S U M M A R YPure shear wave data are only very rarely acquired for offshore site investigations and exploration. Here, we present details of a novel, seabed-coupled, shear wave vibrator and field data recorded by a densely populated, multicomponent ocean-bottom cable, to improve shallow soil characterization.The prototype shear wave vibrator uses vibroseis technology adopted for marine environments through its instalment on top of a suction anchor, assuring seabed coupling in combination with self-weight penetration. The prototype is depth rated to 1500 m water depth, and can be rotated while installed in the seabed. The philosophy is to acquire fully complementary seismic data to conventional P-and P-to-S-converted waves, in particular for 2-D profiling, VSP (vertical seismic profiling) or monitoring purposes, thereby exploiting advantages of shear waves over compressional waves for determining, for example, anisotropy, small-strain shear modulus and excess pore pressures/effective stress. The source was primarily designed for reservoir depths. However, significant energy is emitted as surface waves, which provide detailed geotechnical information through mapping of shear wave velocities in potentially high resolution of the upper soil units. To fully utilize pure shear wave content, a proper analysis of surface waves is paramount, due to the proximity of surface wave propagation speed with shear wave velocities.The experiment was carried out in the northern North Sea in 364 m water depth. Cable dragging was necessary to obtain close receiver spacing (2.5 m effective spacing), with total line length of 600 m. Frequency-waveform transforms reveal both Scholte and Love waves. Up to six surface wave modes are identified, that is, fundamental mode and several higher surface wave modes. The occurrence of these two different dispersive surface wave types with well-resolved higher modes allows for a unique analysis and inversion scheme for highresolution mapping of physical properties in the shallow subsurface as well as anisotropy, which is discussed in an accompanying paper. The data presented in this paper are thus a unique (long and densely populated receiver array allows for multimodal Love and Scholte surface waves from the marine environment) but challenging (marine operations) marine data set.

Skirted anchors or "suction piles" are suitable for moorings in deep waters. The main advantage with the skirted anchor compared to other alternatives in deep water is the fixed-point installation and no need for drag-in operations or proof loading. In this paper, different design and installation aspects such as anchor geometry, load response, equipment and marine operations are discussed for soil and loading conditions representative for deepwater field developments around the world. The length to diameter ratio, called the aspect ratio, is a key parameter for the maximum feasible suction penetration depth. Experience from deep-water installation of suction pile anchors around the world is discussed. New concepts involving suction pile technology provide attractive solutions for deep water. Applications for subsea templates, pipeline supports, well heads and risers are presented. Introduction The suction pile concept was first introduced for the offshore industry some 20 years ago. However, it was only after further research and development in the early 90's that the widespread use in mooring applications for floating production units took place. The first suction pile moorings for permanent use were installed 1995 in 100-200 m water depth and for catenary anchor lines (Ref. 1). Today, there are almost 300 suction piles operative in permanent mooring systems around the world. The first step into deep water using suction piles and fiber rope taut leg moorings was taken in Brazil in 1997 for Petrobras P-19 and P-26 floaters at the Marlin field (Ref. 2). The suction pile concept is versatile and can be optimally designed for almost any loading condition (Fig. 1). Maximum capacity is obtained when the load attachment point and reaction forces from the soil are balanced such that the pile fails without any rotational movement (Fig. 2). Although the focus from the oil industry has been on anchors for deep-water moorings, the most frequent use of skirted anchors in deep waters will perhaps be as foundation systems for templates and well heads. How deep in the water? Water depth is a barrier that the industry is continuously challenging. As far as suction pile technology is concerned, no water depth limitations prevent the technology from following the rest of the deep-water developments. Suction piles have been installed as deep as 2500 m (8400 ft) in the Mississippi Canyon (Ref. 3). Petrobras is planning subsea templates with skirted foundations in 2000 m of water at its Roncador field. Suction piles can be installed from crane vessel or launched over the stern of an anchor handler tug (Fig. 3). For installation in deep water, it is necessary to account for added mass effects and to ensure that the resonant period for the lifting system at the installation depth differs from the dominant wave period at the site. The equipment required for suction pile installation is based on standard ROV technology with its depth limitations. For soft clay, the suction penetration rate can be up to 20 m per hour without any significant rate effects.

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.

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In recent years, there has been an increased focus on environmental issues near oil and gas production sites and pipelines. The trend is likely to continue as future oil and gas production is commencing in environmentally vulnerable areas, and farther north near the polar ice cap. The exploration in more remote areas implies that online in situ monitoring has great advantages over expedition based monitoring.The Norwegian Geotechnical Institute (NGI) is an independent research and consultancy foundation having over 40 years of worldwide offshore instrumentation experience. NGI has recently launched a research initiative to develop new methods for in situ and on line environmental monitoring offshore. In 2012, NGI designed and installed an integrated environmental monitoring system for subsea leakage detection. It was installed on the seabed beneath an operating oil and gas platform in the North Sea. We give here an overview of the instrumentation system and the motivation for the different design choices. The system is an example of an integrated monitoring system, where different types of sensors complement each other and gives a more thorough understanding of methane transport at the site of interest. After installation, NGI has analyzed monitoring data and provided decision support to operator personnel. Nearly two years of production site data collection represents a unique data set that enables a comprehensive analysis of methane transport.From the operator side, the motivation for environmental monitoring usually arises from the need to answer clearly defined questions such as 'Is there a gas leakage in the production system? Where is it? How large is the leakage?' The leakage detection system is expected to answer these questions, preferrably in an unambiguous manner. However, natural gas can be released to the water phase from several sources e.g. organic matter on the seabed. If the leakage detection monitoring data shall be interpreted in a control room that operates with alarm states, these sources of background noise must be identified and separated from a real leak situation. We look at which circumstances to be aware of and how to maximize the operational value of a leakage detection system.