civil-engineering-(土木工程概论英文课件)CHAPTER-NINE

CHAPTER NINE
OFFSHORE STRUCTURAL ENGINEERING
Offshore structural Engineering is a relatively new field of engineering concerned with the design and installation of offshore platforms for various purposes. It dates from about 1974, when the first steel structure was installed in the open waters of the Gulf of Mexico。

It differs from conventional structural engineering mainly in the special problems that have to be considered in the transportation of the structure to the offshore site，its installation，and its ability to withstand the severe environmental loading. The chief driving force behind this new technology has come from the oil industry and its need for fixed platforms for exploitation of the extensive hydrocarbon deposits existing offshore. Its use is，however，not limited solely to this industry and it has important applications also for military and navigational purposes。

This chapter is intended to give an introduction to the offshore structural engineerirtg.
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1 Design of Offshore Structures
The design of offshore structures is parallel in many ways with that of land— based structures but has a very special additional requirement that the offshore structures must be constructed in one area and installed in another。

Although it is difficult to catalog the steps taken in the creative design of an offshore structure % the following sequence of events is usually involved:
1。

Identifying need.
2.Evaluating environmental and local site conditions.
3.Making preliminary design proposals with major emphasis given to the method of installation.
4。

Evaluating these in terms of economics，construction and installationdifficulties，foundation requirements，and so on，and selecting final design form.
5.Sizing and detailing the chosen form to carry required loads and environmental forces。

6.Evaluating the design to ensure that it can withstand the loading associated with transportation to and installation at the offshore site。

The need for a fixed offshore structure is usually specified simply as a requirement at a designated offshore site for an operational deck having a prescribed minimum working area and carrying a prescribed minimum weight and loading。

The choice of the structure to support this deck in an economical way depends primarily on the installation procedure and the environmental and local site conditions。

Environmental conditions normally involve the wind, currents, and waves likely to be experienced by the structure，together with possible hazards from floating ice and earthquakes。

Local site conditions involve the water depth and seafloor characteristics, the latter being important for the foundation requirements of the structure。

With the foregoing information available, various preliminary designs can be developed around possible installation procedures based on the required operational loads and rough estimates of the major environmental loading arising at the site location。

In most cases, it is wave forces that dominate the design，although exceptions can exist if floating ice or earthquakes pose severe hazards to the structure。

From these various preliminary designs, some may easily be rejected as economically unfeasible or impractical from construction or installation considerations. Others may require closer evaluation in terms of many steps involved （including cost) in converting the structure from idea to reality。

Finally, a final design form and installation procedure is chosen but the design should be further detailed to carry the necessary loads. The sizing of the structure to carry operational loads is a relatively direct matter，but the inclusion of environmental loading generally requires several trial-and—error iterations because a change in the size of a structural member changes the magnitude of the environmental loading exerted on it and on the structure as a whole. Finally，the structural design must be examined to ensure that it can withstand the forces associated with transportation and placement of the structure at its intended site。

9.2 examples of offshore structures
By far the most common type of fixed offshore structures in existence today is the template or jacket structures illustrated in Fig。

9. 1。

This types of structures consists of a prefabricated steel substructure that extends from the seafloor to above the water surface and a prefabricated steel deck located atop the substructure. The deck is substructure into the seafloor。

These piles not only provide support for the deck but also fix the structure in place against lateral loadings from wind, waves , and currents。

The construction and installation of a template structure plays a central role in its design。

The
substructure is usually prefabricated at a waterside facility and then placed horizontally on a flat-topped barge and towed to its offshore location。

At the installation site, the substructure is then slid off the barge and uprighted with help of a derrick barge and allowed to sink vertically to the seafloor。

Once the substructure is in place, pipe piles are inserted through its legs and driven into the seafloor to predetermined depths。

Then they are cut off at the top of the substructure and the prefabricated deck stabbed into the piles and connected with field welds。

In its completed form, the deck weight is carried entirely by the piles, with the substructure providing bracing against their lateral movement。

A typical oil drilling and production platform is shown in Fig. 9。

1，This structure is located off Louisiana in the Gulf of Mexico. The deck measures approximately 60ft X 120ft and weighs about 2 million pounds including operating equipment The weight of the substructure is about 4 million pounds。

The eight pipe piles driven through the legs of the substructure have outside diameters of 4ft and wall thickness of about lin。

In addition to these, four skirt piles are placed around the base of the structure. All piles are driven 200 to 300ft into the seafloor。

The structure is designed to withstand a resultant lateral force of about 3 million pounds from wind，waves，and currents during extreme hurricane
conditions。

Because the wave forces are greatest near the water surface，this resultant
force acts near the top of the structure。

The structure is therefore also designed to withstand a base—pounds. These loads and moments are five to seven times those caused by extreme winds on a typical 25-story, 300—ft-tall building on land。

For structures designed for waters greater than about 350ft deep, two variations of the basic eight—leg template structures piles have been considered。

The first has been to increase the number of structural supports so that, with skirt piles, the structure can carry additional deck loads and resist the increased lateral loading and overturning moments, A second modification has been based on the idea that, with taller structures and increased base widths, the interior piles become less effective in resisting overturning moments. As an alternative to the eight - pile structure，consideration has thus been given to eliminating interior piles and placing all piles near the four exterior corners of the structure.
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2.2 Gravity Structures
Template structures，as described above, are especially suited to soft-soil regions such as the Gulf of Mexico, where deeply driven piles are needed to fix the structure in place and carry the required deck loadings。

In regions where hard soil conditions exist and pile driving is more difficult, an alternative structural form has been developed which, in place of piles，relies on its own weight to hold in place against the large lateral loads from wind, waves, and currents。

These structures have large foundational elements which contribute significantly to the required weight and which spread this weight over a sufficient area of the seafloor to prevent failure。

Such structures are generally referred to as gravity structures。

In their more popular form，gravity structures are constructed with reinforced concrete and consist of a large cellular base surrounded by several unbraced columns, which extend upward from the base to support the deck and equipments above the water surface。

Structures of this kind were installed in the North Sea during the mid—1970s。

Fig。

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2 illustrates the main features of these structures. This particular structure is referred to as a CONDEEP （concrete deep—water) structure and was designed and constructed in Norway。

One advantage of the gravity structures over the template type is to reduce the time needed for on—site installation。

This is especially important in extreme weather areas such as the North Sea，where unpredictable weather conditions make it highly desirable to limit the construction time needed to fix the structure in place. Another advantage is the very large deck weights that can be carried by the massive concrete columns。

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2.3 Deep-water Design Forms
For water depths greater than about 1000ft, the weight and foundation requirements make the traditional offshore structures less attractive。

Two other
Fig. 9。

2 Concrete gravity platform used in the North Sea
The guyed-tower concept is illustrated in Fig。

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5. It consists of a uniforrr cross-sectional support structure held upright by several guy lines that run to dump weights on the ocean floor。

From the dump
weights, the lines then run tc conventional anchors to form a dual stiffness mooring system。

Under normal operating loads，the dump weights remain on the seafloor and lateral motion of the structure is restrained. However，during a severe storm，the dump weights are lifted off the seafloor by loads transferred from the structure to the dump weights through the guy lines. This action permits the tower to absorb the environmental loadings acting on it by swaying back and forth without overloading the guy lines。

The guyed—tower concept is presently considered to be applicable to water depths of about 2000ft。

Fig. 9。

4 illustrates the tension—leg concept. In this design, vertical members are used to anchor the platform to the seafloor。

This upper part of the structure is designed with a large amount of excessive buoyancy so as to keep the vertical members in tension. Because of this tension, the platform remains virtually Horizontal under wave action。

Lateral excursions are also limited by the vertical members, since such movements cause them to develop a restoring force。

A nunjor advantage of the tension—leg concept is its relative cost insensitivity to increased
water depths. At the present time，it appears that the main limitation on the tension-leg platform arises
from dynamic inertia forces associated with the lateral oscillations 9.2.4 Mobile Drilling Platforms
The underwater search for new resources has been made possible only by vast improvements in offshore technology. Drillers first took to sea with land rigs mounted on barges towed to location and anchored or with fixed platforms accompanied by a tender ship. Since a wide variety of rig platforms have evolved , some have been designed to cope with specific hazards of the sea and others for more
general works. All new types stress the characteristics of mobility and capability for work in even deeper water.
Mobile platforms can be divided into four main types：self—elevating platforms, submersibles, semi—submersibles，and floating drill ships。

The most widely used mobile platform is the self-elevating or jack—up unit It is towed to location where the legs are lowered to the sea floor，and the platform is jacked up above wave height。

These self—contained platforms are especially suited to wildcat and delineation drilling。

They are best in firmer sea bottoms with a depth limit out to 300ft （90m) of water。

The submersible platforms have been developed from submersible barges，which were used in shallow inlet drilling along the United States Gulf Coast The platforms are towed to location and then submerged to the sea bottom。

They are very stable and can operate in areas with soft sea floors。

Difficulty in towing is a disadvantage but this is partially offset by the rapidity of rising and lowering at the designated site.
Semi—submersibles （Fig. 6) are a version of submersibles。

They can work as
bottom-supported units or in deep water as floaters。

Their key virtue is the wide range of water depths in which they can operate，plus the fact that when working as floaters their primary buoyancy lies
below the waves, thus providing great stability。

The " semis” are the most recent rig-type platforms. Floating drill ships(Fig. 7）are capable of drilling in 60-ft (18—m) to abyssal depths. They are built as self—propelled ships or with a ship configuration that requires towing. Several twin-hulled versions have been constructed to give a stable catamaran design。

Floating drill ships use anchoring or ingenious dynamic positioning systems to stabilize their position。

The latter is necessary in deeper waters. Floaters cannot be used in waters much shallower than 70ft because the special equipment for drilling from the vessel is subject to vertical movement from waves and tidal changes，as well as minor horizontal shifts due to stretch and play of anchor lines. For exploration in deeper waters it is necessary to build more semi — submersible and floating drill ships。

Fig. 9。

6 Offshore semi—submersible drilling platform
Fig。

9. 7 Floating drill ship
9. 3 Analysis of Offshore Structures
The design of an offshore structure progresses through various stages from preliminary design based on rough estimates of environmental loadings to a final design form which must be sized to carry the loadings exerted during installation and operation. This sizing is carried out using a detailed analysis of the structure. The magnitude of the loadings as well as the structural response depends on the size of the individual members, so that the analysis of the structure must first be carried out using initial estimated values. Member sizes that are found inadequate in this first analysis are then changed and the analysts repeated. To keep the eventual fabrication of the structure as simple as possible，individual member sizes are usually not optimized for efficiency in carrying the loads。

For example，with a template substructure，the bracing members between the main legs will normally all have the same dimensions even though some will carry less stress than others.
For steel-framed structures - much of the analysis is conveniently carried out by computers using matrix methods as developed for steel—framed buildings. Stresses within the members are calculated and compared with acceptable levels。

With reinforced-concrete gravity structures, the same methods may also be used for determining the internal forces of the individual members and in turn, the adequacy of reinforcement.
To perform such an analysis it is，of course♦necessary to have a reasonable representation of the loads acting on the structure. In certain cases，such as the lifting and launching of a template substructure，these are defined simply by the weights of various members of the structure. Operational loads and in - place structural weight are similarly well defined. The conversion of given environmental site conditions to loads on the structure presents，however, more of a problem and these must be determined by detailed calculations using appropriate formulas.
The most common analysis of fixed offshore structures under combined operational and severe environmental loadings is based on equilibrium considerations of the structure and the maximum
loadings that can be exerted on it at any instant. This static analysis is normally satisfactory for structures in waters less than about 300ft deep since they are generally rigid enough to allow neglect of inertia forces associated with the back and forth acceleration of the structure under the variable wave loadings. In the case of structures supported by piles on soft sediments, the analysis must take into account the interaction of the structure and piles at the seafloor. The vertical loads are used to determine the depth that the piles should be driven to carry them.
Finally, for structures designed for water depths greater than about 300ft or for structures with considerable flexibility，because of their particular form, appreciable inertia forces can exist。

In this case, the design must then be checked for possible over,stress from dynamic loadings. The same is true when earthquake loadings pose a hazard to the proposed structure，as these can cause a shaking of the base of the structure accompanied by appreciable inertia forces，
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4 Environmental Loadings
Before the design of a proposed offshore structure can be analyzed，it is necessary to have quantitative estimates of all the significant loadings that the structure is likely to experience in the ocean environment. For structural engineering purposes, this environment may be characterized mainly by overwater wind, by surface waves and by currents that exist during severe storm conditions.
Overwater wind during storm conditions is significant in the design of offshore structures. Wind speed during hurricane conditions in the Gulf of Mexico can，for example, exceed 100 mph, causing horizontal forces on a typical offshore structure of 100 kips or more.
Surface waves during storm conditions are also of major importance in the design of offshore structures。

Wave heights (difference between maximum and minimum elevations of the water surface at any instant) in the Gulf of Mexico during storm conditions can exceed 50ft and these can induce
horizontal forces of several hundred kips or more on a typical structure。

Finally, currents at a particular site can contribute significantly to the total forces exerted on the submerged parts of the structure。

Currents refer to the motion of water that arises from sources other than surface waves. Tidal currents，for example, arise from the astronomical forces exerted on the water by the moon
and sun，wind-drift currents from the drag of local wind on the water surface，river currents from the discharge of rivers and ocean currents from the drag of large-scale wind systems on the ocean。

During storm conditions，currents at the surface of 2ft/sec or more are not uncommon，giving rise to horizontal structural forces equal to 10% or more of wave-induced forces.
The following is concerned with details of these environmental elements and with suitable engineering estimates of the forces induced on offshore structures。

9.1 Wind Speeds
The forces exerted on a structure by wind depend on the size and shape of the structural members in the path of the wind and on the speed at which the wind is blowing。

The greatest wind speed to be expected at a particular site can be estimated from analysis of local daily weather records。

Because of wind fluctuations over any measuring time，such records necessarily contain averaged wind-speed measurements over a finite interval of time.
9.4.1 Wind Forces
The wind force acting on an ocean structure is the sum of the wind forces acting on its individual parts. For any part such as a structural member, storage tank，deck house, etc。

，the wind force arises from the viscous drag of the air on the body and from the difference in pressure on the windward and leeward sides. The net force on the object is experimentally described by an equation:
where p denotes the density of the air, A denotes a characteristic area of the body, V denotes the wind speed，and C denotes a dimensionless force coefficient depending on the shape of the body and on the viscosity fx of the air through the value of the Reynolds number NR=pVD/u，in which D is a characteristic dimension of the body。

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4. 3 Ocean Surface Waves
Ocean surface waves refer generally to the moving succession of irregular humps and hollows on the ocean surface。

They are generated primarily by the drag of the wind on the water surface and hence are greatest at any offshore site when storm conditions exist there。

For engineering purposes，it is customary to analyze the effects of surface waves on structures either by use of a single design wave chosen to represent extreme storm conditions in the area of interest or by use of a statistical representation of the waves during extreme storm conditions。

In either case, it is necessary to relate the surface wave data to the water velocity, acceleration, and pressure beneath the waves。

This is achieved by the use of an appropriate wave theory。

Buoyant Forces
Buoyant forces arises from the hydrostatic pressure p given by:
where y denotes the specific weight of water，h denotes the depth of water，and v denotes vertical distance from the seafloor。

This force exists even when wave action is absent and must be accounted for separately.
Ice Loadings
Ice loadings can be important in certain offshore site locations especially in Polar Regions, where thick sheets of ice can move with the tide and strike the legs of an offshore structure with significant force.
The force F exerted by ice crushing against the leg of a structure is determined by the equations
where fc denotes the crushing strength of ice，C denotes a force coefficient and A denotes the area struck by the ice。

Typical values of C range between 0. 3 and & 7 and values of fc range between 200 and 500lb/in2。

In the absence of definite experimental information, a value of C/c=350lb/in2 may be employed for extreme conditions。

9.4.6 Mud Loadings
Mud loadings can arise from mud slides in certain offshore sites，particularly in active delta regions where soft soil is continually being deposited by river discharge。

The force F acting on a unit length of pile embedded in sliding soil is expressible
as:
F=NTD
Where N denotes a force coefficient，r denotes the shearing strength of the soil，and D denotes the diameter of the pile。

Typical values of N used in such calculations range from about 7 to 9。

The shear strength of the soft deposit is customarily determined from laboratory tests of soil samples taken in the area of interest。

Typical values are of the order of 100 to 2001b/ft2.
Questions
How many types of mobile platforms are there?
What are general procedure of construction and installation of a template structure？
What are the materials used in build gravity structures?
Why do we consider other new design forms when water depths are greater then about 1000ft?
How many environmental loadings are there in design a offshore structure？
References
［1］Offshore Structural Engineering，Dawson. Thomas. H. Printed in the United States of 。

America。