A floating storage unit at a buoy in Sakhalin with the Molipaq in the background. Photo Alexander Fedoseev

I wrote this back in 1999, but it still seems relevant. Since the year 2000 a number of further studies have been carried out with, I think, limited conclusions although a lot of statistics.

In October 1997 the HSE, the UK Health and Safety Executive published their own investigations entitled "HSE Concerns Regarding Shuttle Tanker and Offloading Operations at FSPO/FSUs" after two contacts between tankers and FSPOs in the UK sector. Effectively in response to this document the International Marine Contractors Association together with a number of Operators commissioned a report from Global Maritime which was intended to identify hazardous situations which might cause incidents, quantify their frequency of occurrence, and propose methods of mitigating the effects. One assumes that the intent of the report was also to prevent the occurrence of the incidents in the first place, although this is not specifically mentioned in the preamble.

 There may be those who have a somewhat hazy understanding of what offshore offloading is all about and for them there follows a brief explanation. Historically even back in the 1960s it was found that it was not always a good idea to have a tanker approach the shore, tie up the a jetty and take on or offload the product, and in benign areas tankers often tied up to offshore buoys. In those days there were almost always one or two buoys forward and one or two aft and a loading hose which was heaved out of the sea on the port or starboard side.

This process was time consuming and difficult to perform for the ships and the small craft supporting them, but things quickly moved on with the arrival of single buoy moorings which, as the name implies only required the ships to tie up by the bow, connect the loading hose and then weathervane round a single point. Two ship congress was at that time limited to ships lying alongside each other in very calm places and passing crude from one to the other. This process was developed to allow very large ships to approach Europe and then lighten before going into the continental ports.

It was not therefore a massive leap from that point to offshore loading buoys in North Sea fields and then to floating production and floating storage. The very early fields in the UK sector such as Argyll and Montrose used offshore loading by means of single point moorings, the tankers lying at the buoy sometimes for weeks since there was not storage involved. Other fields also used the same technique and the systems gradually moved further north into more hostile waters.

In poor weather there were operational problems which usually revolved round "fishtailing" an activity particularly prevalent during offloading from one vessel to another. In worsening weather the FSPO would begin to oscillate back and forth round its mooring and the shuttle tanker would follow in sympathy until the two would be moving alarmingly, pointing sometimes at 90 degrees to one another. The result was 100,000 ton tankers sailing back and forth across the wind and imposing enormous loads on the hawser connecting the two vessels. The initial means of prevention was for the shuttle tanker to go slowly astern and the next phase would be for a tug to be connected aft to try to keep the whole lot in a straight line.

Part of the Global Maritime report discusses factors in non DP offloading and cites incidents where hawser tensions in excess of 200 tones were experienced, usually in the moments before parting. Of course such high tensions resulted in problems with the release process, and if the Master was hesitant in calling a halt to the operation the crew on the forecastle might experience considerable difficulty. In 1980 a fatality occurred on the bow of a tanker, the cause of which was described in the report as "the disconnection sequence and station keeping".

This accident resulted in improvements to the shut down sequence and then in station keeping by means of DP installations. Over the years up to 1998 the DP capability of the shuttle tankers has increased until they are all now fitted with DARPS - DGPS Absolute Relative Positioning System - however the number of incidents in total has not reduced.

The compilers of the study undertook painstaking research and reported with commendable clarity that there is still work to do, and the fitting of DARPS has not solved all the problems, even though it may have the capability to do so if the tanker personnel are suitably trained and if other factors are considered.    

A second possible interaction between the two vessels is known as "surging" where the connecting hawser becomes tight and the resulting effect propels the two vessels towards each other. Since typical distances between the stern of the FSPO and the shuttle tanker is as little as 80 metres this must be a heart-stopping experience and the report concludes that the movement of the DP control station from the doghouse on the forecastle to the bridge has resulted in the loading activities being terminated in more adverse conditions than before.  This is not unlikely when one considers that the distance between the bridge and the forecastle is likely to be greater than the distance between the forecastle and the stern of the FSPO. The report states that "the likelihood of major collision is reduced by siting the DP control forward as the operator can easily assess, visually, the distance between the two structures".

The Nordic Brasilia at the FPSO P-40. Photo by Tony Poll

Of course in addition to adverse weather problems, which existed in the pre DP days, and which the DP systems have probably reduced, there are the incidents resulting from DP or system failure the worst of which are "drive-ons" resulting from a failure of the reference system.

However in the main, instability of referencing systems can be detected and subsequent to failure the position may be recovered by skilful ship-handling. Of course, in the days before DP there was plenty of ship-handling skill around. Today of course the general effectiveness of the DP systems has reduced the time spent manually driving the ships and therefore the skills are no longer present. The report says that "several incidents have been recorded where a shipmaster's manual manoeuvring was a contributing factor in causing a collision". The report therefore recommends an increase in simulator training which is currently being put in place by the industry in order to teach collision avoidance skills through effective ship-handling.  This should reduce the incidence of, as it states in the report, "lookouts running aft on shuttle tankers".

Another factor in the equation, not easily resolved by DP capability alone is the effect of the turning point of the FSPO on the alignment of the two vessels. The further aft the turret the less likely that the FSPO will weathervane, and apparently some of these units maintain a "wanted heading" by the use of thrusters. At least one incident has been recorded where the installation staff responded to the production department's demands to control the FSPO roll in order to prolong production placing "the burden of alignment" on the shuttle tanker. During the attempts by the latter to re-align itself there was a collision due to an operator error on the shuttle tanker.

Those technically or statistically minded who read this report will probably conclude that if a few more thrusters, a few more referencing systems and a few more procedures are put in place the problem will be solved. Any old mariners who read the report will probably conclude that no amount of cutting edge technology can substitute for the ordinary practice of good seamanship.

 

 
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