bluuebunny/crossref_metadata_2025 huggingface dataset:
Innovative Concepts for Marginal Fields (Advanced Monotower Developments)
Authors: Smith M.T. , Marks V.E.
Published: October 2008

The braced mono tower provides a safe, functional and cost effective solutionfor topsides up to 500 tonnes, with up to 8 wells and standing in water depthsof up to 70 metres. It is both simple in concept and structurally efficient. The superstructure is supported by a single column which is stayed by threesymmetrically orientated legs. A broad mudline base is also provided to limitpile loads. The final concept offers complete protection to the risers andconductors from ship impact, as all appurtenances are housed within the centralcolumn.

The basic design philosophy of the low intervention platform is to minimise theonboard equipment to that vitally needed to produce hydrocarbon. The concepteliminates the life support functions that on a normal North Sea platform cancontribute up to 50% of the topside dry weight. A system of Zero BasedEngineeringis used that ensures each item of equipment contributes more to theNPV of the platform than the fully built-upthrough life cost. This effectivelyeliminates the operator preference factor and the "culture" cost.

1. INTRODUCTION

1.1 General

The design of offshore platforms for hydrocarbon production from the North Seahas recently gone through a period of rapid change. This process has occurredas a response to the increasingly difficult economic conditions over the lasttwo years. The increased competitiveness in design proposals has been matchedby a willingness on the part of Operators to accept more economic solutions tooperational requirements.

Amongst the areas where developments have shown major improvements is in thedesign of shallow water lightweight platforms. This paper traces the designdevelopment of one group of lightweight platforms for usein the NorthSea.

1.2 North Sea Environment

Figure 1 shows the UK sector of the North Sea. Two areas are of specialinterest - the Southern North Sea area which is a gas basin covered by shallowwater less than 50m in depth, and the southern edge of the Central North Seabasin, which contains oil, condensate and gas fields at water depths up to100m.

This paper refers to developments primarily in the Southern Basin area.

These areas are of special importance, as they are mature basins with a largepopulation of marginally economic small fields awaiting low cost developmentschemes before production can be justified.

2. ECONOMIC BACKGROUND

2.1 Gas Demand

The Western European gas market has shown strong growth over the last 15 yearsand is expected to continue to grow at the current rate for some time to come. The development of new high efficiency combined cycle technology andenvironmental concerns have resulted in very strong growth of gas demand forthe power generation industry. This is currently the fastest growing sector ofthe European gas industry.

In the UK, the liberalisation of gas supply from the original British Gasmonopoly position has given a very sharp increase in the proportion of gasfired power generation, as shown in Figure 2.
The Changing Role of the Remotely Operated Vehicle in Offshore Operations
Authors: Mair J. , White J.
Published: October 2008

The ROV's role in satisfying the requirements of the offshore production industry has been one of constant change over the years.

As technical, operational and commercial experience has been gained, new and more demanding challenges have been identified.

This paper demonstrates the capabilities of ROVs and their associated technology in meeting these new demands dictated by deepwater operations, increased safety requirements and low oil prices.

INTRODUCTION

ROVs and their associated intervention tools have been gradually eroding the well established role of the diver.

This is clearly evident by the reduction in the number of divers to date supporting the industry compared with the increase in the number of ROV's and their associated personnel.

Historically a large quantity of equipment and numerous personnel were required to support a simple diving system whose duties on a drilling rig for example were establishing wellhead guide wires and the occasional operation of valves. The introduction of the ROV however was in itself a traumatic period and most divers have their own ROV recovery tale to tell.

These early machines had not been designed for the rigours of offshore use and were therefore generally unreliable.

The ROV's potential was recognised, however, which encouraged companies to invest in these machines and improve their performance to acceptable levels.

In addition to the machines themselves, their launch and recovery systems were also improved. Early launch systems such as articulated cranes. (commonly fitted to trucks) have been replaced with purpose designed "A" frames and towers etc. to cater for the general increased weights of the modern ROV, its tether management system and to provide a contingency for intervention equipment etc.

In addition, systems are now generally designed to comply with offshore lifting regUlations e.g. "DnV Code of Practice for Offshore Lifting Appliances' which ensures their ability to cope with the dynamics associated with their intended use.

CURRENT CAPABILITY

Today it is now widely recognised that the ROV is the primary support system on drilling rigs.

Recently, however. we have seen numerous instances where the ROV has been required to perform activities which may be considered beyond the ROV's commonly perceived capabilities.

Such projects highlight that ROVs can perform tasks that were previously considered unlikely and will encourage new tasks to be considered with confidence.

It also became evident that all work class ROVs do not have equal compatibility and that certain tasks will require the higher specification models to be used.
Pipeline Blockage Location by Strain Measurement Using an ROV
Authors: Rogers L.M.
Published: October 2008

The paper describes an ROV based inspection method for locating a blockage in a marine pipeline. The method measures changes in the hoop strain in the pipe corresponding to changes in the internal fluid pressure. The device latent applied for), converts radial extension or compression of the pipe into axial compression or tension respectively of a load cell. It allows the use of a high sensitivity axial strain sensing element to measure the hoop strain in the pipe. By pressurizing the pipe at positions upstream and downstream of the blockage and measuring the resulting hoop strain, the bounaimies of the blockage can be accurately defined. The device can be installed and recovered by ROK the signals being relayed to the surface via the ROV's umbilical. The method has been used successfully to locate and define the extent of a blockage in a deepwater oil flowline running from a satellite well system to a production platform, allowing the planning of eflective remedial action. i%e results of the strain measurements were found to be fully consistent with the contents of the pipe determined by subsequent sectioning. Key features of the hoop strain &vice include rugged design, high sensitivity, ease of attachment and recovery by ROV with the need for minimal cleaning and for access only to a sector of the pipe, typically Y4 the circumference.

INTRODUCTION

As exploration moves into colder, deeper waters and efforts are made to extend the recoverable resource using existing production facilities, wax dropout in marine pipelines is becoming more problematic. It has been encountered in the North Sea, in deepwater wells off Brazil and in other locations worldwide. The composition of crude oil is a complex mixture ofAlkanes (saturated hydrocarbons) ranging in molecular size from methane (MJV 16) to waxy solids (h4W >100 & MP >90 deg. C)Alkenes (unsaturated hydrocarbons)Aliphatic and Aromatic HydrocarbonsSulphure and Nhrogen CompoundsOther Fluids and Solids (water, grit, metals)

The alkanes are the predominant fraction of most crude oils, the low molecular weight compounds up to C12 (petrol, diesel, kerosene) being the most valuable, Above C12, the alkanes become progressively less volatile and more viscous. Above Cl 6 (or C20 depending on the Authority) the Akanes are collectively called tariffs or waxes.

Fields very enormously in their proportion of high molecular weight compounds and can even vary within the same well. As the molecular weight increases, the paraffin's tend to solidify by forming microcrystalline aggregates which precipitate out of the cnude oil. progressively Initially this solidification is held back by the presence of smaller alkanes which act as parting solvents. However, above C40, these aggregates grow in size and eventually settle onto the solid surfaces of flowlines, separators, tanks, etc.

The down hole temperature in an oilwell can vary from near ambient (in shallow wells) to around 100 deg. C in deep wells. As the oil flows from the well it can cool gradually or rapidly depending on the presence of water.
Sealability of API R, RX, & BX Ring Gaskets
Authors: Fowler Joe R.
Published: October 2008

This report presents work done to determine the sealability limits of API R, RX, and 8X gaskets. The work included finite element modeling and full scale testing.

The research teaches that bolt stresses of 15 to 30 ksi are necessary to seat the gaskets. If these seating loads are not achieved, the gaskets will likely leak at design pressure, particularly for gas service. If the flanges are properly made up, the sealability limits in API Bulletins 6AF and 6AF2 are too conservative and can be eliminated for non cyclic service.

INTRODUCTION

API Spec. 6A (1)* give specifications for equipment with pressures to 20,000 psi and temperatures as high as 650º F. This equipment utilizes metal ring-type joints for all end connectors in the specification. These end seals are of three basic types, RX, R and BX. BX gaskets are generally used for the higher pressures.

Recently API has sponsored a considerable amount of research into the load capacity of the various end connectors in Spec. 6A. This research has resulted in Bulletins (7) which provide the users a prediction of the combined pressure, moment, and tension load capacity of the various connectors, All of this research has been done, however, with the assumption that the gaskets will leak when the applied loads relieve the initialpre-load on the gaskets. The calculations for this were necessarily crude and the assumption has not beenverified. Accordingly, it was desired to perform a general research study on metal gaskets in order to provide users the background of the design of these gaskets to enable one to intelligently assess the degree of safety againstleakage in the end connectors.

In practice, ail of these gasket types have apparently worked reasonably well. BX gaskets were explained in a 1964 paper (2) and were introduced to achieve the benefits of pressure energized gaskets in high-pressure flanges. R gaskets, which are not designed to be pressure energized, and RX gaskets, which are pressure energized, do not makeup with the flange faces face to face. The BX gaskets, however, do generally makeup face to face. Type R gaskets have been in service since the 1930's. (Reference 1, 13th Edition, 1981). ASME (3) publishes procedures for calculated gaskets seating and retaining codes for R type gaskets, but not for RX or BX gaskets.

RESEARCH OBJECTIVE

The objective of this research (13) was to develop general theories which could be used for design and prediction of gasket performance for R, RX, and BX gaskets. Fundamental to this is a prediction of leakage as a function of flange pre-load or flange face movement, Seating load prediction is also an important factor for designers.

The philosophy of this research project was to use the finite element method and full-scale testing to help confirm and validate general theories of behavior. The general theory of behavior which was evaluated was that leakage is likely to occur when the contact pressure between the seat and the seal is less than the applied internal pressure that is being sealed.
Load and Resistance Factor Design Case Histories
Authors: Brand P.R. , Whitney W.S. , Lewis D.B.
Published: October 2008

Casing accounts for 10% to 30% of the total well cost. The oil and gas industry has historically used working stress design (WSD) methods in the design of casing strings, This method requires the load to be defined as an estimate of the maximum anticipated load, while the strength is defined as a lower bound of the materials strength. The designer must assure that the minimum allowable stress of the material is not exceeded, by separating the load and the strength by a safety factor that is fixed by company guidelines.

Load and Resistance Factor Design (LRFD) is a reliability based design philosophy, which explicitly takes into account the uncertainties that occur in the determination of loads and strengths. The LRFD format was first developed in the 1930's in USSR and Europe for use in the Civil Engineering industry. Its development and use has continued in Civil Engineering practices and is now widely accepted in many industry codes.

As a reliability based design philosophy, the engineer is allowed to select a probability of failure that is commensurate with the consequence of failure. This allows the engineer to accept a probability of failure that maximizes the utilization of the casing design, thus improving the economic indicators on the investment.

Beginning in the late 1980's and early 1990's, several efforts were undertaken to develop LRFD for casing design. This paper describes the basic LRFD philosophy and shows two examples of well designs developed using LRFD and compares them to the designs previously developed using WSD. The impact on the well cost and deliverability are discussed for each design. The issue of probability of failure and the impact on the well design are also discussed. The material selection criteria and the qualitysystem that were developed in concert with LRFD are also described

Working Stress Design

WSD is the traditional approach to design that ensures adequacy by computing elastic stresses under maximum anticipated load and comparing them with allowable stresses. The safety of the structure is assured by ensuring that for each element, elastically computed stresses do not exceed an allowable stress, that is a preset fraction of the yield strength of the material. The ratio of the yield strength to the actual stress is known as the safety factor.

The allowable safety factor, is set in an empirical manner and reflects the companies policies with respect to safety and risk. Dueto the uncertainty in the loads, there is not a consistent definition of design loads or minimum factors of safety throughout the industry, except where mandated by government regulations.

Although working stress is conceptually simple and has worked well in most cases, it is not without problems. Some examples of these problems are:it is overly conservative,it does not give the engineer any insight into the degree of risk or safety of the design,it has no risk balanced capabilities,it treats all wells the same,there is little justification in the safety factors.
The Business Impact and Value of 3-D Seismic
Authors: Aylor W.K.
Published: October 2008

3-D Seismic has had a profound and lasting impact on the hydrocarbon exploration and production industry. It is a technology which is often excellent at assessing the risk associated with trap definition, seal and reservoir distribution, the very parameters which are the most critical to optimizing the economics associated with E&P projects, This paperdiscusses Amoco Corporation's experience with 3-D Seismic when used for new field rate acceleration, older field extension, and wildcat exploration. Its emphasis is on assessing the value added by 3-D by reviewing recent E&P experiences in a post-appraisal mode and then in applying the lessons learned from these analyses and case histories to potential new projects.

This work is significant because it first assesses the impact 3-D has had on a large number of business situations at Amoco, that is, it is based on data collected on 159 3-D surveys acquired at Amoco between 1991-1994. Second, it uses the data collected from these surveys and applies the business improvements observed in the data to typical international business opportunities to quantify, in expected value $ terms, the value that the technology brings to an average project. Finally it looks at project economics not only from an oil company perspective, but from the standpoint of a host government, with a discussion of insights and implications of the data, economics and techniques utilized.

3-D can add significant value to exploration and production activity. The value added depends on expected cost, revenue, timing and risk changes, and generally is more valuable the earlier it is applied. If 3-D is appropriate for particular projects (that is if the underlying risk associated with a projeet can be significantly impacted by 3-D), host governments stand to realize much more from 3-D than do oil company partners. Delay in the use of 3-D can significantly erode the ultimate value 3-D can add to the project. Quantified economic examples are used to illustrate all of these conclusions.

Introduction

During the past 5 years, hardly an oil industry technical or business symposium has taken place, or a trade journaI has been published that has not either mentioned or explicitly illustrated the viability of 3-D seismic as a E&P tool. In spite of the enthusiastic claims however, most geoscientists have at one time or another experienced disappointing results with 3- D. Indeed, for some companies, 3-D remains a novelty, an exception to their way of doing business.

A very legitimate question remains then: to what degree has 3-D added value to the E&P situations where it has been utilized? Has value been added when prospects or areas have been condemned by 3-D? How often does 3-D return disappointing results, for example where dry holes are drilled even after 3-D has been used?
Axial capacity of offshore piles driven in dense sand
Authors: Jardine R.J. , Overy R.F.
Published: October 2008

The paper reports a previously unpublished tension load test to failure on a 38.lm long, 660mm diameter steel pipe conductor driven in predominantly dense sands at the Leman BD site in the Southern North Sea. The case history is used, with other pile test data, to evaluate and calibrate new design methods that have been developed from recent research. As an example, the findings are applied to a study into the axial capacity of piles driven at the nearby Leman AD H site.

The Lemon soil conditions are described, and the driving records and test result are presented. The new design methods are then summarised and applied to analyse the conductor's capacity. A comparison with other pile capacity prediction methods is made and the results are used, in conjunction with other data to demonstrate that a modified form of the new method can give accurate predictions for offshore pipe piles.

Applying these approaches the Leman AD 11 platform's foundations indicates axial capacities greater than expected previously. Increases in compressive and tensile capacities of at least 15 and 50Y0, respectively, are predicted. Research is continuing to fizrther refine the new methods for calculating axial capacity.

Introduction

Shell UK Exploration and Production operate groups of offshore jacket structures in the southern North Se% some of which were installed more than 25 years ago. To help assess the effects of a possible reconfiguration of loads applied to the Leman AD 1[ structure, a study was made of the axial capacity of its foundation piles.

Leman AD 11 has piles that were driven in predominantly dense sands, for which standard axial capacity calculation methods are unlikely to offer accurate predictions. Reviews such as those by Focht and O'Neill, Briaud and Tucke, Jardine and Christoulas, Tang, Randolph Chowb or Gavin and Lehane show that conventional design methods for piles driven in sand are relatively unreliable.

New approaches were considered necessary and the study was shaped to:. Interpret the ground conditions and operational soil parameters at Leman AD 11on the basis of existing site investigation data,Assess data from a conductor pull-out test performed at the nearby Leman BD site in 1969 and the recent field tests on driven open-ended pipe piles at Dunkirk described by ChowUse the above to test the reliability of a new Imperial College ([C) method for calculating the shaft resistance of open-ended driven piles in dense North Sea sands and. Apply the new methods, taking account of the recent research concerning base capacity described by Hight et alg and Chowb.
Mini 3D for shallow gas reconnaissance
Authors: des Vallieres T. , Enns D. , Kuhn H. , Parron D. , Lafet Y. , Van Hulle D.
Published: October 2008

The Mini 3D project was undertaken by TOTAL and ELF with the support of CEPM (Comite d'Etudes Petrolieres et Marines) to define an economical method of obtaining 3D seismic HR data for shallow gas assessment.

An experimental 3D survey was carried out with classical site survey techniques in the North Sea. From these data 19 simulations, were produced to compare different acquisition geometries ranging from dual, 600 m long cables to a single receiver.

Results show that short offset, low fold and very simple streamer positioning are sufficient to give a reliable 3D image of gas charged bodies, The 3D data allow a much more accurate risk delineation than 2D HR data.

Moreover on financial grounds Mini-3D is comparable in cost to a classical HR 2D survey. In view of these results, such HR 3D should now be the standard for shallow gas surveying.

Introduction

Advantages of the 3D seismic method compared to classical 2D for oil and gas exploration are obvious, particularly in applications requiring high spatial resolution. However, exploration 3D puts heavy constraints on data acquisition and processing. Costs are consequently high.

High resolution 3D seismic investigation is not commonly used so for such purposes mainly because cost reasons in spite of the technical advantages.

The objective of the Mini 3D project was to investigate economic HR 3D seismic for shallow gas risk assessment. The strategy was to try to define the simplest data acquisition configuration capable of 3D visualization of hazardous shallow gas that is consistent with the equipment currently used in the site survey industry.

The work progressed as follows :

The first step was to acquire a full, state of the art HR 3D seismic and to process the data with full processing sequence. The subsequent 3D volume was by definition the reference.

Further steps were taken to simulate reduced acquisition geometries by means of successive decimations. The results were then compared to the reference volume

Data acquisition

Site selection

To be representative, to possess the following attributes:proven shallow gas occurrencesprevious sumying for shallow gaswell data in the immediate vicinity the experimental site hadlocation in an active oil and gas producing region Such a site was found in the British sector of the North sea, in block 22/7 operated by TOTAL.

Acquisition equipment and recording parameters

The objective was to acquire a HR 3D volume corresponding to a drilling site making use, to the greatest extent possible, of standard 2D HR equipment including : vessel, seismic source, and streamer. The desired highest HR geometry was a 6,25 × 12.5 m bin size to be acquired along lines 12.5 m apart.
Field and Laboratory Data from Pile Load Tests in Calcareous Soil
Authors: Randolph M.F. , Jeer H.A. , Khorshid M.S. , Hyden A.M.
Published: October 2008

Grouted piles are used extensively in offshore deposits of calcareous soils, because of the relatively low shaft capacity of driven piles. Two construction techniques may be used: drilled and grouted piles, where steel pipes are grouted into predrilled oversize holes; and grouted driven piles, where driven pipe piles are pressure-grouted through valves distributed along the pile shaft The paper describes the results of a comprehensive set of field and laboratory tests on grouted piles that were undertaken in calcarenite in Australia, to validate pile design for the calcareous deposits which predominate off the coast of Australia. The experimental results were used to validate and calibrate a numerical load transfer model, that simulates degradation and accumulation of permanent displacement under cyclic loading. The paper provides a recommended design approach for grouted piles in calcareous soil, with due account taken of construction technique, pile diameter and soil strength as indicated by the cone resistance.

Introduction

Drilled and grouted piles are commonly used for offshore foundations in the compressible calcareous soil deposits found off the coasts of countries such as Australia, India and Brazil. Despite their prevalence, design rules remain largely empirical, with a limited database of field and laboratory tests.

Over the last decade, an alternative construction method for grouted piles has emerged. Instead of grouting a steel tubular into an open-hole (a relatively costly and potentially unstable sequence), piles are first driven into the formation, and then pressure-grouted along their lengths through pre-installed valves. Rickman and Barthelemy have described this so called ‘grouted driven pile’ construction method, although the technique has still to be used for a major offshore structure.

EconomicalIy, there appear to be a number of advantagesfor grouted driven piles:The pile may be installed in one piece, using a single installation method (driving), compared with a driven primary pile, drilling out of the primary and hole for the insert pile, and installation of the insert pile.In principle, grouting may start from close to the seabed, rather than from below the primary pile.The pile diameter remains at the maximum that will fit the jacket sleeves throughout its length, with no step down from primary to secondary.

From a risk viewpoint, the potential danger of hole instability for the drilled and grouted pile must be weighed against the uncertainty of grout coverage for the grouted driven pile. From a technical viewpoint the pressurisation of the grout for the grouted driven pile is likely to entail higher shaft capacity than for a drilled and grouted pile. However, the difference may be of secondary importance compared with risk and eeonomic considerations.
3-month GPS Subsidence Analysis in North Sea, ELF/CEA-LDG interpretation
Authors: Flouzat M. , Fourmaintraux D. , Maury V. , Halle G.
Published: October 2008

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

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

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

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

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'Souza Richard , Smith Tom , Nilsen Oyvind
Published: October 2008

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

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.