bluuebunny/crossref_metadata_2025_split huggingface dataset:
3-month GPS Subsidence Analysis in North Sea, ELF/CEA-LDG interpretation
Authors: Flouzat M. , Fourmaintraux D. , Maury V. , Halle G. 
Published: October 2008

Abstract
We process and analyse 3 months of continuous GPS offshore data provided by the operator from 10 single-frequency stations in the Ekofisk field in the North Sea for subsidence monitoring. This analysis is done using the Differential-GPS automatic software and method developed by the EAWCEA-LDG. Themean subsidence rates obtained by the EAP/CEA method shows, over these three months, a maximum rate at the center of the subsidence of about 30 mm/month bowl in agreement with previous determinations. The accuracy of the results can be improved by smoothing techniques. However, the noise is still directly related to the distance between the monitored point and the reference points, For instance, the daily averages lead to an accuracy (root mean squares = r.m. s) of 1.1, 1,4 and 2.4 mm for the East, North and Vertical components of the movements for a 2 km baseline and 1.8, 3.7 and 3.8 mm for the East, North and Vertical components for a 24 km baseline. The choice of the best reference points in the network is discussed, Horizontal measurements are simultaneously detected and may be due either to plate form tilt or to sea-bed horizontal movements. A further analysis (the 2-hours method) indicate that the noise in the results has a periodic daily structure. In addition, results from previous calibration experiments are used in the method to test the result quality, processed data interpretation

Introduction
The aim of the work is to bring a complementary analysis to the current in-situ processing, to contribute of the comprehension of the subsidence mechanisms involved in the reservoir, Thus the best accuracy is searched from the data available which are here spatial geodetic data. Previous studies based on calibration experiments in other sites in 1989 (Ref. 1 and 2) and in 1991 above gas reservoir (Ref. 3 and 4) have shown that after the first data processing step, different ways can be used to interpret the GPS results in a post-processing analysis and improve theaccuracy of the results by using 1) criteria quality associated to the position determination 2) smoothing techniques like daily averages or moving averages on processed data. This later must be adapted according to the scale of the deformation evolution. In addition, a second method relative to the GPS differential data processing itself is also tested in this study and consists in a different way in choosing the observation periods from the 24- hours data files provided.
Another aspect of this study is also purely technical to evaluate parameters like computer time required to process the data. The software developed by the CEA-LDG in a collaboration between EAP and Elf Petroland b.v. is a My automatic software which is adapted to either day to day or delayed processing of a large amount of GPS data required in continous monitoring.
Geological and Technical Aspects of Marine Diamond Exploration in Southern Africa
Authors: De Decker R. , Woodborne M.W. 
Published: October 2008

Abstract
Exploration has indicated the presence of diamonds over a distance of some 1400 km along the southern African continental shelf, and extending from about 100 m above sea level to at least 150 m below sea level. This range could be extended to the 500 m isobath following the granting of new concessions between the 200 m and 500 m isobaths along the South African continental margin. Despite this very wide-ranging occurrence of diamondiferous deposits on the coastal plain and continental Shelf, little is known of the nature of the deposits.
The occurrence of diamondiferous deposits is generallyassociated with high energy, shallow marine depositional environments. The concentration of diamonds and consequent mineral enrichment of these deposits are as a result of the continual reworking of older mineralized deposits, and subsequent redeposition of diamonds preferred trap sites. Reworking was mainly through the agencies wave-induced bottom currents with sea-level changes, as well as fluvial and aeolian erosional processes on emerged marine deposits.
Marine diamond exploration has been under vay since the early sixties. At present there are more than thirty exploration companies holding precious mineral concession rights along the southern African west coast and continental margin, and only about 15% of the entire continental margin area has not been granted Despite this apparent interest, large areas still remain unexplored or surveyed. The current status of knowledge is mostly on the inner shelf and the middle shelf to a water depth of about 150 m. No studies of the surficial geology of the outer shelf have yet been undertaken. Exploration techniques have been developed using detailed Interpretation of high resolution geophysical data and wave refraction programmed to identified the geological features. Combined, these techniques are used to establish the most prospective depositional areas and enable marine diamond exploration is to identify the geological features most likely to be associated with these deposits. It is the aim of this paper to highlight the more salient geological features of marine diamondiferous deposits. This may assist future geophysical exploration programmes in their focus to identify these deposits.

Introduction
The ‘diamond coast’ of southern Africa extends for over 1500 km from St Helena Bay in the south to Conception Bay in the north (Figure l).
The southern African diamond fields have attracted the attention of numerous exploration companies ever since the first diamond was discovered outside Luderitz, Namibia, in 1908. In the following ninety years, over I00 million carats of mostly gem-quality diamond have been recovered from the diamondiferous deposits. The diamonds occur in a number of different depositional environments and these are the subject of this paper. Interest in the marine diamonds grew steadily since active exploration and prospecting started in the early sixties and today the interest is at its highest ever.
The Foinaven and Schiehallion Developments
Authors: Leghorn J. , Brookes D.A. , Shearman M.G. 
Published: October 2008

Abstract
The UK Quad 204 sector between the Shetland Islands and the Fames presents many challenges to offshore oilfield developments. The waters are deep and the weather and currents are harsh. The reservoirs are relatively modest on a world scale and being shallow require multiple drill sites and extensive use of horizontal wells. This paper describes the phased, fast track, parallel approach adopted by BP to substantially reduce historical discovery to production times.
The first project Foinaven uses an FPSO based on a substantially rebuilt Russian submarine carrier, with two subsea well clusters giving an annual production of 85mbd.
The second Schiehallion project is larger at 142mbd and will use a much larger FPSO with multiple subsea well clusters. Both developments will use shuttle tankers for oil export. This paper summarises the technology employed on the two projects and the tendering strategy used to achieve the timetables set.

Introduction
The Faeroes/Shetland Trough area or the Atlantic frontier as it has become known, has some of the deepest waters in the British offshore territory. Environmental conditions in the area are challenging. While wind and wave conditions are similar to those experienced in the Northern North Sea the currents are significantly more severe and complex, almost down to the seabed, due to interaction between the warm Gulf Stream and cold Arctic Stream.
Foinaven, the first development in the Atlantic Frontier area, is located in two blocks, 204/19 and 204124a, which are operated by BP Exploration with Shell UK Exploration and Production as co-venturer. These blocks lie some 180 kilometres west of the Shetland Islands, in a water depth of between 400 and 600 metres - See Fig. 1. This presents a formidable technical challenge, as it is too deep for divers, and any development has to rely heavily on subsea technology for unmanned intervention.
Foinaven is based on the recovery of 200 million barrels. [t is designed to produce early oil efficiently, safely and at low cost. A fast track schedule has been adopted for the project execution plan.
The development centres on subsea wells completed on the seabed, producing oil via a manifold, through rigid flowlines and then flexible risers into a floating production, storage and offloading system (FPSO) which will be permanently stationed in the field. Shuttle tankers will export the crude oil - See Fig. 2. The project was sanctioned late October 1994 with first oil targetted for summer 96.
More information on the reservoir and environmental are given in ref 1-3.
The second development in the area is Schiehallion located some 12km to the East of Foinaven.
As with Foinaven, Schiehallion shares similar environmental challenges with respect to water depths, currents, wave heights and winds. Clearly, therefore the learning from Foinaven has been fundamental in the development of Schiehallion.
A Small Tension Leg Platform for Marginal Deepwater Fields
Authors: Hudson W.L. , Vasseur J.C. 
Published: October 2008

Abstract
This paper describes a small Tension Leg Platform, the Mini TLP, that is configured as an unmanned wellhead platform to support dry trees and minimum topside facilities. The concept has been developed as part of an ongoing process to reduce the overall cost of offshore developments in deep water. The Mini TLP concept presented has been designed for West Africa conditions with tender assisted drilling and work over operations in a water depth of 1000 metres.
In the development of this concept, particular attention has been placed on the drilling operations, well systems and the interaction between the tethers and risers. The Mini TLP is designed to be installed by the drilling contractor using the tender vessel mobilized for the drilling program.
The concept is applicable to small hydrocarbon developments or phased development of larger fields in deep water where a stable platform is required to support surface wellheads. Configurations have been investigated for the West of Shetland environment in a water depth of 400 metres and for depths up to 1 500 metres in the milder conditions of West Africa with tender assisted drilling.
Costs have been reduced wherever possible by keeping the design simple and by using standard components available in the industry. The result is a very low Capex facility based on proven technology.

Introduction
Since 1990, there is a growing interest in deep water prospects on the West Coast of Africa, mainly in the area of the Gulf of Guinea from Nigeria to Angola. This area is known to be similar to the geology in Brazilian waters and a number of discoveries have already confirmed that oil prospects exist in the same way in deep waters.
Production facilities installed in the area can take advantage of the particularly mild environmental conditions, The Gulf of Guinea is mainly exposed to the long swell coming across the South Atlantic Ocean. There are no high winds in the area togenerate large sea states. Consequently the risk of harsh conditions is small and limited to long swells with significant wave heights never exceeding 4 meters for the 100 year return period.
This has a significant impact on the design of the facilities, in particular:a reduction in the height of the deck above Mean Sea Level,a dramatic reduction of wind and wave forces on the structure.
While the overall concept is not new, the wor; presented takes advantages of the weafher conditions in the proposed area of operation to simplify the installation procedure and minimise the development costs.
Concrete FPSO
Authors: Harris R.J.S. , des Deserts L. , Kenison R. , Snell R. , Kerr D. 
Published: October 2008

Abstract
The paper identities the advantages, features and applicability of a concrete hull for an FPSO and the issues related to project execution. For those considering the options for other fielddevelopments, the paper identifies the conclusions and the work on which these were based.
The content of the paper is based on extended studies on the advantages, design features and feasibility of fabrication of thisnew design of FPSO hull.
This work formed part of BPs review of options for the development of their Schiehallion field in 375 mctres water depth West of Shetland, the Atlantic Frontier.
The conceptual design of vessel hull was carried out with the objective of providing the best solution for a concrete FPSO for BPs application. This included consideration of capital cost, system availability, operating costs, HSE aspects.
Decommissioning and disposal, The flexibility of the concept e.g. for addition and modification during service were also examined. The design process was taken to the stage where these matters had been examined and the design has approval in principal for certification. During the design process the mathematical modelling of vessel motions and mooring forces were confirmed in tank tests.
BP participated in the proof of concept, retaining the concept on their final short list of two options for the field development. It was concluded by all parties that this new concept had been full examined and proven technically.

Introduction
This paper has been produced by the Shetland Deepwater Alliance (SDA) with the cooperation and assistance of the BP Atlantic Frontier Programme (AFP) evaluation team. It draws together their shared views and conclusions on the use of concrete as a material for the hull of an FPSO, The following companies formed the alliance SDA for the BP AFP Stage 2 project; DORIS Engineering, Sir Robert McAlpine, Blue water, John Brown and Trafalgar House.
During 1994 BP carried out a detailed evaluation process, in full compliance with the European Union regulations, to select a contractor to develop the concept of a concrete FPSO. Among the responding companies, SDA were placed ‘first in class’ for this work and, during the early part of 1995, were invited to tender this concept for Stage 2 of the Atlantic Frontier Programme in competition with other steel and concrete concepts.
During the tendering process SDA was coached by BP. This allowed the teams working on the development to exchange views with the objective of achieving the optimum solution for this concept. Another of the benefits of this process was that both parties became familiar with the objectives opportunities and constraints in the design, construction, flexibility and service aspects of the concept
A considerable amount of work was carried out by SDA in the development of a concrete hulled FPSO suitable for service West of Shetland. The concept was examined to optimise Capex, Opex, build schedule and flexibility to change during design/constnretion and when in service.
Structural Life Extension of a Twenty Year Old Semisubmersible Driling Unit Converted for Floating Production Service
Authors: Patel Raj , Bation Dave , D&apos;Souza Richard , Smith Tom , Nilsen Oyvind 
Published: October 2008

ABSTRACT
The semisubmersible floating production system converted from an existing semisubmersible vessel has provided the offshore industry with a cost effective option for developing marginal fields. Most of the semisubmersible units available for conversion are second generation designs that average twenty (23) years in age. A key safety, technical, and cost issue is reliably extending the life of a 20-year old structure for an additional 10-20 years depending on the field life. This paper provides the engineer/designer with insight, guidance and systematic procedures to execute such a structural life extension.

INTRODUCTION
Converting a 20 year old semisubmersible vessel to floating production has proven to be a feasible, economic option for offshore fields that must be developed on a tight schedule and low budget. Over forty (40) such units have been employed worldwide since 1977. A critical aspect of the conversion is the restoration of the existing structure to a condition that permits its safe functioning when permanently moored at the development site for the commercial life of the field. There are three major factors to be considered in a structural life extension program.
One is to ensure that the structure has sufficient strength. redundancy, and fatigue life in the prevailing metocean conditions, accounting for previous service life and hull configuration modifications that alter the dynamic loads and structural response of the original configuration.
A second is the renewal of excessively wasted, buckled, deformed or cracked steel and the corrosion system that will protect and minimize future corrosion of restored steel.
A third is the development and implementation of an insituition structural inspection, maintenance and repair program to monitor the condition of structure and coatings.
This paper presents a systematic approach to the structural life extension of a semisubmersible converted to floating production service. Techniques to analyze, modify and refurbish existing structures and critical joints will be provided.

FATIGUE LIFE CRITERIA
The major classification societies, ABS, DnV and Lloyds, generally propose similar procedures and criteria to evaluate fatigue life for converted semisubmersible FPVS, Each requires that the fatigue life of the vessel be reassessed based on site specific metocean data and that existing fatigue damage due to prior service be accounted for in the assessment. The primary structure of the vessel (critical joints, failure of which will result in catastrophic damage) must have a minimum fatigue life that is one to two times the design service life. Higher fatigue life requirements (usually 10 × service Me) are assigned to uninspectable critical joints.
Experimental Investigation of Deep Sea Riser Interaction
Authors: Huse Erling 
Published: October 2008

Summary
In future deep sea field developments [he drag force and corresponding static deflections of the risers due [o current can become quite large. The prevention of mechanical contact (collision) between the risers will need more careful evaluation than in moderate water depths.
The paper describes a series of model experiments in a Norwegian fjord to determine criteria for on-set of collisions between [he risers of a deep sea TLP. The current was modeled using the natural tidal current in the fjord. Results from the tests are summarized and used for verification of numerical calculations of collision criteria.

Introduction
When designing future offshore oil and gas production platforms for very deep waters, 1000 [o 2000 meters, the riser system will be one of the most critical parts. The risers will become heavy and may require some sort of buoyancy arrangement to keep the tension at an acceptable level. More serious is the effect of current on such long risers. Let us for instance consider traditional rigid, vertical risers of the type used in TLP and spar type platforms. With given current velocity, diameter, and pretension the static deflection of the risers due to current drag is proportional to the square of their length. Thus very large deflections can occur, and if the different risers in the array deflect somewhat differently, they may start clashing into each other or collide.

Objective
The objective of this paper is to present the results from a research project, in which the criteria for on-set of riser collisions were determined by model tests in a deep Norwegian fjord.

Wake Method for Prediction of Riser Interaction
The physics of wake interaction, and possibly collisions, in deep sea riser arrays is as follows: A riser sitting in the wake of an upstream riser will, due to the shielding effect, be subjected to a lower inflow velocity than the upstream riser. Other relevant parameters being equal, its static deflection in the current will be correspondingly smaller, and there is the possibility of mechanical contact. In moderate ocean depths there is normally no collision problem since the total deflections are typically of the same magnitude or smaller than the riser spacings at top and bottom ends. In deep waters, however, the deflections may become so large (proportional to the square of the length) that even a relatively small difference in deflection may cause collision between adjacent risers.
In Refs. 1 and 2 is described a simple numerical procedure for predicting static deflection and possible collisions between individual risers due to current. The method is based on the following formulation for the wake field behind a cylinder in homogeneous inflow (see for instance Refs, 1 and 3):
(MATHEMATICAL EQUATION AVALABLE IN FULL PAPER)
For definition of symbols see Fig. 1. The constants k, and k2 are basically empirical constants, but related to each other through the requirement of momentum conservation. In [he present calculations the values k, = 0.25 and k1 = 1.0 have been used.
Development of Pile Foundation Bias Factors Using Observed Behavior of Platforms During Hurricane Andrew
Authors: Aggarwal R.K. , Litton R.W. , Cornell C.A. , Tang W.H. , Chen J.H. , Murff J.D. 
Published: October 2008

Abstract
The performance of more than 3,000 offshore platforms in the Gulf of Mexico was observed during the passage of Hurricane Andrew in August 1992. ?IMs event provided an opportunity to test the procedures used for platform analysis and design. A global bias was inform for overall platform capacity and loads in the Andrew Joint Industry Project (JIP) Phase 1.1 It was predicted that the pile foundations of several platform should have failed, but did not. ?lime results indicated that the biases specific to foundation failure modes may be higher than those of jacket failure modes. ?Ile biases in predictions of foundation failure modes were therefore investigated further in this study. ?he work included capacity analysis and calibration of predictions with the observed behave for 3 jacket platforms and 3 caissons using Bayesian updating. Bias factors for two foundation failure mess, lateral shear and overturning, were determined feat structure. Foundation capacity estimates using conventional methods were found to be conservative y biased overall.

Introduction
Hurricane Andrew was a very intense storm that passed through the Gulf of Mexico in August 1992. While most of the Gulf of Mexico platforms were not adversely affected by Andrew, twenty eight steel jacket platforms were significantly damaged. All of these were installed during 1948 to 1969 and were located in water depths up to 143 feet. In addition, forty seven caissons, nstalled during 1979 to 1991 in water depths up to 113 feet, were also significantly damaged. While foundation failure was identified as the primary cause of damage to most caissons, it was identified as the cause for damage of only one steel jacket platform.
An extreme event such as Andrew provides a unique opportunity to study offshore structures tested under fill scale field conditions. By reviewing the platforms that survived, were damaged or failed during the hurricane it is possible to improve our understanding of the behavior of platform during large storm. In October, 1993, Phase I of a joint industry project, "Hurricane Andrew - Effects on Offshore Platforms,''1'3 was completed. This project established that there was bias in the safety factor (capacity to load ratio) by combining the analytical and observed behavior of 13 jacket platforms. The inform bias is characterized as a correction needed to bring analytical results to agreement with the observed results.
The capacity analysis performed in Phase I of the Andrew JIP indicated that failure of foundation elements should have occurred in a majority of platforms analyzed, Since this was not observed during post-Andrew inspections, it was concluded that the deterministic analysis was conservatively biased. An overall (system) correction factor, with a mean of 1.2 was established. This correction factor was not specific to any particular failure mode in the jacket structure m its foundation but rather included failures of all types.