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Offshore Hydromechanics J.M.J Journée

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or interest during this mating phase of a GBS are:
- the positioning of the top-side relative to the main structure and
- the impact loads between the top-side structure and the main platform structure.
Both these aspects require very mild environmental conditions during the actual mating.
Then the complete platform is towed out to the work location by 4 to 6 tugs. Aspects of
importance or interest during this towing phase of a GBS are:
- the towing force,
- the course stability under tow,
- the ’keel’ clearance at restricted water depths, in‡uenced by vertical platform motions,
- the wave induced motions,
- the motions due to vortex shedding and ‡ow separation around slender columns and
- the current loads.
At the work location, the platform is being ballasted down to the sea bed. Aspects of
importance or interest during this ballasting down phase of a GBS are:
- the positioning accuracy,
- the vertical motions on setting down and
- the horizontal wave, wind and current forces.
Floating Production Units
Floating Production units (FP) are used for oil production at smaller …elds. In very deep
water they are also economically attractive for large oil …elds. The production equipment
and accommodation is placed on a ‡oating structure, permanently moored to the sea bed.
They have no storage capacity. The oil is o¤-loaded to a Floating Storage and O¤-loading
unit (FSO). Figure 1.10 shows an overview of this.
Figure 1.10: Floating Production Unit with External Storage Facilities
Two types of ‡oating production platforms without storage capacity can be distinguished:
the semi-submersible and the tension leg platform (TLP). Production on a TLP in remote
areas - so that a pipe line is uneconomical - requires external storage capacity, for instance
by using a storage tanker moored nearby.
A Semi-Submersible Platform consists of a rectangular deck structure supported by 4
to 8 surface-piercing vertical columns standing on submerged horizontal ‡oaters. These
vessels have good motion characteristics and do not require the heading changed as the
predominant direction of the weather changes. The vessels are moored by means of 8 to
12 catenary mooring lines consisting of chains or combinations of chain and wire. Parts
of the pipe lines transporting the oil to the ‡oater have to be ‡exible to allow for the
wave induced motions of the ‡oater. These ‡exible pipe lines have to be su¢ciently strong
and resilient to withstand high pressures and temperatures of the crude oil as well as the
continual ‡exing due to the ‡oater motions; see …gure 1.11.
The aspects of importance or interest are generally the same as those for drilling semi’s.
However, the production semi-submersible will be permanently moored which means that
- consequently - more stringent demands are placed on the design. Additional aspects of
importance or interest of semi-submersible production platforms are:
- the behavior of bundles of ‡exible ‡ow lines and risers and
Figure 1.11: Semi-Submersible Production Platform
- the stresses in the individual ‡exible ‡ow lines.
A Tension Leg Platform (TLP) consists of a semi-submersible type hull with for instance
four vertical surface-piercing columns standing on underwater ‡oaters and supporting a
large rectangular deck; see …gure 1.12. At each of the four corners of the ‡oater, pre-
tensioned tethers extend vertically downwards to foundation templates which are piled
into the sea bed. Due to the vertical tendons, which are pre-tensioned to such a degree
that they never become slack, any vertical motion of the TLP will be eliminated. This
allows for steel pipe line connections between the wells and the ‡oater, without the need
for ‡exible sections of pipe lines. As a result, it is possible to install the well head control
valves on the deck of the ‡oater instead of on the sea bed. This represents a considerable
advantage from the point of view of ease of maintenance and investment.
The installation phases consist of construction of the ‡oaters in a dry dock inshore, ‡oat
out, deck mating, tow out to the production …eld and the …nal installation. Aspects of
importance or interest during the installation phase of a TLP are:
- the avoidance of vessel impact loads during the deck mating operation,
- the horizontal positioning accuracy during deck mating and hook-up of the tethers and
- the towing resistance.
Aspects of importance or interest in the installed condition of a TLP are:
- the avoidance of wave impacts on the under side of the deck,
- the wave and low frequency horizontal forces and motions,
- the wave and current induced vortices,
- the avoidance of slack tethers,
- the maximum tether tensions,
Figure 1.12: Tension Leg Platform
- the behavior of the bundles of the tethers,
- the super-harmonics in tether tensions (springing) and
- the current and wind forces.
Floating Storage and O¤-loading Units
Permanently moored Floating Storage and O¤-loading vessels (FSO’s) are used to store
the produced crude oil. Periodically, the oil is collected and transported to shore by means
of shuttle tankers. For the o¤-loading operation, the shuttle tanker is often moored either
in tandem with the storage vessel or alongside.
Sometimes the stored oil is piped to a Single Point Mooring system (SPM) some distance
away, to which the shuttle tanker is temporarily moored. A Spar buoy mooring system is
an example of this combined o¤shore storage and mooring facility; see …gure 1.13.
Generally, due to costs aspects, existing tankers - with sizes ranging from 80 to 200 kDWT -
have been used as storage vessels, permanently moored in the neighborhood of a production
platform or a group of platforms. When converting oil tankers for this purpose, special
attention has to be paid to the remaining fatigue life of this older vessel. Nowadays, the
number of suitable tankers on the market is relatively small and a trend toward purpose
built storage vessels is discernible.
A factor of prime importance for the operation of these vessels is the continued integrity of
the mooring system and of the pipe line carrying the crude to the storage vessel. Another
important aspect is the operational limit of the crude oil transfer operation between the
storage vessel and the shuttle tanker. Both these design requirements are mainly deter-
mined by the wind, wave and current induced motions, the mooring forces of the storage
vessel and those of the shuttle tanker.
Aspects of importance or interest of an FSO are:
Figure 1.13: Mooring to an SPM and a Spar Buoy
- design loads of the mooring system for extreme sea conditions (100-year storm),
- the environmental loads on and fatigue aspects of mooring system components,
- the loads in the shuttle-to-storage vessel mooring lines,
- the mean and low frequency environmental forces on the storage vessel,
- the operational limits with respect to shuttle tanker loading,
- the approach manoeuvre of the shuttle tanker,
- the installation aspects of the storage vessel mooring and
- the behavior of (‡exible) ‡owlines in extreme conditions.
Floating Production, Storage and O¤-loading Vessel
A Floating Production, Storage and O¤-loading vessel (FPSO) is generally based on the use
of a tanker hull, which has been converted for the purpose. Such vessels have a large storage
capacity and deck area to accommodate the production equipment and accommodation;
see …gure 1.14. When converting old tankers for this purpose, special attention has to be
paid to the fatigue life of the vessel.
Motion characteristics of such vessels are acceptable as long as the vessel can ’weathervane’
with the predominant direction of the wind and the waves. This requires that a single point
mooring system be used by means of which the vessel is e¤ectively