NO20161645A1 - Slim twisted jacket for unmanned wellhead platforms - Google Patents

Abstract

A steel lattice tower structure for supporting minimum offshore facilities, formed by two parts twisted 45 deg with respect to each other, the upper and lower parts having respectively vertical and battered legs, connected to each other by a transition truss located approximately 40% down of the water depth. The present invention also includes a procedure for fabricating and installing the entire structure as two separate units assembled offshore.

Description

Tubular steel lattice drilling tower structure

Ambit of the invention

The present invention referred to hereafter as SLIMTWIS presents a tubular steel lattice tower structure formed by two parts twisted 45 deg with respect to each other, which is aimed at supporting a minimal offshore topside and guiding vertical conductor pipes in medium water depths typical of the North Sea.

General disclosure of the invention

This invention extends the concept of the twisted base previously implemented on the UKCS in water depths of about 90m (Elgin-Franklin, Shearwater, ETAP, Culzean) to deeper water up to 150m which correspond to the present limits of the most advanced jack-up (JU) drilling units.

Minimal unmanned wellhead platforms (UWPs) have recently been promoted by the Norwegian Authorities as analternative to subsea templates for tapping reserves at the periphery of a main reservoir initially produced via the heavy facilities of a central complex. Whereas a UWP jacket may remain slightly more expensive to fabricate and install than a subsea structure in terms of capital expenditure (CAPEX), it is expected to be cheaper to operate and maintain (OPEX) throughout its lifetime. A UWP facility where all equipment is located above sea surface also

provides a higher degree of reliability and safety compared to a subsea template.

The purpose of this invention is therefore to widen the application range of these UWP structures, allowing themto match the capacities of the most recent harsh environment jack-up units.

Despite, but also because of its simplicity, a minimal UWP platform needs to face the following challenges

1. A UWP must be economical by comparison to a subsea template alternative. Therefore material, fabrication and installation costs of this jacket are kept to a minimum. In particular, its upper part is standardized for all installations on the same field, whereas only its lower part is slightly adapted to the particulars (bathymetry, soil conditions) of its specific location on the field.

2. A UWP is by essence a lightweight structure, by comparison to a more conventional offshore jacket where heavy gravity loads ensure that the foundation experiences essentially one-way compressive cycles during storms. Conversely the environmental loads which prevail on a UWP cause large overturning moments and generate two-way tension-compression cycles, causing a friction fatigue phenomenon which may considerably downgrade the capacity of the soil surrounding the foundation. To that respect it is essential to make the jacket as transparent to the wave as feasible, especially across the splash zone where water particle kinematics combined with the lever arm with respect to seabottom are highest.

3. The UWP must provide adequate protection to the wells against accidental high energy collision by a drifting supply vessel. To that respect sufficient clearance must be maintained between the peripheral belt of the jacket and the conductor pipes (CPs), which are therefore grouped in a compact array at the centre of the jacket.

4. Conversely the slots must remain sufficiently close to the work-over jack-up in order to minimize the outreach of the cantilevered rig. This is achieved by designing the upper part of the UWP jacket with vertical faces, one of them parallel to the hull of the JU.

5. The respective foundations of the UWP jacket and of the JU (spud cans) must remain sufficiently remote from each other in order to avoid interfering. This is achieved by rotating the lower part of the jacket by 45 deg, in order that the foundation facing the JU at one corner of the jacket (pile cluster or suction bucket) is located midway between two spudcans.

6. Keep equipment layout simple. Here the main rows of the topside are aligned with the rows of the upper partof the jacket, also creating wide open bays facing the JU.

STRUCTURE DESCRIPTION

The SLIMTWIS jacket structure essentially consists of two tubular lattice towers, the upper one being twisted 45 deg with respect to the lower one.

Upper Part

The upper part of the jacket is of constant square section approximately 14.0m x 14.0m with vertical legs located in the NW, NE, SW and SE corners of the square running down to the interface level located approximately 40% down of the water depth. The four vertical faces are braced by X-diagonals, which should provide adequate protection for boat impact across the splash zone, whereas horizontal elevations guiding the CPs are diamond shaped.

It is anticipated that this upper structure remains identical for all UWPs to be installed on the same field as long as the properties of the hydrocarbon reservoir are uniform and therefore require identical equipment at topside including risers and umbilicals.

Lower Part

The lower part consists of a traditional jacket characterized by a square plane cross-section with sizes increasing linearly with depth. This part is twisted 45 deg with respect to the upper part and the battered legs are located in the N, W, E and S corners of the square. The four battered faces and the horizontal elevations guiding the CPs are braced by X-diagonals, except at top interface where diamondshaped diagonals are used to provide continuity with the pattern on the upper part.

The top of this jacket constitutes the transition where two large diameter bracings which initiate at the base of one leg of the upper part frame into the top of the two adjacent legs of the lower part, respectively. These bracings are contained in the plan of the battered face of the lower jacket.

An identical batter of about 1/8 is optimally enforced for all structures aimed at the same field, with only the width of the square footprint requiring adaptation to suit the bathymetry at the exact location. At most North Sea locations, the variations of water depth over a same field seldom exceed ±5.0m, therefore lower bases should also be virtually identical to each other.

Foundations

The foundation at each corner of the square footprint may consist of either

− A traditional cluster of 2 to 4 piles, depending on soil characteristics and water depth. Each pile is driven to target penetration by an underwater hammer, then grouted to the jacket sleeve,

− A suction caisson driven into the seabottom by pressure differential.

Under both configurations, the foundation is principally designed to resist the twoway tensile-compressive cycles generated by severe storms which cause severe downgrading of the soil characteristics.

Apputrenances

In a standard configuration, the upper part consists of a 14.0m x 14.0m square and accommodates 10 x 30ӯ CPs in a 4 4 2 arrangement. The CPs are guided by diamond-shaped bracings at the top, mid-height and bottom of this upper part, resulting in freespans of approximately 26.0 m.

A larger variant of this structure would consist of a 21.0 m x 21.0 m square accommodating 16 x30ӯ CPs in a 6 6 2 2 arrangement.

The CPs are guided at the main elevations of the lower part of the jacket by horizontal X-bracings running between the corner legs, resulting to freespans between 28.0 and 36.0m.

The jacket can additionally accommodate one or two caissons and/or J-tubes plus several small diameter risers. Rather than running the risers along the legs at tight spacing to prevent their vortex-shedding vibration, these are clustered around the J-tube(s), forming a riser ladder with the vertical J-tube as spine.

Typical Dimensions

The figures below are representative of a SLIMTWIS structure accommodating 10 slots in 112m water depth typical of Central North Sea. Whereas these figures are believed to be realistic, they are only indicative and would need to be adapted to suit the particulars of a given location in terms of bathymetry, environmental and geotechnical conditions and reservoir characteristics.

− Square upper part 14.0m x 14.0m,

− Interface between the two parts El. –42.0m,

− Square footprint at seabed 32.0m x 32.0m,

− Lower legs batter ≈1/8

− CP array 4 4 2 @ 2.50m spacing

− CP guides El. 11.50m, -15.25m, -42.0m, -73.0m and –106.0m,

− 100-year extreme wave height 28.0m

− Soil conditions dense sand & very stiff clay layers

− Foundation four clusters of 3 x 96”OD piles driven 50m below seabed, − Operating weight topside 1000 mT,

− Not to Exceed (NTE) jacket weight 4200 mT

− J-Tube 1 x 28”Ø

− Risers 1 x 12”Ø multiphase export 1 x 10”Ø gas injection import

− Caisson 1 x 24”Ø open drain

FABRICATION & INSTALLATION

Fabrication

It is anticipated that the upper and lower parts of the jacket will be fabricated separately, possibly on two different construction yards.

Standardization of the upper part in particular permits that specific details are produced in series. In particular some critical nodes like those in the transition zone may be cast instead of welded, which would considerably enhance their endurance to fatigue.

Load-Out & Transportation

The upper and lower parts can be assembled together on the yard and the entire jacket loaded out horizontally by trailers on the transportation barge. A heavy seafastening will be required in order to secure the two pile clusters (or buckets) highest up in the air, as well as cradles to support the upper part rotated by 45 deg.

An alternative to this single-piece load-out and transportation would consist of loading and shipping the two parts separately

− the upper part horizontally and flat, which requires minimum seafastening because of its comparatively small and constant cross-section,

− the lower part vertically, a procedure previously implemented for the “Gyda” jacket which demands less seafastening and also eliminates potential additional permanent bracing on the structure.

Lifting and Upending

It is expected that the entire jacket can be lifted and upended in one piece by the twin cranes of a semisubmersible crane vessel (SSCV) even in its tallest configuration. On-bottom stability may however prove to be critical here due to the comparatively light weight of this structure, therefore tight weather windows must be imposed for the entire operation to proceed safely.

Conversely these on-bottom stability restrictions do not hamper the installation of the lower part as a separate unit, which could even be lowered vertically by the single crane of a monohull crane vessel before piles are driven and grouted, probably without the need of grippers. The upper part is then stabbed on top of it either immediately afterwards, or during a second campaign if the lower part is first used as underwater template for pre-drilling the wells. On-site connection between the upper and lower parts of the jacket can be achieved either by grouting (as successfully implemented earlier on the Kvitebjørn jacket where the interface was much deeper at El. –145.0m) or by mechanical swaging.

Claims (5)

PATENT CLAIMS

1. Tubular steel lattice tower structure formed by two parts twisted 45 deg with respect to each other, which is aimed at supporting a minimal offshore topside and guiding vertical conductor pipes in medium water depths up to about 150m.

2. Upper jacket part of constant square cross-section with vertical legs supporting the CPs by diamond-shape braces.

3. Lower jacket part of square cross-section with sizes increasing linearly with depth and battered legs supporting the CPs by X- braces.

4. Transition between the two parts located approximately 40% down of the water depth, which is constituted by pairs of large diameter braces connecting the base of each leg on the upper part to the top of the two adjacent legs on the lower part.

5. Procedure to fabricate and install the entire jacket, either as a single unit assembled onshore, or as twoseparate parts connected at the offshore site.