THE MOON AND ITS TIDAL EFFECT

ON PIT VOLUMES IN DEEP OFFSHORE DRILLING

By Ukuedojor Kingsley Oghenemeruo

Tide is the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon and the Sun on earth and the rotation of the earth.

To a much smaller extent, tides also occur in large lakes, rivers and within the solid crust of the earth.

What is Tide?

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Causes day and night Contributes to causes of

tide Causes earth to bulge at

the equator due to centrifugal forces

It causes prevailing winds and ocean currents

Causes deflection of moving objects clockwise in the northern hemisphere and anticlockwise in the southern hemisphere (coriolis effect)

Earth’s Rotation

If flight from Nigeria to NY is 6hrs and you leave Nigeria 8am Monday morning, you will get to NY 7am same Monday morning

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If you are travelling from the north pole to the equator (lets say Nigeria) on an airplane and the pilot maintained its course or puts the plane on auto-pilot, by the time you complete your descend, you would have landed the plane in Ghana, this is because, the earth underneath has moved anticlockwise (from west to east) thereby moving your target away from your course. By this, the plane has deflected to the right of the target or clockwise – looking at it from the north pole and thus any activity resulting from air pressures such as hurricane and typhoon will swirl in a clockwise direction.

Now if you are flying from the south pole to Nigeria on the same course, you will also probably land in Ghana The plane would have been deflected to the left of the target (anticlockwise) with respect to the south pole and thus a typhoon will swirl in an anticlockwise direction.

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Understanding Coriolis Effect

To maintain target, the angular velocity must be calculated with respect to the speed of the earth’s rotation. The earth’s rotation is directly proportional to the axis of rotation and therefore it rotates faster at the equator than at the poles.These have to be known for the pilot to maintain longitude and also for a launched missile to hit its target

The earth rotates and revolves around the sun in an anticlockwise direction once every 365 days + 6 hrs. Six hours in four years makes a full day, thereby making 366 days in a leap year.

The moon revolves around the earth at approximately every 28 days (lunar month). Therefore it takes 13 lunar months for the earth to circle the sun.

The earth takes one day to cover 360 degrees about its axis. The moon takes one day to cover approx. 13 degrees around the earth. It therefore means that it will take Nigeria 28 times to pass the moon before the moon complete its orbit.

The pathway for the moon’s orbital revolution is not equidistant from the earth’s centre and therefore makes the moon to be either closer to earth with stronger pull and appear very big or far from earth with weaker pull and appear very small.

The moon gravitational pull on earth is stronger than the sun’s gravitational pull on earth.

Some Basic Facts about Earth-Moon Relationship

• If you toss an object up, it falls back to you due to gravitational pull from earth. You experience this same gravitational pull on a lighter note (about 16.7% that of earth) when you go close to the moon’s orbit. You therefore need heavy suits to walk on the moon’s surface.But once you are out in space, you are weightless (only mass with no gravity); thus when you pour out a glass of water, it never comes down to you.

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All solar eclipse occur at new moons (the earth, the moon and the sun are aligned) but not all new moons are solar eclipse. i.e. if they are not aligned. The sun does not rise or set, the earth rotates on horizontal cardinal planes from east to west in anticlockwise making those in the east to see the sun first and as they gradually begin to go into dark, those in the west gradually begin to see the sun. This is also true for the new moon appearance. When the east would have seen the new moon, the west would have seen nothing.

During a full moon, if the earth, moon and sun align such that the earth obscures the moon’s vision, a lunar eclipse occurs and gradually clears away. All lunar eclipse therefore occur at full moon but not all full moons are lunar eclipse.

Concept of New moon and Full moon

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Moon Cycle and Calendar

High tide occur with rising sea water while low tide occur with falling sea water.

Spring tide: When the moon is full or new, the gravitational pull of the moon and sun are combined. At these times, the high tides are very high and the low tides are very low. They occur when the Earth, the Sun, and the Moon are in a line. The gravitational forces of the Moon and the Sun both contribute to the tides. No moon will be seen at night during new moon, however as the sun rays goes down, you will have a glimpse of the new moon which gradually fades away as the earth turns away from the sun. If this completely aligns in daytime, we will experience solar eclipse.

Neap tide: During the moon's quarter phases the sun and moon work at right angles, causing the bulges to cancel each other. The result is a smaller difference between high and low tides and is known as a neap tide. Neap tides are especially weak tides. They occur when the gravitational forces of the Moon and the Sun are perpendicular to one another (with respect to the Earth).

The Proxigean Spring Tide: Rare and unusually high tide which occurs when the moon is both unusually close to the earth and in the New Moon phase (when the Moon is between the Sun and the Earth).

Types of Tides

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Tides are created because the Earth and the moon are attracted to each other. The moon tries to pull at anything on the Earth to bring it closer. But, the Earth is able to hold onto everything except the water. Since the water is always moving, the Earth cannot hold onto it, and the moon is able to pull at it.

The gravitational force of the Moon causes the Earth to accelerate slightly towards the Moon causing the water to get pulled towards the Moon faster than the solid rock on the side nearest the Moon. On the far side, the solid Earth 'leaves behind' some of the water which is not as strongly accelerated towards the Moon as the Earth is. This produces the bulge on the 'back side' of the Earth. That is why there are two high tides and two low tides each day. The ocean is constantly moving from high tide to low tide, and then back to high tide.

Explanation on Tides

Effect of low tide on a harbor

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How Tide Affects Active Volumes in Drill-ships and Semi-submersible rigsWhenever there is a moon pull on the surface of the ocean, ships and other floating vessels move upward with the rising water. Therefore on a drill ship and semi submersible rigs, this upward movement tend to cause an upward pull of the drill string from the bottom of the hole. This is especially noticeable during in-slips. And more so if the trip tank or active pit is continually circulating the well.

Decreasing pit volumes (High Tides)As the string is being pulled gradually up from bottom, there will be a gradual drop in hydrostatic head, fluid will therefore gradually move from the trip tank into the well to maintain hydrostatic balance.

Increasing pit Volumes (Low Tides)This is like a syringe effect. As the string gradually goes back to bottom with decreasing tide, it displaces its volume back into the trip tank which causes a gradual increase in trip tank volume.

Static Pit Volumes (Highest or Lowest Tidal Peak)The tide rises to its highest peak and stay for some time before dropping. Also at the lowest point, it stays for sometime before subsequent increase.

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From the plots, it was seen that trip tank #1 did not move at all during this monitoring process. This is because it is not in circulation. Only the trip tank #2 was used to circulate the well.

It was also observed that it takes average of six hours to go from high peak to low peak and another six hours to go from low peak to another high peak. Therefore it takes 12 hours to go from one high peak to another high peak.

Conclusion and Recommendation

Using a tide calculator software with Port Harcourt, Bonny river as the reference station, it was easy to predict when the next high tide will occur and by what height. The height was then converted to the maximum volume change expected.Before concluding that volume increase is as a result of tide, it is recommended that discussions must be made with the mud engineer and driller so as to ensure that the mud pit valves are properly tightened.

Mean Tide

Reference Datum

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Thanks for

Listening

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