Environmental campaigners caused a "public nuisance" by staging a protest on a cargo ship, a court has heard.
The ship was carrying animal feed to Bristol
Greenpeace claimed the MV Etoile, which was prevented from heading into Bristol in June 2004, was carrying genetically- modified animal feed, a jury was told.
The 123,000-tonne bulk cargo carrier was eventually able to dock.
Ten men and three women all deny a public nuisance charge at Cardiff Crown Court. The case continues.
Prosecutor Jervis Kay QC told how the ship was en-route from America to Bristol when progress was impeded.
The group was apparently protesting against the nature of the cargo carried on board the vessel which it was thought contained genetically-modified organisms.
The Panamanian-registered vessel was forced to anchor and was unable to use her engine, he said.
"These foolhardy acts amounted to acts constituting a public nuisance," said Mr Kay.
Mr Kay said the ship had been carrying around 99,000 tonnes of animal feed. About half the cargo had already been unloaded elsewhere and the ship was bound for Bristol with about 48,000 tonnes left.
"A great deal of pre-planning and preparation on the part of the members of Greenpeace had taken place," said Mr Kay.
After the defendants were arrested, a variety of documents were seized, including maps, operational orders and an "escape route" for south Wales.
Mr Kay said the impact of the actions was "substantial and wide-ranging".
He said the RNLI and the Maritime and Coastguard Agency had been involved and South Wales Police had deployed more than 80 staff to deal with the incident.
A police helicopter was deployed to the incident on two occasions.
Defending, Edward Rees QC, told the jury: "Sometimes one must do wrong in order to do right."
He said jurors might have to consider whether the defendants' actions in delaying the ship were reasonable in order to prevent criminal offences under the Environmental Protection Act.
The defendants include: Andrew Taylor, 35, of Stroud, Gloucestershire, Janet Miller, 49, and Huw Williams, 38, both of Buxton, Derbyshire, Tim Hewke, 45, of Sittingbourne, Kent, Nicola Cook, 37, of St Cross, Suffolk, and Cedric Counord, 28, of France,
They appeared with the following defendants, all from London: Allen Vincent, 42, of Peckham, Michele Rosato, 33, of Bow, Rachel Murray, 31, of Highbury, Jens Loewe, 36, Richard Watson, 40, and Ben Ayliffe, 28, all of Islington, and Frank Hewetson, 40, of Kensal Rise.