Modelling ships in danger

Stricken ships in danger of breaking up, sinking, capsizing or losing their fuel or cargo need expert help to be rescued. Lloyd’s Register’s Ship Emergency Response Service (SERS) was set up to provide this support. Ingenia talked to Lead Naval Architect, David Prentice; the Manager of SERS, Wijendra Peiris; and Nick Brown, Global Marine Communications Manager, all of Lloyd’s Register Marine in Southampton.

Lloyd’s Register’s Ship Emergency Response Service (SERS) has had several cases of dealing with fire burning alongside a vessel, as a result of oil spill and ignition following collision, contact with quayside or terrorist attack. Issues with fires include loss in structural strength and changes in loading due to outflow and/oringress. Heat damage to steel and pool fires can also be extensive and pass along the ship’s length

When a ship runs aground, catches fire, collides with another vessel, begins leaking oil, or suffers another calamity (including terrorism), someone has to decide what to do. That person is usually the ship’s master.

Tankers, bulk carriers, car ferries and cruise liners are large vessels that have complexities to match. Experience counts for much in handling ships, but, when faced with disaster, the ship’s crew needs extra support.

Ships like the KNOWING WHAT TO DO

When an incident happens, SERS needs to be alerted as soon as possible. The response teams work in groups of three, comprising a naval architect with expertise in modelling, another naval architect with structural expertise, and a team leader (a third naval architect or a master mariner) responsible for coordinating and driving the chosen strategy and, importantly, liaising with the ship’s master.

World map showing locations of some of the incidents dealt with by SERS

The response team will assemble at Lloyd’s Register Marine’s SERS Emergency Response Centre, and establish communication links with the ship and/or client office. Voice, email and fax communication with the ship are usually conducted by INMARSAT or Iridium satellite. Vessel condition and casualty data are then sent to SERS for assessment, the results of which are provided verbally and backed up by written reports. Internet-based web conferencing can be used with clients ashore for interactive assessments, although satellite data speeds currently limit use on board.

The ship’s crew and SERS will both have access to weather, tide and chart data from various providers. SERS is contracted by the ship operator or manager, but will work with third parties such as national, state, port, canal and coastguard authorities, salvors and insurers, as requested or authorised by clients.

The key to providing successful advice lies in the computer modelling of the ship and its cargo. These models can predict what will happen if nothing is done as well as the likely effects of any proposed course of action. SERS computers already hold detailed plans of the structure of each ship subscribing to the service. Software developed over many years can be used to calculate the effects of outside forces bearing on the vessel and, in the light of what is known of its construction, the likelihood of it failing. Stability and floatability can be determined and, as circumstances develop and change, the model can also predict any oil outflow from, and retention within, damaged tanks.

There are 3,200 vessels that are represented in the SERS system. Since its inception, over 5,000 craft have been modelled, representing approximately 150 worker-years of effort. There are ships of all types, ranging from oil tankers, chemical tankers, LPG or LNG gas carriers, container ships and cargo vessels to cruise liners and roll-on-roll-off passenger ferries.

Many factors are at play when a ship gets into serious trouble: the rise and fall of the tide, the local weather and wave action, the location and extent of a grounding point and the measure of any damage to cargo or leakage from fuel oil and ballast tanks. The location and severity of structural damage caused by collision, explosion or fire must also be taken into consideration, as well as the number of flooded compartments of the vessel and whether or not it is listing. Equally influential are the amount, nature and distribution of the cargo; what weight the ship is carrying; if it is liquid, what kind, what quantity and in which tanks; and if in containers, how many and how they are arranged.

INFORMATION NEEDED

SERS assessment procedure follows three established steps: confirmation of its model of the ship condition before the casualty, evaluation of the casualty condition, and consideration of remedial action.

The initial condition of the ship requires a full description of the loading of the vessel. Onboard, the condition status is maintained by the ship’s loading records. Tank levels are generally monitored by gauges that input to the loading computer, but small tanks may be sounded manually and weights then input to the assessment data. Once all weights have been input and the reported draughts agree with SERS calculations, confirmation of the facts known is requested. Confirmation is important as all calculations that follow are based on these data. Then, the vessel’s casualty condition can be modelled – see GROUNDING CALCULATION THEORY

By Archimedes’ principle, the weight of a floating object equals the weight of the volume of fluid displaced. The displaced volume of the fluid, and hence the initial weight of the vessel, is calculated using the initial draughts and the corresponding submerged geometry of the ship. The grounded draughts will determine the new displacement.

The difference in the displacements before and after grounding corresponds to the grounding reaction force, taking into account any flooding weights. The change in trim or heel of the vessel will then determine the location of the line of action of this force – so if the reaction force is aft then the vessel will trim forward (or if to port will heel to starboard). The diagram illustrates how the grounding reaction force is calculated in this way.

COURSES OF ACTION

While the computer can calculate a vessel’s stability, residual strength and resistance to stress, it cannot by itself provide a course of remedial action. Nor is there necessarily only one course of action that could be taken. The SERS team leader has to exercise judgement before advising the crew on what to do with their stricken vessel.

Casualty situations can be complex. SERS technical assessment procedure uses its own nautical aide memoire: ‘SS FOG and Tide’. This assesses:

Strength – considering the hull girder as a ‘beam’ under loadings of weight, buoyancy and wave forces

Stability – the capacity to resist overturning effects of wind, waves, weight and movement of liquid cargo

Floatability – reserve of buoyancy, freeboard and heel/trim

Outflow – loss of liquid cargo

Grounding – reaction force, its distribution and location; and refloating of ship

Tide – which may greatly influence the above and should not be forgotten, such as when aiming to refloat at high water, consider also loads on the ship at low water

Primary assessments are supported by a variety of tools, such as 2D section property modelling of damage residual strength. The range of what might be recommended is broad. It might be to do nothing if the next high tide can be counted on to refloat a grounded vessel. It could be to pump out water ballast or offload part of the cargo. If a tanker is holed near the water line on one side, it might make sense to re-ballast it in such a way as to produce an artificial list to raise damage above the waterline in order to retain cargo and/or allow the compartment to be pumped out.

In the case of grounding in rough seas, a vessel could be battered against rocks or subject to shearing forces from wave action. The first step could be to pump more water into its ballast tanks and so induce it to settle more firmly. This may seem a contrary course of action, but a vessel that is securely grounded and moving less in response to wave action may ultimately fare better than one still bobbing around. Once the storm has abated, the ship’s ballast tanks can be emptied and attempts made to refloat it. Liquid cargo may need to be pumped to adjacent tanks to stabilise a vessel, or alter its orientation in some way – see RUNNING AGROUND

Tanker grounding: analysis of a casualty. The diagram illustrates steps in the assessment of a typical scenario. The vessel has run aground on a rocky pinnacle, which has torn open the bottom of the ship flooding the double-hull (ballast tanks) and the inner bottom allowing some cargo to escape. The graphical interface is used to manipulate the 3D computer model of the vessel. Loadings have already been applied to generate the initial condition model and it has been checked against the reported condition. Now the grounding casualty has been modelled revealing that there is:

1. Flooding - the water ballast tank No.1 Port is open to sea and has flooded with seawater

2. Outflow - cargo tank 1 Port, above, is damaged at the hole marked. The cargo has escaped until there is hydrostatic equilibrium. (Note also the oil-water interface in tank to level of the hole, illustrating how water ingress is also tracked)

3. Grounding - After setting the attitude of the vessel (draughts, heel and trim), and by taking into account flooding of the ballast tank and equilibration of the cargo then the computer will calculate the grounding force at the location shown.

4. Strength capacity - Hull girder bending moment limits (blue): maximum (‘hogging’) and minimum (‘sagging’). The curve is adjusted to take account of grounding damage (dotted line)

5. Strength assessment - The actual still water bending moment is outside (below) the limits. This is due to upward grounding force forward. Note that in this case the greatest exceedence of the limit is midship, where the hull girder is intact and away from the weakened damage area at the grounding point.

This example illustrates the potential complexity of these scenarios. Suppose the vessel is to be re-floated by taking actions to raise the hull clear of the rock. This would normally be timed to take advantage of high water and carried out by movement of cargo internally or by offload, or ballasting/de-ballasting. However, strength and stability must also be controlled during the refloating and for the free-floating condition. Meanwhile pollution must be minimised. The vessel may also need to go through a low tide before the high water or if refloating is unsuccessful other options may be needed.

The team will also advise on the most cost-effective temporary repair arrangements, and the safest route to a harbour where permanent repairs can be effected. The aim throughout is to get the vessel back into normal service as rapidly as possible while at the same time limiting damage to the environment, particularly from leaking oil. However, the final decision on what to do rests with the ship’s master, although SERS recommendations are seldom disregarded.

MARITIME MAISIE

One year ago, SERS had to deal with a catastrophic shipping failure on the southeastern tip of the Korean peninsula. In December 2013, near the South Korean city of Busan, the tanker RISK OF AN ACCIDENT

Although the quantity of world shipping increases year by year and so too does the number of ships in SERS, the number of incidents that SERS deals with remains relatively steady at between 25 and 30 annually. This suggests that the accident rate is dropping, a consequence of improvements in operational and navigational safety and tighter regulation. Environmental pollution by oil in particular has galvanised changes in the last of these.

Following the 1989 REFLOATING

In 2001, the BIOGRAPHIES

David Prentice, Lead Naval Architect for SERS, is responsible for maintenance and development of its systems. David trained at Lloyd’s Register as a ship surveyor and has been a SERS emergency response team leader for 15 years leading on responses to nearly 200 emergencies and 550 exercises.

Wijendra Peiris, manager at SERS, is responsible for its technical leadership and provision of technical expertise and advice to clients. He has been a SERS emergency response leader since 2009 and involved with leading responses in over 100 emergencies and exercises.

Nick Brown, Marine Communications Manager, is responsible for promoting and protecting the interests of Lloyd’s Register’s Marine business. He has led crisis training courses world-wide and, with Brent Pyburn, was co-author of INTERTANKO’s Guiding Principles to Emergency Management and Crisis Communications.