W2A-040 Offshore oil-0.txt
OFFSHORE ENGINEERING

Gerard Keane

1. INTRODUCTION

The Marine environment poses many complex and varied problems to the offshore industry, including offshore production and exploration operators, i.e. the oil industry, design companies and offshore Certification Authorities.

In order to safely, efficiently and economically produce hydrocarbons from offshore oil and gas fields, it is becoming more and more essential to develop a clear definition of the forces expected to exist at the various offshore locations in which production and exploration systems are expected to work.

Oil price ultimately determines the viability of an offshore find. A field may be considered 'Marginal' if the environmental conditions on site, along with the predicted economical return from the reserves, necessitate a close examination of Capital Expenditure, i.e. the cost of the production/exploration unit, operating costs, i.e. transportation of the hydrocarbons, maintenance and repair costs and eventual abandonment costs, in order to determine if, in fact, the proposed development does produce an economical return to the oil operator.

Downtime due to environment may be as high as 30%. The resulting loss in production may render a 'Marginal Field' a failure in terms of expected revenue.

Design companies place great importance on accurate knowledge of the environment, in order to develop systems appropriate to their proposed locations. Certification Authorities need to ensure also that proposed new systems, and those systems already operating, are safe, in terms of personnel and in terms of ensured continuity of supply.

This paper will examine the various aspects of offshore production and exploration activities which are prone to environmental influences, and will comment of the means undertaken to ensure that all available efforts are made to minimise or avoid adverse results from environmental conditions prevailing at offshore locations.

2. ENVIRONMENTAL DATA AT EOLAS

EOLAS has collected wave, wind and current data over many years to assist in its role as Irish Certifying Authority. These data have been collected directly by EOLAS, for example, at the Kinsale Head Production Platform Alpha, with the assistance of Marathon Petroleum Ireland, Ltd., using a programmable data acquisition system. This system collected wave data, concurrently with structural deflection data. The project in question was carried out on behalf of the European Commission.

Other data were obtained from other oil companies carrying out exploration work in the Irish offshore.

A third set of data is that set collected from observations made by ships of passage over approximately 100 years. Such data have been statistically analysed to provide maxima, percentage exceedence and other useful parameters.

A forth set is of an analytical nature, i.e., derived wave data produced by modelling of causative winds. Two models have been accessed, the Irish Meteorological Analytical Wave Model and the UK Meteorological Office Fine Mesh Wave Model. These models use atmospheric pressure data, resulting winds, to create a derived wave file with directionality. The computer produced a grid-wise printout which is referred to actual geographical co-ordinates, thus providing a further independent validation of all other wave data sources.

EOLAS was an Associate Member of NESS, The North European Storm Study, which aimed to standardise European analytical models, or at least to define the differences. The various models are either owned by Government bodies such as meteorological offices or by research organisations. EOLAS wave archives were made available to the study, for use as validation files against the analytical outputs.

All data are available to assist in the accurate determination of structural behaviour in, at times, quite adverse conditions.

The following four sheets detail the extent of data collected to date. These data are augmented whenever new information becomes available.

3. FIXED PRODUCTION PLATFORMS

Fixed production platforms are structures permanently installed in hydrocarbons producing fields. These structures remain in place over the economical life of the field and are varied in design. Following is the most prevalent type.

Steel space frame jackets, supporting production and accommodation topsides. These structures consist of a bracing arrangement, from seabed to sea surface, supporting the topsides which contain process equipment, utilities and accommodation. The main legs of these structures are piled into the seabed.

The support structure is termed the platform jacket, and is described as space frame, in that minimum steel is used to as to present minimum resistance to sea forces, caused by waves and currents. The greater the steel surface presented to the sea, the greater the resulting stresses imposed on the structure. The environmental factors considered in the design of the support structure are: Seabed currents Surface currents Waves Winds

Currents cause a semi-constant force to act on the structure, and are the main consideration at lower depths.

Waves act at the upper portion of the jacket and are critical to fatigue action. The magnitude, direction and frequency of wave action has to be known to an accurate extent in order to predict the future life of the structure. Structural analysis and reanalysis is an ongoing responsibility for engineers.

The results of annual structural inspections are compared with predicted behaviour. Any changes in structural integrity are fed into powerful analysis packages to determine if any defects are of major concern. If so, then repair has to be undertaken, either resulting in grinding out of the defect or clamping of the braces leading into the node with the defect.

The following are the factors examined in general when carrying out a structural analysis: Corrosion allowance for the steel Wave loading directions for the location Water depth Current velocity at depths throughout the structure Wave magnitudes and angles of incidence Topsides loadings

All of the above lead to a determination of remaining fatigue life for all the nodes, joints, of the space frame structure.

Topsides. Topsides are subject to wind loading, which in turn leads to an overturning moment on the jacket support. Crane operations are carried out only when wind conditions are below a critical threshold.

4. FLOATING PRODUCTION SYSTEMS

Floating production systems are, by their nature, susceptible to the environment. This type of production system was developed as a requirement for marginal field exploitation, for example, or in the case where extreme water depth made the more conventional steel jacket impractical.

Floating production systems are either purpose built, e.g., semi-submersible type, or converted vessels, e.g., converted tanker. Operations associated with this type of production are susceptible to environmental influences, where extremes can lead to detrimental consequences, in terms of safety, in terms of damage to the environment, and in terms of lost production.

Marginal field development depends on a determination of all factors influencing the final return on investment. One of the main factors influencing output target is environmental downtime, which can be as high as 30%, experienced at certain Northern Sea fields using floating production.

A semi-submersible production system is connected to the seabed wellhead via a riser and conductor system. The riser transports the hydrocarbon from the reservoir to the unit. The unit, whereas compliant to seastate in a free floating mode, will have restricted movements when connected to the seabed. This is necessary to maintain stress on the piping to an acceptable maximum. Motion limits in heave, pitch, roll, etc., are necessary to minimise stress and bending of the connecting pipework.

EOLAS has been developing a lost-production analysis package since 1987. This package, Downtime Analysis for Marginal Field Production Systems (DAMPS), was developed in co-operation with The European Commission. The package simulates production operations in conjunction with the environmental parameter file for the location and determines downtime caused by conditions above the operating criteria of the various aspects of the operation, e.g., riser/conductor connect/disconnect levels, transport tanker mooring/unmooring levels, production tanker hawser tension limits.

Earlier stages of the work consisted basically of a direct actual environment versus environmental limits comparison. It was considered appropriate to develop the package further to allow comparison between actual structural behaviour and motion limits. Model testing was carried out in UCC's Hydraulics and Maritime Research Centre and at The Danish Hydraulics and Maritime Institutes. Resulting response amplitude operators for various production unit shapes in various seastates were provided to EOLAS to allow conversion from the basic wave data file, for example, to the derived structural response file. The resulting package is now more relevant to offshore personnel, who feel more confident in comparing motion characteristics with motion limits than in studying seastates.

The above analysis routine is applicable to areas other than specifically offshore. Following are examples of areas where downtime analysis can produce cost effective results: Marine works Harbour construction crane vessels Dredging Interconnector power cable installation Fish farm design Oil pollution contingency plans, oil boom choice Seismic exploration vessels Safety/standby vessels

The above are but a few of the areas where EOLAS'downtime analysis package can assist in identifying the most appropriate and cost effective vessel for the project. By choosing a vessel whose performance matches as closely as possible with predicted conditions on site, cost effective marine operations can be realised, and even if the appropriate vessel is more expensive to hire, the resulting uptime most often saves cost over the duration of the project.

EXPLORATION UNITS

Over the last ten years, approximately 50 wells have been drilled offshore Ireland, using 22 exploration units. These wells were drilled in areas of extreme environmental activity, e.g., West Coast Porcupine, and in areas of a more benign climate, e.g., Irish Sea. In each case, the rig chosen for the work was considered suitable in terms of economics and the technical ability to carry out the task.

There exist many varieties of drilling unit. The most common seen offshore Ireland in the semi-submersible unit, which is floated to location and anchored on site. Subsequent ballasting minimises the effects of wave action and provides a relatively stabilised drilling floor.

Another type of unit is the drillship, often a converted tanker. Such a unit is more appropriate to calmer conditions.

A third unit seen offshore Ireland is the jack-up rig, which is floated to site and then stabilised on the seabed foundation using steel bracing of solid legs which are cranked downwards to the sea floor, thus allowing the unit hull to be raised out of the water.

All of the above have operating limits, similar to the floating production units considered earlier.

EOLAS has used the DAMPS package to assist oil companies determine the most suitable unit from an environmental uptime perspective.

Environmental suitability is then added to the assessment list, which will include actual unit hire costs, to allow the operators decide acceptable downtime risk in the light of the overall cost of the unit.

Obviously, a unit with high environmental criteria, i.e., a unit which is likely to continue operating under highly adverse conditions, will have little or no downtime, but will also most likely be the more expensive of the options being considered.

An assessment by EOLAS on the probably conditions on site during the drilling programme, assists the final decision. Environmental assessment is made from the knowledge of previous years for the proposed location.

To provide future information, weather data collected by the rig crew during a drilling season if often passed on to EOLAS to augment existing archives. Thus, a rapport has developed over the years between EOLAS and the oil companies/drilling unit owners. Contact is maintained with the operator during the well to allow comparison between predicted rig behaviour and actual motions experienced.

The aim of this process is to help confirm the accuracy of the study and also, where necessary, to fine tune the software even further to best reflect the information requirements of the offshore crews. Quite often, different drilling companies, i.e., those companies who lease rigs to the oil operators, may have different methods of determining environmental suitability. Even though we are dealing with similar seas, rigs will respond differently dependent on their makeup and original designs. For example, one rig may have a more accommodating conductor/riser slip joint arrangement than its neighbour. Heave compensation may be more effective on one rig than on another.

The EOLAS package takes all these specifics into consideration, thus simulating the actual unit as closely as possible.

The overall aim of this type of study is to identify the most suitable rig for the well to be drilled.
