Dangers of Working Offshore

Author: Gary Humphrey,    P.E. VP, Geotechnical Engineering

The Jukes Group is pleased to provide some insights to some experience of our engineering staff based on personal experience. The most important of our core values is Safety. This short article emphasizes some important safety considerations in working offshore.

I was working on a Mobile Offshore Drilling Unit (MODU) during a tow to a new location. This was somewhat unusual as we typically didn’t come onboard to do our work until the MODU was already on location. We had three tugboats connected to the triangular shaped rig; one on each side. Below is a picture of a jack-up MODU that has been jacked up on a location and is prepared for drilling.

It was windy and there were strong currents. As we approached the location where the MODU was to jack-up, it required us to make a 90-degree turn into the wind and currents. When we made the turn, the tow lines became very taut, especially one of them. The tow lines were attached to the MODU via bits or equivalent to 8-in pipe approximately 2-ft tall that were welded to the deck. The wall thickness of the bit was approximately 1/2-in.

As we turned toward the location, one of the bits sheared off completely causing the tow line to go slack. But the broken bit was thrown over 100-ft out into the sea. It sheared the bit and the bit wiped out the railing around the outside of the deck. We were very fortunate that nobody was standing nearby as they would have surely perished. It was a stark reminder to put safety first.

Another danger with MODU’s, particularly jack-up rigs, is the chance for sudden penetration of one or more of the legs. This is known as “punch through”. It is standard practice to take a soil boring either prior to rig arrival on a location, or sometimes, from the rig itself.

The soil borings are done to predict the leg penetration during the preloading sequence. If done prior to the rig arrival, the initial penetration prior to preloading is also predicted. As the rig is jacked up, the soil fails beneath the footings/foundations until it becomes strong enough to support the rigs weight while semi-buoyant and when it clears the water surface and the full initial load is applied to the footings. This is called the initial load or lightship plus variable deck load. Once the soil can support this initial load, the jack-up rig pumps millions of gallons of water into ballast tanks in the hull to “preload” the footings to the maximum load expected in an extreme event like a hurricane.

Only one soil boring is typically done (especially in the Gulf of Mexico) to determine the penetration during preloading. This is because it is relatively expensive to collect the soils data and determine the engineering properties. In the case of taking a soil boring directly from the rig itself, you have no choice as the drilling rig is positioned between the two back legs. In the case of a cantilevered rig, it can only move overboard along the main axis of the rig (straight overboard, perpendicular to the back of the hull). Yet, this can cause problems because this can leave one with borings that are far away from the legs/footings. Usually, anywhere from 75 to 200-ft from the legs/footings.

Therefore, sometimes the soil boring results are not indicative of what soils are beneath one or more of the legs/footings. One of the first times I was on a jack-up rig to do a soil boring, we experienced a “punch through” such as indicated in the picture below (although not the same rig).

The punch-through was sudden and one leg penetrated 19-ft more than the other two legs in just a few seconds. There was no way to keep up with the sudden penetration with the leg jacks. It was frightening as I had gone to take a shower and get some rest, so I could get up and relieve my colleague who was monitoring the penetration during the preloading sequence. We had been up for several hours at that point and needed to divide responsibilities. I had just laid down and was about to go to sleep when the unpredicted punch though occurred. I was literally standing on my head in the bunk afterward.

Everyone on the rig went into full alert mode and grabbed life jackets and went to their muster points. The rig stabilized but was unable to jack up any further to level the structure. This was due to broken chords in the truss-style legs. In 5 years of going offshore, approximately 200 days/year, this was one of only two times I had to put on a life jacket and prepare for abandonment. This stresses the importance of your safety and survival training and how it must be taken very seriously.

Stay tuned to The Jukes Group posts on our social media pages. We are pleased to offer some insight into working in an offshore environment and some insights of personnel with over 36 years of working offshore. Among the core values of The Jukes Group are: safety, integrity, trust, excellence, people, technology, giving back and gratitude. Note that Safety is First.

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