Physical Parameters to Take Into Consideration When Operating Booms

The entrainment failure is caused by too high velocity of the oil/water perpendicular to the boom wall. This means that the oil is going under the boom wall due to turbulence. If the boom formation is in open sea it could be corrected by lowering the speed of the formation in relation to the water velocity. The velocity of the oil/water must never exceed 0.7 knots (0.36 m/sec). If the boom formation is static it may be solved by angling the boom in relation to the current. 

The angle required for correcting the entrainment failure can be calculated by using cosinus/sinus relations. However, from a practical point of view it is more relevant to calculate the length of boom required in relation to the width to be covered with the boom formation. 

Example: A river with a width of 50 m should be closed with a boom formation. The current is 2 knots.

Length of boom required:  2 : 0.7 ≈ 3. 3 x 50 = 150. So 150 metres of boom is required for making the required angle of boom formation and not causing entrainment failure.

Drainage failure cannot be corrected. It is caused by too much oil recovered by the boom, again causing oil escaping under the boom wall. The best way to solve the problem is having a skimmer placed in the area where the drainage failure takes place. 

Splash-over failure is caused by oil splashing over the freeboard of the boom. It can be caused by too low freeboard and too low buoyancy of the boom. Use a boom with larger freeboard which again normally will give higher buoyancy as well. 

Submergence failure is caused by too high towing speed and too low buoyancy of the boom. Reduce the speed and again use a boom with a higher buoyancy. 

Planing failure means that the boom is forced parallel to the water surface. It could be caused by a combination of wind from one direction and current in the other direction. The boom should have correct ballast for keeping it in the right position. Especially the Ro-Boom is unique due to a scientific calculation of the chain length and chain weight compared to the specific boom type. Further, a boom with cylindrical buoyancy chambers performs better than other buoyancy shapes. 

Structural failure means that the boom is exposed to forces exceeding the tear strength of the boom material. It is the ultimate failure as the boom normally cannot be used in operation anymore. There are various precautions to be taken:

-          Ensure that the towing line has a tensile strength lower than the tear strength of the boom wall. If the boom has a max strength of 200 kN the towing line will burst at approx 130 kN.

-          Ensure that the velocity of a boom formation never reaches a level where it can cause structural failures of the boom.

RO-CLEAN DESMI has carefully designed our towing equipment for each size and quality of boom. As it is much cheaper and easier to change a rope this professional approach ensures our customers that their boom systems will always be ensured against structural failures. 

Further it is fairly simple to calculate the forces on a boom system in order to evaluate the maximum speed of a system:

F = 26 x A x V² (kp or kg). 
F is force
26 is a constant (empiric).
A is the area in m² below the water directly exposed to the current
V is the speed of the boom formation in relation to the water in knots

Example: A U-formation of a total of 400 metres of boom. The gap opening of the formation is 150 metres. It is a Ro-Boom 2000 with a draft of 1.1 metres. The towing speed of the boom formation is 1 knot.

F = 26 x (150 x 1.1) x 1² = 4.290 kp (note that it is only the 150 metres of boom wall exposed to the current used in the formula).

If the speed is increased to 2 knots: F = 26 x (150 x 1.1) x 2² = 17.160 kp!!

If the speed is increased to 3 knots: F = 26 x (150 x 1.1) x 3² = 38.610 kp!!!

It should be noted that this simple formula will give a figure which is approx 20 – 30% higher than in reality, but it is good enough to give an estimation of the forces which could be expected during operation.  

The speed is critical in any boom formation as the increase is in square of the speed. By choosing a professional supplier the structural failure will be avoided due to correct design of towing equipment in relation to the boom strength as previously mentioned. 

The expected forces on the boom system are also of interest when looking at the vessels to be used for towing the booms. A simple rule of thumb is that 1 bhp can pull 10 kp. A fast going propeller, ie on an outboard engine, can pull perhaps 3 kp per 1 bhp. A water jet driven vessel is designed for high speed, but has very limited bollard pull, less than 3 kp per 1 bhp.