of transferring the load of the OWEC at open sea from the transport barge tothe installation ship. However this means that the installation ship has to be purpose built for thissolution. (Fig 3.3.1.1)

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3.3.2 Assembly on location by other ships

From the previous piling solutions it became clear that one vessel is not able to do all the work.Compared to the base case this was caused by the extended sailing times to the wind park location.

To overcome this problem the concept of the work method has to be altered to a system whereby thesailing time is reduced. With ships like the Mayflower or A2Sea several turbines can be transported atthe same time thus effectively reducing the sailing time per turbine. After arriving at the buildinglocation the turbines can be erected with a rate of a turbine per 1,5 days (this being the 3 mWturbines). Suppose the erection time for a 6 mW turbine is 2 days then 80 turbines will take 80 * 2 daysis 160 days which is just sufficient to fit into the schedule. With no spare time for contingencies. Theseships however are not able to do the piling works for the reference mono pile because it is too heavy.For the A2Sea ship the reference depth of the Site VII (19-27 meters acc. Page 10 Terms ofReference) is too deep because the legs can extent only to water depths of 15 meters or less!. TheMayflower can operate in water depths up to 35 meters.From the above it can be concluded that this work method is only correct if a second ship is used. It isfair to conclude that one ship can place a maximum of around 4416 hours / (48 hours * 1.2) = 76turbines.

3.3.3 Assembly on location by Self elevating PlatformsInstallation of the OWEC’s by SEP’s has the advantage that the OWEC is broken up into smallerelements so the hoisting operation is less complicated. If several SEP’s can be used the workflow isdivided over several work fronts. Installation of 80 OWEC’s per season is then a matter of usingenough SEP’s.The main problem with using the SEP is the lifting of the Nacelle of 203 tons at a elevation of 95meters. Even if the deck of the SEP can come at 10 meters then still the crane has to reach 95-10+spreader beam and sling length (10 meter) = 95 meter. For this a Liebherr LH 1800 or LH 11250 canbe used. Problem with these cranes is that the total weight of the crane with counterweights consumesalmost the total deck capacity of the SEP. Therefore this is not a good solution.

3.4 Offshore assembly location

3.4.1 Installation processThe installation process is more or less the same as the process described in WP1 task 12 “installationof structure”. The only difference is that the activities, which would be carried out on an onshoreinstallation location, will be carried out on an offshore assembly location.

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System

Factory

Europort

Installation islandStorage on

barge / quay

Installation

location

Superstructure

Transshipment

seagoing barge

I n s t a l l a t i o n w i t h

c r a n e

Wind farm

Transshipment to Svanen

Rotor, nacelle,

generator, turbine

tower

Monopile,

transitionpiece

Factory

Europort

Installation island

Storage on barge

Transshipment

seagoing barge

Transshipment crane

to Svanen / SEP


S v a n e n / S E P

OWECWind farm

Element Component


S v a n e n

Foundation

S e a g o i n g b a r g e

Road, water, rail

Transshipment crane

Inland navigation

Figure 3.4-1: Installation procedure offshore assembly location

3.4.2 General requirements An offshore assembly location will consist of the following items:

- Breakwater;- Quay;- Work area;- Storage area;- Infrastructure;

- Installation equipment;- Housing facilities;

- Helicopter platform;

- Mooring facilities;

- Traffic control.

In the following paragraphs the above-mentioned items will be described in more detail.

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The italic marked items are not taken into account.Breakwater

1

General design informationFor the design of a breakwater the following information is required:

- Water depth;- Wave heights;- Frequency of occurrence of waves;

- Wind data;- Soil data.

Because this is a rough design for an indication of the cost of an offshore assembly location,

FunctionalityBreakwaters are built along the offshore assembly location for different purposes namely:

- Provide protection against waves;- Provide quieter water for ships to navigate and moor;- Guidance of currents in order to reduce the amount of scour protection.

Types of breakwatersGenerally speaking two different types of structures are used for breakwaters:

- Rubble mound;- Monolithic.

Many variations are possible based on the above-mentioned structures. Some of these variations arelisted below.

Composite – Rubble mound frontPermanent structure consisting of some form of monolithic vertical breakwater with a rubble mountform placed before and against it.

Advantages:- Low reflection of waves;- Moderate material use;

- Impervious to water and sediment;- Can provide quay facilities on lee side;- Can be built working from structure itself.

Disadvantages:- Expensive form of new construction since it requires multiple construction techniques to be

built.

1 Coastal Engineering, volume III Breakwater Design; edited by W.W. Massie, P.E.; January 1986;

Technical University of Delft; The Netherlands.

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Figure 3.4-2: Composite - rubble mound front

Composite – vertical monolithic top A permanent structure consisting of a rubblemound base, surmounted by a monolithicvertical structure.

Advantages:- Moderate use of material;- Adapts well to an uneven seafloor;- Provides a convenient

promenade.

Figure 3.4-3: Composite - vertical monolithic top

Disadvantages:- Suffers from impact forces of largest waves; Reflects the largest waves that can damage

the lower rubble mount portion;- Rubble mound must be carefully constructed in order to provide a good foundation for the

monolithic top;

- Destroyed when design conditions are exceeded.

Monolithic – porous front A permanent monolithic structure having a porous front wall which acts to absorb the oncoming waveenergy.

Advantages:- Uses relatively little material compared to rubble mound;- Less wave impact and reflection than conventional monolithic structures;- Needs little space;- Provides quay on lee side.

Disadvantages:

- Difficult to construct;- Need high quality concrete and workmanship;- Even seafloor needed;- Intolerant of settlement;- Foundation problems on fine sand;- Severe damage when design conditions exceeded.

Monolithic – sloping front A monolithic structure with the upper portion of the vertical face sloping back at an angle of about 45degrees.

Advantages:- Economical of material;

- Rather quickly constructed;- Less wave impact and reflection if compared with conventional monolithic structures;- Needs little space;- Quay facilities can be provided on lee side.

Disadvantages:- Needs even seafloor;- Intolerant of settlement;- Can have foundation problems on fine sand;- Severe damage if design condition exceeded.

Monolithic sunken caisson A temporary structure floated into place and sunk and ballasted to form an initial breakwater. Often

used to cut off currents so that it can then be buried in a permanent breakwater.

Advantages:- Very quickly placed on the site;

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- Can provide quay facilities on the lee side;- Occupies little space;- Uses little material;- Provides promenade;- Provides work road for later construction phases.

Disadvantages:

-Size limited by towing limitations;

- Easily damaged (often by moderate storm);- Foundation difficulties on fine sand bed;- Requires smooth seafloor.

Rubble mound – Pell – mell artificial armour units A permanent breakwater consisting of layers of stone and gravel protected on the exposed surface bya layer of randomly placed artificial armour units. A massive structure may be incorporated in the crestto save material.

Advantages:- Durable;- Flexible (accommodates settlement);

- Easily adapted to irregular bathymetry;- Needs no large natural units;- Functions well even when severely damaged.

Disadvantages:- Need factory for production of armour units;- Large quantities of material needed;- Needs under layer if built on sand;- Unsuited to soft ground.

Rubble mound – placed unitsPermanent structure similar to pell-mell unit placement, except that units are now individually placed ina precise pattern. A monolithic crest construction is usually used.

Advantages:- Flexible (adapts to settlement);- Uses least material of rubble mount types;- Adapts well to irregular bathymetry;

Disadvantages:- Armour units must be fabricated;- Needs much skill in construction;- Impossible to place armour under water;- Unsuited to very soft ground;- Needs under layer if it is built on sand.

Rubble mound – stonePermanent structure consisting of successive layers of stone. The exposed surface is covered withheavy armour stones.

Advantages:- Very durable (resists severe attack well);- Functions even when severely damaged;- Adapts to ground settlement;- Uses natural common available materials;- Easily adapted to irregular bathymetry;- Construction possible with limited skilled labour;- Uses common construction equipment;- Materials are usually inexpensive;

Disadvantages:- Uses the most material of all types;- Must be adapted for construction on sand;

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- Unsuited to very soft ground.

Conclusions:

- Rubble mount structures are the most durable, and as such are best suited to extremelyheavy wave attack;

- Monolithic structures use less space and material; this is especially true in deeper water.For the design of the breakwater only these two types will be taken into account.

3.4.3 ConceptsFor the design of the island two different concepts are presented. For both concepts breakwaters needto be constructed before the actual island can be built. Concept I is a permanent island and is made ina rather traditional way. Concept II is a temporary work island and built up out of prefabricatedelements. This speeds up the building time and makes it easier to demolish or even replace the islandat an other location.

Work area

I II

Breakwater

Quay

Figure 3.4-4: Schematic presentation concepts

3.4.4 Concept IConcept I is a traditional made island.Work order:

- Construction of breakwater;- Construction of quay;- Reclamation for work area;- Subsidence work area;- Infrastructure;- Installing building equipment.

BreakwaterIt may be considered to use two different types of breakwater constructions. Directly adjacent to thework area a semi overtopping breakwater is required. Semi overtopping means that during theinstallation period from February till October it should be non overtopping. In wintertime wheninstallation activities do not take place, a severe storm may overtop the breakwater. However the loadacting may not damage the breakwater during this storm. The strength of the breakwater is designedto be able to withstand winter storms. The height however can be designed for wave heights occurringduring installation periods.The other breakwater, providing quieter water conditions for mooring and transshipment activities, maybe overtopping. Both breakwaters are of the type of rubble mount breakwaters.

Rubble mount breakwater Almost every rubble mound breakwater is constructed in layers. Each layer of the breakwater must bedesigned in a way that the adjacent layer of finer material cannot escape by being washed through itsvoids. This also applies to the natural bottom material layer underneath the breakwater. If the bottommaterial consists of fine sand then a filter layer must be constructed. The outer layer must be designedto withstand the expected wave attacks. The choice of the construction material is largely determined

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by the availability in the quantities needed. A rather indescribable core material can be used to supportthe outer layer. Usually the cheapest available material is thrown in.

Depending on the function on the leeside of the rubble mound breakwater an overtopping or a non-overtopping breakwater can be used. The less critical or important the activities on the lee side themore overtopping the breakwater may be.

Figure 3.4-5: Rubble mount breakwater

Table 3-1: Cost estimate of rubble mount breakwater per meter length

Fraction Type ofplacement

Volume[m

3/m

1]

Unit price[€]

Total price[€]

Primary armor 16 t Over crest 380 58 22,040

Secondary armor 1-6 t Over crest 100 58 5,800

Barge placed 575 46.5 26,737

Quarry run 300-1000 kg Over crest 30 70 2,100

Barge placed 1,240 54 66,960

Filter gravel Barge placed 385 31 11,935

€135,572

Monolithic breakwater

As described in paragraph 0 there are a few different types of monolithic breakwaters. For a costestimation it is decided to built up the monolithic breakwater out of caissons. These are built in a drydock and floated to the site where they are ballasted and sunk in place. The cost, stated below, do notinclude the transport cost and the cost for the use of a dry dock facility.

1 0 m 1 0 m

2 0 m

3 0 m

6 0 m

Dimensions caisson:

Length 60 [m]

Width 20 [m]

Height 30 [m]

Cell width 10 [m]

Wall thickness 0.5 [m]

Amount concrete 6,300 [m3]

Mass 15,750 [tons]Cost per caisson 4.25 [mln €]

Cost per meter 70,800 [€]

Figure 3.4-6: Global dimensions caisson

QuayBecause the work area consists of pumped up sand the quay construction should provide anembankment. Therefore the quay construction should be of a solid form. Because transhipment andmooring activities take place, the boundary at the waterside should be vertical. The following types of

constructions can be used as a quay:- Cofferdam (a);- Caisson (b).

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a b

Figure 3.4-7: Quay constructions

Ad. a: ProductionTwo combi-walls (sheet piles + tubular piles) are placed in the wet. After connecting them with anchorsthe space between is f illed with sand.Installation platformFor the foundation of the installation platform there still needs to be constructed a pile foundation.

Ad. b: ProductionThe concrete caisson is built in a prefab yard somewhere onshore. It is towed to the site, ballasted andsunk in place.Installation platformThe caisson is designed to provide a platform on which the installation procedures can take place.

Conclusion: A breakwater built up out of caissons is, for this water depth, cheaper than a rubble mount breakwaterbecause it uses to much material. Therefore the breakwater will consist of caissons.It is not likely to built the quay with a cofferdam for the following reasons:The length of the tubular piles and sheet piles of the combi-wall need to be approx. 45 m. For thetubular piles this is no problem but the length of the sheet piles is limited to 30 m. This means thatscour protection is needed at the toe of the cofferdam as shown in Figure 3.4-7a;To prevent extreme deformations of the combi-wall there need to be at least two levels of anchors for aheight of 30 m. The lower level is situated below water level. Installing these anchors is extremely

difficult. Therefore the quay will also be constructed out of caissons.

Cost concept 1Dimension Amount

Total work area: 240 x 120 28,800 [m2]

Backfill sand 220x100x30 660,000 [m3]

€1/m3 0.66 [mln €]

Quay: 360 [m1]

Caisson 6 #25.5 [mln €]

Breakwater: 1,000 [m1]

70.8 [mln €]

Total cost Approx. 100 [mln €]

Cofferdam Caisson

Advantages: Low cost;No even seafloor needed;Small equipment needed to produce.

No extra foundation required forinstallation platform;Immediately functional after placing;Easy to dismantle and reuse.

Disadvantages: Large amount of offshore pile placement;Limited construction height;Extra foundation required for installation

platform;Hard to dismantle.

Even seafloor needed;Large equipment needed to produce;Prefab yard needed;

High production cost.

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3.4.5 Concept IIWork order:

- Construction of breakwater;- Placing jackets;- Placing deck boxes;- Installing building equipment.

a b

Figure 3.4-8: Island consisting of prefabricated elements

There are two different procedures for installing the deck bock:a. Floating;b. Lifting.

In the following paragraphs a concrete deck caisson will be dimensioned for the floating installationprocedure and an steel deck box will be dimensioned for the lifting procedure.

Design calculations concrete deck caissonIn order to get a global idea of the dimensions of the deck some design calculations are made for adeck caisson that is floated in place. The following aspects are taken into account:

- Floatability;- Strength.

Ad. 1 FloatabilityIn order to float the deck caisson on top of the jacket a substantial free board is required as show inFigure 3.4-9.

Free board (F)

Clearance (C)

Quay heigth (Q)

HAT

LAT

Deck heigth (H)

Figure 3.4-9:Required free board and clearance for floating installation

The difference between HAT and LAT is 2m according to the terms of reference. Assuming that:- Installation of the deck caisson is carried out during a sea level of 50% of HAT (MSL

+0.50m);- Minimum clearance 1.00 m;- Minimum quay height to HAT 2.00 m.

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Page 22 of 25

2-3 hours

Figure 3.4-10: Installation windows floating installation

Freeboard = Clearance + quay heightFreeboard = 3.00 m.In order to create a freeboard of approx. 3 meters the following dimensions for the deck caisson aredetermined.

6 m

4 0 m

10 m 10 m

30 m

0.4 m0.5 m0.5 m

0.4 m

Dimensions concrete deck caisson:

Length 40 [m]

Width 30 [m]

Height 6 [m]

Wall thickness 0.4-0.5 [m]

Amount concrete 1,400 [m3]

Mass 3,500 [tons]

Freeboard 3 [m]

Cost 0.95 [mln €]

Ad. 2 StrengthIt is assumed that some type of crawler crane or other crane carries out the installation on top of thedeck caisson. The mass of the crane including its load is about 600 tons.

Own weight: ]kN/m[87540

10500,3 14

=⋅

;

Crane load: kN][6000 ;

30 m5 m 5 m

Max. bending moment: [kNm]000,14530600030875412

81

=⋅⋅+⋅⋅ ;

Moment of resistance: ][m2232.527630 43

1213

121

=⋅⋅−⋅⋅ ;

][m7465.0

223 3=

⋅

Stress: ][N/mmW

Mσ

2= ][N/mm2.0

74

10145,000 23

=⋅

.

Deflection: l[m]0.052231030,00048

30106,000

48EI

Fl60014

333

⋅≈=⋅⋅⋅

⋅⋅

=

The deflection by own weight and the positive stress are compensated by post tensioning. Theremaining deflection of 0.05m by the crane is acceptable for the span of 30m.

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Design calculation steel deckThe main span of the steel deck will be built up out of trussed girders as shown in Figure 3.4-14. Ontop of the main girders, tangential to the main span, deck girders will spread the load of the crawlertracks. The load of each crawler track is spread over two main girders.

3 m

10 m

1.5 m

8.8 m

Steel trussed girders

Figure 3.4-11: Top view crawler crane

Crane load: kN][6000 ;

During lifting operations this load is not equal spread over the two crawler tracks. It is assumed to be

75% and 25% per track. The maximum load per track is 4500 kN.

Deck girders

Max. bending moment: [kNm]255 ;

Allowable stress: ][N/mm150 2;

Moment of resistance needed: /m][mm107.1150

10255][mm

σ

MW 36

63

⋅=⋅

== ;

Profile choice: 4 HE180 B per meter;Mass deck: 2.05 kN/m

2; 245 tons in total.

Main girders

30 m5 m 5 m

3 . 7

5 m

3.75 m

HE 450 B

HE 450 B

` ` ` ` ` ` ` `

Figure 3.4-14: Steel trussed girders

Own weight girder: Assume: Top and bottom girder profile HE450B. Diagonals 75% of mass of top and bottom girder.

][kN/m.67]Ton/girder [5.30]kg/m[125]m[135]kg/m[171]m[80 111==⋅+⋅ ;

Max. bending moment: [kNm]100,14125,285.030)05.236.7( 2

81

=⋅+⋅⋅+⋅ ;

Plain stress truss: ][mmAσz2

M 2=

⋅⋅

;

0.75 m 1.5 m 0.75 m

300 kN/m' 225 kN 225 kN

225 kN

225 kN

255 kNm

Figure 3.4-12: Critical load case deck girder

10 m 10 m 10 m

450 kN/m'2,250 kN 2,250 kN

2,250 kN

2,250 kN

28,125 kNm

Figure 3.4-13: Critical load case main girder

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][mm500,28150450)-(3750

1014,100 26

=

⋅

⋅

z

HE 450 B

HE 450 B z

H

h F

F

Bending top girder:][mm102,333

150

10350

σ

MW

[kNm];3502.5450M

336

2

81

⋅=⋅

==

=⋅⋅=

Profile choice: 2x HE 300B (For top girder and bottom girder)Mass: 235 kg/m

][kN/m.610]Ton/girder [5.42]kg/m[175]m[135]kg/m[235]m[80

111==⋅+⋅

Total mass steel deck

Deck girders 245 [ton]Main girders 605 [ton]Extra 10% 75 [ton]

Total: 885 [ton]

Cost 1.75 [mln €]

Design calculation steel jacket The dimensions of the jacket are indicative and based on experience with these type of offshore

constructions.

Support reactions: [tons]49025060088561

21

61

=⋅+⋅+⋅ ;

Stress: ][N/mm55)1460(1500π

10490 2

22

41

4

≈

−⋅

⋅;

Cost concept 2

AmountBreakwater:Caisson 1,000 [m

1]

70.8 [mln €]

1 0 m

10 m 10 m

2 2 - 2 7 m

20 m

Dimensions jacket:

Ø 1500.30 160 [m]

Ø 1000.20 120 [m]

Ø 400.10 180 [m]

Amount steel 32 [m3]

Mass 250 [tons]

Cost 0.35 [mln €]

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Quay: Floating 160x30 [m1]

Deck caisson 4 #Support jacket 5 #

5.55 [mln €]Quay: Lifting 160x30 [m

1]

Steel deck 4 #Support jacket 5 #

8.75 [mln €]Finger piers 3 #

Support jacket (3) 1.0 [mln €]Steel deck (3) 1.0 #

2.0 [mln €]

78.4 / 81.6 [mln €]

Total cost (min) 78.4 [mln €]

3.4.6 CostComparing the two concepts the following can be concluded:

- Concept 2 seems to be less expensive than concept 1- The material cost for an offshore assembly area are estimated to be approx. 80 mln Euro;

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