Deepwater production jack-up offers ample storage and stability

Following the debut application of the TPG 500 platform on BP UK's Harding Field, Technip-Geoproduction has now extended the concept to waters 3,300 ft deep or beyond. The TPG 3300 is a fully integrated floating, self-installing, catenary moored platform with ample space for drilling and oil production facilities and living quarters. It combines the main advantages of the jack-up technology used on the TPG 500 with those of a semisubmersible platform, without the disadvantages of the complex anchoring system required for a tension leg platform. It can also be moved easily to another field when production is exhausted.

Technip-Geoproduction is offering the concept in three or four-legged mode. However, the latter formed the basis of the company's presentation last November at DOT 95, Rio de Janeiro.

Above its legs, the TPG 3300 has a hollow base and a watertight hull which supports all necessary equipment. The hull can be jacked up and down the legs.

The four square columns are composed of a lattice structure and buoyancy tanks to ensure platform stability. The legs are secured to the hull structure by the locking/jacking systems. Depending on the length of these buoyancy tanks, oil can be stored in their lower part.

In the center of the steel base is an open space, likened to a square donut. This hollow base enhances platform stability, and by adjusting its dimensions, allows optimization of heave behavior (effect pile/pontoon). Due to the combination of the four legs and the immersed base, heave, roll and pitch response is low, which in turn means that wellheads can be placed on the deck.

The platform is anchored to the seabed by a conventional catenary mooring system. It can be adapted easily to varying environmental conditions and can be designed to carry heavy or medium weight topsides.

During operation, the TPG 3300 floats on its buoyancy tanks and the base is completely ballasted. Distance between the hull and base is constant and independent of water depth. Environmental conditions determine the airgap clearance.

The platform has either ordinary columns with storage inside or columns consisting of a closed section in the upper part and an open lattice structure in the lower part. A deep draft floater is relatively insensitive to the increase in topside loads and can also withstand variations in the location topside of the center of gravity, as there is ample space for ballast in the lower part of the columns/pontoon.

For favorable environmental conditions, the design with oil storage inside the legs is an attractive option. If a deck weighing 21,000 tonnes is specified, motion optimization leads to a platform with the following dimensions:

base volume: 33,000m3

waterplane area: 1,156m2

immersed length of buoyancy tanks: 80 meters

draft: 86 meters

total displacement: 127,46 1t.

Storage capacity for this design is approximately 270,000 bbl of oil.

Motions obtained with a high frequency simulation in a 100-year storm condition, as encountered in the Gulf of Guinea (Hs=3.7m, Tp=16s) are:

Heave:

double significant amplitude = 0.36 meters

natural period = 30 sec.

Roll:

double significant amplitude = 0.64

natural period = 44s.

For a 100-year storm in North Sea conditions (Hs=14.4m, Tp=18.2s), wave frequency motions are:

Heave:

double significant amplitude = 0.66m

natural period = 32s

Roll:

double significant amplitude = 2.15

natural period = 65.48s.

In both cases, the low frequency effect remains weak owing to the value of the natural periods.

General arrangement

According to Technip-Geoproduction, the platform arrangement has to meet the following design conditions:

highly integrated safety features to allow for drilling, production and living quarters on a single platform
optimization of the deck area
installation inside the hull structure of all equipment which is field independent and non-hydrocarbon processing
easy installation of main equipment on the top main deck, and easy access of production/process equipment for future modifications.

Typically, all industrial fluids are stored inside the hull at the capacity deck level. On the machinery deck are situated all general platform and drilling utilities (mud treatment).

On the main deck, on the platform stern, process equipment for both oil and gas is arranged around the moonpool which is surrounded by a U-shaped fire and blast wall. A general fire blast wall completely separates this area from the bow side where non-hazardous facilities are installed such as power generation, living quarters, helideck and lifeboats.

Space located inside the legs is also used. The living quarters, for example, are placed on top of one leg. The helideck is installed on the very top level.

Over the stern area above the moonpool is the drill floor, and a pipe rack is attached. The process area is located at the top view level where the flare is also installed. Hydraulic packages for the locking/jacking mechanisms are fixed along the jack-houses.

The closed leg columns are subdivided by bulkheads to provide stability should damage occur. Pumps and ballast tanks are installed in the lower part of the buoyancy tanks. Compensatory ballast compartments are used to counterbalance changes in weight caused by variable loads.

A typical four-legged TPG 3300 with a 21,000-ton deck would be configured for oil production of 80-100,000 b/d; water injection at 151,000b/d; gas compression (lift and export) at 71 million cf/d; and a living quarters for 60 persons. However, the concept can also be modified for lighter facilities to meet specific project requirements.

To export oil, the platform with oil storage facilities will be connected by flexible risers to a subsurface buoy hooked up to a tanker. The advantage of storing oil in the lower part of the legs is to decrease the frequency of the shuttle service. For a TGP 3300 with the lattice structure, oil will be exported by a sea line.

The proposed locking/jacking system has already been deployed on the TPG 500 on BP's Harding Field. The locking system allows structural redundancy, avoids operating difficulties experienced by classical locking systems, and to a large extent simplifies and reduces mechanical pieces for easier maintenance.

Dynamic forces on the jacking system are eliminated, with better stress distribution. The locking system also ensures a rigid connection between legs and hull with a smooth stress distribution, while the axial load is absorbed in the leg chords.

Mooring system

The station keeping system consists of 12 mooring lines arranged symmetrically around the platform perimeter. Each corner of the platform accommodates three catenary lines.

Depending on environmental conditions, these lines can be fixed either to the hull or to the base (if access on all sides is required). Alternately, they may be fixed to the legs to limit static inclination.

For a deepwater field, an optimal solution is to use a catenary line composed of a chain/wire combination with drag-embedded anchors. But pre-installed anchor lines may also be used.

The platform's allegedly excellent behavior would allow placement of the wellheads on the hull and would limit the stroke of the riser tensioners. This configuration eases maintenance and is significantly cheaper than installing them on the seabed at such depths.

Tensioning loads are designed for up to 20 wells with the tensioning system located on the machinery deck. Tensioners are designed to maintain a constant tension in the riser whatever motion occurs.

A hydro-pneumatic system is used to accommodate movements between the platform and the riser. Tensioner stroke depends on the horizontal motions of the platform and on heave, roll and pitch motions. Most other effects impacting riser stroke, such as tidal motions, would occur relatively slowly.

Construction & tow-out

A key advantage of the TPG 3300 is that it does not require a deepwater construction site, and it can be built at any yard with sufficient acreage to construct the base and hull. The latter are an assembly of square pontoons fabricated from flat panels which can be automatically welded.

Skid beams are installed on the quay and on the floating base. When fabrication of the upper hull has been completed, and all equipment has been installed, the hull can be transferred onto the base.

The weight of the hull with a small part of the legs is supported by jacks and transferred onto the base. Compensation for weight transfer is achieved by deballasting the base which will in turn maintain the top of the base level with the quay during transfer of the hull onto the base.

The lower part of the legs is welded to the base while the upper part is installed gradually, adding weight and thus increasing the draft. The erected legs are secured to the hull structure by the locking systems.

Prior to departure from the yard the platform may be tested and pre-commissioned. It can leave the yard with a draft of less than 10 meters. Upon reaching deeper water, the base is partially ballasted to reach the towing draft.

TPG 3300 has also been designed to allow towing with a draft of less than 12 meters. The legs are secured in elevated position by the locking system which increases the strength, leading to improved safety during towing. The jacking systems are pre-stressed to strengthen the connection between the base and the hull and to relieve loads on the locking systems.

On arrival at the field, the anchors are positioned and pre-tensioned; the locking system is then released and the base slowly ballasted. At the same time the legs are lowered and guided into the sea by the jacking mechanism, located within a jack-house on the hull.

To avoid over-dimensioning the jacking system, the apparent load of the immersed structure is kept low and constant by balancing with ballast in the base first and then in the legs. The base will be completely full at a limited water depth, and consequently does not have to be designed to withstand the high pressures encountered at deeper draft.

When the required distance between the bottom of the hull and the base is reached, the legs are fixed in that position by the locking system in the hull structure. By deballasting the legs, the draft of the complete structure is reduced whereby the hull is raised out of the water to a pre-determined height which keeps it out of range of the highest waves.

During installation, the TPG 3300 should not suffer any instability as occurs with a regular deep draft semisubmersible during installation. Upon demobilization at the end of the field life, the installation procedure is simply reversed, and the platform may be towed to a yard for modifications for re-use or for salvage purposes.