INTRODUCTION Working underwater is a lot more difficult than working above water or on land, therefore the incentives to do so must be found in some aspects of project realization, such as reduced cost, advantageous schedule, improved technology In this chapter these aspects will be considered for a number of cases of floating systems and subsea developments which have been carried out The North Sea provides examples of floating systems, starting with Hamilton Brothers'/Argyll, and the development of the buoyant tension-legged platform of Conoco Hutton TLP in production in 1984 Subsea systems are complementary to existing fixed structures and increase the recoverability, for example, BP Magnus has seven satellite wells Subsea production began with the Zakum Subsea Production Scheme (1969–72) and now subsea systems are used m field developments in the North Sea, for example, the Shell UMC, Texaco Highlander and North-East Frig This chapter also discusses some prospective developments in applications, techniques and equipment. Other chapters in this volume will elaborate on many of the topics which will be raised. Early practitioners of the art of working underwater include marine salvagers and offshore drillers, and techniques which were developed for these activities are being elaborated and unproved upon for those more complex activities of offshore oilfield development Experience, and common sense, has taught that complicated construction work should be done as completely as possible on land before taking units offshore for installation, and that only minima1 assembly of pieces be required subsea Examples of subsea construction activities include well templates, manifolds, pipelines, multiple flowlines, anchor foundations, use of divers and remotely operated vehicles Prospects for the future of subsea activities are assured by the present keen interest in floating production systems and subsea satellite developments to be tied-back to existing installations The future North Sea development m the UK sector will be primarily smaller fields, satellite fields, condensate/sour gas Deepwater developments present greater challenges but, with the discovery of sufficient reserves, there is no doubt that these developments wtl1 also stimulate new requirements as well as new capabilities for underwater construction. ARGYLL FIELD The Argyll Field layout is represented in Fig 1, and the key information summary is presented m Table I The production riser system is of particular interest Production riser system The production riser system at Argyll is made up of standard drilling components assembled in what was a unique system, in 1975 The(Fig 1 is available in full paper) system consists of five basic elements from the sea bed upwards - the mass anchor, permanent base, manifold, risers and flexible connections. The central riser is of 10 in * nominal bore and serves as the main supporting member and export (or shipping) riser. This element consists of a stab sub-assembly, universal joint and SLX 40 ft* joints of 10 in riser pipe connected with standard marie riser joints.

INTRODUCTION The Occidental Consortium's newly developed "Scapa" field has now come on-stream as planned, some 14 months after Department of Energy approval The Scapa development consists of a major subsea facility connected to the Claymore platform, 3 miles* away by two pipeline bundles and associated control umbilicals (see Fig 1) The new topside facilities are connected in turn to the pipelines by a large bundle riser, carrying the produced oil and lift gas, with a facility for a utility if required The field is estimated to have recoverable reserves of 40 million barrels of oil with a gravity of 32 5 ° API to be extracted from a Lower Cretaceous reservoir The Scapa development had a considerable engineering content The main production riser and template each required a large proportion of this engineering for the installation design It was a requirement that all of the subsea work should be completed with one season. To accommodate this the contracts were broken down into elements template - fabricate and install, bundle - fabricate and install (Fig 1 is available in full paper) riser-fabricate riser-install These packages therefore gave the flexibility to maximize the offshore work programme involving vessel availability and installation capacity This chapter gives a short overview of Scapa's facilities and installation difficulties, together with a fuller description of the riser and template It should be noted with interest that the total weight of Scapa additions to the jacket is approximately 800 tomes, or 2 3% of Claymore's jacket and topside weight The structure's capability of accepting the extra load required verification FACILITIES Pipelines and umbilicals The template is connected to the Claymore platform by an extensive pipeline system, consisting of two 28 m * multi-line composite bundles, an 8 in water injection line and a 3 in bulk chemical treatment line, as shown in Fig 1 Each pipeline bundle is approximately 2 8 miles long and contains a 10 in. production line, a 6 in test/service line and three 3 in gas lift lines. The bundles are connected to the template manifold and the platform production riser by flexible spools. The two pipelines, which are the longest of their type to be towed to date, were installed using the mid-depth tow method. Using the natural buoyancy of the pipeline to raise the line off the sea bed and chains to control this flotation, the lines were towed using two tugs These tugs kept a set tension on the line during tow and the level of tow was varied depending upon the weather conditions. The towlines were attached to the bundles by specially designed tow heads These tow heads also served to splay the integral lines for accommodating the flange tie-in points. To confirm manual calculations, tests were carried out on a scale model of the tow head to determine.

INTRODUCTION For a conventional multiwell offshore field development, traditionally drilling could only commence after a fixed platform had been installed and commissioned. This meant that the gas or oil was not recovered until well into the development programme, and that the cost of developing fields m deepwater or marginal fields became, if not unpractical, then certainly less financially attractive It is usually cost-effective for an operator to begin production at the earliest opportunity, because recovery of capital outlay cannot commence until the first gas or oil flows. Earlier production can be achieved by pre-drilling the wells through a drilling template previously installed on the sea bed. The wells can then be tied back to surface production facilities typically installed on a fixed platform or a tension-leg platform. The early production of gas or oil must, of course, be offset against the costs of the template, pre-drilling and tie-back operations Where the cost of providing a fixed structure proves to be uneconomic, then the use of a template containing the production facilities should be considered. It can provide the drilling facilities in addition to accommodating Christmas trees, production piping, well killing and pigging facilities. The template can act as a gathering point for satellite wells, and export by pipeline to either a jacket located in shallower water or via a riser to a floating structure within the development Its use m this fashion allows the economic development of a marginal field and even small pockets of a large field not accessible by deviation drilling from a central platform The technology used and the expertise gamed from such operations will also prove useful in exploiting the reserves in the deepwater fields which are currently being investigated. The major considerations governing the design and installation of these structures form the basis of this chapter. DESCRIPTION General When a fixed or tension-leg platform is to be used m conjunction with pre-drilled wells it is first necessary to drill some or all of the wells through a previously installed template. The platform is subsequently installed, the subsea wellheads are tied back to the production trees on the deck, and any further wells can be drilled from the platform Such a template structure, which is only required during the drilling operation, is generally referred to as a drilling template, a typical example of which is shown in Fig 1. As an alternative, it is possible to locate the production trees on a subsea template and then tie-in to a floating production facility (e g a modified tanker or drilling vessel) or a remote platform The oil or gas is exported to the floater via a riser or to the platform by flowline Such a structure is commonly referred to as a production template (a typical example is given in Fig 2), and is expected to act as a guide and support for the drilling operations and to house the Christmas trees throughout the life of the installation

INTRODUCTION Monohull vessels make up by far the most wdely used hull form on the waters of the world, and not least m the field of subsea construction The workhorses of the offshore industry, the monohulls, have been designed, modified and often converted to facilitate a multitude of tasks. It is acknowledged that the semi-submersible hull form is playing an increasing role in subsea construction, but it is the numerically superior and often more versatile monohulls upon which so much still depends. It is not the purpose of this presentation to analyse comparisons, but rather to illustrate the abilities of monohull vessels in subsea construction DESIGN AND SPECIFICATION Subsea construction vessels tend to fall into two categories The first are those which are designed and constructed to perform a task or tasks, with diving very much a secondary capability. The second category consists of vessels that have a major diving capability but are also able to carry out a number of other tasks. Vessels forming the former group are the crane barges, lay barges and general-purpose construction vessels, for example, Reel Ship Apache , and the newly built DB Challenger The latter group are the diving support vessels, for example Arctic Seal and newly built Seawell One Both categories must be able to perform to meet high specifications and the following considerations must be made during design and budding dimensions, power; positioning systems, subsea construction facilities. Dimensions Length, beam and draft make up the vital statistics which dictate the actual size of a vessel However, it is the way in which these measurements are arranged that is important. The monohull must be full enough in the beam to provide a stable and sea-kindly work-platform, but must be an efficient profile to enable good transit speeds to be achieved and economically maintained. The draft of a vessel must be considered, not only as a component of stability but also in order to keep a wide choice of ports and harbours available, as one of a monohulls strengths is its ability to go alongside a jetty or quay for load-out and mobilization. When considering a vessel for specific workscopes, size plays an important part in the selection process For the most part, larger vessels will be utilized for the heavier tasks and when large numbers of personnel are required. This does not by any means preclude the smaller vessels, as they come into their own for lightweight tasks and where access may be restricted Often a large vessel may carry out a single part of a project and a smaller vessel may then take its place to complete the work. Deck and working areas are proportional to the overall dimensions of a vessel and the accommodation requirement. The scope of work will dictate how much deck is required, and therefore vessel selection. The loading capability of a deck must also be considered, as specialist construction equipment may cause problems requiring the fabrication of additional grillage and sea fastenings.

INTRODUCTION The continued development of offshore installation equipment has allowed new methods and techniques to be employed for offshore installation. This is true for both subsea developments and conventional platforms, and has enabled large-scale underwater construction to be carried out more efficiently. This chapter looks at the development trends of offshore construction vessels and their effect on subsea installations, and expands on some of the areas where heavy-lift vessels have an important role to play. INSTALLATION VESSEL DEVELOPMENT When the search for hydrocarbon resources first moved offshore, the floating construction equipment necessary for installation consisted mainly of flat-bottomed barges with relatively small cranes This was all that was required for the initial development areas of the US Gulf, and similar mild environs When more-hostile areas, such as the southern and central North Sea, were developed ship-shaped vessels of larger crane capacity proved far more capable and quickly emerged as the minimum requirement for working at such locations Crane vessel lift capacity soon increased from less than 800 tons* to over 2000 tons, as it became obvious that the bigger vessels with greater capability were the most cost-effective method of installation. However, when very hostile waters such as the northern North Sea were encountered even the more sophisticated monohull crane vessels found it took a whole season to complete a major installation As operators could not afford these project delays, the industry started looking for ways of reducing installation tunes, extending workable weather windows and enlarging the scale of offshore operations. The semi-submersible hull form, long known as one of the most stable platforms from which to operate in hostile environments, combined with dynamic ballasting systems, proved to be ideal for heavy-crane lifting operations in harsh conditions. Workability unproved dramatically, giving the possibility for the majority of offshore tasks to be undertaken year-round (Fig 1) and allowed more-efficient subsea operations to be performed, especially when remote or diverless operations were employed (Fig 1 is available in full paper) Individual offshore crane capacities have increased tenfold in the past 15 years, with total vessel lifting capacities growing from 700 tonnes on monohull ships to 14 000 tonnes on SSCVs during this period (Fig 2) Whilst this increase in capacity was aimed mainly at jacket and module installations, allied with it was the ability to maintain station accurately, to provide a very stable platform for offshore operations and to operate a large range of equipment designed specifically for deep underwater operations such as large-capacity subsea hoisting blocks, high-energy hydraulic underwater hammers and deep-dive systems These have proved invaluable for a number of underwater constructions, and most subsea templates and manifolds over 300 tons in weight have been installed with heavy-lift vessels (HLVs). HLV INVOLVEMENT IN UNDERWATER CONSTRUCTION A certain amount of subsea construction has been associated with most field developments and has usually involved HLV operations This ranges from the installation of subsea items associated with conventional platforms, such as pipeline tie-ms, to construction of complete subsea production systems

INTRODUCTION Most forms of subsea installations have, as a common feature, a requirement of physical contact directly or indirectly with the sea bed The nature of that sea bed greatly influences the design and installation technique adopted, and consequently the economics of the project Furthermore, incorrect assumptions about the sea bed can necessitate expensive remedial measures, since little flexibility is available to adjust the design at the tune of installation Despite these factors, site-specific knowledge of the sea bed is often unavailable for subsea installations. Designs based on sod conditions at platform sites many kilometres away are not uncommon This chapter describes some of the influences that the geotechnical conditions can have on the design and operation of subsea installations, and demonstrates the desirability of the acquisition of site-specific soil data FOUNDATION LOADS For the purposes of classifying their basic foundation loads, subsea units can be divided into three categories units which are set completely below sea-bed level, units which are set at or slightly below sea-bed level, units which float above the sea bed and are anchored to the sea bed In each case the purpose of the foundation member is to support the loads Imposed by the unit and any external forces applied to the unit safely, effectively and at an acceptable deformation. The first stage in deciding on the type of foundation unit to be adopted (and hence on the soil data requirements) is to establish the magnitude of the loads for the various options under consideration Fully recessed sea-bed units are currently only adopted where local conditions render surface units vulnerable to damage Typically, silos may be adopted where iceberg scouring of the sea bed can occur Such units, by their nature, do not attract high accidental lateral loads, and silos are generally able to support their own weight and that of the unit by external-skin friction very adequately They therefore provide their own foundation, and the only geotechnical areas for particular study are those governing their installation. Surface units represent the vast majority of existing and planned subsea installations The first generation consisted of a wellhead with a rigid riser and a production tree affording well control. The next stage was the development of manifold units to permit collection from a number of satellite wells and to allow transportation to adjacent topside production facilities. Current and future developments include sea-bed separation units, thus making possible long-range sea-bed transportation and the use of modular 1 atm units serviced by submersible Unless requiring the special protection afforded by category (a) above, these units are all normally placed on the sea bed and require support by the sea bed. Vertical loads generated by these units and their protection cages are generally much lower than the range of loads normally associated with platforms. Vertical components of foundation loading arise from

INTRODUCTION The sea bed is not the most accessible of places for dealing with breakdowns or the retrieval and replacement of defective equipment, nor does it permit frequent and easy maintenance operations. Subsea installations are therefore designed for long trouble-free periods of operation of many years' duration The necessity for quality assurance when purchasing equipment is very obvious. The purpose of this chapter is to give an insight into the setting up and operation of quality assurance and control activities relevant to the provision of equipment for subsea installations Additionally, some examples are given based on experiences during the last 3 years, of incidents which demonstrate the need for precise communication of quality requirements and the value of dedicated surveillance during manufacture and testing. Quality assurance and quality control are not the same, and an appreciation of the difference is necessary. In the past there has been a considerable degree of misunderstanding, and it was proclaimed by some sceptics that both of these were merely fancy new names for inspection - and an excuse for delaying progress of work by introducing requirements for vast quantities of documentation and certification to satisfy the whims of customers and regulating authorities. No doubt the objectives of a quality system as being the identification and specification of relevant "good engineering practice" with subsequent confirmation by monitoring, recording and certification are better understood throughout industry today than several years ago, and also the difference of meaning between "assurance" and "control", but never-the-less it might be prudent to commence by defining these terms again, and by briefly summarizing quality assurance philosophies and the formal quality system which results in the establishment of a quality plan setting out the degree of control which must be exercised QUALITY ASSURANCE PHILOSOPHY AND QUALITY SYSTEMS Quality assurance is the systematic performance of all planned activities and functions necessary to provide confidence that an item or facility will be satisfactory in service. Quality control consists of those actions which provide a means to control and measure the characteristics of an item or facility to specified requirements Most companies now profess the philosophy of operating a quality system in accordance with BS 5750 or similar standard, to whatever degree is suitable for their activities and product. This philosophy will recognize that the assurance of quality must be fundamental for all work carried out within the organization, and must be practised by all personnel in their dally activities The existence of a QA/QC department within the organization does not diminish the individual quality related responsibilities of other personnel The philosophy will also recognize that quality is enhanced by working in a disciplined manner to formal procedures designed to eliminate the occurrence of deficiencies, and that the evidence of quality, before the self-evidence of satisfactory performance in service, is provided by records generated during any activity undertaken towards provision of the product

INTRODUCTION This chapter is concerned with the suitability of long and wide piles for different subsea applications A brief overview of the development of large hammers for driving long jacket piles is presented, and the suitability of wide piles for other applications is discussed Methods of interconnecting wide piles are described, and finally there is an account of the development and testing of a launching system for piling off small vessels. Note In the context of this chapter, a "long" pile has an aspect ratio (length breadth) in excess of 20 1, whereas a "wide" pile has an aspect ratio of nearer 5 1 APPLICATIONS FOR OFFSHORE PILES Applications for offshore piles may be divided into three broad categories Jacket piles to support production platforms. Pile loads are principally axial compressive, supporting the structural weight, with some lateral and tensile loading due to the overturning effect of storm conditions on the platform Subsea structures such as templates and protective structures Pile loads are much smaller than for jacket piles, principal loads being horizontal or vertical, compressive or tensile, caused by snagging or deflection of fishing gear Self-weight compressive loads are generally small by comparison. Anchor piles for permanent moorings of floating production systems, loading buoys and circumstances where sea-bed flowlines do not permit conventional moorings. Pile loads are principally horizontal, with some vertical tensile in extreme conditions Since engineering solutions are predicated by available techniques and the mainstream of offshore hammer developments has been towards large hammers for jacket piles, a brief overview is appropriate OVERVIEW Mainstream piling techniques The two principal techniques for installing piles offshore are (a) Impact driving and (b) drilling and grouting Other techniques such as jetting, vibration or jacking may be advantageous or of assistance in certain circumstances, but are of application to a more limited number of soil types, and are unreliable in others Offshore construction projects tend to be of much shorter duration than their onshore equivalents, employing more-expensive equipment The penalties of downtime are therefore more severe, so that driven or drilled piles have been most frequently used. The principal application for offshore piling has been to support the weight of steel jackets and production platforms on sedimentary sods Bearing capacities up to 2000 tonnes/pile may be required, with typical pile diameters of 1–2 m to transmit the load. Driven piles are better suited to such large-scale applications, because the reaction to the energy input is provided by the hammer itself, and because final pile sets provide a recognized measure of the capacity of the installed pile By contrast, a drilling technique necessitates satisfactory torque reaction, and this presents problems at large scale In addition, the installed grout envelope around the pile is often irregular and unpredictable in shape, so that the pile must be proof-tested in-situ unless it is of very conservative design

INTRODUCTION Subsea Production Systems generally incorporate a template or satellite structure which has the following primary functions location and verticality of wells, support of subsea components and maintenance equipment, limitation of deflections between components of the system, protection of subsea equipment The first three of these requirements are normally driven by system parameters, such as number of wells, method of maintenance or method of flowline pull-in and connection. Protection of the system, however, depends largely on the local environment and can have far-reaching consequences on the layout, installation and maintenance of the subsea system In particular, if a subsea system is installed in a fishing zone, it must be decided at an early stage which of the follouring solutions is to be chosen: a structure which will protect equipment by snagging any trawl gear on an outer bumper frame, thus preventing snagging or snarling of equipment in the template the trawl gear is, of course, usually destroyed, a structure which will deflect trawl boards and nets without damage to either subsea equipment or fishing gear This chapter discusses options for alternative forms of protection, and describes and compares various structures already installed or taken to detail design as examples of the different philosophies PROTECTION REQUIREMENTS General It is general practice to protect equipment on a template or satellite to some degree, m order to limit damage from typically, the following sources fishing gear, dropped objects, anchor snags Because the wells on a subsea production system will be protected by subsea safety values, it is unlikely that damage from these causes could result m hydrocarbon spillage. Damage could, however, result in the following inability to re-enter or kill the well, intervention required to replace damaged equipment, additional inspection requirements and removal of tangled nets It is interesting to note that the NPD Regulations now require that all subsea installations m the Norvegian Sector of the North Sea be designed so that fishing gear will not be harmed. This requirement may not apply to fishing exclusion zones, which could be requested on the grounds of low fishing activity m the area, proximity to permanent platforms No such requirements yet exist in the UK sector Protection against fishing gear There are various trawling techniques commonly used in the North Sea, and among these are white-fish trawling, industrial trawling, beam trawling Since the white-fish trawl arrangement can apply the largest snag load to a subsea structure, only this form of trawl will be described Trawlers also deploy anchors which can cause similar loadings to trawl gear snag loads. White-fish trawling (Fig 1) A white-fish trawl consists of a net with a mouth up to 25 m wide and 9 m high. The net is connected to the two towing warps by a ground line with bobbins and a headline with floats.