FIP State of Art Report:
Foundations of concrete gravity
structures in the North Sea
FIP COMMISSION ON CONCRETE SEA STRUCTURES
Chairman: J.A. Derrington, UK
Members:
E. Wulff, Denmark
L. Plislcin, France
P. Xercavins, France
P.F. Daly, Republic of Ireland
J.G. Bodhe, India
M. Caironi, Italy
J.A. van Loenen, Netherlands
J.C. Slagter, Netherlands
E. Gjglrv, Norway
S. Hafskjold, Norway
K. Hove, Norway
D. Alfredo Paez, Spain
K. Christenson, Sweden
K. Eriksson, Sweden
K . H . Brittain, UK
A.A. Denton, UK
T. Ridley, UK
H. Haynes, USA
E. Hognestad, USA
K.H. Runge, USA
C. Finsterwalder, FRO
A. G.F. Eddie, Australia
D. J. Lee, UK
S. Shiraishi, Japan
S. Inomata, Japan
D.K. Mzareulov, USSR
I . Foss, Norway
B. Contarini, Brazil
G.E.B. Wilson, New Zealand
J. Rutledge, New Zealand
Technical Secretary: W.F.G. Crozier, FIP
15.706
ISBN 0 7210 1150 0
Published, designed and printed f o r the Fédération Internationale de la Précontrainte by the
Cement and Concrete Association, Wexham Springs, Slough SL3 6PL.
First pubhshed 1979
Although the Fédération Internationale de la Précontrainte does its best to ensure that any
information it may give is accurate, no hability or responsibihty of any kind (including
habihty f o r negligence) is accepted in this respect by the Federation, its members, its servants
or agents.
F I P S T A T E O F A R T R E P O R T : F O U N D A T I O N S O F C O N C R E T E G R A V I T Y S T R U C T U R E S I N T H E N O R T H SEA.
F O R E W O R D
This State o f tlie A r t report details w i t h the experience f r o m the f o u n d a t i o n s o f the concrete gravity o i l or gas p l a t f o r m s w h i c h have recently been installed i n the N o r t h Sea. The r e p o r t was prepared b y a W o r k i n g G r o u p f o r m e d i n November 1976 b y the FIP Commission o n Concrete Sea Structures. Members o f the group represent a cross-section o f the f o u n d a t i o n engineering interests w h i c h have been involved i n this recent experience.
By a number o f w o r k i n g sessions and the preparation o f original papers the W o r k i n g G r o u p has distilled the state o f the art i n t o f o u r m a i n themes:
Geotechnical investigations Design procedures
Installation methods Operational experience.
The b e n e f i t o f this w o r k has been considerable f o r those involved i n its preparation and FIP has published the r e p o r t f o r general i n f o r m a t i o n t o other engineers who may be interested i n this experience.
Since the installation o f the E k o f i s k concrete o i l storage tank b y Phillips Petroleum i n 1973, there has been a r a p i d development o f design and c o n s t r u c t i o n technology i n a p p l y i n g concrete structures t o other locations i n the N o r t h Sea. I n dealing w i t h these o f f s h o r e locations there have been many severe problems t o resolve, c h i e f l y arising f r o m the severity o f the w i n d and waves i n this area. F o r the f o u n d a t i o n engineers this has particular problems due t o the innovative concepts o f design and their l o c a t i o n on deep water unprepared seabed sites f o r w h i c h there was l i m i t e d experience o f soils response t o this t y p e o f structure.
The soil conditions at the sites, consisting o f s t i f f clays and dense sands, are being observed to be capable o f supporting the loads i n t r o d u c e d b y concrete gravity p l a t f o r m s . I t is satisfactory to be able t o report t h a t the f o u n d a t i o n s o f all the concrete structures are p e r f o r m i n g w i t h o u t
d i f f i c u l t y .
The highhghts o f the conclusions contained i n this r e p o r t relate to the f o l l o w i n g p o i n t s :
The q u a l i t y o f i n f o r m a t i o n obtained f r o m geotechnical investigation techniques under severe offshore conditions has i m p r o v e d w i t h the development o f new equipment.
Many o f the theoretical analyses o f the geotechnical design problems related to the f o u n d a t i o n behaviour have been c o n f i r m e d b y observed experience.
The r e h a b i l i t y o f installation procedures has been demonstrated, and careful m o n i t o r i n g has obtained f u r t h e r data w h i c h has i m p r o v e d p r o d u c t i o n methods.
The i n t e r a c t i o n between the f o u n d a t i o n s and the instaUation o f operational p l a t f o r m equipment f o r c o n d u c t o r and well drilling has been observed, w i t h methods devised to a t t e m p t t o minimise the disturbance t o seabed f o u n d a t i o n soils.
The r e p o r t has been divided i n t o sections and each section has been w r i t t e n b y i n d i v i d u a l members together w i t h the collaboration o f colleagues i n their organizations. The complete report has been reviewed b y all members o f the W o r k i n g G r o u p w h o have endorsed i t i n the f o r m t o be published. The Chairman wishes to acknowledge the considerable v o l u n t a r y e f f o r t made by all concerned. Their co-operation i n dealing w i t h the preparation o f original w o r k f o r this report has led t o a significant advance i n pubhshed i n f o r m a t i o n being made available to others f r o m the unique experience they have gained i n the N o r t h Sea.
T o m R i d l e y
Chairman, W o r k i n g G r o u p o n Foundations
Technische Hogeschool Bibliotheek
Afdeling: Civiele Techniek ^ Stevinweg 1
C O N T E N T S Page
1 F O U N D A T I O N S O F C O N C R E T E G R A V I T Y S T R U C T U R E S I N
T H E N O R T H S E A Tom Ridley 5
1.1 I n t r o d u c t i o n 5 1.2 Concrete gravity structures i n the N o r t h Sea 5
1.2.1 E k o f i s k tanlc 7 1.2.2 B e r y l ' A ' P l a t f o r m 7 1.2.3 B r e n t ' B ' P l a t f o r m 7 1.2.4 F r i g g C D P l P l a t f o r m 7 1.2.5 Frigg TP 1 P l a t f o r m 7 1.2.6 Frigg M C P O l P l a t f o r m 7 1.2.7 B r e n t ' D ' P l a t f o r m 7 1.2.8 S t a t f j o r d ' A ' P l a t f o r m 7 1.2.9 D u n l i n P l a t f o r m 7 1.2.10 Frigg TCP2 P l a t f o r m 14 1.2.11 N i n i a n P l a t f o r m 14 1.2.12 B r e n t ' C ' P l a t f o r m 14 1.2.13 Cormorant P l a t f o r m 14 1.3 Summary o f the state o f the art 14
1.3.1 Background i n f o r m a t i o n 14 1.3.2 F o u n d a t i o n design problems 14 1.3.3 Site investigations 15 1.3.4 Design procedures 15 1.3.5 I n s t a l l a t i o n methods 16 1.3.6 Operational performance 16 1.4 Conclusions 17 1.5 References 17 2 G E O T E C H N I C A L I N V E S T I G A T I O N P R A C T I C E S Richard A. Sullivan 20 2.1 I n t r o d u c t i o n 20 2.2 Planning o f investigation 21
2.2.1 PreHminary area survey 21 2.2.2 Detailed f o u n d a t i o n investigation 21 2.2.3 Position f i x i n g 21 2.3 Geophysical survey 22 2.3.1 B a t h y m e t r y 22 2.3.2 Seafloor topography 22 2.3.3 Continuous sub-bottom r e f l e c t i o n p r o f i l i n g 23
2.3.4 Shallow penetration sampling 23
2.4 Geotechnical investigation 24 2.4.1 R o t a r y d r i l h n g and wirehne sampling 24
2.4.2 Cone penetrometer testing 25 2.4.3 Other i n situ testing 27
2.5 L a b o r a t o r y testing 27 2.5.1 Shipboard testing 27 2.5.2 Onshore testing 27 2.6 Evaluation o f design soil parameters 28
2.6.1 I n situ testing 28 2.6.2 L a b o r a t o r y testing 29
2.7 Conclusions 30 2.8 References 31
3 F O U N D A T I O N D E S I G N M E T H O D S F O R G R A V I T Y S T R U C T U R E S
Ivar Foss and Rune Dahlberg 33
3.1 I n t r o d u c t i o n 33 3.2 Design principles 33 3.3 Stability 36 3.3.1 Problems 36 3.3.2 A n a l y t i c a l methods 37 3.3.3 Soil properties 39 3.4 Long-term d e f o r m a t i o n 4 0 3.4.1 Problems 4 0 3.4.2 A n a l y t i c a l methods 4 0 3.4.3 Soil properties 4 2 3.5 D y n a m i c m o t i o n s 4 4 3.5.1 Problems 4 4 3.5.2 A n a l y t i c a l methods 4 4 3.5.3 Soil properties 45 3.6 Penetration resistance 46 3.6.1 Problems 46 3.6.2 A n a l y t i c a l methods 47 3.6.3 Soil properties 48 3.7 Reaction forces on base structure 49
3.7.1 Problems 49 3.7.2 A n a l y t i c a l methods 50
3.7.3 Soil properties 50 3.8 E f f e c t s o f repeated loading 51
3.8.1 Problems 51 3.8.2 Basic mechanism o f soil behaviour 51
3.8.3 A n a l y t i c a l methods 53 3.9 Centrifuge m o d e l tests 54
3.10 Summary 54 3.11 References and b i b l i o g r a p h y 55
4 I N S T A L L A T I O N O F C O N C R E T E P L A T F O R M S
Ove Eide and Oddvar Kjekstad 60
4.1 I n t r o d u c t i o n 60 4.2 Installation manual 60
4.3 T o u c h d o w n 60 4.4 Skirt penetration 62
4.4.1 D i f f e r e n t i a l water pressure across skirts 62
4.4.2 Skirt penetration resistance 62 4.4.3 Skirt and soil capacity f o r h o r i z o n t a l loads 63
4.5 Base contact 64 4.6 G r o u t i n g 65 4.7 Installations t h r o u g h the base slab 66
4.8 Data aquisition system 66 4.9 References and b i b l i o g r a p h y 67 5 O P E R A T I O N A L P E R F O R M A N C E O F C O N C R E T E P L A T F O R M S K.L. Taylor 68 5.1 I n t r o d u c t i o n 68 5.2 Conductor installation 68 5.3 F o u n d a t i o n performance 68 5.4 Use o f i n s t r u m e n t a t i o n f o r performance m o n i t o r i n g 69 5.5 Seabed scour 69 5.6 Operational records 69 5.7 References 76
1 F O U N D A T I O N S O F C O N C R E T E G R A V I T Y S T R U C T U R E S I N T H E N O R T H S E A
1.1 Introduction
Thirteen large concrete structures have been installed since 1973 on the seabed i n the N o r t h Sea. The experience gained f r o m the site investigations and design o f the f o u n d a t i o n s f o r these
structures can n o w be reviewed, together w i t h the installation and i n i t i a l performance over the last f o u r years. These structures have the f u n c t i o n o f providing o f f s h o r e p l a t f o r m s f r o m w h i c h the various operations o f d r i l l i n g , p r o d u c t i o n , storing and t r a n s p o r t a t i o n o f o i l or gas can safely take place.
A l l o f the structures being used have adopted the same t y p e o f f o u n d a t i o n concept. This consists o f the e l i m i n a t i o n o f u p l i f t forces b y the provision o f structural selfweight and ballast using large caisson-type f o u n d a t i o n s . This weight provides continuous contact w i t h the seabed and resists the very large forces generated b y the w i n d and waves i n o f f s h o r e water depths vary-ing f r o m 70 t o 153 metres.
The f o u n d a t i o n loading is transferred f r o m the structure at the seabed interface, and this takes the f o r m o f vertical load, bending moments and shear forces. High local forces can also arise during installation, due t o local highspots on the seabed. These can combine w i t h the t e m p o r a r y d i f f e r e n t i a l water pressures on the p l a t f o r m walls and base w h i c h exist d u r i n g i m m e r s i o n o f the structures t o cause critical soil and structure f o u n d a t i o n loading. Projecting skirts have usually been provided below the f o u n d a t i o n structure t o increase the e f f i c i e n c y o f transfer o f forces t o the u n d e r l y i n g seabed soils. These also prevent u n d e r m i n i n g o f the f o u n d a t i o n due to scour action and are convenient compartments f o r sub-base g r o u t i n g .
The very large forces thus generated i n the seabed soils have posed f o r m i d a b l e problems f o r the f o u n d a t i o n engineer, requiring the rapid development o f new knowledge and techniques t o provide judgement o f safe f o u n d a t i o n conditions. This development has been associated w i t h the design and installation o f each o f the d i f f e r e n t structures. The operating companies are also n o w beginning t o judge the performance characteristics f r o m the structures i n service i n the N o r t h Sea.
1.2 Concrete gravity structures in the North Sea
The t h i r t e e n concrete structures w h i c h are being used i n the N o r t h Sea at this t i m e are i d e n t i f i e d i n Table 1.1, and are given i n historic sequence o f their installation. A b r i e f description o f each structure w i l l assist an understanding o f the general points w h i c h are made i n the rest o f the State-of-the-Art r e p o r t on their f o u n d a t i o n design principles. Their locations are indicated o n the N o r t h Sea map as shown i n Figure 1.1.
Figure 1.1: L o c a t i o n o f concrete structures in the N o r t h Sea ( 1 9 7 8 ) .
Table 1.1: Concrete structures i n the N o r t h Sea.
Site data Base
Type Name Operator Year Water
depth m
Foundation soil
Slab Skirts Dowels
Doris Ekofisk Phillips 1973 70 Dense fine sand Flat A = 7400 0.4 m concrete ribs None
Condeep Beryl A Mobil 1975 120 Dense fine sand
over clay Conical domes A = 6200m2 3.0 m steel 0.5 m concrete 3
Condeep Brent B Shell 1975 140 Stiff clay w i t h
sand layers Conical domes A = 6200 4.0 m steel 0.5 m concrete 3
Doris Frigg CDPl Elf 1976 98 Dense fine sand Flat, ring-shaped
A = 5600m2
None None
Sea Tank Frigg TPl Elf 1976 104 Dense fine sand
over clay
Flat A = 5600 m^ 2.0 m concrete None
Doris Frigg MCPOl Total 1976 94 Dense fine sand Flat ring-shaped
A = 5600m2
None None
Condeep Brent D Shell 1986 140 Stiff clay with
sand layers Conical domes A = 6300 m^ 4.5 m steel 0.5 m. concrete 3
Condeep Statfjord A Mobil 1977 145 Stiff clay Conical domes
A = 7800 m^
3.0 m steel 0.5 m concrete
3
Andoc Dunlin Shell 1977 153 Stiff clay with
sand layers
Flat A = 10 600 m^ 4.0 steel 4
Condeep Frigg TCP2 Elf 1977 102 Dense fine sand
over clay Conical domes A = 9 3 0 0 m^ 1.2 m steel 0.5 m concrete 3
Doris Ninian Chevron 1978 136 Stiff clay with
sand layers
Flat A = 15 400 m^ 3.5 m steel None
Sea Tank Brent C Shell 1978 140 Stiff clay with
sand layers
Flat A = 10 300 m^ 3.0 concrete None
1.2.1 Ekofisk Tank (Figure 1.2) ^^'^^
This 1 m i l l i o n barrel o i l storage tank was designed b y C.G. Doris and constructed i n Stavanger, N o r w a y , f o r the PhilUps Petroleum Company's E k o f i s k f i e l d . The t a n k was t o w e d i n t o p o s i t i o n i n June 1973 and located i n 70 m water depth. The f o u n d a t i o n base is nearly circular i n plan w i t h a mean diameter o f 95 m , and when completed this concrete structure had a t o t a l weight o f 215 000 t .
1.2.2 Beryl 'A' Platform (Figure 1.3)(4-10)
This was the f i r s t concrete p l a t f o r m structure t o be constructed t o the Condeep design, and is used f o r the o i l p r o d u c t i o n f a c i l i t y o n M o b i l ' s B e r y l f i e l d . I t was b u i l t i n Stavanger, N o r w a y , and t o w e d i n t o p o s i t i o n i n July 1975 t o be installed i n 120 m water depth. The f o u n d a t i o n base consists o f 19 c y l i n d r i c a l tanks w i t h steel skirts giving a mean overall diameter o f a p p r o x i -mately 90 m and a t o t a l p l a t f o r m weight o f 330 000 t.
1.2.3 Brent 'B' Platform (Figure 1.4)(^ ^^
This Condeep structure was also b u i l t i n Stavanger, N o r w a y , and installed i n 140 m water depth during August 1975 f o r use b y the Shell/Esso group o n their Brent o i l f i e l d as a p r o d u c t i o n p l a t f o r m . The f o u n d a t i o n base was similar t o the B e r y l ' A ' p l a t f o r m w i t h 19 c y l i n d r i c a l tanks using steel skirts giving a mean overall diameter o f a p p r o x i m a t e l y 90 m and a t o t a l p l a t f o r m weight o f 330 000 t .
1.2.4 Frigg C D P l Platform (Figure 1.5)^^2, 13)
Designed b y Howard-Doris and constructed i n N o r w a y b y the Norwegian Contractors G r o u p , this p l a t f o r m was installed i n 98 m water depth i n September 1975 and is used f o r the d r i l h n g , p r o d u c t i o n and processing o f gas. The p l a t f o r m is a prestressed concrete structure, and closely f o l l o w s the design principles estabhshed w i t h the E k o f i s k t a n k . The f o u n d a t i o n base is circular w i t h a diameter o f 102 m and the t o t a l p l a t f o r m weight ( i n c l u d i n g some ballasting) is 183 000 t .
1.2.5 Frigg T P l Platform (Figure 1.5)(^4, 15)
Designed and b u i l t b y the McAlpine—Sea Tank group at A r d y n e Point i n Scotland and installed in 104 m water d e p t h i n June 1976. This p l a t f o r m is used f o r the retreatment o f the gas f r o m the C D P l p l a t f o r m . I t consists o f a caisson w i t h 25 cells giving a f o u n d a t i o n base o f 72 m square i n plan using concrete skirts and w i t h a t o t a l p l a t f o r m weight o f 176 000 t .
1.2.6 Frigg Platform M C P O l (Figure 1.6)*^^
Designed b y C.G. Doris and constructed first i n a d r y d o c k and later i n a f j o r d at Kalvic, Sweden, this barrel structure is located some 174 kilometres f r o m the Scottish mainland in 94 m water depth. The p l a t f o r m was installed during June 1976 and its principal f u n c t i o n is t o boost the f l o w o f gas d o w n the pipeline "from Frigg t o the St. Fergus terminal. The circular base is 102 m i n diameter and the t o t a l p l a t f o r m weight (including sand ballasting) is 183 000 t.
1.2.7 Brent 'D' Platform (Figure 1.4)^^ ^ )
This structure is v i r t u a l l y the same as the Brent ' B ' p l a t f o r m described i n paragraph 1.2.3 being a Condeep design b u i l t i n Stavanger, N o r w a y . I t was installed i n July 1976 in 140 m water depth.
1.2.8 Statfjord 'A' Platform (Figure I J p ^ '
This p l a t f o r m is the tallest p l a t f o r m so far constructed and was installed i n a water d e p t h o f 145 m i n May 1977. I t is t o be used f o r o i l p r o d u c t i o n facilities b y the operating c o m p a n y , M o b i l . Based o n the Condeep design the p l a t f o r m was b u i l t at Stavanger, N o r w a y , b y Norwegian Contractors G r o u p . The f o u n d a t i o n base has a diameter o f 110 m and consists o f 19 c y l i n d r i c a l tanks w i t h a t o t a l p l a t f o r m weight o f 350 000 t .
1.2.9 Dunlin 'A' Platform (Figure 1.4)(20-23)
This p l a t f o r m was designed and constructed b y the A N D O C group, using a dry-dock near
R o t t e r d a m f o r the f i r s t stage w i t h c o m p l e t i o n o f the structure i n a f j o r d near Stavanger, N o r w a y . I t was t o w e d t o l o c a t i o n i n 153 m water depth d u r i n g May 1977 f o r use i n the Shell/Expro D u n l i n o i l f i e l d east o f Shetland Islands. The f o u n d a t i o n base is 100m square w i t h steel skirts and a t o t a l p l a t f o r m weight o f 250 000 t.
1.2.10 Frigg Platform T C P 2 (Figure 1.6)(4)
Designed b y the Condeep G r o u p and b u i l t i n Aldalsnes, N o r w a y , b y the Norwegian Contractors G r o u p , this p l a t f o r m was located i n 102 m water d e p t h i n June 1977. I t is used t o process the recompress gas f r o m the Frigg f i e l d . The f o u n d a t i o n consists o f 19 c y l i n d r i c a l cells on a hexa-gonal base slab, w i t h a t o t a l p l a t f o r m weight (including sand ballasting) o f 306 000 t.
1.2.11 Ninian Platform (Figure 1.2)(24-26)
This p l a t f o r m was designed b y the Howard-Doris group and was completed at t h e i r L o c h K i s h o r n site i n Scotland f o r installation i n 136 m water d e p t h d u r i n g the summer o f 1978. I t is the largest concrete structure t o be b u i l t t o date f o r N o r t h Sea use, and w i l l have a t o t a l p l a t f o r m weight (including ballasting) o f 600 000 t . The p l a t f o r m w i l l be used b y Chevron f o r the o i l p r o d u c t i o n facihties on the N i n i a n o i l f i e l d east o f Shetland Islands. The f o u n d a t i o n base is circular o f 140 m diameter and uses steel skirts.
1.2.12 Brent ' C ' Platform (Figure 1.4)^27)
This p l a t f o r m was designed and constructed b y the McAlpine—Sea T a n k group at A r d y n e P o i n t , Scotland and is being completed near Stavanger, N o r w a y , f o r i n s t a l l a t i o n i n 140 m water d e p t h . I t w i l l be used b y the Shell/Esso group as an o i l p r o d u c t i o n p l a t f o r m f o r their Brent o i l f i e l d east o f Shetland Islands. The f o u n d a t i o n base is 91 m square w i t h 64 cells, using concrete skirts and a t o t a l p l a t f o r m weight o f 282 000 t .
1.2.13 Cormorant Platform (Figure 1.4)(27,28)
Similar t o the Brent ' C ' p l a t f o r m this structure has been designed and constructed b y the McAlpine—Sea Tank G r o u p at A r d y n e Point, Scotland, and is being completed near Stavanger, N o r w a y f o r installation i n 150 m water d e p t h at the Shell/Esso C o r m o r a n t o i l f i e l d east o f Shetland Islands. The f o u n d a t i o n base is 100 m square w i t h 64 cells, using concrete skirts, and a t o t a l p l a t f o r m weight o f 343 000 t .
1.3 Summary of the state of the art
1.3.1 Background information
There is l i t t l e accumulated knowledge and experience o f the t o p layers o f seabed soils i n the presently explored o f f s h o r e areas o f the w o r l d . The site investigations f o r o f f s h o r e sites are a m u c h more demanding task t h a n onshore. The q u a l i t y and amount o f data w h i c h is available t o the designer is considerably less f o r an o f f s h o r e gravity structure t h a n f o r a structure o f similar size and importance onshore. This deficiency has t o be compensated f o r b y choosing the design parameters on the safe side, t a k i n g i n t o account the variation i n results between borings.
A n o t h e r f a c t o r is the p r o b l e m involved i n placing a large structure, about 100 m i n diameter, on an unprepared seabed site. Local variations i n soil conditions and b o t t o m t o p o g r a p h y have to be taken i n t o account. I t is also an advantage, f r o m the operator's p o i n t o f view, t h a t the design o f the structure is s u f f i c i e n t l y adaptable t o allow installation o n several d i f f e r e n t sites. This arises f r o m the possibihty that the reservoir appraisal indicates t h a t the o p t i m a l p l a t f o r m position is d i f f e r e n t f r o m the one f i r s t envisaged. The requirements f o r l o c a t i o n f l e x i b i l i t y are thus very i m p o r t a n t .
The actual dimensions o f the base structure are n o t o n l y decided b y the f o u n d a t i o n engineer. More than f o r most other structures he wiU w o r k hand i n hand w i t h the structural engineer and other members o f the design team. Considerations i n f l u e n c i n g the base dimensions include requirements f o r hydrostatic stability i n relation t o deck load d u r i n g t o w - o u t , the need f o r o i l storage, the structural system selected f o r the caisson and the admissible d r a f t i n shallow water along the t o w - o u t r o u t e , as well as the geotechnical considerations n o r m a h y applied.
This means that the f o u n d a t i o n engineer must w o r k very closely w i t h the other designers during all stages o f the design programme, so that there is c o n t i n u i t y o f i n t e r p r e t a t i o n o f all factors arising f r o m the seabed soil conditions and t h e i r behaviour d u r i n g the i n s t a l l a t i o n and w o r k i n g h f e o f the structure.
1.3.2 Foundation design problem
The basic requirement f o r the f o u n d a t i o n o f an offshore concrete p l a t f o r m is similar t o t h a t f o r any landbased structure, i.e., a s u f f i c i e n t f a c t o r o f safety against u l t i m a t e f a i l u r e and no
intolerable settlements or movements under the applied loads. However, the o f f s h o r e c o n d i t i o n s and operational use o f the p l a t f o r m structures means t h a t there are special problems arising w h i c h combine w i t h the placing o f these structures on unprepared seabed sites t o require new f o u n d a t i o n procedures t o be i n t r o d u c e d . As the economics o f the structure depend greatly o n the design o f the f o u n d a t i o n , i t is clear t h a t unnecessary conservatism must be avoided.
I n deahng w i t h the o f f s h o r e conditions the f o u n d a t i o n engineer has t o face severe l i m i t a t i o n s based o n existing knowledge f o r the f oho w i n g reasons:
(1) For the extreme loading f r o m wave forces there are m u c h larger h o r i z o n t a l forces t h a n are t y p i c a l f o r land based structures, and the f a i l u r e modes t o be considered are therefore d i f f e r e n t f r o m n o r m a l experience.
(2) The dynamic nature o f the wave loading introduces the special p r o b l e m o f the effects o f repeated loading on the geotechnical properties o f the soils.
Most structures are equipped w i t h skirts designed t o penetrate a certain depth i n t o the seabed. Unless the submerged weight o f the structure is s u f f i c i e n t t o achieve the required p e n e t r a t i o n depth, the safety o f the f o u n d a t i o n s may be less than required. I f , o n the other hand, a larger penetration is obtained local high spots or hard points on the seabed may i n t r o d u c e excessive local loads o n the base structure. N o r m a l l y , these loads govern the design o f the base slab.
The skirts also provide p r o t e c t i o n t o the f o u n d a t i o n f r o m the effects o f seabed scour, and skirt lengths o f 1 t o 5 m are used depending on the structure and soil conditions. The l a y o u t o f the skirts must be chosen t o aUow the submerged weight o f the structure t o provide adequate penetration i n t o the soils during installation. T o achieve this result may require a number o f innovative design features such as c u t t i n g edges, j e t t i n g locating dowels and grouting o f the underside o f the base slab.
1.3.3 Site investigations
A comprehensive site investigation f o r a concrete structure i n the N o r t h Sea can cost a p p r o x i -mately £ 5 0 0 000 o f w h i c h about 10-15% relates t o geophysical survey. This means t h a t soils i n f o r m a t i o n is obtained progressively d u r i n g the early stages o f a project, and a detailed geo-technical survey is n o t usually carried o u t u n t i l the result o f p r e l i m i n a r y feasibility surveys is completed.
To undertake the f o u n d a t i o n design requires measurement o f soil properties t o a d e p t h b e l o w seabed o f about 150 m . Under offshore conditions this has considerable practical d i f f i c u l t y i n planning and executing site investigations. The i n t e r p r e t a t i o n o f results f r o m i n situ tests and laboratory tests needs m u c h skill, judgement and experience t o allow the evaluation o f design parameters t o represent soil conditions d u r i n g installation and the w o r k i n g l i f e o f the structure.
Successful o f f s h o r e site investigation involves the use o f a large number o f techniques f o r investigating various aspects such as regional and local geology, seabed topography, soil types, their genesis and variation, and the mechanical properties o f the ground under static and cyclic load. The measurement and i n t e r p r e t a t i o n o f all these features is far f r o m s t r a i g h t f o r w a r d . F o r example, the characteristic i n situ static strength o f a given stratum is a f u n c t i o n o f m a n y factors, such as soil f a b r i c , structure, stress h i s t o r y and imposed stress changes, and may d i f f e r significantly f r o m the strength derived f r o m r o u t i n e l a b o r a t o r y tests. These and may other d i f f i c u l t i e s and l i m i t a t i o n s must be bourne i n m i n d at all times during the design o f the f o u n d a t i o n s and other elements o f an offshore structure.
Using a seafloor jack, push sampling can be carried o u t t o obtain better q u a l i t y samples. Operationally reliable i n situ testing equipment, such as the cone penetrometer and remote vane, have been developed and have proven t o be o f value. Better q u a h t y high-resolution geophysical data can be obtained and integrated w i t h geotechnical data t o provide c o n f i d e n t site i n f o r m a t i o n .
1.3.4 Design procedures
The m a i n geotechnical problem.s associated w i t h the f o u n d a t i o n design f o r offshore concrete structures are:
L o c a l base contact stresses Stability
Settlements
D y n a m i c displacements
E f f e c t s o f cyclic loading o n soils H y d r a u l i c behaviour on soils Scour action o n seabed
The l i m i t state m e t h o d is n o w being accepted f o r the f o u n d a t i o n design o f concrete structures, the most i m p o r t a n t checks being o n stability i n the u l t i m a t e Hmit state and displacement i n the serviceability l i m i t state.
The effects o f repeated loading lead t o i m p o r t a n t changes i n the stress—strain—strength properties o f b o t h sand and clay, w h i c h have t o be accounted f o r i n stability analysis as w e l l as calculations o f d y n a m i c m o t i o n s and settlements. I n a d d i t i o n t o a conventional quasi-static stability analysis, the risk o f failure i n cyclic loading has t o be considered as a separate failure mode i n the u l t i m a t e l i m i t state. E m p i r i c a l guidehnes and simple methods o f analysis using l a b o r a t o r y specimens as representative o f average stress conditions are w i d e l y used t o assess the effects o f repeated load-ing o n the soil properties. F i n i t e element methods f o r complicated cases and centrifuge m o d e l tests f o r studying mechanisms and behaviour are valuable supplementary tools.
Analyses o f penetration resistance o f dowels and skirts, as w e l l as reaction forces o n t h e base structure t o be expected d u r i n g installation, are i m p o r t a n t considerations i n the design o f a concrete structure. The methods o f analysis developed f o r these purposes, usually r e l y i n g o n soil data f r o m cone penetration tests, appear t o give results i n reasonable agreement w i t h f i e l d observations.
Analysis o f settlements m a y be based o n l a b o r a t o r y consoHdation tests, b u t f o r the sands and s t i f f clays most c o m m o n i n the N o r t h Sea i t appears t h a t empirical relations between settlement parameters and the index properties o f the soils produce equally good results.
1.3.5 Installation methods
The f o u n d a t i o n design must pay particular a t t e n t i o n t o the critical installation phase as the concrete structure touches d o w n , penetrates, makes base contact and yet preserves the f o u n d -a t i o n soils. D u r i n g penetr-ation o f the skirts the structure c-an experience l-arge eccentric soil resistance forces w h i c h must be compensated b y eccentric ballasting moments w i t h i n the caisson t o m a i n t a i n a vertical p o s i t i o n . The p r e d i c t i o n o f these forces is therefore an i m p o r t a n t part o f the geotechnical design p r o b l e m , and methods have been f o u n d w h i c h give g o o d correlation w i t h the experience obtained f r o m presently installed structures.
I n dealing w i t h the contact pressures experienced b y the base during installation i t has been necessary t o i n t r o d u c e load-measuring instruments so t h a t the calculated pressures are n o t exceeded. Reasonably good agreement has been observed between measurements and pre-dicitions. G r o u t i n g o f the space between seabed and the base o f the structure has been successfully carried out i n order t o :
Keep the p l a t f o r m level
A v o i d p i p i n g below the structure A v o i d overstressing
A v o i d f u r t h e r skirt penetration Secure an even soil reaction.
The experience t o date has c o n f i r m e d t h a t this t y p e o f concrete structure can be installed w i t h -o u t seri-ous d i f f i c u l t y , and each structure has taken -o n l y a f e w h-ours t -o achieve its seabed f o u n d a t i o n t o provide f o r f i x e d stability w i t h good l o c a t i o n a l accuracy.
1.3.6 Operational performance
A l l structures are reported t o be p e r f o r m i n g t h e i r f u n c t i o n s satisfactorily f o l l o w i n g some i n i t i a l d i f f i c u l t i e s i n the installation o f o i l - w e l l c o n d u c t o r pipes where they have been needed. Since all
the concrete structures are on hard or dense f o u n d a t i o n soils, i t has been f o u n d necessary t o install c o n d u c t o r pipes b y a c o m b i n a t i o n o f d r i l l i n g a p i l o t hole and driving the c o n d u c t o r pipe. Each operator has evolved d i f f e r e n t techniques t o minimise the disturbance o f the f o u n d a t i o n soils.
None o f the structures has suffered critical settlements t o date. As an example, the settlement o f the E k o f i s k p l a t f o r m f r o m the installation i n 1973 t o 1977 has been 26 c m . I t experienced the m a j o r i t y o f its settlement during the f i r s t year, i n c l u d i n g a movement o f 4 c m d u r i n g one day, b e f o r e reaching the maxiimum value.
Assumptions about f o u n d a t i o n behaviour made i n design are being c o n f i r m e d relating t o stability, soil f o u n d a t i o n pore pressures and d y n a m i c m o t i o n s w i t h extension i n s t r u m e n t a t i o n installed o n v i r t u a l l y all concrete p l a t f o r m s . A l t h o u g h wave forces equivalent t o o n l y 45% o f the m a x i m u m N o r t h Sea design conditions (100-year s t o r m ) have so f a r been experienced, observations indicate t h a t pore pressure have been w e l l w i t h i n allowable l i m i t s .
Conclusions
The state-or-the-art o n the various engineering aspects o f the design and construction o f safe f o u n d a t i o n s f o r concrete gravity structures i n the N o r t h Sea has advanced over the last f e w years to give confidence using current procedures.
There are clearly m a n y requirements t o be met before b o t h safe and economic design solutions can be f o u n d f o r each l o c a t i o n . This leads t o each structure relying on considerable i n n o v a t i o n i n its development f o r use, and a close l i n k is needed between all o f the skills i n the design team. I t is essential f o r the f o u n d a t i o n engineer t o be included at all stages o f the project t o give c o n t i n u i t y o f i n t e r p r e t a t i o n on f o u n d a t i o n problems.
The rapid period over w h i c h these very large structures have proceeded f r o m concept t o execution has led t o considerable demands f o r technological development being imposed o n all concerned. This phase o f the N o r t h Sea o i l and gas o f f s h o r e activity has given an unparalleled o p p o r t u n i t y t o demonstrate t h a t concrete sea structures do provide rehable o f f s h o r e p l a t f o r m s f o r d r i l l i n g , p r o d u c t i o n and storage o f o i l or gas.
The r e l i a b i l i t y o f these concrete p l a t f o r m structures wiU continue t o depend o n their f o u n d a t i o n performance. Predictions o f l o n g t e r m behaviour w i l l be c o n f i r m e d f r o m the data obtained during m o n i t o r i n g i n use.
R E F E R E N C E S
1. M A R I O N , H . and M A H F O U Z , G.
Design and construction o f the E k o f i s k a r t i f i c i a l island. Proceedings of the Institute of
Civil Engineers, V o l . 56, Part 1, November 1974.
2-. M A R I O N , H . A .
E k o f i s k storage tank. Proceedings of the R.I.N. A. Symposium on Ocean Engineering, T e d d i n g t o n , 1974. pp. 83-94.
3. F E R R Y M A N , P.J.
E k o f i s k I ' s new role i n the N o r t h Sea. Paper OE-75 212 presented t o O f f s h o r e Europe Conference, Aberdeen, 1975.
4. G E R W I C K Jr., B E N , C.
C o n d e e p - i n 400 f t water concrete caisson requires no piles. Civil Engineering, ASCE,
Vol 4 6 , N o . 4, A p r i l 1976.
5. BLOCK, M.L.etal
B e r y l ' A ' M o b i l chooses concrete f o r B e r y l f i e l d p l a t f o r m . Petroleum International 14, ( 4 ) , A p r i l 1974. pp. 73-75.
6 A N O N
7. A T T F I E L D , K .
B e r y l ' A ' beats problems t o land o n the coast. Offshire Engineer, August 1975. p p . 43-44.
8. A N O N
K e y component i n $ 3 5 0 - m i l l i o n complex installed i n N o r t h Sea. Ocean Industry. 10 ( 8 ) , August 1975. pp. 44-47.
9. A N O N .
Concrete giant B e r y l ' A ' begins N o r t h Sea service. Gas World, 180, ( 4 6 7 8 ) , August 1975. pp. 412-415.
10. GANSEAL,E., etal
Concrete o f f s h o r e structures, strains measured o n f u l l scale shells. Paper 2435 presented t o the O f f s h o r e Technology Conference, H o u s t o n 1976.
1 1 . E I D E , O V E . T. and L A R S E N , L E I F . G.
InstaUation o f the Shell/Esso Brent ' B ' condeep p r o d u c t i o n p l a t f o r m . Paper 2 4 3 4 presented t o the O f f s h o r e Technology Conference, H o u s t o n , 1976.
12. F R I G G C O N S O R T I A .
The Frigg Gas Story. Frigg Consortia 1977. ( T o t a l , E l f A q u i t a i n e , Norske H y d r o and
Statoil.)
13. M I C H A E L , D . and H A M A R D , J.C.
Frigg CDPl—The intermediate p l a t f o r m f o r Frigg. I n d . Petrol. Europe Cas-Chim. 42 ( 3 ) , M a r c h 1974. pp. 49, 5 1 .
14. D E R R I N G T O N , J.A.
T P l — T h e construction o f gas treatment p l a t f o r m N o . 1 f o r the Frigg Field f o r Elf-Norge A / S . The Structure Eitgineer, V o l . 55, N o . 2, February 1977. pp
15. D E R R I N G T O N , J.A.
T P l — C o n s t r u c t i o n o f M c A l p i n e sea tank gravity p l a t f o r m s at A r d y n e Point, A r g y l l . Paper presented at Conference o n Design and C o n s t r u c t i o n o f O f f s h o r e Structures. L o n d o n , 1976. pp. 121-130.
16. A N O N .
MCP-01 Special Report: Frigg: tears and t r i u m p h . Noroil, V o l . 5, N o . 10, October 1977. pp. 57, 59, 6 1 .
17. A N O N .
Brent ' D ' platform—Second concrete p l a t f o r m set i n U K ' s largest o i l f i e l d . World Oil, 183, ( 4 ) , September 1976. pp. 7 4 , 76.
18. A N O N .
S t a t f j o r d A - T h e largest ever. Noroil, V o l . 5, N o . 1 1 , November 1977.
19. A N O N .
S t a t f j o r d group f i g h t s f o r single p l a t f o r m approach. Offshore Engineer, September 1977. pp. 69-74. 20. A N O N . D u n l i n P l a t f o r m . Noroil, V o l . 3, N o . 7, June 1975. 2 1 . A N O N . D u n l i n P l a t f o r m : f l o a t - o u t i n R o t t e r d a m . Noroil, V o l . 3, N o . 7, July 1975. pp. 5 1 , 53. 22. A N O N .
Scotland may miss o u t on D u n l i n deck placing. Offshore Engineer, September 1975. p p . 1 1 .
23. A N O N .
D u n l i n Concrete P l a t f o r m - s t a r t to f i n i s h . Petroleum Review, V o l . 3 1 , N o . 3 6 7 , July 1977. pp. 10-12.
24. A N O N .
N i n i a n C e n t r a l - N i n i a n Central P l a t f o r m t o be the w o r l d ' s largest concrete gravity structure.
Oil and Gas Journal, V o l . 74, N o . 26, June 1976. pp. 133-138.
25. A N O N .
Howard-Doris carve niche at K i s h o r n . A^ew^ Civil Engineer, February 1976.
26. M U T C H R.
Ninian F i e l d : w i t h delays aside, the central p l a t f o r m awaits t o w i n g hook-up. Offshore
Engineer, V o l . 37, N o . 1 1 . October 1977. pp. 66-70.
27. D E R R I N G T O N , J.A.
C o n s t r u c t i o n o f McAlpine/Sea Tank G r a v i t y Platforms at A r d y n e Point, A r g y l l . Paper presented at the I C E Conference o n Design and C o n s t r u c t i o n o f O f f s h o r e Structures, L o n d o n 1976.
28. A N O N .
Tallest p r o d u c t i o n p l a t f o r m s set f o r N o r t h Sea. Oil and Gas Journal, V o l . 7 2 , N o . 2 1 , M a y 1974. pp. 34.
2 G E O T E C H N I C A L I N V E S T I G A T I O N P R A C T I C E S
2.1 Introduction
O f f s h o r e concrete structures, resting o n the seafloor, rely u p o n the underlying soils t o provide adequate support against f o u n d a t i o n instability,- and t o undergo small deformations so t h a t h o r i z o n t a l and vertical structure movements are w i t h i n tolerable operating limits. I n a d d i t i o n to investigating geologic s t r a t i f i c a t i o n o f the supporting soil and rock strata, geotechnical
properties o f the strata must be determined f r o m i n situ testing and laboratory testing o f core samples, i n order t o predict soil-structure response under anticipated structural and environ-mental loads.
The E k o f i s k o i l storage t a n k was the first concrete gravity p l a t f o r m t o be installed i n deep water i n the N o r t h Sea i n the summer o f 1973 ( M a r i o n , 1974). The water d e p t h is 70 m and the near circular structure is 90 m t a l l and about 93 m i n diameter. The base w i d t h o f concrete gravity p l a t f o r m s generally ranges f r o m 75 to 125 m , so t h a t soil and r o c k conditions should be explored t o penetrations o f about 150 m . These large and heavy structures have been placed i n the N o r t h Sea o n strong overconsohdated clays and dense sands to m i n i m i z e p o t e n t i a l structure deformations.
I n the Pleistocene glacial period, the northwest c o n t i n e n t a l glacier advanced and retreated several times across the N o r t h Sea continental shelf ( L o k e n , 1976). The m o v i n g ice sheets eroded irregular surfaces i n t o the underlying sediments t h a t were i n f i l l e d w i t h glaciomarine and fluvioglacial sediments during interglacial periods, w h i l e m u c h o f the southern N o r t h Sea remained above sea level. The repeated advances and retreats o f the glaciers produced c o m p l e x sequences o f glacial, marine and f l u v i a l sediments w h i c h are overconsohdated f r o m the weight o f the ice sheets. Since the end o f the Pleistocene period, the N o r t h Sea basin has been submerged below sea level and Holocene deposits o f silts, clays and f i n e sands are relatively t h i n , except f o r i n f i h i n g o f erosion depressions remaining f r o m the last glacial retreat. The Holocene sands are i n a dense c o n d i t i o n f r o m the compacting e f f e c t o f ocean wave induced pressures o n the seafloor ( B j e r r u m , 1973) that have no beneficial e f f e c t u p o n the strength characteristics o f clays. Figure 2.1 shows t y p i c a l soil conditions i n water depths greater t h a n 150 m i n the n o r t h e r n N o r t h Sea and i n the central N o r t h Sea where water depths are about 75 m .
Undrained shear strength l<N/m^ Undrained shear strength kN/m^ (0
1
1
8 ^50-0) o CL g75-100-i
Very stiff to hard silty clay Dense silt Hard sandy clay Dense sand 200 400 600 25 CO 0) E uT O O «50-COI
CD XI ^75-100-1
1
0 200 4 0 0 6 0 0 Dense fine sand Hard sandy clay Dense fine sand Hard clay(a) Northern North Sea
(b) Central North Sea
2.2 Planning of investigation
A comprehensive f o u n d a t i o n investigation o f a gravity p l a t f o r m site i n the N o r t h Sea m i g h t cost between £ 4 0 0 000 and £ 6 0 0 0 0 0 , o f w h i c h 10 t o 15% relates t o the geophysical survey Consequently, the investigation o f a p o t e n t i a l site is generally undertaken i n progressive stages, so t h a t structural concepts can be developed w i t h due regard t o soil conditions. The shallow geological environment o f the area is examined f r o m a geophysical survey, together w i t h one or t w o soil borings f o r correlating stratigraphic boundaries interpreted f r o m the seismic p r o f i l l i n g data. The geotechnical investigation is ideally delayed u n t i l the results o f the shallow geophysical survey have been studied and the actual p l a t f o r m l o c a t i o n has been selected, w i t h due consideration t o the development o f the o i l or gas f i e l d .
The feasibility o f placing a concrete gravity p l a t f o r m on the seabed is f r e q u e n t l y assessed while e x p l o r a t i o n d r i l l i n g o f the prospective f i e l d is still i n progress. I n a d d i t i o n to gathering b a t h y m e t r i c data and topographic i n f o r m a t i o n o f the seafloor, the shaUow seismic survey yields data f o r mapping o f geologic s t r a t i f i c a t i o n and features such as b u r i e d channels i n f i l l e d w i t h materials o f d i f f e r i n g compressibility, faults across w h i c h d i f f e r e n t i a l displace-ments m i g h t occur, and areas o f seafloor slides ( M i l l i n g , 1975).
Geotechnical properties o f soils and rocks are n o t determinable f r o m acoustic measurements, w i t h the result t h a t soil properties f o r use i n feasibihty studies must be developed f r o m laboratory tests o n samples recovered f r o m one or t w o borings, d r i l l e d t o penetrations o f about 100 m . Seismic survey vessels are highly manoeuverable w i t h o u t anchoring capabihties to stay o n l o c a t i o n , so soil borings must be drilled w i t h larger anchored coring vessels or b y placing a mobile r o t a r y d r i l l i n g rig o n the jack-up or semi-submersible o i l well p l a t f o r m during e x p l o r a t i o n d r i l l i n g .
Development o f soil parameters f o r analysing p l a t f o r m stability, skirt penetration and soil-structure response usually involves d r i l h n g and sampling o f f o u r or more borings, t o pene-trations o f 100 t o 150 m and conducting 10 t o 15 cone penetrometer tests, o f w h i c h f o u r or five tests should penetrate t o about 30 m or less depending u p o n stratigraphy and the remaining tests t o shallower penetrations o f about 10 m . The areal extent o f weak surface soils can be examined r a p i d l y b y a d d i t i o n a l cone penetrometer tests. Other types o f i n situ testing such as gamma logging, plate loading, and pressure meter have been attempted at N o r t h Sea sites t o i m p r o v e soils i n f o r m a t i o n f o r f o u n d a t i o n design.
Detailed topographic data on an intended p l a t f o r m l o c a t i o n are generally obtained w i t h smaU submarines (Hitcliings et al, 1976), f i t t e d w i t h d i f f e r e n t i a l pressure sensing devices and having a surface support vessel. Topographic maps can be developed t o vertical accuracy o f ± 10 c m and seafloor features such as bolders and marine debris are recorded o n videotape and photographic f i l m .
2.2.3 Position fixing
Marine surveys demand accurate navigating o f seismic vessels along pre-selected survey Unes and p o s i t i o n i n g o f anchored coring vessels on soil b o r i n g and i n situ testing locations. F o r ocean navigation, vessels are equipped w i t h some t y p e o f l o w f r e q u e n c y shore-base radio positioning system, such as Omega, b u t this is n o t s u f f i c i e n t l y accurate f o r survey w o r k ( M c Q u i l l i n and A r d u s , 1977). Positioning o f geophysical vessels is usually achieved b y one o f the several m e d i u m frequency chains (Decca-Hi-Fix, T o r a n , or L o r a n ) having an accuracy between 5 and 30 m or b y the more accurate microwave system (Trisponder, A u t o t a p e or H y d r o d i s t ) w h e n w i t h i n radio line o f sight o f fixed reference points.
Positioning o f coring-vessels is usually c o n t r o l l e d b y an array o f f o u r acoustic transponders laid o n the seafloor providing relative accuracy o f ± 3 m . L a y i n g o f the transponders is done w i t h a vessel positioned b y one o f the m e d i u m frequency chains, w h i c h can be operated w i t h satelhte navigation giving an accuracy better than 10 m w h e n the vessel is held stationary f o r a period o f about 24 hours.
2.2.1 Preliminary area survey
2.3 Geophysical survey
Sub-bottom geologic features are explored w i t h a multi-sensor acoustic system o f t o w e d devices having d i f f e r e n t f r e q u e n c y responses ( M c Q u i l l i n and A r d u s , 1977), and other acoustic devices record simultaneously b a t h y m e t r i c data and seafloor topography. A t y p i c a l array o f t o w e d devices is illustrated i n Figure 2.2. Survey vessels i n the N o r t h Sea are t y p i c a l l y 45 t o 55 m long, driven b y a slow revolving single screw t o m i n i m i z e acoustic noise and having t w o conventional b o w anchors. Deck winches and an A f r a m e are required f o r handling the various seismic t o w devices over the stern, and one or t w o 3 t cranes f o r lowering shallow penetration sampling devices o n the side.
Radio positioning
Figure 2.2: Diagram o f t o w e d seismic devices.
Geology o f the area surrounding a prospective p l a t f o r m site is generally explored over an area o f 2 t o 4 k m o n a coarse spacing o f survey lines at 300 t o 500 m apart i n b o t h n o r t h -south and east-west directions. M o r e detailed i n f o r m a t i o n o n geologic features w i t h i n the immediate l o c a t i o n o f a p l a t f o r m is obtained b y reducing the grid spacing to 75 or 100 m in one survey d i r e c t i o n t o f o r m an area o f 1 k m b y 1 k m . Seismic data gathered along the survey lines are r o u t i n e l y interpreted t o construct maps showing b a t h y m e t r y , shallow geologic features, shallow sediment isopachs, and construction hazards at, or slightly b e l o w , the seafloor.
2.3.1 Bathymetry
Water d e p t h is n o r m a l l y measured b y a h i g h precision echo-sounder e m i t t i n g h i g h f r e q u e n c y acoustic signals o f about 40 k H z t h a t are reflected back t o the transducer as an echo. A n echo-sounder is generally m o u n t e d o n a seismic survey vessel t o produce a seabed c o n t o u r map o f the area, or o n an anchored d r i l l i n g vessel t o measure water d e p t h and t i d a l
variations. Echo-sounders operating at higher frequencies up t o 200 k H z are used t o detect gas seeps at the seafloor.
2.3.2 Seafloor topography
A side scan sonar f i s h w i l l provide a sonar picture o f irregularities at the seafloor and • augments b a t h y m e t r i c data acquired along track lines. The f i s h transmits h i g h f r e q u e n c y pulses o f 50 t o 200 k H z i n a t h i n , fan-shaped pattern i n a plane perpendicular t o its t o w
path, and receives echoes f r o m the seafloor. The acoustic beam can scan as far as 500 m t o each side o f the f i s h b u t is h m i t e d t o about 150 m when the f i s h is t o w e d f r o m 20 m t o 40 m above the seafloor t o achieve good resolution. Steel or i r o n objects ranging f r o m sunken ships, pipelines, anchor, and telephone cables can be detected w i t h a marine p r o t o n magnetometer b o t t l e w h i c h measures the t o t a l magnetic f i e l d intensity i n gammas along the t o w line.
2.3.3. Continuous sub-bottom reflection profiling
H i g h resolution seismic p r o f i l i n g i n r o u g h sea states that prevail f o r l o n g periods i n the N o r t h Sea, requires deep t o w i n g o f the t o w f i s h to m i n i m i z e m o t i o n o f the sound source and
receiving sensor, otherwise seismic p r o f i l i n g operations are l i m i t e d t o relatively calm sea states. The deep-towing technique also reduces acoustic noise f r o m the vessel and loss o f acoustic energy f r o m absorption and divergence i n deep sea water, and improves resolution o f t h i n soil strata. The raw seismic data are preferably digitised and recorded o n magnetic tape o n the vessel or recorded i n analog f o r m f o r processing t o improve vertical r e s o l u t i o n and t o m i n i m i z e multiples at the seafloor. Data f r o m deep seismic surveys f o r o i l and gas e x p l o r a t i o n can be used t o complement the high r e s o l u t i o n data i n better understanding the geologic environment.
A n appropriate seismic p r o f i l i n g system depends u p o n the required d e p t h o f penetration the desired degree o f resolution, and the acoustic opaqueness o f the shallow f o r m a t i o n s . Seismic survey vessels usually carry t w o or three types o f sound sources i n order to provide a b r o a d range o f frequencies, and selection o f this equipment is made o n the advice o f an experienced geophysicist. I d e n t i f i c a t i o n o f soil and r o c k materials cannot usually be made based o n sub-b o t t o m r e f l e c t i o n p r o f i l i n g alone so one or t w o soil sub-borings are required f o r geologic correlation o f the seismic data.
Table 2 . 1 . High resolution p r o f i l i n g systems.
Acoustic Source Energy Frequency Resolution Penetration
Joules Hz m m Stacked sparker 4000-10 000 80-200 ± 10 350-900 Sparker 20-200 500-1200 3-4 15-100 Deep t o w sparker 200-800 1000-3000 0.5-1.5 15-60 Multi-electrode sparker 200-1000 300-3000 1.5-3 15-120 Boomer 500-1000 300-3000 2-4 30-120 Deep t o w boomer 400-600 800-1000 0.5-1.5 15-60 Precision boomer 100-500 400-15 000 0.5-1 15-75 M i c r o p r o f i l e r 1-100 2000-12 000 ± 0.5 max. 30
Generally l o w frequency, high energy systems produce deep penetration w i t h l o w resolution whereas high frequency systems y i e l d l i m i t e d penetration w i t h h i g h resolution. The c o m m o n types o f seismic p r o f i l i n g systems used i n the N o r t h Sea are listed i n Table 2.1 together w i t h t y p i c a l operating characteristics such as energy, frequency, r e s o l u t i o n , and penetration. H i g h energy systems used i n engineering surveys are operated at frequencies between 100 and 400 Hz. capable o f producing penetrations o f 200 t o 300 m w i t h r e s o l u t i o n i n the order o f 6 m below penetration o f 10 t o 20 m . Higher frequency multi-electrode sparkers achieve less penetration t o about 150 m w i t h i m p r o v e d resolution o f 3 m while boomers operating at about 2 k H z can penetrate 50 to 75 m w i t h resolution o f 1 t o 1.5 m . L o w energy m i c r o -profilers and pingers e m i t t i n g a sound pulse w i t h a f r e q u e n c y o f 5 k H z give r e s o l u t i o n o f about 0.5 m and can penetrate about 25 m i n t o s o f t clay b u t o n l y a f e w metres i n t o dense sand and gravel.
2.3.4 Shallow penetration sampling
Seabed sampling w i t h gravity corers f r o m seismic survey vessels produces soil samples suitable f o r classification testing t o map shallow soil stratigraphy, b u t generaUy unsuitable
f o r providing reliable soil shear strength data. Piston or open-barrel gravity corers are lowered to the seafloor o n a wireline and the samplers f r e q u e n t l y have a triggering weight, so t h a t the sampling tube falls f r e e l y over a pre-determined distance before penetrating the seafloor ( N o o r a n y , 1972). The diameter o f the sampling tube o f these one-shot devices ranges f r o m 50 t o 150 m m , and penetrations greater t h a n 5 m are seldom achieved i n s o f t to f i r m clays. The length o f undisturbed clay samples recovered i n a sampling tube is restricted t o 15 t o 20 times the inside diameter o f the sampler, while i n sands t h e r a t i o is o n l y about 10. C o n s e q u e n Ü y , w h e n the penetration o f a 75 m m sample exceeds about 1.25 m , the recovered samples suffers some disturbance.
A n o t h e r shallow penetration technique is t o drive a t h i c k - w a l l tube, w h i c h can range f r o m 100 t o 2 7 0 m m i n diameter, i n t o the seafloor w i t h a v i b r a t o r y hammer. The vibro-corer is lowered o n a steel cable t o the seafloor w i t h a tether line f o r f l o w o f compressed air to the mechanical vibrator. Penetration ranges f r o m 8 m i n s o f t - t o - f i r m clays and loose sands t o
o n l y 0.5 t o 2 m i n t o strong clays, because o f damping o f the v i b r a t o r y energy b y the surrounding clay.
A remote controUed seabed sampling device, designed t o carry o u t push sampling t o 10 m , has undergone sea trials i n the N o r t h Sea. The seabed u n i t consists o f a r o t a r y d r i l l i n g device w h h a r o t a t i n g supply disc carrying 10 sampling tubes o f 55 m m diameter and 9 0 c m long. A tube is pushed f r o m the seafloor, retracted and stored i n the supply disc, and t h e n the unit drihs a hole to 0.9 m before another sample tube is pushed f r o m 0.9 t o 1.8 m . This push sampling and d r i l l i n g sequence is contintied t o penetration o f 9 m .
2.4 Geotechnical investigation
Vessels f o r drilhng borings and c o n d u c t i n g i n situ testing must be capable o f remaining i n p o s i t i o n f o r periods o f several days i n moderate sea states. These vessels are usually 65 to 75 m long, driven b y t w i n screws w i t h a b o w t h r u s t e r so that three b o w anchors and three stern anchors can be laid w i t h o u t assistance f r o m an anchor handling boat. A n c h o r i n g i n 200 m o f water requires at least 1750 m o f wireline attached t o each 3 t anchor and the d r u m o f the anchor winches should h o l d about 2250 m o f wireline. The vessels have a 4 m b y 4 m m o o n p o o l t h r o u g h w h i c h seafloor devices are lowered and raised, and are f i t t e d w i t h a permanent d r i l l i n g derrick and draw works.
2.4.1 Rotary drilhng and wirehne sampling
The N o r t h Sea vessels utilize 115 t o 130 m m d r i l l pipe w i t h d r i l l collars and an open-centre drag b i t at the b o t t o m o f the string, and t h e t o p o f the string is connected t o t h e m o t i o n compensator i n the crown o f the derrick so that the d r i l l string is constantly h e l d i n tension. The d r i h string is internal f l u s h o f 100 m m i n t e r n a l diameter, allowing wire line tools u p t o 90 m m i n diameter t o be r u n t h r o u g h the d r i l l pipe. Borings are advanced b y r o t a t i n g the d r i l l pipe w i t h a power swivel or power tongs and the d r i h remains i n the d r i l l e d hole u n t i l the boring is completed. The d r i l l i n g procedure shown i n Figure 2.3 requires a continuous re-supply o f drilling m u d o f desirable viscosity and fluid weight f o r stability o f the borehole, since all the d r i l l i n g f l u i d s exist f r o m the hole at the seafloor. D r i h i n g and sampling oper-ations are generally closed d o w n w h e n vertical vessel m o t i o n approaches 3 m .
The most c o m m o n l y used sampling technique is percussion sampling. Percussion samphng, illustrated i n Figure 2.3 is accomplished b y driving a 75 m m t h i n - w a l l tube i n t o the soü below the b o t t o m o f the b o r i n g b y b l o w s o f a 130 k g hammer dropped a p p r o x i m a t e l y 1.5 m_. The sliding hammer is attached t o the sampler and is operated o n a wire line. The technique is fast and effective i n p r o c u r i n g samples, b u t produces significant disturbance o f clay samples and no quantitative measure o f the i n situ c o n d i t i o n o f sand samples. Sample disturbance is reduced b y push sampling b u t is restricted t o relatively calm sea conditions, because the d r i h pipe produces the necessary reaction load. This technique involves latching the sample tube i n t o the d r i l l b i t that is pressed i n t o the ground b y reducing the tension load i n the d r i h string.
ControUed push sampling i n s t i f f clays and sands has been conducted i n N o r t h Sea borings t o penetrations o f 125 m , w i t h the seafloor j a c k i n g u n i t o f Stingray. The 20 t seafloor j a c k i n g u n i t has hydraulically operated h o r i z o n t a l clamps, w h i c h grip the driU pipe and the vertical
hydraulic rams, having a stroke o f 1 m , push the latch-in 75 m m sampling tube i n t o the soil. The weight o f the seafloor j a c k prevents vessel m o t i o n being transferred to the sampling tube.
> Prepared "^drilling fluid
in
1/Full-hole bit with blade reamer
2in
(D
Wire-line •sampler and jars
2'/2in.O.D. thin-wall tube
®
(D
Drilling
Before sampling
Sampling
Figure 2.3: Wire-line percussion sampling.
The t w o l i f t i n g lines t o the seafloor u n i t serve as a guide system to lead the b i t t h r o u g h the jack and back i n t o the driUed b o r i n g , thereby f a c i l i t a t i n g replacement o f the open-centre drag b i t w i t h a r o c k b i t , whenever r o t a r y r o c k coring is necessary. B y using a wire line core barrel latching i n t o the d r i l l b i t , cores o f 40 to 50 m m i n diameter can be recovered inside a plastic liner.
Push sampling has been successfully p e r f o r m e d w i t h Stingray i n strong, over consohdated N o r t h Sea clays w i t h undrained shear strength approaching 500 k N / m ^ . Percussion wire line sampling was also conducted i n the same clays and revealed t h a t the sampling technique had no marked e f f e c t u p o n the shear strength, b u t the solid stiffness was reduced b y 25 t o 50% (Sullivan, 1978) Comparative sampling tests i n a moderately sensitive clay, w i t h undrained shear strength
increasing f r o m 40 t o 100 k N / m ^ , have shown that percussion sampling produced s u f f i c i e n t disturbance t o lower the strength b y 25 t o 40% below strengths measured o n push samples ( E m r i c h , 1970). Push sampling provides better q u a l i t y samples, thereby a l l o w i n g greater c o n f i d -ence i n selecting design soil parameters t h a t i n t u r n reduces conservatism i n f o u n d a t i o n design.
Recovered pushed or hammered samples are carefuhy extruded f r o m the 75 m m sampling tube, aUowing visual-manual i d e n t i f i c a t i o n o f the soils before packaging i n cardboard or plastic tubes and selected samples can be retained and sealed i n the steel sampling tube f o r transport t o an onshore laboratory. A sample is f i r s t wrapped i n plastic f o i l , then i n a l u m i n i u m f o i l and the annular space between the soil specimen and the tube is f i l l e d w i t h non-shrink m o l t e n wax. Disturbance o f s t i f f soils f r o m extrusion is m i n i m i z e d b y immediate packaging i n cardboard tubes, w h i l e samples retained i n the sampling tubes are exposed t o d r i l l i n g f l u i d trapped i n the t o p p o r t i o n o f a sample t h a t can cause swelling o f s t i f f clay w i t h associated loss o f strength.
This i n situ testing technique is w i d e l y used i n the N o r t h Sea t o explore soil c o n d i t i o n s , since a test can be p e r f o r m e d q u i c k l y giving a continuous record o f resistance. I n t e r p r e t a t i o n o f strength and stiffness o f clay and sand f r o m cone results is based o n empirical relationships and requires mature geotechnical judgement.
The seafloor cone penetrometer u n i t Seacalf has been used considerably i n the N o r t h Sea (de R u i t e r , 1975) and a diagram o f the u n i t is given i n Figure 2.4. I t is a 20 t u n i t lowered t o the seafloor t h r o u g h the m o o n p o o l o f a drihing vessel and provides the reaction f o r t h r u s t i n g a
10 cm^ cone i n t o the soil, at a rate o f penetration o f 2 cm/s. The cone test rods extend above the t o p o f the seabed u n i t r e q u i r i n g support f r o m a m o t i o n compensated wire line and cone penetration is l i m i t e d t o 6 t o 8 m i n hard clays or dense sands. The Wison wire line cone pentro-meter was developed t o conduct cone testing at a greater depths i n c o m p a n i o n borings b y latching i n t o the d r i l l b i t . I t relies u p o n the d r i l l string to provide reaction, w i t h the result that the rate o f cone penetration is n o t closely c o n t r o h e d and cone penetration i n t o s t i f f clays is l i m i t e d t o about 1 m , because the reaction load f r o m the d r i l l string seldom exceeds 3 t . 2.4.2 Cone penetrometer testing
A successful seafloor j a c k i n g device is Stingray (Ferguson et al, 1977) w h i c h has p e r f o r m e d cone testing f r o m the seafloor t h r o u g h hard clays t o 25 m penetration. Cone penetrometer testing w i t h the versatile seafloor j a c k i n g u n i t is shown i n Figure 2.5. The vertical h y d r a u l i c rams force penetration at 2 cm/s o f the 10 cm^ cone i n increments o f 1 m u n t i l the 5 m r o d l e n g t h is f u l l y u t i l i z e d or refusal is reached. The cone and r o d are retrieved b y the wire line procedure and the d r i h pipe is advanced b y conventional r o t a r y d r i h i n g t o a level j u s t short o f the cone's m a x i m u m penetration. Thereafter, the sequence o f pushing the cone and r o t a r y drihing is repeated t o any desired depth.
2.4.3 Other in situ testing
Soil conditions are investigated w i t h other c u r r e n t l y available i n situ testing devices. Since continuous soil sampling i n deep penetration borings is n o t standard practice, gamma logging has been used ( G u y o d , 1963) t o o b t a i n a continuous p r o f i l e o f soil s t r a t i f i c a t i o n . The measuring sonde is r u n t h r o u g h the d r i l l pipe since natural gamma rays f r o m clay materials have good transmission t h r o u g h steel. Density and porosity logging t h r o u g h the d r i l l pipe is n o t effective because o f the influence o f the d r i h pipe and the m u d f i h e d gap between the d r i h pipe and the side o f the b o r i n g u p o n the transmission o f neutron-gamma and neutron-neutron rays t o the measuring sonde.
Load testing o f a 30 cm diameter plate has been carried o u t o n the seafloor i n the N o r t h Sea t o assess contact stresses and penetration resistance o f domes on the b o t t o m o f the concrete base o f a p l a t f o r m . The plate loading test was p e r f o r m e d using a remote c o n t r o l l e d seabed device such as the Seacalf u n i t .
A h h o u g h i n situ vane shear tests have n o t been conducted i n the N o r t h Sea, this t y p e o f test is c o m m o n l y used i n the G u l f o f M e x i c o ( D o y l e et al, 1971), to determine the undrained shear strength o f f i r m - t o - s t i f f clays. A remote wire line vane is available that is implanted i n t o the soils below the b o t t o m o f the b o r i n g under the weight o f the d r i h pipe or pushed i n t o the soil b y the Stingray seafloor j a c k to measure undrained shear strengths less than 125 k N / m ^ . The remotely c o n t r o l l e d 65 m m diameter vane operates independently o f the d r i h pipe and has a set o f reaction blades positioned above the test vane t o provide the necessary t o r q u e reaction.
Pressure meter testing has been p e r f o r m e d i n oversized driUed holes (Menard, 1957) and w i t h devices j e t t e d or vibrated i n t o s o f t clays and sands. These techniques t o insert the pressure meter i n t o soils alter the stress-strain response o f the surrounding soil due t o stress relief, or disturbance, or densification. More recently the stress-strain properties and lateral stresses i n soils have been successfully investigated o n land w i t h self-boring pressure meters (Baguelin et al, 1972) and
pressure meters pushed i n t o soil over an undersized p i l o t hole. These installation techniques ensure i n t i m a t e contact between the t o o l and the surrounding soil, and m i n i m i z e disturbance w h i c h influences the i n situ soil measurements. M o d i f i c a t i o n s t o these devices f o r o f f s h o r e investigations are being undertaken and a successful sea t r i a l was made i n 1977 t o insert a 90 m m push t y p e pressure meter i n t o s t i f f clay w i t h the Stingray seafloor jack.
2.5 Laboratory testing
2.5.1 Shipboard testing
Soil testing o n board the d r i l l ship is l i m i t e d t o r o u t i n e classification tests and d e t e r m i n a t i o n o f undrained cohesive shear strength on about 25% o f recovered samples, thereby leaving ample material f o r detailed testing at an onshore laboratory. U p o n e x t r u s i o n f r o m the sample t u b e, visual-manual description o f colour, plasticity, grain size, f a b r i c and structure are recorded f o r the sample i n a d d i t i o n t o percentage o f sample recovery and b l o w count, w h e n sampling w i t h a wire line hammer. N a t u r a l moisture content and density d e t e r m i n a t i o n is made o n most clay samples and the consistency is estimated f r o m hand penetrometer, miniature vane, and f a l l cone tests. Sieve tests are p e r f o r m e d o n selected sand samples f o r grain size i d e n t i f i c a t i o n and shear strength o f clay samples is determined f r o m unconsohdated undrained t r i a x i a l tests.
2.5.2 Onshore testing
Testing o f samples i n an onshore l a b o r a t o r y is directed towards evaluating soil shear strength, soil stiffness under static and d y n a m i c loading, loss o f shear strength under repeated wave load-ing, and consoHdation under the weight o f a p l a t f o r m . Moisture content and density deter-m i n a t i o n are deter-made o n ah undisturbed sadeter-mples and soil deter-macro-structure is recorded b y t a k i n g colour photographs o f split samples. Classification tests o f l i q u i d and plastic h m i t s are p e r f o r m e d
