The open-space deck is one of three topsides concepts Saga and Aker are studying as part of the triangular TLP project.
Recent model tests of the triangular tension leg platform (TLP) concept being developed by Saga Petroleum and Aker Engineering have provided encouraging confirmation that the project is proceeding along the right lines.
The model was subjected to a fortnight of tests in Marintek's deepwater tank, including simulated 100-year storms, in simulated water depths up to 1,250 meters. "The results were very close to what we expected," commented Jan Muren, a senior marine engineer with Aker Engineering. The project is an attempt to overturn the TLP's reputation as a high-cost solution, and capitalize on its major benefit - the location of wellheads on the platform rather than on the seabed - in a deepwater context. Saga does not yet have applications for the triangular TLP, but it moved a step closer to finding one with the award earlier this year of three deepwater licences off mid Norway, including one which it operates in waters 1,200-1,400 metres deep.
Together with Aker, which did its first study on the TLP concept 20 years ago, Saga set out to identify the high-cost factors and see what could be done about them.
The companies' main points of reference are the Snorre TLP installed on the Norwegian shelf in 1992, and the Heidrun TLP installed last year. Snorre has a steel hull, Heidrun a concrete hull, both platforms have four columns and both involved an expensive and lengthy fabrication process.
The tether system turned out to be the major high-cost element, and the triangular shape the solution to it. With a three-legged structure, an equal distribution of tension naturally takes place, giving the platform a stability which is well suited to rough sea states, Muren says. There is no need, as there is with four-column TLPs, for a complicated, costly and time-consuming installation operation to achieve the required distribution of tension. For the same reason, the platform is also less sensitive to foundation settlement and creep effects.
Modifications have also been made to the top of the tether which considerably simplify the task of attaching it to the leg. All in all, the installation of a triangular TLP should prove to be much simpler, faster and cheaper than Snorre or Heidrun was.
Other benefits also flow from the triangular concept. It has no need, for example, for the diagonal stiffeners required by the four-leg hull. According to Muren, this leads to a 5-15% saving on structural steel. Optimization of the topside design has also provides savings in structural steel. A TLP with the same production capacity as Snorre would require a deck weighing 10-15,000 tonnes less, according to Saga staff engineer Tom Marthinsen. The overall conclusion so far is that a triangular TLP with a production capacity twice that of Snorre's would cost about half as much.
A further round of optimization will now take place, with the aim of cutting fabrication time by 50% compared with the time taken to build the Snorre platform. In conjunction with Maritime Hydraulics, the design team are looking at a light drilling derrick which would weigh only half as much as a conventional derrick. Its exposed area would also be much reduced, an important factor given the high wind loads off mid Norway.
Three topside concepts are under study: an open-space concept, a wet truss deck (with two decks), and a dry truss deck. The open-space concept, in which all the topside equipment is located over the column areas, and the interior part of the triangle is left open, provides the most savings in structural steel.
It is also most limited in terms of the topside weight it can bear. But this is true of the triangular design in general, in comparison with the four-legged concept. The present configuration probably has an upper limit for the topsides weight of around 50,000 tonnes. But since this configuration has a process capacity of 60,000 cm/d (375,000 b/d), it is clearly more than sufficient for most applications, and certainly more than, say, current production ship concepts can achieve. There is also a limit on the number of risers that can be accommodated.
Interest in the triangular TLP has recently been expressed by BP, itself in the throes of preparing for deepwater production. Its priority, however, is the concrete-hulled version. This was examined by Saga and Aker at an early stage in their project, as was a hybrid TLP with steel columns and a concrete pontoon.
The conclusion was that for Saga's purposes, the steel-hulled concept was most appropriate. According to Marthinsen, concrete could provide the best solution in circumstances where field life was long and the environment rough, but the water depth not extreme. "The concrete hull has attractive features in a very fatigue-dominated environment," he says. "In very deep waters the steel hull shows more predictable behavior due to its lower weight and shallower draught."
Copyright 1996 Offshore. All Rights Reserved.
