The drilling industry is moving towards dynamic positioning (DP) for semisubmersible drilling rigs. But it should not be assumed that drilling without a mooring system always constitutes the best solution .
Semisubmersibles make up nearly one-third of the world’s entire rig count. There are 205 of them. Of these units, 122 rely on mooring systems for station-keeping; the remainder have DP systems. It is significant, however, that of 23 reported new-build semisubmersibles due to enter service in the years up to 2018, most, 19, fall into the second category.
The preference for DP is easy to understand: there is real chance of making valuable time savings when the rig is being moved between jobs. But, according to Martin Kobiela, InterMoor’s operations director, DP may not always be the optimum solution. He says, “It is clearly easier and faster to position a semisubmersible when there are no moorings to deal with. But the gains in rig time have to be weighed against a range of other costs and operational risk factors. One of the biggest issues has become the cost of the fuel used to constantly drive the DP system. Then there is the impact of the emissions generated as a consequence.”
Kobiela argues that, in some cases, mooring systems, especially when pre-installed, can offer economic and operational advantages over the use of the rig’s DP system. The problem is deciding when it makes sense to moor the rig and when it is best to rely on the DP system. “There is an analysis to be carried out each time and careful judgements to be made,” he says.
Water depth is one of the first things on Kobiela’s list of decision criteria. He would generally be reluctant to promote the case for moorings over DP in seas much deeper than 1500 m, and this figure would be less in parts of the world prone to extreme environments. And there would be a mooring design analysis and a risk assessment involved in each case, along with a check on the local availability of skilled personnel, anchor-handling vessels and mooring equipment.
Assuming that moorings are a realistic option, there then needs to be a look at the various costs involved. The costs of hiring mooring equipment, people and the anchor-handling vessels needed to install and recover the moorings have to be compared with the cost of fuel and, if necessary, of the offshore refuelling operations. Kobiela has constructed a simple model to perform this comparison for a given location and mooring requirements. He points out that the outcome of the analysis inevitably depends on day rates, fuel costs and the time the rig spends on location, as the mooring costs are dominated by the installation and recovery costs, whereas DP system costs rise linearly with time.
“What we tend to find,” he says, “is that DP works out cheaper than moorings for periods of up to 35–45 days at a given single location. Beyond this period, moorings often start to display economic advantages.
“The analysis is not always straightforward,” he continues. “There are operational and cost risks to be considered, and different operators’ perceptions of these will vary. Lastly, there is the emissions question; there are political pressures in various areas of the world for operators to be seen to be minimising the emissions from drilling activities. In some countries, for example, Norway, there are taxes to be paid on carbon dioxide and nitrogen oxide emissions, and so there is a further cost factor to be included in the economic analysis for DP.”
Kobiela is keen to stress that this is not about trying to promote moorings services instead of DP. Rather, it is case of highlighting the issues involved in making good station-keeping decisions from both the operational and the economic standpoint.
A current drilling campaign in China, which is supported by InterMoor, perfectly illustrates Kobiela’s assertion. The client has hired a new DP semisubmersible (also equipped with 12 mooring winches) to drill 12 wells at a site roughly 24 hours from the nearest port. After InterMoor and the client jointly assessed the operating environment and the local mooring equipment availability and anchor-handling vessel capabilities, a decision was taken to drill six wells in water depths of less than 1000 m using pre-laid moorings and six wells in deeper water using the rig’s DP system.
Kobiela concludes, “We were able to formulate an overall plan for this campaign that minimised the operational risks involved, made best use of local resources and provided the optimal economic solution under the circumstances. It pays to keep an open mind. This is not about whether moorings are better than DP; it is all about ensuring efficient rig operations through a thorough examination of the options available on each occasion.”