Classification of support structures

Support- or substructures used for the exploration for or the production of oil and gas may be grouped into specific categories according to a number of distinguishing features. Without claiming completeness five possible distinctions are:

• their function

(e.g. drilling, production, compression, flaring, accommodation platforms) • their degree of permanency

(e.g. permanent v. temporary or mobile structures)

Note that instead of permanent the word fixed is also often used to indicate the opposite of mobile; however, the term fixed structure will get a specific meaning in the classification scheme that will be presented and should therefore be avoided as the counterpart of mobile.

• the material from which they are constructed (steel v. concrete structures)

• the type of foundation supporting them

(deep and piled v. shallow and bearing foundations)

• the way in which the loads on the structures are accommodated (e.g. bottom supported v. floating structures).

Classifications of the first and the second type are closely related to the purpose that the structures are intended to serve. The second type combines this with aspects that have a major impact on their design. In the previous section we have chosen this very feature to distinguish two main clusters of man-made structures. Classifications of the third and fourth type are associated with the civil and marine engineering aspects of their design and construction. Classification according to the fifth and final type of distinction is on the other hand based on the structural mechanics features of their behaviour in service. Categorisation on the basis of load carrying mechanisms is a very useful and robust one and is our preferred choice for the classification of substructures. The loads on offshore structures are very large and the way in which these loads are accommodated must therefore be a major distinguishing feature.

The other options to distinguish the various types can, of course, also be used whenever these are considered useful to highlight particular aspects for a particular purpose. Using these options will naturally result in different classification schemes that are, however, equally valid. It should further be recognised that, whatever scheme is chosen, distinctions will not always be pure. In several cases there will be combinations and overlaps of features. If this is the case, then the predominant feature should be used for the classification.

The loads on an offshore structure derive from its function, from its own weight including all the facilities and equipment that it carries, and from the environment. All loads are either vertical or horizontal (which is also called lateral), and they may be essentially constant (i.e. static) or they vary appreciably as a function of time (i.e. they are dynamic). In this regard, variations in time

Chapter 2 – A classification of offshore structures 2-4

Figure 2.3. Full matrix of possibilities to accommodate load types by support types

are usually measured with reference to the periods of the waves, which are between some 2-3 and some 25 seconds, because waves tend to be the predominant dynamic loading source on an offshore structure. The static vertical loads on the structure due to weights always act downwards, while buoyancy forces always act upwards.

In view of the definition of offshore technology, all structures are associated with an approximately fixed location. Therefore, they must be in a state of equilibrium. The equilibrium can be a static equilibrium or a dynamic equilibrium. In the case of static equilibrium all applied loads must be transmitted through the structure to "supports", where they are resisted by reaction forces through spring actions. In the case of dynamic equilibrium the applied loads can also be balanced by mass inertial forces, which are self-generated as a result of induced movements. The net effects of the applied loads and the mass inertial forces together (the resultant of which may either be larger or smaller than the applied loads) are then similarly resisted by the "supports", again through spring actions. Clearly, static loads will always need to be balanced by static reaction forces in a static equilibrium. Dynamic loads, on the other hand, may be balanced either by dynamic reaction forces at the supports, or by mass inertial forces, or by a combination of the two. The distribution between dynamic reaction and mass inertial forces depends on the dynamic characteristics of the structure and the effectiveness of the "supports". In practice, a combination of static and dynamic equilibrium is often found, e.g. depending on the frequency content of the environmental loads.

In the above, the term "supports" is used in a generic sense. For example, for a structure standing on the sea floor the foundation is an obvious support. However, in a very similar manner the surrounding water provides a buoyancy force equal to the weight of the displaced volume of water for both floating structures and structures standing on the sea floor. This is also a "support" in the generic sense of the word. Other supporting elements are e.g. cables or struts, which may be attached to the structure and thus provide a reactive force. Such elements are components of the total structural system, but they do not form an inherent part of the structure itself. The structure may be thought of as an organised assembly of connected elements that provide some measure of rigidity. These elements can normally not be separated without severely affecting the rigidity of the structure, up to the point that this is likely to damage or even destroy the original assembly. However, the removal of one or more supports will, of course, affect the equilibrium when external loads are present, but it will not damage or destroy the rigidity of the assembly of elements making up the structure proper.

All reaction forces to the applied loads must eventually, directly or indirectly, be transmitted via one or more of the supports to earth, which provides the only fixed point they can go to. As the applied static vertical loads always act downwards they can only be supported by the sea floor or by buoyancy, or by a combination of the two. Cables or similar slender constraints in tension can additionally resist a resultant upward vertical force that may exist in some special cases. The applied static lateral loads can also be resisted by the sea floor; alternatively, lateral restraint can be provided by cables or similar slender elements acting in tension and anchoring the structure in a fixed average position relative to earth. There is one possible extension to these options. That is when thrusters, mounted on a floating platform, provide the counteractive force to keep the platform in position instead of cables. Although such a system is called 'dynamic positioning', the thrusters can only be expected to resist the

Chapter 2 – A classification of offshore structures 2-6 (quasi-) static lateral loads. This is a result of both the magnitude of the dynamically applied loads and the time constant of the dynamic positioning system.

The applied dynamic loads arise from the environment, with waves being the predominant cause. They can act both horizontally and vertically, the latter perhaps being less commonly appreciated. The best examples of dynamic vertical loads are the heave excitation of floaters and the fluctuating vertical loads on the base (the caisson) of a concrete gravity structure. The variable vertical wave pressures on the top of the caisson also create a dynamic moment, which is an additional contribution to the overturning moment that is often wrongly associated with the existence of lateral loads on the vertical columns and the vertical sides of the caisson only. For steel structures the variable vertical wave pressures on horizontal members are much smaller but have in principle the same effect.

The dynamic loads can be resisted by the sea floor, by cables or by similar constraints to which the structure may be attached, or they may be balanced by inertial forces resulting from structure movements. These are the only three possibilities, and although strictly speaking combinations are usual, one of these three is invariably the most important mechanism.

In summary, there are 4 different types of loads and 5 different types of supports, together forming a matrix of 20 possible combinations; see figure 2.3. Not all these combinations are physically possible, however. For example, buoyancy always acts vertically and can therefore never accommodate horizontal loads. Similarly, inertial forces can never balance static loads. These boxes must therefore be excluded and are shaded in the matrix in figure 2.4.

Following these principles, figures 2.5 (a) and 2.5 (b) can be constructed. In this way five different types of structure are distinguished. These are:

• the fixed structure • the compliant tower • the guyed tower

• the tension leg platform (TLP)

• the (catenary) anchored floater (CAF or AF).

The first three of these are bottom founded, the last two are floating structures.

Using the principles underlying this classification system we can conclude the following. If all four load categories are transmitted to the sea floor the structure is a fixed platform. If inertial forces balance the dynamic lateral loads the platform is called compliant. However, the mechanisms that are associated with the dynamic vertical loads create different types. For instance, when these too are balanced by inertial forces due to vertical motions the platform is a floater (catenary anchored or dynamically positioned). On the other hand, when cables or tethers resist them, thus suppressing the tendency of the platform to move up and down, it is a tension leg platform (TLP). If the dynamic vertical loads are transmitted to the sea floor the structure is a bottom founded compliant platform.

Similarly, the basic distinction between a guyed tower and other bottom supported compliant towers is the way in which the static horizontal loads are accommodated. For the guyed tower

Inspection of figures 2.5 (a) and (b) will show all the differences between the five basic types of structure thus distinguished. One note of clarification concerning the static vertical loads should be added. Submerged members of all structures will experience buoyancy forces. Therefore, the static (downward) vertical forces will always be supported by a combination of buoyancy and the sea floor. This is true for fixed structures as well as for compliant and guyed towers. Fixed structures will mainly transmit the vertical loads to the sea floor. Compliant and guyed towers are invariably much more slender and will in many cases need greater buoyancy support. That is the reason that in figure 2.5 (a) (and the subsequent figures 2.6 (a), (b) and (c) to be discussed hereafter) the fixed structure is marked as transferring the static vertical loads to the sea floor while the compliant and guyed towers are shown to transfer the loads partly to buoyancy and partly to the sea floor. However, as explained, this is a soft rather than a hard distinction.

It is believed that this breakdown with respect to load carrying mechanisms is complete and that any proposed structure concept, now or in the future, will belong to one of the five types identified.

Projecting the appropriate combinations of load and load carrying mechanism onto the matrix scheme results in the figures 2.6 (a) to (e), one for each type of structure. Superimposing these five figures onto one schematic gives figure 2.7. This contains 4 empty boxes which, if filled, could potentially result in additional structure types. However, it is easily argued that this is very unlikely ever to occur. The reasoning goes as follows.

Flexible cables or similar slender constraints cannot take up (downward) static vertical loads. In what manner the dynamic vertical loads are reacted will depend on the relative stiffnesses involved. Bottom founded structures will transmit these to the sea floor, because the structure and the foundation represent much stiffer springs than the surrounding water. A TLP will transmit them to the taut and vertical anchoring system for the same reason. The vertical motions of a more or less free floating structure (a CAF) will hardly be affected by the soft spring action of a catenary anchoring system and, therefore, the dynamic vertical loads will be entirely balanced by inertial forces.

Chapter 2 – A classification of offshore structures 2-8

Chapter 2 – A classification of offshore structures 2-10 TYPE OF SUPPORT/ RESISTANCE TYPE OF LOAD TYPE OF SUPPORT/ RESISTANCE TYPE OF LOAD

STATIC DYNAMIC STATIC DYNAMIC

VERT. HOR. VERT. HOR. VERT. HOR. VERT. HOR.

BUOYANCY BUOYANCY PARTLY BUOYANCY PARTLY SEABED PARTLY BUOYANCY PARTLY SEABED CT SEABED FS FS FS FS SEABED CT CT CABLES OR SIMILAR CONSTRAINTS CABLES OR SIMILAR CONSTRAINTS BALANCED BY INERTIAL FORCES BALANCED BY INERTIAL FORCES CT

Figure 2.6 (a). Fixed Structures Figure 2.6 (b). Compliant Towers

TYPE OF SUPPORT/ RESISTANCE TYPE OF LOAD TYPE OF SUPPORT/ RESISTANCE TYPE OF LOAD

STATIC DYNAMIC STATIC DYNAMIC

VERT. HOR. VERT. HOR. VERT. HOR. VERT. HOR.

BUOYANCY BUOYANCY TLP PARTLY BUOYANCY PARTLY SEABED GT PARTLY BUOYANCY PARTLY SEABED SEABED GT SEABED CABLES OR SIMILAR CONSTRAINTS GT CABLES OR SIMILAR CONSTRAINTS TLP TLP BALANCED BY INERTIAL FORCES GT BALANCED BY INERTIAL FORCES TLP

Figure 2.6 (c). Guyed Towers Figure 2.6 (d). Tension Leg Platforms

TYPE OF SUPPORT/ RESISTANCE TYPE OF LOAD TYPE OF SUPPORT/ RESISTANCE TYPE OF LOAD

STATIC DYNAMIC STATIC DYNAMIC

VERT. HOR. VERT. HOR. VERT. HOR. VERT. HOR.

BUOYANCY AF BUOYANCY TLP AF PARTLY BUOYANCY PARTLY SEABED PARTLY BUOYANCY PARTLY SEABED CT GT SEABED SEABED FS FS CT FS CT GT FS CABLES OR SIMILAR CONSTRAINTS AF CABLES OR SIMILAR CONSTRAINTS GT TLP AF TLP BALANCED BY INERTIAL FORCES AF AF BALANCED BY INERTIAL FORCES AF CT TLP GT AF

Figure 2.6 (e). Anchored Floaters Figure 2.7. Superimposition of figures 2.6 a-e

Similar reasoning clarifies the situation with respect to the dynamic horizontal loads. For bottom founded structures the structural stiffness and the foundation stiffness represent two horizontal springs in series. As long as the combined spring action is stiff enough, the dynamic horizontal loads are transmitted through these springs to the sea floor. However, when the combined spring becomes weak, significant motions with the associated inertial forces will develop, creating a largely self-balancing system in a state of dynamic equilibrium; any differential dynamic horizontal force will be accommodated by the sea floor. For floating structures and practical designs of anchoring systems, either of the vertically taut or the catenary type, the horizontal springs are always weak and hence these structures will tend to be self-equilibrating in a state of dynamic equilibrium.

Consequently, the empty boxes in figure 2.7 are almost certain to remain empty forever. They are effectively excluded from the range of possibilities on practical grounds, as opposed to the shaded boxes that are excluded as a matter of principle.

Chapter 2 – A classification of offshore structures 2-12

Permanent installations Platforms

Structure type Structure Fixed Compliant

Tower

Guyed

Tower Tension Leg

Platform Anchore d Floater Total USA – Gulf of Mexico 4000 2 1 5 4000

USA – West Coast 45 45

Central/South

America 340 8 360

Europe – North Sea 400 3 20 425

Europe/Africa – Medit.

100 3 100

Africa – West Coast 380 9 390

Middle East 700 700 Asia 950 19 975 Australia/New Zealand 30 7 40 Total ~ 7000 2 1 8 66 ~ 7000

Table 2.1. Approximate numbers of different types of structure in the platforms group of the permanent installations (1999)

Temporary installations Platforms

Number

Jack-ups 460

Drilling Submersibles 25

Floaters Drilling Semi-

submersibles

200

Drill Ships 50

Drill Barges 35

Total 770

Tabel 2.2. Approximate numbers of different types of structure in the platforms group of the temporary installations (situation around 1998)