Shallow caisson foundations are considered as alternative anchors for permanent mooring of a process barge at an oil and gas field developed by ELF Congo in the Gulf of Guinea. The 12 mooring lines are scheduled to be installed on site in July 1995 and the process barge will be connected in 1996 for start of production. b14tha water depth of about 170m, this field will be the deepest one offshore Western Africa to date. The soils at the site consist of soft normally consolidated clays. The paper presents the results of a comparative study of potential anchor solutions for this production barge, including high capacity drag anchors, driven piles, and shallow caisson foundations installed by under pressure. This comparative study, including design, construction, and installation of the mooring system, has shown that the caisson foundations represent the best suited solution, technically as well as on an economical point of view, because of the following advantages:vertical load capability, thus allowing to reduce the anchoring pattern,oncepositioned on the seabed, the location of each anchor is fixed and known with accuracy, andmore simple and shorter installation procedure, since caisson anchors, as piles, do not require to be proof-loaded on site. INTRODUCTION With oil exploration wells and development of offshore fields coming into deeper water depths, innovative concepts of structures and foundations are considered, with the aim to develop alternative solutions that can yield cost savings, but with similar or improved reliability. The NKossa oil and gas field presently developed by ELF Congo in 170m of water depth will be the deepest field in the Gulf of Guinea to date. It includes two steel jackets (installed early 1995) with tender-assisted drilling, one process and accommodation barge (presently under construction and scheduled to be at the site in 1996 for start of production), and two floating storage and offloading tankers, one for oil and one for LPG (see Figs. 1 and 2). The present paper gives the results of a comparative study on potential anchor solutions forpermanent mooring of the process barge, including high capacity drag anchors, driven piles, and shallow cylindrical steel caissons. The shallow caisson foundations, penetrated into the seabed by means of under pressure in the water inside the caisson (also called "buckets", "skirted foundations" or "suction anchors" in the literature), were first introduced by Senpere and Auvergne (1) as alternative anchors for mooring of a storage tanker at the Germ field offshore Denmark. More recently, and due to its simplicity and reliability, this foundation concept has found a renewed interest in the offshore industry. The geotechnical analysis of skirted foundations is based on the experience gained from large North Sea gravity structures. Model tests have been performed in clays by the NGI for application to the Snorre TLP (Dyvik et al, 2; Andersen et al, 3), as well as for studying the performance of anchors under horizontal loading (Keaveny et al, 4). Since then, it has been applied for different types of structures.
Ccwwht 1995 O&hue Tec.hfm!mgyCanferewdThis papa was Pmsanbd al the 2Sth Annual OTC m I-buskx, Texas, U S A, &9 May 1G96 Thrn pap was malectad fci prewnlatmn by the OTC Program Cmnmti k4kwiw Wow cd infonmat,on con!amed !m an qMracl subin!lted by the author(s) Contents cd the paper, ac presented have nol been re.wwed by the OIMore Technobg y Conference and are s@@I M correction by tfw wthof(s). % material, qs p-enied, does MI neceuwtly Ad any postin al the Offshore Technology Conlermce w th .&kers Permms!on to c.apy IS rdncled to qn abstwl ot motmore than XX wrd8 Illustrslmrm may K4 be c++ad The ehlracl shwld co@aIn conswucws achmwladgmant C4tire and by -me paper m pfasenwd AbstractInnovative shallow suction anchor piles have been used for permanent mooring of a process and accommodation barge at an oil and gas field developed by ELF Congo in the Gulf of Guinea. The 12 mooring lines, equipped with 5m diameter and 13m high steel suction anchor piles, were successfully installed at the site in July/August 1995. The process barge is scheduled to be connected earfy 1996 for start of production. With a water depth of about 170m, the Nkossa field is the deepest in offshore Western Africa to date. The soils at the site consist of sofl normally consolidated clays, representative of deep water sites. The anchoring system design is presented, together with the description of the suction pile installation by underpressure. The monitored performance of the suction piles during installation is evaluated, including orientation and tilt at touch-down, and penetration rate during suction penetration. The installation behaviour appeared to be very smooth, and the measured penetration resistances were in perfect agreement with the predictions, thus demonstrating the reliability of the design installation method.
This paper presents a method for evaluating the soil resistance to driving in the hard clays, very dense sands and rock strata encountered offshore Angola. The method is calibrated on typical case histories where 48" O.D. platform piles and 30" .D. conductor pipes were instrumented during driving. Results are applicable on most of the oil fields located in the southern part of the continental shelf of the Gulf of Guinea. INTRODUCTION The Gulf of Guinea is a zone of intense oil field developments. In the southern part of the Gulf, i.e. roughly south of the equator line, the continental shelf consists of a narrow band (50 to 100km wide) extending from Cap Lopez (Gabon) to Moss Medes (Angola) (Fig. 1). The water depth increases gently from 20 to 200m from east to west then dips suddenly to 1000m. The zone is characterized by a common geological history. The submarine canyon of the Congo river is the only outstanding topographic accident. In the past few years, several oil companies had to face serious platform installation problems due to major difficulties encountered during pile driving operations. Three types of problems have been identified:development of strong set-up effects in hard clays during add-on welding,premature refusal of piles in very dense sands,collapse of pile toes on limestone layers. The paper addresses these different topics and presents a procedure to predict the soil resistance to driving in the specific soil conditions of the region. The method is based on the experience gained during the first phase of development of oil fields located offshore Angola and operated by ELF AQUITAINE. Its application may be extended to the southern part of the continental shelf of the Gulf of Guinea, wherever quite similar subsurface soil conditions prevail (South Gabon, offshore Congo, offshore Cabinda). EVALUATION OF DRIVING HAZARDS The development of Block 3, offshore Angola, started in 1985. Four jacket-type platforms have been installed so far in water depths ranging from 65 to 90m. They were founded on 48" O.D. steel pipe piles driven by hammers with rated energies up to 700 kJ (500 000 ft.lbs). The wall thickness of the piles was comprised between 1" and 2". The 30" O.D. conductor pipes were also installed by driving. Premature refusals of piles and/or conductor pipes were observed during the installation of the first two platforms. Obviously, these difficulties were due to local specificities in the soil conditions: high degree of heterogeneity of the soil formations, presence of limestone layers with a random distribution, unusual response of some materials to dynamic sollicitations
The geotechnical history of the gravity base structure CDPl on the Frigg Field in the North Sea is retraced in this paper. Erosion problems were encountered early in the platform life. Remedial measures were taken. The structural and foundation monitoring results, complemented with the results of in situ testing and sampling, provided the information necessary to verify that the structure was operating under safe conditions.
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