Riser tensioning system

Details Riser tensioning system Riser tensioning system

2002-06-19

2004-02-17

Pezzuto, Robert E. (Department: 3671)

Wells

Submerged well

Means removably connected to permanent well structure

C166S350000, C166S346000, C166S367000, C405S224400, C175S008000

Reexamination Certificate

active

06691784

ABSTRACT:

TECHNICAL FIELD OF THE INVENTION
The invention relates to a riser tensioning system for a floating oil or gas production platform.
In particular, the invention relates to a riser tensioning system for use on a deep draft floating production facility of the type illustrated in PCT Application WO99/10230. However, the invention could also be used on other floating platforms.
BACKGROUND OF THE INVENTION
Oil and gas production is taking place in progressively deeper water. In water depths up to about 300 m in the North Sea and about 400 m in the Gulf of Mexico, fixed platforms have been used. In deeper water depths, floating platforms are necessary. Production has taken place from ship shaped vessels, column stabilised semi-submersible vessels, floating spars and tension leg platforms (TLPs).
In all cases, near vertical pipelines bring the oil (or gas) up from the sea bed to the floating platform for processing and onward transmission. These near vertical pipelines are known in the offshore industry as ‘risers’. A problem exists in that risers need to be held constantly in tension against vertical motions (‘heave’) of a floating platform. If the risers are allowed to go into compression, buckling may occur. Thus it has been necessary to use heave compensators to keep the risers under tension.
In water depths greater than 1500 m, the heave period becomes a problem for TLPs. Deep Draft Floaters (DDFs) have smaller motions than conventional semi-submersible vessels, but larger motions than TLPs.
In some floating platforms, such as in ‘spar’ platforms, it has been known to use external buoyancy cans to tension the risers. This technique is described in U.S. Pat. No. 4,702,321. Tensioning with external cans has several drawbacks. The risers are confined in a central vertical duct. Damage from fatigue may be experienced by the risers due to uncontrolled ‘piston’ actions from buoyancy cans and excitation of various modes of vibrations, as well as uncontrolled sticktion phenomena. This may lead to rupture and consequential leakage, fire and explosion with resulting damage to the topside facilities and to other risers. This makes caisson type vessels especially vulnerable. In these vessels, the leakages pass up through the caisson well into the middle of the topside deck installation. TLPs do not have this disadvantage as their risers are suspended freely in the water. In most cases a leakage in the riser system will be dispersed from the TLP by water currents and winds at the surface.
In principle, it is possible to extend (lengthen) tensioner systems developed for TLPs to accommodate the larger heave motions which are likely to be experienced by risers on DDFs and other vessels. However, this creates practical difficulties.
DDFs have slightly less air gap than TLPs between their lowest deck and the sea surface, because there is no “pull-down” from the tethers as the TLP moves off its nominal position. The same effect increases the need for riser “pay-out” for a DDF for the same displacement Additionally, for DDFs, there is a contribution from their significantly larger heave motions. To allow for this larger pay-out/pay-in of risers, (often referred to as heave compensation), the traditional ‘pulling cylinder’ design of heave compensator would become so long that under normal operation, the ‘tensioner ring’ would be partly below water level. The tensioner ring is an assembly connecting the tensioner rods of the heave compensator to the riser. If the tensioner ring is partly below water, this critical connection is difficult to reach for inspection.
To raise this critical connection to above sea level, it would be necessary for the tensioner rods to be longer, so that they would extend up through the deck opening. This would lead to a complex arrangement, with a risk of potential clashes, or loss of valuable area on the production deck or drilling equipment deck. Another expedient for raising the tensioning ring would be to invert the tensioner system, so that it had a ‘rods up’ configuration. This would increase the Xmas tree height above the tree deck; lead to instability in shear and torsion; and possibly lead to a compression/buckling problem with the inverted tensioner rods.
In any case, the tensioner stroke necessary for such longer tensioner systems could be beyond what is practical, reliable and cost effective.
Thus there is a requirement for a riser tensioning system which would be applicable to vessels with larger heave motions than TLPs, and which would avoid the practical difficulties outlined above.
DISCLOSURE OF THE INVENTION
The theoretical background to this invention is described in OTC Paper 11904 (published at Houston Tex. in May 2000).
The invention provides, in a substructure for a floating oil or gas production platform, an arrangement to tension a plurality of risers extending from the sea bed up to the substructure, the arrangement comprising:
i) a conventional hydraulic tensioner/heave compensator for each riser, in which there is a soft spring formed by a piston cylinder combination acting against an accumulator, the heave compensators for the risers being disposed to compensate for vertical oscillations of relatively short period (e.g. from 1 second to about 5 minutes) between the risers and a vertically adjustable Xmas tree deck, and
ii) a vertical position adjustment system capable of intermittent operation to adjust the vertical position of the Xmas tree deck relative to the floating substructure to compensate for longer term changes which would otherwise cause the individual riser's tension or stroke position to depart from its target value/range; the Xmas tree deck vertical position adjustment system being normally located in one particular position within its range of movement to compensate for the longer term changes.
In the foregoing, examples of the relatively short period oscillations referred to in i) are the first order wave motions and normal operational state surge, sway and pitch slow drift oscillations. Examples of the longer-term changes referred to in ii) are an extreme quasistatic horizontal offset caused by severe storm conditions, extreme overlaid oscillations at the critical surge/sway period of the moored substructure (slow drift), or inadvertent flooding of one of the buoyant compartments of the substructure.
It is preferred that the vertical position adjustment system includes stiff hydraulics (in which pistons may be hydraulically locked) which Interconnect the Xmas tree deck and the substructure.
It is further preferred that hydraulic oil is supplied from pressurized accumulators when raising the Xmas tree deck, and bled to a tank when lowering the Xmas tree deck.
In one preferred form the Xmas tree deck has counterbalance means, such that its vertical movements to compensate for longer term changes are counterbalanced, and only minimal force is required to effect vertical movement.
In this form it is preferred that the Xmas tree deck vertical position adjustment system comprises at least three piston cylinder and accumulator combinations acting between the Xmas tree deck and the floating substructure, and in which the three combinations are synchronised to avoid excessive tilt of the Xmas tree deck relative to the substructure.
It is further preferred that the cylinders in the combinations are connected to a single accumulator, so that the Xmas tree deck is sensibly horizontal, and in which there is a rack and pinion mechanism which engages with the substructure to maintain parallellity of the moving X-mas tree deck with the substructure at all times, where rack and pinions engage at least two faces of the deck, at right angles.
In this form it is alternatively preferred that the vertical position adjustment system comprises at least three pulley systems acting between the Xmas tree deck and the substructure, and in which the pulley systems are powered to compensate for longer term vertical changes.
It is further preferred that a part of each pulley system is engaged by a further piston cylinder combination.
The pulley systems m

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