ABSTRACT The paper describes the main features and initial underwater field-testing results of a remotely operated submersible drill rig designed to obtain continuous cores of soil and/or rock up to 6m long in water depths of 200m. The rig mast angle is hydraulically actuated covering 90° of altitude. This enables both a fully horizontal position used for deployment onto, and recovery of the rig from, the seabed, as well as a fully vertical position during coring. The frame is equipped with positioning and inclination sensors for accurate register of core orientation. Coring equipment with diameter of up to 133mm can be mounted in the rig, thus enabling the recovery of relatively large-diameter cores. The coring operation is fully instrumented and all the drill operation parameters such as torque, thrust, advance, injection rate and pressure are recorded. The equipment is deployable from a variety of platforms. 1. Introduction The need for marine geotechnical soil investigations is clearly increasing. Apart from the already seasoned drivers of this industry (e.g. harbour and coastal developments), or the unrelenting trend towards underwater resource exploitation by the extractive industries, development of marine renewable energy resources, particularly for wind, is quickly approaching maturity. The costs associated with their deployment and mobilisation are usually the main item in any marine soil investigation campaign (Gourvenec and Randolph, 2011), and the choice of which type of vessel/platform to use requires careful consideration. Obviously important amongst the factors that need to be considered are the required investigation depth, the water depth at the site and the expected nature of the soils under investigation. Perhaps even more essential factors to establish are the quality level of the data and the ability of the vessel to deploy the necessary equipment and, therefore, the nature of that equipment.

Abstract The axial sliding resistance between a surface-laid pipeline and the seabed is an important design parameter that influences the pipeline end expansions, as well as lateral buckling and cyclic walking responses. This paper describes a new framework for quantifying axial pipe-soil interaction and reports data from interface shear box testing of three carbonate soils in support of this approach. The framework uses concepts from critical state soil mechanics, and encompasses the influences of stress level, pipeline roughness and shearing rate - including drainage and viscous effects. Each of these effects is independently quantifiable and relevant to design. Typical pipeline expansion rates span drained and undrained behaviour on deepwater mud, and typical pipeline coatings are often in the transitional range between fully rough and fully smooth. In addition, at the stress levels relevant to pipelines, the effective stress failure envelope is non-linear. The laboratory data comprises interface shear box tests on a range of carbonate sediments spanning from sand to fine silt. Novel procedures are used to replicate the history of loading and consolidation experienced by soil beneath a pipeline, revealing the consequent changes in the interface strength. The new framework provides a basis for integrating characterisation data from soil element tests and model tests of pipe-soil interaction. This has led to an improvement in design practice, through the use of recommendations that are more robust. 1. Introduction The axial resistance between a surface-laid pipeline and the seabed affects the in-service end expansions and cyclic axial walking, as well as the initiation and growth of lateral buckles. This paper describes a new theoretical framework that captures the various mechanisms that control axial resistance. Results from interface shear box tests on three different carbonate soils are used to illustrate some of the mechanisms that control axial pipe-soil resistance.

ABSTRACT Accurate depth registration and high quality continuous sampling of seabed sediments at multiple locations from mundline are essential for shallow foundation design. This is particularly true for the siliceous silts and sands of the Taranaki basin seabed off New Zealand's west coast. OMV New Zealand recently commissioned the use of Benthic Geotech's Portable Remotely Operated Drill (PROD) to carry out a detailed confirmatory site investigation at the Maari platform site. The PROD was able to make use of two short weather windows to complete successfully the required scope of work. This paper describes how the PROD was use to deliver the site investigation results and illustrates how accuracy and quality were assured. INTRODUCTION The Maari Field is situated in the Tasman Sea offshore the west coast of Teranaki, New Zealand, as shown in Figure 1. The field is approximately 80k m from the coast in a water depth of 101m and is the most south westerly of the other existing developments in the area. The Maui B platform is 36km to the weather events direct from the Southern Ocean, or funneled through the Cook Straits. The development, currently in the project execution phase, comprises a not-normally manned wellhead platform (WHP) with associated floating production and storage off-loading facilities and flow lines. The WHP supports 12 conductor slots for the first eight wells that will be drilled from a jack up drilling rig. Maari is a marginal field in a challenging environment. There are no precedents for platforms on shallow gravity foundations in this region. The site investigation had to deliver sufficient quality evidence to ensure verifiable design for a skirted shallow foundation without another investigation. This paper describes how quality and accuracy were achieved in the geotechnical site investigation. Prior Regional Experience and Selection of Platform Concept The Taranki Basin experiences challenging metocean conditions, which have combined either directly or indirectly with ground conditions to impact adversely on a number of projects. While these are not often publicly documented, Rennie et al. 1 have given detailed account of experiences at Maui A, while four pile were successfully drive to 70m, 24 piles met refusal in sand layer at around 30m. hammer setup on site led to less efficient performance, higher blow counts and ultimately pile refusal. The complex sequence of sand and silts encountered at Maui are shown in Figure 2 in term of pore pressure ratio, B q and cone resistance, q c A similar geology of inter-bedded sequences of sands and silts were anticipated for Maari, and the potential for similar shallow pile refusals was considered high. In addition, the high costs of mobilizing large pile driving equipment to this remote region, couple with the lack of suitable weather windows for the area, contributed to the decision for Maari to favour a self-installing gravity-based platform. The platform concept chosen is from Arup's range of ACE self installing steel gravity based platforms 2 and is similar in concept to the Yolla A Drill ACE platform installed in the Bass Strait, Australia 3

ABSTRACT The Yolla A Platform incorporates a new foundation concept, defined as an un-ballasted raft foundation, and was successfully installed in the Bass Strait of Australia in 2004. Soil conditions at the site comprise a complex sequence of normally consolidated calcareous silt, calcareous sand and calcareous clay, and required extensive soil interpretation to support the design process. The foundation is subject to extreme combinations of environmental loading and was designed using a combination of analytical and numerical design tools. Soil preloading using underbase suction was incorporated in the design to facilitate increases in foundation capacity and to limit post installation long term foundation displacements. Skirt penetration and subsequent suction preloading were issues requiring close attention during detailed design, as well as extensive monitoring and interpretation both during and after installation. This paper presents a general overview of the design and installation processes, and includes a summary of observations made during foundation installation. INTRODUCTION The Yolla A Platform was installed in the central Bass Strait (Figure 1a) of Australia in March 2004, as part of the BassGas Project for client/operator Origin Energy and venture partners AWE, CalEnergy and Wandoo (note that Wandoo has since sold its share in the platform to a new venture partner). The main contractor for the construction and installation of offshore platform was Clough Engineering Ltd, and the substructure design was undertaken by Arup Energy. The Yolla A Platform is a self-installing steel gravity base platform comprising three primary components: the foundation base, the jacket and the deck (Figure 1c). The platform uses the buoyancy of the deck to provide floating stability for the entire platform during installation, with the deck and base initially clamped together during wet tow to the platform site (see Figure 1b, which shows the deck on top of base prior to departing the fabrication yard in Batam, Indonesia). Once on site, a jacking system is used to lower and install the base on to the seabed and raise the deck clear of the water after the base is installed. Once the deck is in the final position, it is welded to the jacket. The Yolla A Platform base comprises a 50m x 50m skirted foundation, with skirts penetrating 5.4m into a seabed comprising primarily soft to very soft calcareous silt. The skirts subdivide the base into 12 individual compartments, and installation is achieved using ‘suction’ within the skirt compartments. This paper outlines two separate aspects of the project, namely foundation design and foundation installation. Foundation Design Water depth at the Yolla A Platform site is approximately 80m. The foundation design is novel in many aspects, including: The Yolla A foundation is believed to be the first foundation of this type (with deep skirts and no ballast) to be installed in calcareous soil Consolidation was incorporated in the design to ensure adequate foundation stability Underbase suction was used to for base installation, which had not previously been undertaken at this scale in calcareous soils