ENVIRONMENT/SAFETY TECHNOLOGY Collision resistance of double-hulled tankers

Studies indicate a double hull provides 4-5 times the protection of a single hull in a collision. Shown above is the 85,000 DWT tanker Isola Blu during fabrication in Fincantieri's Ancona yard. (Photo courtesy Fincantieri)

• Hull depth contributes significantly to energy absorption as girders buckle under penetration in this top view.
• The double-hull tanker's inner hull is 3,740 mm from the outer hull shown at point of penetration in this end view.
• Table:All double hussl are not equal, as this comparison of collision resistance between single-hulled and double-hulled tankers (290,000 DWT) indicates. Energy absorption capacity, measured in megajoules, before an oil leak.
• Table:This formula provides the critical condition for leaking of cargo oil from a double-hulled tanker during a collision.

Tanker disasters occur on a large scale and make a deep negative impression on the public. The tanker Braer grounded and broke up near the Shetland Islands in The United Kingdom in January 1993, which was followed by the Maersk Navigator collision accident near Sumatra in the same month.

In the United States, the "Oil Pollution Act of 1990," became law in August 1990. It requires that all tanker vessels delivered after January 1, 1994 and operating in US waters must have double-hulled structures. IMO-MEPO agreed that the oil tanker must have a double-hulled structure or an equivalent alternative in March 1992.

What happens when an 11,000-metric-ton cement carrier impacts double or single-hulled oil tankers? The double-hull tanker proves its merits in avoiding oil spills.

The collision resistance of a double-hulled tanker was examined quantitatively by a team of NKK engineers. The depth of the double hull contributed significantly to the energy absorption capacity of the ship's side structure before leaking oil. The structure under examination is shown in the two images you can select.

Selected tanker

A 290,000 DWT type double-hulled tanker was chosen for study. The midship section has a 3740 mm double hull depth. A cement carrier with a displacement of 11000 metric tons was chosen for the colliding ship. The relative vertical position of both ships is shown on page 77, which indicates where the load water line levels coincide.

The load-penetration relationship obtained from the analysis until the stem reaches the inner hull is shown by the solid line and dotted line, which represent the case of DH = 3000 mm and 3740 mm, and the case of DH = 2000 mm respectively. The point of rapid decrease in load corresponds to rupture of the side shell. The stepwise increase of load results from the crushing of columns of stringers, where the column consists of a stiffener attached to the stringer and a certain effective part of the stringer plate by direct action of the stem.

The strength during collision varies significantly, depending on the collision location. The calculation result for collision of the stem at the center point between webs (abbreviated as between webs), where the solid line and dotted line represent the case explained above.

In this case, the stem does not touch any structural members except the side shell for a period of time after the side shell ruptures because the transverse web spacing (6000 mm) of this ship is fairly large compared to the stem size. Therefore, the total energy is much smaller than for the case of a collision on a web.

The energy-penetration relationships were obtained by integrating the curves. In this analysis, critical damage is defined as the condition when the stem reaches an inner hull.

The energy absorption capacity of a single-hulled tanker is assumed to be the energy of the double-hulled tanker until the side shell ruptures. Likewise, the energy absorption capacity of the double-hulled structure is estimated to be the summation of the energy until the side shell ruptures, plus the original energy obtained by the present analysis method. The energy absorption capacity of the inner hull was calculated using a correction factor equal to the thickness ratio of the inner hull plate to the side shell plate.

The table on page 77 is a summary of the comparison. This table clearly shows that the change from a single hull to a double hull is remarkably effective.

These figures clearly indicate that the effect of the installation of a double-hulled structure is small for a between-webs collision, where the transverse web spacing is very large, compared to the stem sire of a colliding ship, as in a very large crude carrier. Therefore, the depth of the double hull is very important.

This table indicates that gives the total energy absorption capacity including the effect of an inner hull. It was found that the energy absorption capacity of the NKK double-hulled tanker is much higher than that required for a double hull depth of two meters, as specified by the IMO-MEPC.

During collision

The critical condition for leaking of cargo oil from a double-hulled tanker during a collision is evaluated from the accompanying equations. The energy absorption capacities of tankers are given below.

It was found that the collision strength of the 290,000 DWT type double-hulled tanker designed by NKK is remarkably high, compared to a single-hulled tanker of the same size. The NKK design is also much stronger than a 2-meter deep double hull design that complies with the minimum value stipulated by IMO-MEPC.

AUTHORS

Takaaki Goto is manager of the Ship & Offshore Structures Basic Design Dept. with NKK.

Toshihide Hagihara is a naval architect in the Hull Structures, Ship & Offshore Structures Basic Design Dept. with NKK.

Haruhiko Komiya is manager of the Ship & Offshore Structures Planning Dept. with NKK.

Kimio Kitano is a naval architect and general manager with Ship & Offshore Structures Planning Dept of NKK.

Hirokazu Sato is a general manager in the Ship & Offshore Structures Engineering Dept of NKK.

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