Full-Scale Measurements of Containership Sinkage, Trim and Roll

We report on a recent project to accurately measure full scale sinkage, trim and roll of 16 deep-draft containerships entering or leaving a major container port. Measurements were performed using high-accuracy GPS receivers and a fixed base station. Results were used to calculate the sinkage of the bow, stern and bilge corners, as compared to the static condition. Overall dynamic draft increase and the governing factors affecting underkeel clearance of containerships are discussed. Introduction In early 2005 the Centre for Marine Science and Technology was contracted by Hong Kong Marine Department to perform a detailed study into sinkage and dynamic draft of containerships entering and leaving Kwai Chung, the busiest container port in the world. Part of this study was to carry out full-scale trials on some of the largest containerships, ranging in overall length from 277m to 352m, in order to accurately measure sinkage, trim and roll. It has only been through the continually increasing accuracy of GPS receivers that full-scale squat measurements have become viable in the past decade. Whereas in model tests, the towing carriage acts as a fixed vertical reference, at full scale no such simple fixed reference exists. Inertial heave sensors are useful for performing seakeeping tests, since amplitudes rather than absolute motions are important in that case. However these sensors tend to “drift” over longer periods of time, so that they cannot accurately give absolute vertical motions. GPS receivers have the advantage that they give absolute vertical motions relative to a fixed vertical reference on the earth (usually the WGS84 ellipsoid). For ship trials, these vertical heights can then be adjusted to be relative to nautical chart datum. In this way, once the tidal height is known, the height of the receiver above the static waterline can be calculated. Carrier-phase GPS receivers now have vertical accuracy in the order of centimetres, and can operate in kinematic mode to record the continually changing latitude, longitude and vertical height of points on a moving ship. Measuring vertical motions using GPS To measure sinkage and trim of a ship, two GPS receivers are positioned on the ship’s centreline, with one forward and one aft. Often the forward unit is mounted at the forecastle and the aft unit atop the bridge. To measure sinkage, trim and roll, three GPS receivers are needed onboard the ship. In this case, the units can be mounted at the forecastle, port bridge wing and starboard bridge wing. Sinkage, trim and roll are then calculated by assuming that the ship is a rigid body. For high accuracy, an external reference GPS receiver is also required. This may be positioned either ashore or on a small escort vessel. Once the onboard GPS receivers are fixed in place, a stationary reading of each receiver’s vertical height is taken at the berth. This is then used as a reference value with which to compare the vertical height measurements whilst under way. Shore-based receiver method The shore-based receiver method is described in Feng & O’Mahony (1998). In this case, a GPS receiver is mounted ashore and kept in a fixed position for the duration of the measurements. Cross-correlating GPS signals between the moving receivers and the shore-based receiver then allows accurate position fixing of the moving receivers. However, ship sinkage measurements must be taken relative to the instantaneous static waterline around the ship, and this changes as the tidal height changes. Therefore, results must be corrected for the fact that the static waterline height (tidal height) is changing with time and position. Also, the difference between the GPS vertical reference and the mean sea level (geoid undulation) must be accounted for. Escort vessel method The escort vessel method is described in Härting & Reiking (2002). In this case, the escort vessel first does its own sinkage and trim test using the shore-based method described above. This allows the vertical position of the escort vessel to be known for any given vessel speed. Then the escort vessel acts as a reference unit for the larger vessel. The use of an escort vessel removes the error associated with estimating the tidal height at each time and position in the transit. This is because the escort vessel moves up and down with the tide by the same amount that the ship does, so any tidal height effects cancel out. In a similar way, geoid undulation may also be neglected when using this method. Choosing the appropriate method In choosing between the shore-based receiver and escort vessel methods for a particular application, the main deciding factors are: availability of tidal data; length of transit; sea state; and cost. These factors are discussed below with reference to performing trials in Hong Kong harbour. Availability of tidal data: Using a shore-based GPS receiver requires accurate knowledge of the tidal height at each point in the transit. If this cannot be calculated sufficiently accurately, then the escort vessel method should be used. In the case of Hong Kong harbour, tidal variations are not too large (around 1 – 1.5m) and tidal heights are measured continuously at several points around the area. Length of transit: If using a shore-based receiver, errors increase as the baseline length (distance between shore-based receiver and moving receivers) increases. The baseline length always stays small when using the escort vessel method. For the Hong Kong trials, the area of interest was around 4 nm (nautical miles) in length, and during this part of the transit the ship was never more than 2.5 nm from the shore-based receiver, so that baseline errors remained small. Sea state: The escort vessel method requires a fairly calm sea ahead of the ship, to have a stable vertical reference. Any swell, sea or wake from other vessels will cause an oscillatory vertical motion of the escort vessel, which may be difficult to average out. Hong Kong has a fair amount of wave wake, generated by the very frequent shipping going into and out of the port. In addition, the easterly trade winds in winter cause high-frequency wind waves, while the summer monsoon brings significant southwesterly swells. This would have caused difficulties with gaining stable reference heights from the escort vessel. Cost: The use of an escort vessel entails additional costs as well as logistical difficulties. All things considered, it was thought best to use the shore-based receiver method for these trials. For longer transits in calm water with little tidal data available, the escort vessel method is suitable and has been used successfully in Europe, as described in Härting & Reiking (2002). Description of the trials Trials location Trials were conducted on vessels transiting to or from Kwai Chung, the Hong Kong container terminals, via the East Lamma Channel (see British Admiralty chart 3280 for details). For vessels leaving Kwai Chung, measurements were taken from the berth until the vessel passed approximate latitude 22°16’N. This transit includes an initial course towards the WSW, followed by a slow turn to port with final course SSE. The inward transits were the reverse of above. The charted depth for this section of the transit varies from 15 35m. The transit is in essentially unrestricted water, with no channel blockage effects. The channel is wellsheltered from the prevailing swell. Equipment Trimble 5700 real-time kinematic, carrier phase GPS receivers were used. Three of these (with Trimble Zephyr antennas) were mounted on the vessel, with one at the bow and one on each of the bridge wings. A fourth receiver (with Trimble Zephyr Geodetic antenna) was mounted on-shore, at a secure location close to the container berths. This formed the base station with which to correlate the moving receiver results. This equipment setup gives a root-mean-square accuracy of 20mm for each moving receiver’s vertical position, as stated by the manufacturer for the baseline lengths used in these trials. Sampling rate was 1 Hz. Figure 1: Typical GPS receiver locations on vessel Figure 2: Typical bow receiver mounting Arranging transits Before commencing the trials, Hong Kong Marine Department and CMST spent considerable time liaising with pilots, container terminals, shipping lines and ship masters, organizing everything so that the trials could proceed smoothly. It was important that there was no interruption to any of the ships’ usual operations. Although ideally each individual transit should be arranged well in advance, this was impossible due to the changing ship schedules, as well as the safety requirement of only performing daylight transits. Instead, potential target ships were notified in advance and final confirmation of the transit given a few hours before each transit through the Pilots’ Association. The Pilots’ Association was invaluable in providing up to the minute schedules, liaising with ships’ masters, and getting the CMST personnel to and from the ships with the pilots. Through prior planning, round the clock monitoring of ship schedules, and a flexible approach to arranging transits, 20 transits were performed on 16 deep-draft containerships over a 9-day period. The vessels chosen were among the largest containerships currently in operation. Trials procedure Two CMST personnel performed the trials on the vessels. Both incoming and outgoing trials were conducted. The procedure for incoming transits is described below: • CMST personnel board vessel with pilot, carrying equipment up ladder in backpacks • Report to bridge, then set up equipment (5 – 10 minutes) • Log data through to the berth, then take stationary reading at berth • Remove equipment and disembark with the pilot For outgoing transits the procedure was the reverse of above. Data analysis Trimble Geomatics software was used to take in the data from the three moving receivers and base station, and output (time, latitude, longitude, height)