The Toisa Paladin - a new DP3 diving ship operated by Sealion.  

Curiously, while the marine industry knows much about the technicalities of DP vessels, they are less aware of the capabilities of these craft, and indeed why there are now so many of the them about. Another article in this category discusses the capabilities of platform ships to remain in a precise position on the earth's surface, with the assistance of an electronic referencing system, some interfacing by computer and the provision of engines, rudders and thrusters; the last probably at both ends of the craft. The purpose in this case was to remain within range of an offshore crane, or within the scope of a bulk loading hose.

Of course platform ships were not the first vessels to be provided with DP. The very first were diving ships which usually maintained position by means of clump weights on the seabed connected to a taut wire. The referencing system in this case was the sensor which detected changes the angle of the wire from the vertical and restored the angle to what it was before by moving the ship.

The tendency for these rather simple systems to fail in some way and leave the divers on the seabed has resulted in more and more elaborate specifications for diving ships, in the hope that single failures or a catastrophic event in a single compartment will not result in loss of position, or even worse, drive-off. Owners of the structures alonside which these vessels may lie for long periods of time are also sensitive to drift off and more particularly drive off, since many of the craft doing this sort of work are extremely powerful and well found, and could do a lot of damage.

The possibility of a catastrophic event being caused by DP failure has resulted in a well defined set of standards for vessels so equipped, ranging in IMO terminology from Class 0 to Class 3. These are fully described in IMO Guidelines for Vessels with Dynamic Positioning Systems, MSC Circular 645 6th June 1994, which determines that a DP system "consists of components and systems acting together to achieve sufficiently reliable position keeping capability". The document also states that the larger the consequences of failure the more reliable the system should be.

Basically Class 1 notation states that loss of position may occur in the event of a single fault, and Class 2 states that loss of position may not occur in the event of a single fault, in both cases the fault occuring in an active component. Active components include generators, thrusters, switchboards and remotely operated valves. When this article was first written most supply vessels fitted with DP were Class 1, but today in 2016 very many support vessels of all sorts are fitted with Class 2.

Class 3 notation is a little more onerous in that as well as requiring that loss of position will not occur in the event of a single fault in an active system, it also requires that a single fault in any static component will not result in loss of position. In addition to conform with Class 3 requirements loss of position should not occur due to loss of all the components in a single watertight compartment, or in one fire subdivision, due to fire or flooding. This last item requires that the vessel be provided with 2 engine rooms, each one containing enough prime movers to power the thrusters. Diving ships, Constructions vessels and modern ROV support craft are usually Class 2 or Class 3.

The IMO Guidance also states that for Class 2  and 3 vessels a single inadvertent act should be considered as a single fault if such an act is reasonably probable. In simple terms this means to it should not be possible for some-one to press the wrong button and cause a disaster.  In most control systems, where pressing a single button may result in a major change to current status, typically such items a engine emergency stops, it is common to fit flip-up covers, however most software can be configured so that such possibilities can be avoided.

Use of dynamic positioning is no longer restricted to supply vessels and diving ships. An increasing number of subsea completions are installed and operated without a diver going anywhere near them, due to the considerable increase of what are becoming known as "Construction and Life of Field Support Vessels", which are craft of considerable dimensions fitted out for laying flexibles and installing, operating and maintaining subsea completions.

Earlier versions might be converted diving ships or platform ships particularly UT705s whose deck area make them suitable for such tasks, but today there are numerous purpose built ships entering the market. The main weapon in their arsenal is the ROV which can be configured to carry out an amazing number of tasks, in addition to the inspection activities which was their only job when they first appeared in the 1970s. Indeed the ROV vessel is no longer likely to be a converted trawler following a pipeline, on the lower edge of sophistication. Au contraire, it is more likely to be the latest and most expensive vessel in the port.

Another task undertaken with the assistance of DP is that of extended well testing, the two most sophisticated exponents of which are the vessels constructed for Brovig in the last decade of the 20th century, the Crystal Sea and Crystal Ocean. Brovig is long departed but these vessels are still out there, fitted with what is effectively a process plant on the deck. Instead of  the well test sytem being on board the oil rig, the gas, water and oil being separated, and the results flared off, all of the product is transfered by pipeline to the vessel which carries out the processing. The product is stored in its tanks and then discharged later at a suitable terminal. This activity is becoming more popular as the planet becomes more environmentally sensitive. It definitely avoids the misfortune of the oil being discharged straight into the sea from the end of the flare boom if it has proved to be difficult to light. This process is described by the wonderfully euphamistic oilfield term of "cold flaring".

Outside the oil industry DP is being used for many other tasks, possibly the most exciting being maintaining very large passenger ships at a specific location adjacent to a port while the passengers of ashore for excursions, and assisting with the operation of the rocket launching semi-submersible the Oddyssey. The latter has links with the oil industry in that the vessel was formerly the Ocean Oddyssey. It motors off in the company of its support vessel the Sea Launch Commander to a point on the equator where every body leaves the rig and goes on board the support vessel while the two are 10 meters apart, both operating on DP. Therafter the rig maintains its position on DP and the rocket is launched remotely from the support vessel. This is an amazing concept which surely tests marine systems to their limits. The support vessel is the size of a large ferry - 30,000 tonnes. The semi-submersible was converted for its new role by Kvaerner in Norway, while the support vessel was constructed by Kvaerner Govan.

Co-incidentally, during an International Offshore Marine Support Conference, at which Ian Giddings, then  of the Nautical Institute presented a paper on Dynamic Positioning, and from which some of the information presented here is taken, Lloyds List announced the first launch from the Oddyssey. For some-one a dream has become a reality.

All this is quite difficult to grasp for those who were in the oilfield in the seventies when virtually every semi-submersible was fitted with a diving habitat, a diving bell and all the acoutriments. Then the divers were required to make connections between the Blowout Preventer and the Wellhead, and the Blowout Preventer and the marine riser and the Blowout Preventer an all sorts of high pressure control and power cabling, without which it it not possible to drill for oil.

But regardless of the latest hardware available, the wonders of the ROV and the skills of their pilots, who incidentally might be the real heros of this technological quantum leap since it is they who actually fly the little submarines at the bottom of the sea, the fields which were put in place using divers still need divers to keep them going.

Because of this requirement there are still a number of ageing diving ships operating in the North Sea, and an increasing number of new diving ships are being built. In this new undersea world ROVs are sometimes pressed into service as gophers, taking tools down from the surface to the divers or peering over their shoulders so that the management on the ship knows precisely what is going on down there. And it is not completely beyond the realms of possibility that the ROV cables could could accidentally connect the ship to the structure, sp it is little wonder that the masters of diving ships generally look like worried men.      

As the offshore industry moves into deeper and deeper water the possibility of diver intervention is reduced and the likelihood of the fields being operated through platforms with their feet on the seabed becomes less. There was some development in the mid seventies of dry habitats on the seabed, which it was felt, would solve some of the problems relating to work with divers. Such habitats would allow workers to descend to the seabed at surface pressure, enter the habitat, also at surface pressure, carry out the work which was required and then return to the ship. No endless days in a decompression chamber would be required.

Today there is talk of drilling wells from the seabed, and as the systems on board drilling rigs become more automated so such ideas get closer to reality. Such a drilling system would be carried, launched from and operated by a surface ship on DP. If this seems like fantasy one only has to give a moments thought to the Sea Launch programme. Sea Launch is science fiction but it has become a reality because of the availability of DP.

 

 
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