Figure 4 Examples of types of offshore structures: (a) is a jack‐up placed alongside a jacket structure, (b) is a jacket structure, (c) is a concrete gravity‐based structure and (d) is a semi‐submersible unit.
Source: Sundar, V. (2015), Ocean Wave Mechanics: Application in Marine Structures, © 2015, John Wiley & Sons.
Figure 5 Various forms of wind turbine substructures: (a) suction pile caisson, (b) gravity‐based concrete foundation, (c) monopile, (d) tripod, (e) fixed steel structure (jacket), (f) tension leg platform, (g) spar buoy.
Source: Bhattacharya, S. (2019), Design of Foundations for Offshore Wind Turbines © 2019, John Wiley & Sons.
Fixed platforms typically are of the type:
fixed steel structures (jacket structures), which are mostly pile supported or suction anchor supported;
concrete gravity‐based structures;
jack‐ups; or
monopile.
Floating platforms are in general dependent on their watertight integrity and station keeping in addition to their structural integrity to function. The most used types of floating structures in the offshore industry include:
semi‐submersible platforms (mostly in steel, but one in concrete exists);
tension leg platforms (mostly in steel, but one in concrete exists);
ship‐shaped platforms and barge‐shaped platforms (mostly in steel, but a few in concrete have been made); and
spar platforms.
Structures must adhere to regional regulations (stationary structures) or to flag state regulations in combination with class society rules (mobile units). So‐called mobile offshore units (MOUs) are typically at a location for a limited time and inspection and repair are often performed at a yard at regular intervals. These yard periods allow for inspection and repair to be performed in controlled circumstances, often in dry conditions or benign conditions. In comparison, inspection and repair of stationary structures requires inspection and repair to be performed in an offshore environment and often underwater by an ROV. Hence, inspection and repair are rather different for these two types of structures, both with regards to the regulatory regime and the environment in which the inspection and repair is performed.
1.3.1 Fixed Steel Structures
Fixed steel structures (jackets) consist of a steel spaceframe piled to the seabed (Figure 4 b), supporting a deck with space for drilling rigs, production facilities and crew quarters. Fixed steel structures are also used as a substructure for wind turbines. Steel jackets are usually made of tubular steel members. A typical six‐legged steel platform is shown in Figure 4 b. Historically, the piles were driven directly through the legs and into the seabed. In more recent platforms the piles are typically connected to the legs by pile sleeves where the annulus between the sleeve and the pile is grouted. In a few cases, a specialized suction pile (bucket foundation) has been used, for example the Norwegian platforms Draupner 16/11‐E and Sleipner T. In addition to the basic structure there are frames for the conductors, j‐tubes, risers and caissons needed for production and operation. J‐tubes are typically used to enable small diameter flowlines, electrical cables or pipeline bundles to be connected to the topside facilities. Steel structures are subject to ageing processes such as fatigue and corrosion and hence life extension is a key issue with regard to the structure itself.
Jack‐ups (Figure 4 a) are self‐elevating units with a buoyant hull and several legs which when on location can be lowered to the seabed and raise the deck above the level of the sea thus creating a more stable facility for drilling and/or production. During operation in an elevated situation, jack‐ups behave like a fixed platform. Jack‐up rigs have been primarily used for exploratory drilling, but there are a few instances where they have also been used for production.
Monopiles (Figure 5 c) support the deck on a single pile / tower. These types of structures are particularly used for offshore wind energy production.
1.3.2 Floating Structures
Semi‐submersible units (Figure 4 d) have hulls, together with columns and pontoons, with sufficient buoyancy to enable the structure to float. Semi‐submersible platforms change draft by means of ballasting and de‐ballasting (changing the water level in seawater tanks). They are normally anchored to the seabed by mooring systems, usually a combination of chain and wire rope. However, semi‐submersibles can also be kept on station by dynamic positioning (DP), commonly used for semi‐submersibles as drilling units and flotels in deep water. The hull supports a deck on which various facilities for drilling and production can be installed. Semi‐submersible platforms are also being used for wind energy production, such as on the Kincardine wind farm in Scotland.
A tension leg platform (TLP) is of similar form to the semi‐submersible platform but is vertically moored by tension legs, which decrease the vertical motion significantly. A TLP is designed with excess buoyancy to ensure that the tension in the legs remains in all operational conditions.
A spar platform consists of a single moored large‐diameter vertical cylinder that is supporting the topside or a wind turbine. The cylinder is ballasted in the bottom (often by solid and heavy material) to provide stability. Spar‐type of structures are increasingly being used for floating wind energy production.
A ship‐shaped structure is a floating vessel similar in shape to a conventional ship, primarily used by the production and processing of hydrocarbons. These are often called a floating storage unit (FSU), a floating storage and offloading unit (FSO) or a floating production, storage and offloading unit (FPSO). The steel hull typically consists of plates in a main deck, side shells, turn‐of‐bilge and a bottom shell in addtion to tank tops and longitudinal bulkheads (wing tank bulkhead and centreline bulkhead) and transverse bulkheads. These main platings together form the ship beam. In most cases these plates are stiffened by girders and stiffeners. The plates are exposed to the loading from hydrostatic and dynamic pressure, which is transferred to the stiffeners and girder beams and into the structure. Ship‐shaped structures are usually moored to the seabed by chains or wire ropes, although they can also be held on station using a dynamic positioning system. These mooring systems will require underwater inspection and possibly repair.
Most of the details of floating structures can be inspected from the inside in near dry conditions and are, as such, not a part of this book. However, underwater inspection and repair of the mooring, anchors, outer skin, valves and attatchments are relevant and