Management Approach to Weight Engineering

44 Pages·1984·3.225 MB·44\44

by  Craddock, P.A., Bell, C, Miles, J

REPORT UR24 1984
A management approach to weight engineering
Price f40 (f8 UEG Members)
0 ClRlA 1984
ISSN: 0 305 4055 ISBN: 0 86017 220 1
UEG
6 Storey’s Gate Westminster London SWlP 3AU Telephone [phone]
THE RESEARCH AND INFORMATION GROUP FOR THE UNDERWATER AND OFFSHORE ENGINEERING INDUSTRIES
A PART OF CIRIA-THE CONSTRUCTION INDUSTRY RESEARCH AND INFORMATION ASSOCIATION
Foreword
The project leading to this report was carried out under contract to UEG by Ove Arup
& Partners, where the staff engaged on the work were Mr P A Craddock, Mr C Bell and
Dr J Miles. The work was financed through UEG by a joint venture of five oil
companies and the project managed by Mr R J Simpson. The report has been prepared
with the assistance of a Project Steering Group.
The Group comprised:
Mr R J Simpson (Chairman) UEG
Mr P A Craddock Ove Arup & Partners
Dr N Dick Britoil Plc
Mr D G M Eggar BP International Ltd
Mr B Fowler McDermott Engineering London
Mr A W Gilfillan YARD Ltd
Dr J Miles Ove Arup & Partners
Mr A Quennelle Total Exploration and Production
Mr V D Stiggins Phillips Petroleum Company UK Ltd
Mr P Teymourian Gulf Oil Corporation
Mr A M Thompson Britoil Plc
Mr R K Venables UEG
Summary
This Report reviews the working practices of some 30 companies which have a particu-
lar concern in the control of weight in structures on or offshore, and records their views
on the origins and solutions of the problems involved. It is evident that, although the
starting points are similar, the methods are not, and that although effective manage-
ment of weight is possible, it is not always achieved.
Apparent causes of poor weight control are identified, and a scheme of weight
management is described which can overcome defects in existing systems of control.
Emphasis is given to the view that weight control, to be effective, must be treated as an
independent engineering discipline, and given due importance and authority within
the overall project.
The use of computers in weight control and existing weighing techniques are discussed
in the Appendices.
2 UEG Report UR24
Contents
Page
FOREWORD 2
SUMMARY 2
EXPLANATION OF TERMS 4
INTRODUCTION 5
1.1 Background and objectives 5
1.2 Scope 5
CURRENT PRACTICE IN THE OFFSHORE INDUSTRY 5
2.1 Introduction 5
2.2 Terminology 5
2.3 Weight control procedures 6
2.4 Observations 9
CURRENT PRACTICE IN OTHER INDUSTRIES 10
3.1 Introduction 10
3.2 Shipbuilding 11
3.3 Automotive 11
3.4 Aircraft 12
3.5 Observations 12
CONCLUSIONS 13
RECOMMENDATIONS FOR EFFECTIVE WEIGHT CONTROL 13
5.1 Introduction 13
5.2 Justification 14
5.3 Organisation 15
5.4 Co-ordination 15
5.5 Data processing 15
5.6 Data acquisition 16
5.7 Communication 16
5.8 Implementation 16
ACKNOWLEDGEMENTS 32
BIBLIOGRAPHY 32
APPENDIX A THE USE OF COMPUTERS IN WEIGHT CONTROL 34
APPENDIX B REVIEW OF WEIGHING TECHNOLOGY 37
UEG Report UR24 3
1.1 BACKGROUND AND OBJECTIVES
Within the offshore industry, the value of accurate prediction and monitoring of the
weights and centres of gravity of structures from conception through to operation is
well acknowledged. However, a general appreciation of the subject of weight control
and its effect on costs appears to be lacking.
The objectives of the present study were:
1. To review the procedures for weight control in the offshore and other industries;
2. To determine the value and effectiveness of weight control and especially isolate
and identify the deficiencies that exist;
3. To formulate and recommend basic principles upon which an efficient weight
control system can be based.
This report is the result of those studies and is intended to form a guide to weight
control for project management.
1.2 SCOPE
This report reviews the procedures adopted both on and offshore where weight is sig-
nificant. It is based upon discussions with representatives of a wide cross section of
industry and draws upon their experiences to make recommendations for the control
of weight and centres of gravity on offshores tructures. During the course of the investi-
gation, the varying degrees of importance attached to weight by different industries
have been considered.
This report puts forward the foundations upon which good weight engineeringc an be
based in order to achieve the target weights set for a project, or to allow target weights
to be modified with confidence.
2. Current Practice in the Offshore Industry
2.1 INTRODUCTION
This Section is intended to convey an overall picture and does not attempt to go into
detail. Information concerning the details of particular projects is not freely available
within the offshore industry, and weight control is a particularly sensitive area.
Companies are naturally unwilling to discuss weight control procedures that have
failed to work and which may have jeopardised the success of major projects. In some
cases, the information is not available because the records have not been kept.
2.2 TERMINOLOGY
No weight control terminology is universally accepted throughout the offshore indus-
try. This fundamental shortcoming has given rise to misunderstanding and error in
communication during the course of projects.
The lack of common terminology extends right through the scale of platform com-
ponents from relatively small items of equipment to items of major structure. Even
such commonly used terms as ‘topsides’ or ‘deck structures’ have different meanings
within different companies.
The most common area in which misunderstandings have arisen is in equipment
supply. Vendors’ literature sometimes does not differentiate between dry weights
(without lubricants or other liquids) and operating weights (in the working condition
with lubricants, fuel and other normal contents included). For power plants, it is not
always clear whether quoted weights include exhaust ducting, mountings and other
essential ancillary structures. The introduction of a standard weight classification sys-
tem would help to make clear exactly what items are included in the weight quoted by
the supplier or manufacturer.
UEG Report UR24 5
1.1 BACKGROUND AND OBJECTIVES
Within the offshore industry, the value of accurate prediction and monitoring of the
weights and centres of gravity of structures from conception through to operation is
well acknowledged. However, a general appreciation of the subject of weight control
and its effect on costs appears to be lacking.
The objectives of the present study were:
1. To review the procedures for weight control in the offshore and other industries;
2. To determine the value and effectiveness of weight control and especially isolate
and identify the deficiencies that exist;
3. To formulate and recommend basic principles upon which an efficient weight
control system can be based.
This report is the result of those studies and is intended to form a guide to weight
control for project management.
1.2 SCOPE
This report reviews the procedures adopted both on and offshore where weight is sig-
nificant. It is based upon discussions with representatives of a wide cross section of
industry and draws upon their experiences to make recommendations for the control
of weight and centres of gravity on offshores tructures. During the course of the investi-
gation, the varying degrees of importance attached to weight by different industries
have been considered.
This report puts forward the foundations upon which good weight engineeringc an be
based in order to achieve the target weights set for a project, or to allow target weights
to be modified with confidence.
2. Current Practice in the Offshore Industry
2.1 INTRODUCTION
This Section is intended to convey an overall picture and does not attempt to go into
detail. Information concerning the details of particular projects is not freely available
within the offshore industry, and weight control is a particularly sensitive area.
Companies are naturally unwilling to discuss weight control procedures that have
failed to work and which may have jeopardised the success of major projects. In some
cases, the information is not available because the records have not been kept.
2.2 TERMINOLOGY
No weight control terminology is universally accepted throughout the offshore indus-
try. This fundamental shortcoming has given rise to misunderstanding and error in
communication during the course of projects.
The lack of common terminology extends right through the scale of platform com-
ponents from relatively small items of equipment to items of major structure. Even
such commonly used terms as ‘topsides’ or ‘deck structures’ have different meanings
within different companies.
The most common area in which misunderstandings have arisen is in equipment
supply. Vendors’ literature sometimes does not differentiate between dry weights
(without lubricants or other liquids) and operating weights (in the working condition
with lubricants, fuel and other normal contents included). For power plants, it is not
always clear whether quoted weights include exhaust ducting, mountings and other
essential ancillary structures. The introduction of a standard weight classification sys-
tem would help to make clear exactly what items are included in the weight quoted by
the supplier or manufacturer.
UEG Report UR24 5
2.3 WEIGHT CONTROL PROCEDURES
There are several major phases in the development of an offshore project. Current
procedures make it very difficult to differentiate clearly between each phase, as
activities overlap in certain instances, and companies tend to identify these phases by
different, but similar, terms. There are, however, several discrete stages common to all
projects which for the purposes of this report, are identified as follows:
1. Feasibility studies
2. Conceptual design
3. Detailed design
4. Fabrication
5. Installation and hook-up
6. Operation
7. Decommissioning.
2.3.1 Feasibility studies Preliminary calculations are made to assess the viability of the project. Little detailed
engineering design is done at this stage and overall concepts only are considered. Con-
sideration is given to the alternative types of installation which could be adopted. As
final cost is related to platform weight, the estimation at this stage of process plant and
accommodation weights is important. Since data often is not readily available from
previous projects, most of the weight estimates will be based on the personal experi-
ences and expertise of the study leader. It is most unusual to find any formal weight
control technique being adopted at this early stage.
Once the project is shown to be feasible, the engineering outcome of this stage is a
statement of the preferred type of structure and the overall weight targets within which
the final structurem ust be realised. No details of weight breakdown would be available
at this stage.
2.3.2 Conceptual design The process systems are defined together with items of major equipment; weights will
be assessed for each part of the platform. Weights will, if information is available, be
based upon previous experience. If the design is innovative or the design team inex-
perienced, the error bands on the estimated weights can be quite large.
It is at this stage in a project that a serious weight problem may start to develop. What is
done to prevent it varies markedly throughout the industry. There may be a complete
absence of any formal weight control mechanism or there may be some form of com-
puterised data processing for weight control. In the latter case, during the conceptual
design stage, information would be entered into the data bank for each item of equip-
ment and structure identified. This information would include an estimated weight, a
contingency allowance and a three-coordinate location for the centre of gravity. Pro-
cessing of this data would give weights and centres of gravity for each module and the
topside as a whole, together with the reactions at the deck support frame. The end
product of the conceptual design studies will be the engineering scheme for the struc-
ture and a definition, in some detail, of the process systems and other important plat-
form features including accommodation, powerplants etc. It will also include revised
weight targets from the feasibility study, which should be subdivided into a number of
different categories, each weight component having an estimated weight, tolerance
and a contingency allowance. The naval architect and the lifting contractor should be
brought in at this stage, to ensure that all the requirements of construction, transport
and final installation can be met.
Heavy lift craneage, in particular, has a very long lead time and the necessary cranes
need to be reserved at a very early stage, usually years in advance. Large cranes are
used most economically when working near their maximum capacity which is deter-
mined both by the weight and radius of a lift, and here there will be severe restrictions
on the margins which can be associated with the weight targets which are set. The qual-
ity of the weight estimates adopted, and the margins associated with them can be criti-
cal to the success of the project. As a general guide to the tolerances associated with
various different disciplines at concept stage, the following examples have been sug-
gested:
Structural items: 10%
Mechanical and process equipment: 15%
Ancillaries and rigging: 50%
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If the weight control is correctly applied at this early stage, then in the later stages it
need amount to no more than monitoring the weight of the structure as the project pro-
ceeds. If, however, the weight estimates are poor and low, then the weight control
effort for the remainder of the project will be much more arduous and will require very
positive ‘control’ in addition to ‘monitoring’. This will inevitably lead to much ad-
ditional design work by all disciplines to save weight and to a reduction in choice of
equipment and solutions to problems. This can attract a time penalty and may have
serious cost implications.
2.3.3 Detailed design Once the conceptual design has been agreed, detailed design work is started. It is cur-
rent practice for any formalised weight control/monitoring to commence at this stage.
As the design proceeds, the weight estimates can be refined and broken down into
greater detail. This may not, however, improve the accuracy of the estimates. There are
two reasons for this:
1. The source of new information may be poor. Equipment suppliers, for example,
are often not able to supply accurate weight data for their goods. This particularly
applies to small items of equipment where it is not unknown for errors to be in the
order of 50% on 5 tonnes, but it can also be true for large items. A desire for
greater ‘weight consciousness’, coupled with an interest in minimum weight
design, which should not be confused with weight control, has led in recent years
towards aerospace developed equipment being used for offshore structures.
2. The process of collating all the data and presenting it in such a way that technical
and management decisions can be taken swiftly becomes a formidable task. This is
complicated by the fact that minor alterations are constantly taking place in all
parts of the project, many of which interact and have a knock-on effect in appar-
ently unrelated areas or disciplines. The process of establishing, and then up-
dating, a comprehensive database covering every item of weight on the project is
known as weight monitoring.
At this stage the use of a computer aided weight control scheme becomes essential. It
may be necessary in a large project to refer to as many as 15 000 or more items, each
having four or more weight status levels, each needing regular revision as information
becomes more accurate. These weight status levels would typically be:
1. Current Estimated Weight
2. Weight Currently on Drawings
3. Weight Assessed Missing
4. Final Weight.
Current Estimated Weight is equal to the Weight Currently on Drawings plus Weight
Assessed Missing, which will include a suitable contingency. As the detail design pro-
gresses, item (2) improves, item (3) decreases and items (1) and (4)s hould converge.
Further weight conditions may also be applied. For example, it is necessary to allow for
‘dry’ and ‘operating’ weights of plant and ‘test’ weights associated with tanks and
accumulators. In some cases, items will be temporary and some indication of the times
at which they are added and deleted may need to be included.
Apart from the storage of data, simple processing facilities to give total weights and
centres of gravity for each condition are usually required of the weight control pro-
gram. Further to this it is generally acknowledged that a sophisticated editing program
is required so that a concise summary, preferably of a single page, of the weight con-
dition of the platform can be produced which focuses on the major trends in weight
build up. More information regarding the performance and selection of weight control
computing facilities is given in Appendix A.
Once the weight schedules have been produced there must be management pro-
cedures to check and correct deviation from the schedules. If it becomes apparent from
the reports that weight targets are being exceeded, or centres of gravity are becoming
unacceptably displaced, then corrective action is required either by modifying the
structure to accommodate the change or by requiring the increased weight to be distri-
buted or repositioned. The consequences of either decision can be far reaching in terms
of cost and schedule and this emphasisest he importance of forecasting and monitoring
of weight trends.
UEG Report UR24 7
At this stage of the project it will also be necessary to submit to the certifymg authority
the standard calculations to prove not only that the structure being designed can with-
stand the ‘live’ loads to which it will be subjected by the environment, but also the
‘dead‘ loads that result purely from the weight of the topsides being mounted on the
deck and jacket. Errors, discrepancies or even an insufficientlys ubstantiated estimate
may lead to the certifying authority witholding preliminary approval of the design.
Weight control therefore plays an important part in the detailed design phase. At the
end of this phase the final weight targets and weight breakdowns within which the
structure should be constructed will be set out. Failure to meet these targets may, if the
design of the supporting structure has been fixed, require stripping out or other drastic
measures prior to transport and lift. These are invariably expensive and time consum-
ing. Unfortunately, weight control managers have not generally been given the degree
of authority which is required and weight problems are often encountered during
fabrication, even on recent projects.
2.3.4 Fabrication Because of the size of offshore structures and the specialist skills involved in their fabri-
cation, it is inevitable that the constituent parts of large platforms are built in different
places. It is therefore difficult for one fabricator to hold an overall view on weight as the
project progresses to completion. It becomes all the more important, as a result, for the
designer (or operator) to keep a careful check on the actual weights and ensure that
their totals are within the design targets.
Similarly the client should be closely involved in the weight control aspects of the pro-
ject and be prepared to define specific responsibilities within the project team.
Particular care must be taken during fabrication to ensure that the components are
built to the drawings and that no undue accumulation of potentially heavy items e.g.
cabling and pipework occurs. It is equally important to weigh items both complete and
as sub-assemblies in order to check that quoted weights and predictions are accurate
and that the monitoring programme reflects the true weight of the structure.
Often the weight control team will be required to comment on, or give approval to, a
proposed weighing procedure. Short descriptions of the techniques that might be
employed and their relative advantages and accuracies are given in Appendix B.
The weight monitoring and control effort is usually practised by a team which is based
on site. In many cases, this team will be part of the service which the fabricator pro-
vides, but occasionally the client or his design contractor may undertake this role.
On-site weighing is particularly important in the case of very heavy items (e.g.
modules, deck frames etc.) since it is usual for lift contractors to demand that weights
and centres of gravity are known to within 5% prior to lift.
It is a fairly well accepted practice to weigh modules at least three times during fabri-
cation. This allows weight growth to be monitored over a period of time and allows cor-
rective action to be taken should it prove necessary. Typical stages at which a process
module, for example, might be weighed are:
Completion of structure frame
50% completion of plant and equipment
90% completion of plant and equipment.
In addition to these weighings, the module would, of course, be weighed on completion
of fabrication and immediately prior to load-out.
It is rare for radical design changes to be made at this stage of a project. More often a
module, if too heavy, would be stripped of some of its equipment prior to lift, and then
have it reinstated after the installation on the platform. Stripping and refitting is a very
costly and time-consuming operation but its occurrence has not been infrequent.
2.3.5 Installation and Installation and hook-up covers the phases between the completion of onshore fabri-
hook-up cation and the start-up of offshore oil or gas production. Typically the intensity of
activity means that it is too late to take corrective design action if a severe weight
problem exists. It is not usual, therefore, for weight control procedures to continue in
this phase. However, the following points are worth noting:
1. Any problems associated with the attitude and draft of a structure at load-out can
be catastrophic, especially if, say, a barge launched steel jacket has had to be re-
designed as a self-floater.
8 UEG Report UR24
2. A very significant proportion of platform commissioning costs are incurred dur-
ing installation and hook-up, therefore the aim is to minimise the amount of off-
shore work. In Section 2.3.2 it was noted that lifting efficiency is critical to cost.
Both overweight and grossly underweight conditions at this stage can reflect
enormously in overall costs.
2.3.6 Operation Other than for floating structures, it is rare to find any formal weight control activities
practiced in the operation of an offshore structure.
Because everything will have been weighed prior to lift, the total weight of the platform
at the time of installation should be known to within k 5% (often less). However, once
the platform starts operation, the total weight supported by the structure inevitably
increases as goods are taken aboard and occasional modifications to equipment and
structure are made. After a period of operation,t he structure’sw eight is almost certain
to have increased, perhaps by a significant amount. However, logs relating to ‘weight
accumulation’ are rarely kept and the only means of retrospectively identifymg an
accumulation is by carrying out a visual inspection. This procedure is often followed,
but it is frequently inaccurate.
For fixed structures operating in the UK sectors the licence to operate issued by the
Department of Energy has to be renewed every five years. Re-certification includes
confirmation that the topside weights have not increased to a point where the support-
ing structurei s loaded beyond pre-defined limits based upon recognised codes of prac-
tice and their associated factors of safety. Usually it will be sufficient to demonstratet o
the certifying authority that no significant changes have been made to the topsides and
that previous weight estimates are still applicable. Should, however, any discrepancies
become apparent or doubts be raised on the submission, especially where the jacket or
deck have only a small margin or reserve strength, then a complete weight audit may be
insisted upon.
The procedure on floating structures is quite different. Self propelled semi-submers-
ibles are classified as ships and, as such, are required to maintain a day-to-day log of all
weights taken on and off the structure. Regulations relating to such exercises are con-
tained in Det norske Veritas Rules for Classification of Mobile Offshore Units, 1982.
The fact that this can be, and is, done, indicates that it would be possible on fixed plat-
forms, if sufficient effort were put into making the procedures work.
It is interesting to note that even with rigorously enforced weight monitoring schemes
as applied to naval ships there still results an inexplicable weight growth of typically
0.5% of the displacement tonnage of the vessel each year.
2.3.7 Decommissioning Little serious thought has yet been applied either to the decommissioningo f redundant
items of equipment during operation, or the ultimate complete removal and disposal of
the installation at the end of its useful life. Weight will obviously play a significant role
in such an exercise and estimated weights of equipment and the accuracy of prediction
will be critical.
2.4 OBSERVATIONS
There is no reason why the weight control procedures so far described should not pro-
vide a satisfactory means of controlling platform weights. Many weight problems on
first generation platforms were not even detected by the rudimentary weight control
systems which were operated during the design phases at that time. This situation has
been improved with second and third generation platforms, but the problem of serious
overweight at completion of fabrication has by no means been eliminated and errors in
weight of greater than 20% have occurred.
The following reasons may contribute to the continuing occurrance of serious weight
errors:
1. Each platform or installation is effectively unique and without the opportunity for
appraising prototypes or using ‘production-run’ techniques.
2. There is a general reluctance within the industry to admit openly that problems of
poor weight control still exist. The lack of co-operative exchange of information
has led to the frequent repetition of similar or even identical errors.
3. Misunderstandingsa rising from the use of an inconcise and confused terminology
often raise questions on, or may invalidate, the weight data supplied.
UEG Report UR24 9
4. The quality of existing databases, on which early weight estimates can be based,
are poor. Information is sparse and generally badly documented. The most
important information is often carried in people’s heads.
5. With large projects the design team is built up to a peak and then dispersed on
completion. There is a poor record of continuity of personnel from project to pro-
ject. There is also unfortunately a lack of personnel continuity even within a single
project. This means that the lessons learned from mistakes are seldom practised in
subsequent projects. Even more serious, people may not be aware of their mis-
takes.
6. There is a lack of disciplined ‘weight thinking’ within the industry. Weight control
is often regarded as an unnecessary and unfulfilling task and therefore insufficient
effort has been taken to make the systems work effectively. Weight control per-
sonnel often have insufficient authority within the design team to carry out their
true function, and, as a result, the methods are ineffectively applied. This is parti-
cularly apparent in projects which involve innovative concepts where, without the
benefit of previous experience, an undisciplined approach to weight control has
very little hope of being successful. A disciplined approach, on the other hand, can
be very successful and this is witnessed by the aerospace industry which has a good
record in weight control.
7. Successful weight control depends on having competent experienced personnel
to operate it. They can not only obtain the correct data in the first place, but can
also make good engineeringj udgements of what action is necessary to curb weight
growth once it has been identified.. There are few ‘dedicated’ weight engineers
within the industry.
8. The economic climate and the political demand for North Sea oil in the past has
tended to make the extraction of oil at the earliest conceivable date the major goal
of any North Sea project. Other considerations except safety suffered in con-
sequence.
The climate has changed markedly since the early platforms were designed. How-
ever, old practices are difficult to change and weight control is still regarded by
many as of secondary importance.
9. The operational requirements of a platform often change during the design
period. Legal requirements may also change and often bring with them severe
weight penalties (for example, accommodation regulations). No weight control
system can predict quantitativelyt hese sorts of change in weight, but if weights are
known a better appreciation of the significance of such changes can be achieved.
In conclusion, it is not unusual with the current state-of-the-art for a ‘well defined‘
module to escalate in weight by a factor of about 1.8 between conceptual design and
hook-up. (This escalation results from all sources, including poor initial estimates and
changes in equipment specification during design.) The weight of a ‘poorly defined‘
module (e.g. a utilities module, which comprises a large number of relatively small
components) may escalate by a factor of 3 or more over the same period. This shows
what can be achieved by weight estimation and weight control as practised by the
industry today.
3. Current Practice in Other Industries
3.1 INTRODUCTION
A survey of weight control methods in the shipbuilding, aircraft and automotive indus-
tries has been carried out, and a summary of the findings is presented below.
Shipbuilding is most like construction for the offshore industry. The aircraft and auto-
motive industries are less comparable because there is a disproportionatep enalty (in
terms of performance of the finished article) associated with weight and, also, aircraft
and cars are mass produced with many prototypes rather than being one-off designs.
For these reasons it is unlikely that the methods of the aircraft and automotive indus-
tries could be directly applied within the offshore environment, although several valu-
able lessons may be learned from them and these are noted.
Weight control in these other industries has not always been as effective as it is today.
Tremendous strides in both the car and aircraft industries have been made since the
10 UEG Report UR24