Conceptual Design of a
Time-Efficient Method for the
Instal-lation of Mono-piles exceeding
Crane Capacity
Amber Vellekoop
Master
of
Science
Thesis
Conceptual Design of a Time-Efficient
Method for the Installation of
Mono-piles exceeding Crane Capacity
Master of Science Thesis
For the degree of Master of Science in Floating and dredging
Engineering at Delft University of Technology
Amber Vellekoop
October 30, 2015
Supervisor:
Prof. dr. ir. R. H. M. Huijsmans
Thesis committee: Ir. K. Visser,
TU Delft
Ir. J. den Haan,
TU Delft
M Kershaw
Van Oord
Faculty of Mechanical, Maritime and Materials Engineering and Civil Engineering · Delft University of Technology
Abstract
The offshore wind energy industry is growing rapidly. Parks and turbines grow in size and capacity. Contractors are hereby forced to develop installation methods, capable of handling large size and weight substructures. These new methods are preferably executed with already existing installation equipment. This research develops a new mono-pile installation method for existing equipment, in which the pile to be installed exceeds the crane capacity.
A four phase approach was used to conceptual design a time-efficient method for the installa-tion of mono-piles exceeding crane capacity. First, a study on current existing methods was performed. The most important criteria, and factors that influence the level of these criteria were defined. The most important criteria are: safety, time, workability and costs, in which costs is mainly a measure to reach a certain criteria level. It appeared that methods with a high level of safety, workability and a short duration were basically always higher on costs. Furthermore, safety was identified as the most important criterion.
These criteria were used to evaluate the different concepts generated in phase two of the design process. It came forward that there are a lot of possibilities regarding the stated challenge. To channel all these possibilities to one most suitable solution, a four step approach is established. In the first phase, as many as possible concepts, which satisfy the objectives, are conceived. Second, the concepts were eliminated on basis of safety. Fundamental unsafe concepts were excluded. The third step was the elimination on basis of practicability. Three upending concepts and three load reduction concepts were identified as suitable solutions at the end of phase three. The last step was a multi criteria analysis to define the most suitable concept. The most suitable installation concept regarding crane capacity exceeding mono-piles is by upending the pile in water and to use trapped air as load reduction mechanism in the crane. The pile is upended in water and subsequently positioned in the gripper. Then, the pressure inside the pile is increased so that the bottom plug can be removed. However, the pile remains its floating capacity due to ’trapped air’. In this way, the weight in the crane can be controlled and does not exceeds its capacity. The pile is then lowered to seabed whereby the pressure in the pile is maintained. Since the volume decreases, a valve needs to be applied which regulates the pressure and volume within the pile. Once the pile is lowered on the bottom, water is inserted to reduce the probability of soil parts to flow in when pressure is decreased. When the water level and pressure inside correspond to outside values, the top plug can be removed.
ii Abstract
to a higher level. The feasibility of the concept was evaluated on the three main criteria of this thesis. Safety was examined by a Risk Assessment and the largest risks were identified in plug handling and plug design and, the dynamic motions of the pile during upending. A time schedule for one cycle of pile installation was established and it appeared that the mean installation time was only 1 hour and 5 minutes longer. For all installation steps the main operational limits were determined. However, it occurred that there was currently not enough insight in the dynamic motions of the pile during upending to establish the operational limits of this operation. For this reason, a dynamic motion analysis on the upending process has been executed.
In the dynamic analysis it came forward that there are two natural modes that influence the motions of the pile during upending. One of the natural frequencies of these modes approaches to the wave frequencies. The dynamic load on the crane was not expected to exceed the crane capacity due to buoyancy forces working on the pile. It was on the other hand expected that side lead angles of the crane line exceed their limits.
Non linear time domain software OrcaFlex was used to model the upending process in different environmental conditions. In the model, the mono-pile is upended from horizontal to vertical position. Three load cases were studied to establish the operational limits of the upending procedure in which wave heights, periods, and approach angles are varying. The operational limits were subsequently established and a scatter workability- and persistency workabilty analyses was executed for one specific location.
This thesis concludes with an advise whether the ‘Floating Upending, Trapped Air Concept’ is a feasible concept to install mono-piles exceeding crane capacity.
Table of Contents
Abstract i Acknowledgements xv 1 Introduction 1 1-0-1 Wind energy . . . 1 1-0-2 Aeolus . . . 1 1-1 Problem Definition . . . 3 1-1-1 Objective . . . 4 1-1-2 Approach . . . 42 Phase 1: Existing method study 7 2-1 Criteria . . . 7
2-1-1 Costs . . . 8
2-1-2 Workability . . . 8
2-1-3 Safety . . . 9
2-1-4 Time . . . 9
2-2 Features of an installation method that influence the score on the criteria . . . . 10
2-3 Aeolus Oppportunities . . . 11
3 Phase 2: Concept Generation 13 3-1 Brainstorm . . . 15
3-2 Elimination Gate 1: Safety and Costs . . . 16
3-3 Elimination Gate 2: Practicability . . . 17
3-3-1 Upending concepts . . . 18
3-3-2 Crane load reduction concepts . . . 19
3-4 Multi-Criteria analysis . . . 21
iv Table of Contents
3-6 Identify options . . . 23
3-7 Criteria . . . 23
3-8 Score Options . . . 23
3-9 Weight of Criteria . . . 26
3-9-1 Ratio score of criteria . . . 26
3-10 Calculate the results . . . 27
3-11 Sensitivity analysis . . . 28
3-11-1 Weight factor sensitivity . . . 28
3-11-2 Score sensitivity . . . 29
3-11-3 Results and Conclusion . . . 30
4 Phase 3: Feasibility and practicability study 33 4-1 Installation steps . . . 33
4-1-1 Transport and Hook-in - Figure 4-1 . . . 34
4-1-2 Pile upending - Figure 4-2 . . . 34
4-1-3 Pump water in pile - Figure 4-3 . . . 35
4-1-4 Supply compressed air to pile - Figure 4-4 . . . 35
4-1-5 Lower Pile and insert water - Figure 4-5 . . . 36
4-1-6 Removing the top plug . . . 36
4-2 Closing plugs . . . 37
4-3 Statics of the upending proces . . . 38
4-3-1 Pile specifications . . . 38
4-3-2 Pile Upending Phases . . . 38
4-4 Pressure development . . . 42
4-5 Feasibility . . . 43
4-5-1 Safety . . . 44
4-5-2 Risk Assesment for the Operation . . . 44
4-5-3 Time . . . 45
4-5-4 Workability . . . 46
4-5-5 Feasibility conclusion . . . 47
5 Dynamics of upending a mono-pile 49 5-1 Natural frequencies . . . 49
5-1-1 Natural frequencies and inclination angles . . . 50
5-1-2 Mode 1 . . . 51
5-1-3 Mode 2 . . . 52
5-1-4 Mode 3 . . . 54
Table of Contents v
5-3-2 Mono-pile . . . 58
5-4 Design Limits . . . 59
5-5 Model verification . . . 61
5-5-1 Upending (without waves or current) . . . 61
5-5-2 Natural frequencies . . . 63
5-5-3 Conclusion . . . 65
5-6 Load cases . . . 65
5-6-1 Waves . . . 65
5-6-2 Current . . . 66
5-6-3 Wave heights and periods . . . 67
6 Results 69 6-1 Load case 1: Waves . . . 70
6-1-1 Time domain analysis . . . 70
6-1-2 Batch analysis results . . . 71
6-2 Load Case 2: Waves approach from 90 degrees and current direction changes . . 72
6-2-1 Time domain analysis . . . 73
6-2-2 Batch analysis results . . . 74
6-3 Load Case 3: Waves and current, waves approaching from 135 degrees . . . 76
6-3-1 Time domain analysis . . . 76
6-3-2 Batch analysis results . . . 77
6-4 Upend duration . . . 78
6-5 Analysis on different pile masses . . . 79
6-6 Discussion on Damping . . . 80
6-6-1 Batch analysis results . . . 81
6-7 Pile Motions . . . 83
6-8 Conclusions of Dynamic analysis . . . 84
6-8-1 Crane Load . . . 85
6-8-2 Side-Lead . . . 85
6-8-3 Off-lead angle and pile position . . . 85
7 Workability Analysis 87 7-1 Operational conditions of upending operation . . . 87
7-2 Operational limits total operation in waves . . . 88
7-3 Environmental conditions . . . 89
7-3-1 Wave scatter . . . 89
7-4 Workability . . . 90
vi Table of Contents
8 Discussion and Conclusion 93
8-1 Discussion . . . 94 8-1-1 Concept generation . . . 94 8-1-2 Concept . . . 95 8-1-3 Dynamic analysis . . . 96 8-2 Recommandations . . . 99 8-2-1 Dynamic analysis . . . 99
8-2-2 Waves approaching in angle of 45 degrees with pile . . . 100
8-2-3 Dynamic motions in gripper . . . 101
A Existing method study 103 A-1 Introduction . . . 103
A-2 Analysing installation methods . . . 105
A-2-1 Installation by Excalibur . . . 105
A-2-2 Installation by Taklift . . . 105
A-2-3 HLV Svanen . . . 106
A-2-4 installation method by jack-up vessels like Aeolus . . . 106
A-2-5 Pile installation by Oleg Strassnov . . . 107
A-2-6 Installation by a method performed with Lisa-A . . . 107
A-2-7 Conclusion . . . 108
A-3 Design Blocks . . . 108
A-4 Criteria . . . 109
A-5 Evaluation of the design blocks on workability, safety and time . . . 109
A-5-1 Criterium: Workability . . . 110
A-5-2 Criterium: Safety . . . 114
A-5-3 Criterium: Time . . . 122
A-6 conclusion . . . 126
A-6-1 Method features that influence criteria . . . 126
A-6-2 Relation between workability, safety and time . . . 126
A-6-3 Best solution per block . . . 128
A-6-4 Solutions for Aeolus . . . 130
A-6-5 Next phase . . . 131
B Brainstorming 133 B-1 morphological analyses . . . 134
B-2 Block: Upending . . . 134
B-3 Vessel positioning . . . 137
B-4 Load reduction on crane . . . 137
Table of Contents vii
D Gate 2: Practicability and costs 145
D-1 Design parameters . . . 145
E Sensitivity analyses 151 E-0-1 Weight factor sensitivity . . . 151
E-0-2 Score sensitivity . . . 152
E-0-3 Upending . . . 154
E-0-4 Load reduction . . . 156
F Duration op the operation 161 G Risk Matrix 165 H Equation of Motion of a floating object hanging in a crane 167 H-0-5 Forces on floating structures . . . 168
H-0-6 Forces on floating slender structures . . . 169
H-0-7 Inertia forces . . . 169
H-0-8 Drag forces . . . 170
H-0-9 Linear Damping forces . . . 170
H-0-10 Equation of motion for floating (semi) slender structure hanging in crane 171 I Hydromechanics 173 I-0-11 Irregular waves . . . 174
J OrcaFlex 177
List of Figures
1-1 Van Oords offshore installation vessel Aeolus . . . 2
1-2 Cranes upending a mono-pile from deck . . . 2
1-3 Aeolus lifts mono-pile from deck and lowers it to the seabed . . . 3
1-4 Mono-pile hammered into seabed . . . 3
1-5 Approach . . . 5
2-1 Model safety vs risk [Suddle, 2002A] . . . 10
3-1 Approach in flow chart diagram . . . 14
3-2 Concept 1: upending of floating pile . . . 19
3-3 Concept 2: upending of pile with seabed support . . . 19
3-4 Concept 3: upending floating pile with barge support . . . 20
3-5 Concept 1: Trapped Air . . . 20
3-6 Concept 2: Pigging Tool . . . 21
3-7 Concept 3: Floaters inside pile . . . 21
3-8 Probability density functions of the total scores of the 3 upending concepts with a deviation in weight factors . . . 30
3-9 Probability density functions of the total scores of the 3 load reduction concepts with a deviation in weight factors . . . 30
3-10 Probability density functions of the total scores of the 3 upending concepts with a deviation on score . . . 31
3-11 Probability density functions of the total scores of the 3 load reduction concepts with a deviation in score . . . 31
4-1 Hook in pile in Crane . . . 34
4-2 Pile Upending by Aeolus . . . 34
4-3 Pile positioning in gripper and water supply . . . 35
x List of Figures
4-5 Lower Pile and Insert water . . . 36
4-6 Closing mechanism of plug for flanged pile and plug with trunions for specific project 38 4-7 Inclination angle due to crane line tension in Phase 1 . . . 40
4-8 Sum of moments around hook point . . . 41
4-9 Van Oords Risk Assesment Matrix (RAM) . . . 44
5-1 First, second and third natural modes . . . 50
5-2 2 degrees of freedom for mode 1 in vertical pile position . . . 52
5-3 Visualisation of the second mode . . . 53
5-4 Dynamic system Mode 3 (left) and Fundamental periods of Mode 1,2,and 3 (right) 55 5-5 OrcaFlex model of floating pile upending operation by Aeolus . . . 57
5-6 Model input data . . . 58
5-7 Mono-pile input data . . . 58
5-8 Drag coefficient for fixed circular cylinder for steady flow (left) and Static Load chart of main crane[22] (right) . . . 60
5-9 Static crane load and inclination angle during upending calculated by OrcaFlex and analytical . . . 62
5-10 Load case 1, 2 and 3 . . . 66
6-1 Pile motions in mode 1 and mode 3 . . . 70
6-2 Y position topplug over time . . . 71
6-3 The most probable maximum tensions for waves approaching from 90 and 135 degrees . . . 72
6-4 Most probable maximum offlead- and side angle and minimum y position . . . . 72
6-5 Time domain simulations of pile bottom tip for current approaching from 135 degrees 73 6-6 time domain simulations for current 135 and 180 degrees . . . 74
6-7 Most probable crane load for different current angle approaches . . . 75
6-8 Most probable side-lead angles for different current angle approaches . . . 75
6-9 Most probable maximum offlead angle and minimum y position . . . 76
6-10 Time domain simulations of x and y position for current approaching from 90 in waves approaching form 135 degrees . . . 77
6-11 Crane Load for waves approaching from 135 degrees for 10t and 40t pull force . 78 6-12 Side-Lead angles for waves approaching from 135 degrees for 10t and 40t pull force 78 6-13 Maximum offlead angles and Y positions of bottom tip for waves approaching from 135 degrees for 100KN and 400 KN pull force . . . 79
6-14 Crane load for 595 and 600 seconds of crane wire pull-in for current approaching from 270 degrees . . . 79
6-15 Most probable maximum crane loads and offlead angles for 4 different pile masses 80 6-16 Y and Z motions of pile for the old and new damping . . . 81
List of Figures xi
6-19 MPM of Z velocities and pile bottom position for the two different damping
coef-ficients . . . 84
6-20 MPM of Z velocities and pile bottom position for SMALL damping and different current situations . . . 84
7-1 Operational limits of upending for waves approaching in line with crane . . . 88
7-2 Operational limits of the pile floating . . . 89
7-3 Wave scatter data of project . . . 89
7-4 Difference between workability based on scatter and based on persistency . . . . 91
7-5 Persistency workability analysis of the installation of one mono-pile . . . 91
7-6 Scatter- and Persistency workabilities of the installation of one mono-pile . . . . 92
8-1 Pile motions in mode 1 and mode 3 . . . 93
8-2 Y motions of pile top of 3 seeds for different pull in velocities . . . 96
8-3 Most probable maxima of the velocities for Hs=1.2m and Tz=3.02 3.61 and 4.19 sec and a vavriating added mass Ca . . . 98
8-4 OrcaFlex hydrodynamical estimations . . . 99
A-1 Overview process . . . 104
A-2 Mono-pile installation by Excalibur . . . 105
A-3 Taklift upending a mono-pile . . . 106
A-4 Svanen upending a mono-pile . . . 106
A-5 Pile installation by Aeolus . . . 107
A-6 Oleg Strassnov upending a mono-pile . . . 107
A-7 Lisa-A upending a mono-pile . . . 108
A-8 Overview of score on time . . . 113
A-9 Suitability of solutions of building blocks on safety . . . 121
A-10 5 factors that influence safety . . . 122
A-11 Overview of score on time . . . 125
A-12 Framework criteria . . . 127
A-13 Overview of score on time . . . 128
A-14 Legend . . . 128
A-15 Overview of score on time . . . 129
B-1 Concept 1 and 2 for XL pile installation . . . 135
B-2 Concept 3 and 4 for Xl pile installation . . . 135
B-3 Concept 5 and 6 for XL pile installation . . . 136
B-4 Pile installation concepts 7 and 8 . . . 136
B-5 Pile installation concepts 9 and 10 . . . 137
B-6 Load reduction concepts 1 and 2 . . . 138
B-7 Load reduction concepts 3 and 4 . . . 138
xii List of Figures C-1 overview on elimination . . . 140 C-2 overview on elimination . . . 143 C-3 Pipeline pig . . . 143 D-1 overview on elimination . . . 146 D-2 Guided by slings . . . 147
D-3 Concept: trunion tool . . . 148
D-4 overview on elimination . . . 149
D-5 Load reduction concepts 1 and 2 . . . 149
D-6 Load reduction concepts 1 and 2 . . . 150
E-1 Probability density functions of the total scores of the 3 upending concepts with a deviation on weight factor . . . 155
E-2 Probability density functions of the total scores of the 3 upending concepts with a deviation in criteria score . . . 155
E-3 Probability density functions of the total scores of the 3 load reduction concepts with a deviation in weight factors . . . 157
E-4 Probability density functions of the total scores of the 3 load reduction concepts with a deviation in weight factors . . . 158
“You can paint with all the colors of the wind” — Pocahontas
Acknowledgements
This report contains the results of my thesis project performed at Van Oord Marine Contrac-tors. The thesis forms the final phase of the Offshore and Dredging Engineering Master at the Delft University of Technology.
On the first of January 2015, I started my thesis at Van Oord in Gorinchem. Now, 10 months later, I am looking at a the results of my research. I had never expected that I could write such a report as this one. I am therefore very grateful for the chance that is given to me to work on such an inspiring and creative subject.
I would like to thank Prof.dr.ir. Huijsmans for the guidance during my thesis and pointing out some interesting academic aspects of my research. Furthermore I would like to thank Ir. K. Visser for his help on the design process and Ir. Joost den Haan for taking the to review my reseacrh.
I would like to thank Martin Kershaw for his encouraging words and the faith he had in me through the process. Additionally I want to thank my colleagues at Van Oord for the help they always gave me when I asked. They made working on my thesis inspiring and fun. Especially I want to thank Daan Scheltens, for teaching me OrcaFlex and the patience that comes with that...
Last but not least I want to thank my parents, for listening to stories which they did not understand. I do not tell them enough how glad I am with them.
Chapter 1
Introduction
“At a time when fossil fuel prices are spiralling, the threat of irreversible climate change is on everyone’s minds and serious questions are being raised over the cost and safety of nuclear, wind energy is considered more widely than ever a key part of the answer.”(European Wind Energy Association, Kjaer, Zervos [1])
1-0-1 Wind energy
Wind energy has now established itself as an essential renewable energy source of today and tomorrow. Present day, wind turbines on land and sea generate about 3% of the global electricity supply [2]. In Europe this share is up to 5.3 percent and the trend is still growing. Due to political set goals and the general awareness on renewable energy, wind energy is gaining in popularity. For the EU only, an annual production of wind energy is expected to be 24.8 GW in 2020. This is twice the annual production of 2010. 17 % of this annual production is expected to be from offshore wind (EWEA 2011 [1]). Obviously, contractors as Van Oord want to play a part in the development of Offshore wind energy.
Offshore Wind turbines consist of a foundation, a tower, a generator and blades. For shallow water, long cylindrical shaped piles (mono-piles) are the most commonly used foundation. This is mainly due to their easy installation. The tower can be put straight on top of the mono-pile, or on a transition piece (TP). The mono-pile continues via the water and penetrates into the seabed. Aeolus, Van Oord’s Offshore Installation Vessel, is designed to install these mono-piles and transition pieces.
1-0-2 Aeolus
Aeolus is a jack-up vessel, which means that the vessel can elevate, or jack, itself out of the water. The vessel is in jacking-mode totally supported by its legs and is not floating any more. An illustration of Aeolus in the two modes is provided in Figure 1-1. A large advantage of
2 Introduction
this jacking possibility is that the vessel does not experience any motions due to waves and current during installation. This ensures more precise installation of offshore structures.
(a) In sailing mode (b) In Jack-up mode Figure 1-1: Van Oords offshore installation vessel Aeolus
This thesis focusses on mono-pile installation by Aeolus. A small description of the common installation method is provided in the following section:
The piles are horizontally stacked on main deck for easy upending by the main crane. The tailing crane supports the operation by managing the other side of the pile. Once upended, the pile is lifted out of the tailing crane and moved to the other side of the vessel. There, it is slowly lowered through the gripper into seabed. The gripper controls the lateral motions of the pile during lowering and later, also during installation. Once the pile is lowered into the soil, the lifting tool of the crane is changed for a hammering tool. The pile is subsequently hammered into the seabed, while small corrections to the pile position can be made by the gripper.
An illustration of the installation is provided in Figure 1-2, 1-3 and 1-4.
(a) (b)
1-1 Problem Definition 3
(a) Crane circles monopile from deck to gripper (b) Crane lowers mono-pile to seabed Figure 1-3: Aeolus lifts mono-pile from deck and lowers it to the seabed
(a) Crane picks up hammer (b) Mono-pile hammered into seabed Figure 1-4: Mono-pile hammered into seabed
1-1
Problem Definition
Van Oords offshore installation vessel Aeolus has a main crane lift capacity of 900t. Currently, wind turbine capacity, and thereby weight, has risen to such levels that mono-piles exceeding the crane capacity have to be installed.
Also, Aeolus can only bring a small number of piles to the installations site. When these piles are installed, new piles have to be picked up in the harbour. These pile pick-up trips are an economical disadvantage since they are time consuming.
Class rules do currently not allow an upgrade of the crane capacity on the vessel. However, Van Oord still would like to participate with the Aeolus in this XL mono-pile installation market. By developing a time efficient alternative installation method to install mono-piles exceeding the 900t crane capacity, Van Oord can compete in this market.
4 Introduction
1-1-1 Objective
The objective of this thesis is to design a safe and adequate mono-pile installation method that can be used for lifting mono-piles that exceed the current maximum crane capacity of 900t of Van Oords offshore installation vessel Aeolus.
Literature studies, inter collegial discussion, Risk analyses, dynamic motion models will all be used to verify the applicability and feasibility of the defined installation method.
1-1-2 Approach
This thesis is a research to the possibilities to solve the stated problem regarding crane capacity exceeding mono-piles. The goal of this thesis is not to analyse one specific aspect of the installation process. It is a study to all the different components and factors that influence such an installation method.
First, a literature study to the current mono-pile installation methods is executed. The most important design aspects for a mono-pile installation method are distinguished in this phase. These design aspects will be used in phase two to generate new concepts and evaluate them. By means of a step-wise elimination approach, the most suitable concept regarding the design is identified. Thereafter, the concept is specified to a higher level, feasibility and practicability will be analysed, and the main questions regarding these aspects are identified. By means of a risk assessment it is determined which challenge or question requires more attention to evaluate the feasibility of the concept. This feasibility question is consequently researched in the final phase of this thesis. At the end of this thesis the feasibility and practicability of the designed concept should be clear.
1-1 Problem Definition 5
Chapter 2
Phase 1: Existing method study
In the literature study of this thesis, existing mono-pile installation methods were analysed to indicate the most important design aspects regarding mono-pile installations. The installation methods are analysed to investigate if there are currently (parts of) solutions available to establish a new mono-pile installation method for Aeolus which reaches the set objectives about time-efficiency and crane capacity exceeding piles. Three goals were achieved in this study:
1. The most important criteria on which a mono-pile installation method should be eval-uated were determined
2. The most important features of a mono-pile installation method that influence the score on the main criteria were identified.
3. Design opportunities for time-efficient XL mono-pile installation with Aeolus were dis-tinguished
The following sections, represent these goals. First, the most important criteria are discussed. Then, the most impactful features that influence these criteria are described. Finally, the opportunities with regard to this thesis goal are illustrated. This section of the report provides an executive summary of the literature study. The total literature study is provided in Appendix A.
2-1
Criteria
The most important criteria to evaluate a mono-pile installation method on, are workability, safety and time. Costs are considered a measure to reach a certain level of score on the criteria. The challenge for an engineer in the design of an installation method is to search for the right balance between the different criteria plus costs.
The following sections discuss why and how the criteria, costs, workability, safety and time, are important.
8 Phase 1: Existing method study
2-1-1 Costs
Van Oord is a for-profit company and consequently needs to deliver results. Cost management is therefore important to reach set goals. Most work is tendered to off-shore contractors. Apart from pricing, quality and following set requirements are to be determined to select the best contractor. Most economical offerings have best chance to survive the tendering process, but als workability, safety measures and timing are important to consider in awarding the work. The challenge therefore is to find the optimal balance in cost versus criteria.
2-1-2 Workability
According to van der Wal and de Boer (2004) [3] workability is the percentage of time that the environmental conditions at a given location meet the operational conditions for a certain procedure.
The operational conditions, are the maximum environmental conditions whereby the proce-dure can be satisfactory executed. If, at a certain location, the environmental conditions are mostly smaller then the operational limits of a procedure, then the workability is high. A high workability consequently leads to a higher cost-efficiency of the installation method since there is a low probability on downtime during operation.
Workability is in general statistical determined by use of wave scatter diagrams. These diagrams are based on historical data and show the probability of a certain wave height to occur as a function of direction and period. By comparing these diagrams with the operational limits the workability can be determined.
The operational limits of procedures are generally described by ’downtime lines’: the maxi-mum allowable wave height as function of primary wave direction and wave period [3]. When sea states exceed this downtime line, the considered procedure may, or can not be safely exe-cuted. The operational limits are determines by the response criteria, the maximum allowable response values of a procedure. These response criteria can for instance be the maximum ves-sel motions or crane capacities.
First, the response criteria for a procedure are determined. Then, the thereby belonging environmental conditions which excite these response criteria are establish. Finally, these operational limits are compared to the wave scatter data and consequently the workability can be determined.
Persistency
According to Verwey, Serraris and Huijsmans [4]there is a significant influence of the duration of the procedure and the duration of the environmental conditions on the workability of the procedure. This can be analysed by a time-domain persistency analyses. The workability is in this analyses determined by "Checking for each subsequent time step which operational mode is applicable and if this mode can be carried out" As discussed by van der Wal and de
2-1 Criteria 9
A procedure with a short duration needs a smaller time gap then a procedure with a long duration. The number of workable time gaps will therefore be higher. To investigate the workable time gaps of a large procedure, the procedure is split up into small procedure steps with their own operation criteria.
Combining all these steps into one procedure, will lead to the time domain operation criteria for the total of the procedure. With these time domain operation criteria, the workability can be determined. For example, if a procedure exists of 3 time phases which subsequently require a maximum significant wave height of 1, 0.2 and 0.8m then, the percentage of time where these environmental limits are actually achieved (for the prescribed duration) is the workability.
The duration of certain phases should therefore be taken into account in workability analyses.
2-1-3 Safety
Safety is described by Vrouwenvelder et al (2001) as ’a state of being adequately protected against hurt or injury, free from serious danger or hazard’.
To describe or determine the level of safety, the level of risk is analysed. Risk is described by the UK’s Health and Safety Executive (2001) as ’the chance that someone or something that is valued will be adversely affected in a stipulated way by the hazard’ [5]. More straightforward is the Oxford dictionary explanation of risk: the possibility that something unpleasant will happen.
Risk is a function of probability and consequence. The consequences can be expressed in different occurrences like injuries, material damage or environmental harm. Probability of an undesired event is often described in terms of frequency per year. It should be clear that determining this probability is not easy or straightforward.
According to Suddle (2003) [6] safety is complementary with the level of risk. A safe situation has a low level of risk and an unsafe situation has a high level of risk. In his study, consequences and therefore also risks, are expressed in terms of costs. To lower the risk, investments for safety measures should be made. Suddle states that the higher the investments, the lower the level of risk will be, seen in figure 2-1. On the other side, when the risk is substantial high, the costs will rise as well, as the hazards are likely to occur. An optimum level of safety should therefore be estimated. There will always be a residual risk, since not all risks can be eliminated. Norms and regulations limits the acceptable risk. By engineers choice, this risk can be eliminated even more. In practice, this risk assessment is often done following the ALARP principle, As Low As Reasonbly Practicle.
2-1-4 Time
"Remember that time is money" is a popular business quote once written by the famous statesman and scientist, Benjamin Franklin. He illustrates that when no work is carried out, time is wasted, and therefore money is wasted in two ways: by not earning money and by spending money at the same time. A procedure which is short in duration compared to others saves money. Less people have to get paid, less fuel is used and there is less devaluation of the equipment. Of course all these aspects are dependent on the used equipment and material
10 Phase 1: Existing method study
Figure 2-1: Model safety vs risk [Suddle, 2002A]
but generally seen, a procedure which takes less time is lower on cost then a procedure with a longer duration, under the same conditions.
Also, a short procedure has a lower probability of incidents. There is less time that the procedure is exposed to eventual hazards. On top of that, a short procedure has a higher workability in terms of persistency. Since the procedure is shorter, a smaller gap in the time analyses has to be found to do the installation, and there will consequently be more of these suitable gaps.
After all, time is an important criteria because it can reduce costs, lower the probability on incidents and ensure a higher workability.
2-2
Features of an installation method that influence the score on
the criteria
The score on a certain criteria is determined by different features or factors. The following sections discuss the factors that influence the score on the three criteria workability, safety and time. As discussed, cost is considered a mean to reach a certain level of the criteria and will therefore not be discussed.
The workability of a method is determined by the sensitivity of the pile to dynamic wave loads. Operations are harder to complete when great motions are involved. When a pile is floating in water it is more sensitive to wave loads than when it is positioned on a vessel. Also,
2-3 Aeolus Oppportunities 11
Safety is dependent on probabilities and consequences. Generally, the amount of control determines the probability of an incident. If an operation is totally controlled the risk on accidents is low. But, absolute control on an operation is almost never achieved. Three factors ensure the probabilities on accidents. First, the sensitivity or motions of the pile (due) to wave loads. Motions during operation increase the complexity and cause extra forces in hoist-lines and structures. Due to the motions of vessel, barge, or pile dynamic forces rise in the lifting and mooring lines, hull of the vessel, crane and other equipment. This altogether increases the risk of the operation.
Second factor that influences the probability on hazardous occurrences is the human factor of a process. As discussed by Kohn LT, Corrigan JM and Donaldson MS, the human factor is the relation between the human being and the system of which they interact [7]. Humans are sensitive to mistakes and errors due to their brain capacity, emotions, feelings and memory. They are not like computers, which are very predictable and reliable. The possibility on a mistake in an operation is therefore larger when human beings are involved. On the other side, Runciman, Bill and Walton, Merrilyn discuss that [8] human beings are very creative, self-aware, imaginative and flexible in thinking, which is required in complicated offshore operations.
Third factor influencing the probability on hazardous occurrences is the complexity of a process. Complicated operations are more sensitive to accidents since the different operational steps are more difficult. As discussed above, difficult operations often require a human factor which consequently leads to a larger risk on mistakes.
The main consequences of incidents and accidents are human injury and damage of equipment. The magnitude of the consequence should be decreased as much a reasonably practicable. Toe bumping is for example a much less severe consequence of human injury compared to a human fatality. Offshore contractors and companies should always aim to reach the smallest consequences and lowest probabilities with regard to risks.
Time is mostly dependent on the number and duration of the time taking steps. Operations always exist of multiple steps. The duration and the number of steps consequently influence the total duration of the operation. Besides the actual duration there is also the probability of a delay. If the operation is for example complex there is a large probability on delay. The probable duration is consequently larger than the predetermined duration. The risk attached to an operation has therefore also a large influence on the duration. A riskfull operation has probable a larger duration due to delay. But also because risk can be better managed when the operation is slowed down.
2-3
Aeolus Oppportunities
The goal of this thesis is to design a time-efficient mono-pile installation method for piles exceeding crane capacities for offshore installation vessel Aeolus.
Two challenges are to overcome: To install piles which exceed the crane capacity and to time-efficiently supply the piles. After analysing the existing methods a number of opportunities for XL-mono-pile installation for Aeolus arised.
12 Phase 1: Existing method study
1. Since the weight of the pile exceeds the crane capacity, the load of the pile on the crane should be reduced. Therefore, measures should be taken whereby the load on the crane becomes less then the piles dry weight. Buoyancy force on the pile can reduce the load which the crane has to lift. Buoyancy force is well understood and widely used in offshore installation processes, for example with jacket installation, and is therefore an interesting solution to research.
2. Crane capacity exceeding mono-piles can also be lifted with other equipment than the main crane of Aeolus. In this way, the limiting capacity of the crane is avoided. An example of such a method is a pile-gripper-can. This tool is attached to Aeolus and performs the lift.
3. Installation of piles by Aeolus in floating configuration can provide many possibilities with regard to risk reducing and Time-efficiency. When Aeolus is jacked up, the legs are exposed to bumps, collisions and other hazards. The legs of the Aeolus are part of the most valuable equipment. When the legs are damaged, the vessel is not able to operate any more. If the legs are not exposed, so when Aeolus is floating, this risk is largely decreased. Installation of piles in floating configuration should therefore be investigated.
4. Time efficient supply of piles can be performed by tugs. The piles are closed and towed to site by tugs. After this analyses it is clear that the supply of piles in floating configuration causes some difficulties regarding workability and safety. This is mostly due to the sensitivity of the floating piles to wave motions and the risk of collision between jack-up legs and other vessels. When tugs are closely sailing around jack-up legs, risks arise due to possible vessel collision. When one of the legs is damaged, the consequence of this incident is, as discussed, large.
5. Another way to time-efficiently supply piles is by placing them on a barge. But the main challenge here is that Aeolus’ crane can not lift the piles from the barge since they are exceeding crane capacity. If another way is found to reduce the piles weight, then pile supply on barges may be a thinkable solution.
Chapter 3
Phase 2: Concept Generation
Engineering is not a straightforward process. Often, earlier made decisions are recalled, evaluated and adapted. Dr S. P. Tayal (2013) states that engineering "is a decision making process (often iterative) in which the basic sciences, mathematics, and engineering sciences are applied to convert resources optimally to meet a stated objective [9]".
The main objective of this thesis is to design a time-efficient, crane capacity exceeding, mono-pile installation method for the offshore installation vessel Aeolus. A main aspect of this research is consequently to funnel all wishes, constraints and existing knowledgege regarding the objective in such a way that the most suitable concept results from the analysis. A common method to analyse the concepts and ideas is by means of the evaluation on criteria. The suitability of a concept is determined by the level the concept meets the set of criteria. Since the objective of this research is complex, there is a variety of different possibilities to meet the design criteria. There is not one specific solution for one established criterion. To gradually funnel all the different ideas and concept through the design process with the different criteria, the design process is divided in four steps. In this way, non-desirable concepts are eliminated in an early stage and costly time is saved.
The criteria and goals of the different steps are listed beneath and Figure 3-1 provides a visual illustration.
1. Ideation or brainstorming. Identifying as many as possible solutions for the given problem. During this phase, the only criteria that have to be met are the thesis ob-jectives. Objectives are mandatory for the concepts, they define the goal of this thesis. All solutions which meet the objectives are permitted. It is after this phase where the undesired concepts are eliminated.
2. Gate 1: Safety and costs. The concepts generated in step one are evaluated on the first set of criteria: safety and cost-efficiency. The concepts are evaluated on their achievability with regard to safety. Concepts with high risks or high risk mitigation costs are eliminated. The concepts that are fundamentally unsafe or unfeasible are eliminated here.
14 Phase 2: Concept Generation
Figure 3-1: Approach in flow chart diagram
Main question: can the method be safely executed in a cost efficient way?
3. Gate 2: practicability and costs. The residual concepts are evaluated on their operational efficiency and costs. The concepts are detailed to operational level and the non-efficient ones are separated from the efficient ones. Concepts will for instance be evaluated on operational limits, time and simplicity. The guidelines for the elimination in gate two is provided in chapter 3.
Main question: Is the method operational efficient?
4. Gate 3: Multi Criteria Analysis. The detailed concepts are evaluated on criteria by means of a multi-criteria analysis. If required, concepts are even more detailed. The methods are evaluated on safety, costs, workability and time.
Main question: Which concept is the most suitable concept for a time-efficient XL mono-pile installation with Aeolus?
3-1 Brainstorm 15
with a sensitivity analysis. The goal of this chapter is to define the most suitable concept to install mono-piles exceeding crane capacity in a time-efficient manner.
3-1
Brainstorm
The first step of concept generating is brainstorming. All solutions for a time-efficient instal-lation method of XL mono-piles for Aeolus are identified in this process. But, the solution has to meet the objectives to limit the number of options and to efficiently design a concept. The solution has to meet the following 4 terms:
1. The designed method should be more "economical" efficient with regard to the pile supply than the existing method.
2. The designed method is for mono-piles only. The new method increases Aeolus market position in the mono-pile installation industry. The method will therefore only be designed for mono-piles.
3. The weight of the mono-pile exceeds the capacity of the crane for normal installation. These piles are referred to as XL-monopiles.
4. No adjustments can be made to the main crane with respect to crane capacity. Due to class regulations there is no possibility for a crane capacity upgrade.
A morphological analysis is often used during brainstorming. The analysis is developed by the Swiss astronomer Fritz Zwicky designing complicated jet engines and rockets. Ritchey (1998) defines a morphological analysis as "a method for identifying and investigating the total set of possible relationships or "configurations" contained in a given problem complex" [10].
The design of a time-efficient method for XL mono-pile installation encounters many dimen-sions and design aspects. It is, just as the design of jet engines and rockets, a complex design system. It is therefore that this method is chosen to approach the design of the mono-pile installation method.
The opportunities discussed in the literature study are used to identify the different dimensions of the morphological analysis. There are generally three dimensions with their own design space based on the following opportunities.
1. Mono-pile weight handle. The large mass of the pile can be handled by the crane if buoyancy force is introduced. An other pile lifting tool should be used if buoyancy can not be applied to the pile.
2. Vessel configuration. Aeolus may be able to upend the pile in floating modus instead of jack-up mode.
3. Time-efficient supply of piles can be achieved by transportation on barges or by self floating piles.
16 Phase 2: Concept Generation
The three design dimensions are consequently: ‘Upending Method’, ‘Vessel Modus’ and ‘Pile Load Reduction’ if crane is used. The last dimension is not always necessary to be met. There where ten concepts identified for upending method, two for vessel modus, and five for pile load reduction. The total brainstorming process with the different concept can be found in Appendix B.
3-2
Elimination Gate 1: Safety and Costs
The first elimination gate is about safety and costs. All concepts can in principle be safe, if they are proper engineered and the circumstances are safe. But, engineering and fabrication are costly and circumstances are always varying. This gate represents therefore the balance between safety and engineering, fabrication and operational limits. If the operational costs to reach a certain safety level are high due to engineering, fabrication or operational limits, the concept is not cost efficient.
This gate represents the first evaluation moment of the concepts. The concepts that are fun-damentally unsafe or infeasible are eliminated or changed. The remaining concepts continue to the next elimination gate.
The main question to evaluate the concepts is “can the method be safely executed in a cost efficient way?” To answer these questions, four criteria (with subcriteria) have been established.
1. The loads on equipment should always be lower than its save working load limit. All equipment should be able to withstand the forces executed in the mono-pile installation procedure without any affliction of the equipments practicability.
(a) Aeolus should be able to withstand forces and motions of the designed method. Aeolus may not be damaged in such a way that it can not perform its task any more.
(b) Maximum crane capacities should not be exceeded. The crane has a safe working load limit of 900t on its main hoist with an outreach of 18-30m. The maximum side-lead angle of the crane is 5 degrees.
(c) The method should have as less as reasonable practicable negative influence on the structural integrity of the pile. The method should not carry out any damage to the pile.
(d) Excessive engineering work to provide safety should be avoided. Equipment and materials should be handled smart, properly and in the way they were designed for. In this way engineering cost can be kept low.
.
3-3 Elimination Gate 2: Practicability 17
(a) Incidents: The probability of incidents during the installation procedure should be as low as reasonably practicable. Incidents can cost human life, money, time, companies reputation etc. They should therefore be avoided.
(b) Environmental harm: The probability on environmental harm as a result of the in-stallation procedure should be as low as reasonably practicable. The consequences of this hazard are strongly negative for the world, society, climate etc. They should therefore be avoided.
(c) Harm to workers: The probability on harming workers as a result of the installation procedure should be as low as reasonable practicable. Harm to workers should be avoided since no man should get harmed by the work he carries out.
(d) Damage to equipment: The probability on damage of equipment should be as low reasonable practicable. If equipment is damaged or broken it cannot fulfil its task properly and risk may arise.
3. The method should describe an installation procedure conform the consisting regula-tions regarding this procedure. In this way, probabilities on incidents, faults or other inconveniences can be avoided. Also, regulations are binding. A procedure may not be executed when it is not according the existing rules and regulations. Existing regulation standards are for example DNV, ISO and IMCA.
4. Besides this, Van Oord and the client work together with safety advisors or, Marine Warrenty Surveyors (MWS). In the offshore business field it is common practice to acquire a third party opinion with regard to safety aspects. These Surveyors provide "independent third-party review and approval of high value and/or high risk marine
construction and transportation projects, from the planning to the execution stages.for approval by the marine warrenty surveyor. [11]" (LOC, Marine Warrenty Surveyors
(2008)) Procedures have to meet the criteria to reach the safety level adopted by the MWS.
Three upending methods where eliminated based on a large momentum on the vessel and the risk of jack-up leg damage. Installation of piles by Aeolus in floating configuration is eliminated on basis of low lifting capacity, complexity and time. Besides this, installation of a pile is executed via a gripper to ensure the precise vertical position of the pile. One can imagine that when Aeolus is floating this precise vertical position is much harder to achieve. The total analysis of the first elimination gate can be found in appendix C.
3-3
Elimination Gate 2: Practicability
In the third phase of concept generation the solutions are evaluated on their practicabil-ity. The concepts where detailed to operational level and consequently evaluated on their operational efficiency. Operational efficiency is measured alongside the following guidelines.
1. The method should be designed for as much as possible variable environmental con-ditions. In other words, the method should have high operational limits. The cost-efficiency of the method is consequently high, the method can be used for a long time and may be used in different projects without the necessity to make fundamental changes.
18 Phase 2: Concept Generation
Waterdepth 45 m Maximum depth Jacked Aeolus Mass Pile 1500t Weight of XXL mono-pile, as large
as reasonable practicable Diameter Pile 8 m Likely diameter of 1500t piles
Table 3-1: Design parameters for concept evaluation
2. Available resources should be smartly used. Aeolus capabilities should be deployed smart so that cost-efficiency rises.
3. The method should be time efficient.
4. Simplicity of the design reduces costs and risks. Less material, man and equipment will be used. Also, the probability on mistakes during design, manufacturing and procedure is lower.
5. The use of proven technology reduces the engineering and manufacturing costs and reduces the level of risks during operation.
The design parameters chosen to evaluate the concepts on operational level are shown in Table 3-1.
Five pile upending methods where eliminated in this phase, mostly due to non smart use of available resources and their sensitivity to waves. One load reduction option was changed, and one was eliminated. At the start of the multi criteria analysis there where three upending concepts, and 3 load reduction concepts. The total analysis with regard to workability is given in appendix D. The following subsections discuss the different concepts which where analysed in the multi-criteria analysis.
3-3-1 Upending concepts
Concept 1: Floating upending - Figure 3-2
The pile is transported in floating modus to the installation site where it is upended in the water by the main crane. This concept is simple and straight forward. On the other hand, thoughts need to go out to pile motions since the pile is completely exposed to environmental loads.
Concept 2: Seabed support - Figure B-2b
The pile is floating next to a barge in two surrounding slings which guide the pile when it is lowered. The pile is ballasted by inserted water and the pile consequently sinks to the seabed. Aeolus main crane upends the pile while it is supported on the seabed. The motions of the pile are constrained since the bottom of the pile is supported by the seabed. Once the pile is vertical it can be moved to the installation location.
3-3 Elimination Gate 2: Practicability 19
(a) Pile transportation (b) Pile upending Figure 3-2: Concept 1: upending of floating pile
(a) Lower pile to seabed (b) Pile upending Figure 3-3: Concept 2: upending of pile with seabed support Concept 3: Guided by barge - Figure 3-4
The pile is positioned along side the barge. Two winches attached to the barge are winded around the pile. When the pile is upended these slings keep the pile close to the barge.
3-3-2 Crane load reduction concepts Concepts 1: Trapped Air - Figure 3-5
The pile is closed on both ends by means of plugs. After upending, by means of the three upending concepts, compressed air is supplied to the pile and the pressure in the pile increases. When the pressure is large enough, (larger than the water pressure at the bottom end) the bottom plug can be removed. Because the pressurised air is trapped in the pile, the water can not flow in. Since there is no bottom plug in the pile, the pile can directly be installed
20 Phase 2: Concept Generation
(a) Pile transportation (b) Pile upending Figure 3-4: Concept 3: upending floating pile with barge support
on the location. The buoyancy of the pile is retained by means of trapped air and the load in the crane is therefore not equal to the total pile mass.
(a) Pile transport (b) Compressed air pushes water out pile Figure 3-5: Concept 1: Trapped Air
Concept 2: Pigging Tool - Figure 3-6
The lower pile plug is implemented as some sort of pigging tool. A pigging tool is a cylindrical shaped tool which is used in the oil and gas industry to clean the pipeline. This pigging tool for mono-piles, keeps the water out of the pile, and can move upwards before installation on the seabed.
Concept 3: Floaters Inside Pile - Figure 3-7
3-4 Multi-Criteria analysis 21
(a) Pile transport (b) Pigging tool pulled up and water enters pile Figure 3-6: Concept 2: Pigging Tool
(a) Pile transport (b) Floaters decrease due to water depth Figure 3-7: Concept 3: Floaters inside pile
3-4
Multi-Criteria analysis
The final phase of concept generation is a multi-criteria analysis. The remaining concepts are evaluated on multiple pre-defined criteria. The most suitable solution according to these criteria, retrieves the highest score and is consequently the most appropriate solution for the problem statement.
A multi-criterion-decision making analysis is often used to approach complex problems with interfering criteria or objectives in engineering. There is no optimal solution for a problem statement when there are conflicting criteria. For example, if a friend wants to buy a car that is fast, low on costs and spacious he will find multiple cars that partly satisfy his needs. But he will not find a really cheap car which is also spacious and fast. He has to make a trade-off. If space is the most important to him he does not mind if the car is a little bit more expensive and less fast. In daily life we make these choices everyday without realising them. But for
22 Phase 2: Concept Generation
more complex problems a systematic approach is desired to keep a clear overview over all the possibilities.
In order to choose the most suitable solution with regard to multiple criteria a Multiple Cri-teria Decision Analysis (MCDA) can be executed. A MCDA is widely used by policy decision makers to integrate preferences and judgements to derive a solution for a problem. Keeney (1982), one of the first explorers of the MCDA illustrated the MCDA as “ a formalization of common sense for decision problems which are too complex for informal for common sense ”[12].
MCDA’s are extensively studied and used through the years, starting at 1970 (Köksalan, Murat and Wallenius 2011[13]). A stepwise MCDA manual, published by the UK Government [14] is provided in Table 3-2 and is used for the structure of this MCDA.
Step nr Stage Actions
1. Decision context establishment Identify key players
2. Identify options Select concepts to consider
3. Select Criteria Select criteria which the concepts need to satisfy
4. Score the options Score the options on the criteria
5. Weighting Determine the importance of the criteria
with respect to each other
6. Calculate results Multiply the score of each option with the weighfactor of the criteria. Give a total score per option
7. Sensitivity analysis Analyse the sensitivity of the scores on changes in weight factor or score (step 4 and 5)
Table 3-2: Stepwise approach for MCDA published by UK government (2009)
3-5
Decision context
The objective of this MCDA is to identify the most suitable solution for mono-pile installation where the piles are time-efficient supplied and where the weight of the mono-piles exceed the current crane capacity of 900t of Van Oord’s Offshore Installation Vessel Aeolus.
The most important key players in this analysis are van Oord and the Client. Van Oord wants a cost-efficient and safe installation method which is attractive for a possible client. Furthermore, there is a third party safety advisor present to enhance safety of a project or procedure. One of the most common safety advisors and provider of offshore certificates and guidelines is DNV-GL. Finally, the employees, subcontractors and other personnel working at site require safe working conditions and achievable tasks.
3-6 Identify options 23
Concept number Upending concepts Crane load reduction concepts
1 Floating upending Trapped Air
2 Seabed support Pigging Tool
3 Guided by barge Floaters Inside Pile
Table 3-3: Design space of Multiple Criteria Decision Analysis
3-6
Identify options
The concepts taken into the MCDA are the final concepts of the third phase. The two blocks considered for the evaluation are: ‘Upending’- and ‘Load Reduction Concepts’. These blocks will be separately evaluated since all options of one block can be combined with all solutions of the second block.
3-7
Criteria
The most important design criteria for a mono-pile installation method identified in the literature study are safety, workability, time and cost. These four criteria and the aspects that influence the score of these criteria are the basis of the concept evaluation and are listed below.
1. Safety. Is determined by probabilities and consequences. the probabilities are mostly influenced by sensitivity to wave loads, the human factor and the complexity of a process. The main consequences of incidents are human injury and damage of equipment. 2. Operational limits. The operational limits together with the site specific
environ-mental conditions determine the workability of a method. The operational limits are determined by the pile sensitivity to wave loads.
3. Duration. The duration of the method depends on the number and duration of the time taking steps, probability of delays and the risk of a process.
4. Operational costs. These are the main costs of the designed method and encounter for example design, fabrication and wages. A cost efficient method is more likely to be chosen in a tender phase. And, on top of that, Van Oord is commercial company and consequently needs to make a profit.
3-8
Score Options
According to Roozenburg and Eekels (1996) [15], there are 2 judgement methods to evaluate designed concepts, ordinal (qualitative) and cardinal (quantitative). Ordinal evaluation is the relative score of the concepts, the best and the worst. Cardinal evaluation gives a number of points to the concepts, related to the amount the concept meets the considered criteria. To simplify the scoring process, these two methods will be used. First, the concepts will be ranked. Then, points will be given to what level the concept meets the criteria. The concept
24 Phase 2: Concept Generation
can acquire a score that ranges between 1 and 5. Low if the concept does not meet the criteria, high when the concept does meet the criteria. Table 3-4 illustrates the criteria to earn a certain score.
Score Criterion
1 The method does not meet the set criteria at all 2 The method meets the set criteria partly
3 The method meets the set criteria moderate, not good, not bad 4 The method meets the set criteria sufficient
5 The method meets the set criteria perfectly
Table 3-4: Criteria for a concept to earn a score between 1 and 5
The score for the upending methods are shown in Table 3-5. The ‘Floating Pile’ upending method scores high on safety since the pile can freely rotate without any obstructions. The pile can simply follow the environmental forces without obstruction of barge or seabed. The median concept is the ‘Guided by Barge Concept’. Pile and barge are in this concept connected whereby a ‘floating two body system’ is created. Due to irregular waves the bodies start to move, forces and motions between the two bodies arise. This solution makes things more complicated compared to the first upending concept. The last concept is the ‘Seabed Support Concept’. Since there is still air present in the pile, there is a probability of non-desired fast upending. The air will search the fastest way up whereby the pile can easily rotate up. This ensures uncontrolled motions and consequently unmanageable risks.
The ‘Floating Pile Concept’ has the highest score on operational limits, the pile can easily follow the wave forces. The ’Guided by Barge Concept’ is the mean concept. Due to the ‘floating two body system’, forces and motions will interact. This is less controllable and less safe in higher waves. The last concept, with regard to operational limits, is the On Seabed concept. The pile is lowered to the seabed by ballasting tanks on the top and bottom of the pile. In the first meters of lowering the pile is sensitive to wave motions and forces. The waves can not be too high since the pile is positioned next to a barge. But also, the probability of undesired uncontrolled upending motions is apperent.
The most expensive option is the ‘On Seabed Concept’. A lot of small vessels, other equipment and extra research is needed to execute this method. This increases the costs. The middle concept is the ‘Guided by Barge Concept’ due to the rental of a working barge and designed and fabrication of the special designed upending slings. The cheapest option is the ‘Floating Upending Concept’. No other equipment is required to execute this procedure. It is simple and straight forward.
The concept with the longest executable time requirement duration is the ‘Guided by Barge Concept’. The slings need to be slacked together with upending of the pile. This is complex and therefore requires time. The upending procedure is executed in small steps so that there is time for the slings to adapt. The middle concept is the ’Floating Upending Concept’. The pile is gradually upended vertical. The fastest concept is the on seabed concept. Due to the water and air inside the pile, the pile upends fast. The air searches the fastest way up and
3-8 Score Options 25
Criteria Floating Upending Guided by Barge On Seabed
Safety 4 3 2 Operational Limits 4 3 2 Operational Costs 5 3 1 Duration 3 2 4
Table 3-5: Score of the upending concepts methods on the different criteria
force of the bags decreases with an increasing pile depth. The ‘Trapped Air Concept’ and the ‘Pigging Tool Concept’ are very much a like. The largest disadvantage of the tool is its complexity and design. The largest disadvantage of the ‘Trapped Air Concept’ is the high pressure and consequently high forces that are encountered. Due to its simplicity of design, and therefore more easy to mitigate risks, the ‘Trapped Air Concept’ is considered number one. At a short distance followed by the ‘Pigging Tool Concept’.
The workability of the ‘Floating Bags Concept’ is low. The bags lose their buoyancy volume when depth increases. They can therefore take up less buoyancy force than the other two methods. In high waves, the crane does not have enough capacity to encounter the large change in the buoyancy force of the pile. The ‘Trapped Air Concept’ and the ‘Pigging Tool Concept’ have the same workability. Since they reduce the weight of the pile in the crane sufficient.
The concept with the shortest time consumption is the ‘Floating Bags Concept’. The bags are pulled out, together with the plug. Second is the ‘Pigging Tool Concept’. Once the pile is installed at position, the pigging tool is pulled back. The longest duration is for the ‘Trapped Air Concept’. The pile is closed on both sides and upended. Then, the pile is pressurized and the lower plug is released. The pile positioned in the gripper and the pressure lowered. The lowest cost solution is the ‘Floating Bags Concept’. The bags can easily be bought and installed. Second is the ‘Trapped Air Concept’. The equipment required are the compressors and a closing plug. The design and construction of the closing plug will be very costly. The most expensive solution is the ‘Pigging Tool Concept’. The inner diameter is varying so the tool should be capable of adapting itself to this. This complicated the design and construction. Costs will therefore rise.
Criteria Trapped Air Pigging Tool Inside Floaters
Safety 4 3 2 Operational Lim-its 4 4 2 Operational Costs 4 3 5 Duration 3 4 5
26 Phase 2: Concept Generation
3-9
Weight of Criteria
The weight factors of the criteria illustrate the relative importance of these criteria. A high weigh factor means that the belonging criteria is important and that this considered criteria has a large influence on the total score of a concept.
Weight factors of criteria can be determined by weight obtaining methods. The method used in this thesis, often used and discussed by Rogers and Duffy (2012) [16] is the normalised ratio system. In this system, the least important criteria receives a score of 1. Then, the other criteria receive scores relative to this one. So, as later is explained, duration is the least important criteria. Duration consequently receives a score of 1. If safety is three times important as time, safety receives a score of 3. The justification of the ratio scores of the criteria is provided in subsection 3-9-1.
The weight factor is determined by Equation 3-1. The ratio scores of the criteria with corre-sponding weight factors are provided in Table 3-7.
wi = zi n X i=1 z1 (3-1)
Where, wi is the normalised weight factor and zi is the ratio score.
Rank Criteria zi Ratio score wi
1 Safety 3 0.4 2 Operational Limits 2 0.27 3 Costs 1.5 0.2 4 Time 1 0.13 P = 7.5
Table 3-7: Ratio scores and corresponding weight factors of the criteria
3-9-1 Ratio score of criteria
This section discusses the relevance of the individual.
Safety is the most important criteria. No damage on equipment or human injury or other incidents may take place.
Time is the least important criteria. Even though time influences the workability, the opera-tional costs and limits are more decisive. So time receives a score of 1.
Workability determines for a large part the cost efficiency of a procedure. But, the equipment, engineering or fabrication costs are also important to take into account. Operational costs
