Mooring, subsea advances making rigs more competitive for deepwater

Semisubmersibles versus Spars and TLPs

Jul 1st, 2000
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The recently completed Visund semisubmersible for Norsk Hydro. Drilling and intervention as well as production can be carried out from the platform.
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GVA Consultants in Gothenburg, acquired by the BMT Group last year, probably possesses more experience in the design of semisubmersibles than any other organization in the world. The company has developed a series of standard designs for different conditions, which have been used to build a total of 15 semisubmersibles ranging from 13,000 tons up to 85,000 tons displacement. These include five of the nine purpose-built semisubmersible production vessels currently in operation around the world. Recent projects include two Umoe/GVA 8000 semisubmersibles (semis) for Norsk Hydro's Visund and Troll C fields, as well as a Kvaerner/GVA 70 for Statoil's Åsgard B development, all in the Norwegian sector of the North Sea.

However, as the focus on deepwater has intensified over the last few years, semisubmersibles have not been at the top of everyone's list of options. In terms of deepwater field production facilities, it has been the TLPs and Spars installed in the Gulf of Mexico that have been grabbing the headlines.

While GVA Consultants itself has been heavily involved with TLPs in the past - it developed the TLP-1000 for Petrobras in the early 1990s - the company has recently been restating the case for the semisubmersible. GVA Consultants' managing director, Lars Felix, believes that some parts of the industry have had a block with regard to one very basic issue. "Naturally people want a unit capable of being used both for drilling and production. Some companies seem to think that this is a problem with semisubmersibles. Quite simply, this is not the case. Semis, TLPs and Spars have similar deck loading capabilities; in fact, the ratio of payload to the weight of the structure for semisubmersibles is generally the best of the three. Combined drilling and production semisubmersibles are perfectly feasible - the latest projects in the North Sea are all cases in point.

Cost reductions

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"Moreover, during the last few years we have seen developments in riser design, the introduction of taut-leg mooring systems using lightweight polyester ropes and significant reductions in the costs of subsea equipment, all of which have made it possible for semisubmersibles to compete on economic terms with the TLPs and spars in deeper water."

The advantage of TLPs and Spars has been that their low heave motion characteristics enable them to be used with rigid risers and dry trees. Semisubmersibles experience more movement in the water and therefore the trees need to be placed on the seabed with flexible risers to the surface. It was the cost of subsea christmas trees particularly that accounted for the poor competitiveness of semisubmersibles. However, as Felix is keen to point out, their cost has now come down considerably.

Data presented by GVA-C, which was supplied by Kongsberg Offshore, shows the cost of developing a field using subsea trees was a staggering $40 million per well in 1985. This included the cost of the associated subsea architecture - template, manifolds and other items. By 1990, this had come down to $10 million and now it stands at $4 million.

With this very significant information to hand, GVA-C recently decided to carry out a larger cost comparison of semisubmersible drilling and production units with TLPs and Spars. The company has based its comparisons on specific structures in order to give the results maximum impact. In one such exercise, it has compared the Genesis Spar in the Gulf of Mexico with the Balmoral Field semisubmersible in the North Sea - the world's first purpose-built drilling and production unit delivered by GVA in the mid-1980s.

Genesis vs Balmoral

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The main features of the two platforms are shown in Table 1. Immediately apparent is the fact that the Spar is in eight times the depth of water as the semi. However, GVA-C considers this not an issue. "Balmoral is in the harsher environment; it could encounter waves of 100 ft in the North Sea, rather than the 70 ft maximum expected in the Gulf of Mexico. Also, the Balmoral semi is perfectly capable of being stationed in the same water depth as Genesis using the same mooring technology as for the Spar and flexible risers, which are suitable for use down to 6,500 ft. The additional weight of the risers and moorings would not have a significant effect on the deck payload and consequently the size of the semi is more or less independent of water depth. Technically there is no depth limitation for a semi compared to a Spar," Felix claims.

In terms of the production volumes, the two units carry out very similar tasks. Not surprisingly, therefore, the topsides operating weights are very similar (see Table 2). Both also have drilling and intervention access to the reservoir, although intervention is via a workover riser in the case of the semi-submersible.

The main difference comes in the weight of the structures. As shown in the table, the Balmoral hull is far lighter than the Genesis Spar, which also carries a substantial amount of fixed ballast. GVA-C estimates that at $3/kg for steel structure construction and $0.5/kg for the Spar's fixed ballast, the total saving on the semi is in the region of $61 million.

Despite the reduced costs of subsea trees mentioned earlier, the subsea costs for a development using a semi are still higher than for the Spar. Subsea trees are unlikely ever to come down to the same cost as dry trees and GVA-C has calculated that the Spar has a saving over the semi of $0.8 million per well. This includes the associated costs of the risers, assuming the semi operating in the same depth of water as the Spar - 2,600 ft. Felix points out that this means that the Spar would have to support production from an unrealistic 76 wells before it made up for its higher construction costs.

For the semi, Felix stated that further cost reductions were possible in the future, particularly in the area of subsea templates and riser systems, where the introduction of hybrid and steel catenary risers held great promise. Furthermore, he drew attention to the simpler and less costly installation of the semi compared to the Spar. Whereas in the case of the semi, the topsides can be added at shore and hook-up largely completed before tow-out, the Spar needs to be up-ended in deeper water before its topsides can be installed and hook-up carried out. Also, the Spar cannot be relocated, like other floating units, due to its unwieldy draft. In total, GVA-C claims that the semi will be $75-100 million cheaper than the Spar and yet capable of performing the same role.

Ursa vs Visund

GVA-C has carried out a similar analysis involving the Visund semi and the Ursa TLP, and again come out with a significant cost advantage in favor of the semi. The company concedes that analyses of this type will always be called into question, since they are not comparing like with like. However, the company is convinced that at least the data demonstrates that operators should not be ruling out semis when looking at options for deepwater field developments.

In fact, the arguments might already have begun to make an impact. GVA-C understands that there are two major operators who were both previously set on dry-tree solutions, seriously looking at semis for very considerable field developments in the Gulf of Mexico. Felix also notes that the first phase of the development of the Snorre Field in the North Sea in 1985 employed a TLP, Snorre A. The second phase now underway will use a semi, Snorre B, with the capability for drilling and production. "We know that the views of the operator have been changed by the way costs of subsea systems and risers have fallen," said Felix.

GVA-C is keen to avoid the charge that it is pushing the case for semis too hard. It accepts that they are not the answer to all field developments. "We simply want people to keep an open mind," Felix insists.

For more details contact Robert Ludwigson, GVA Consultants. Telephone +46 31 10 67 70, fax +46 31 10 67 10 or e-mail

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