Marine operating manuals explained

Added to the archive in 2026 · Reference note, not an original Bennett Offshore page

The design drawings of a rig live in an engineering office. The operating manual lives on board. It tells the crew how much the deck can carry, where the tanks are and what is in them, how far the legs may be jacked in a given sea, and what to do when the unit starts to lean for no obvious reason. The restored marine operating manuals page shows that Bennett Offshore sold this work as its own line of business, reformatting and updating manuals for jack-ups, semi-submersibles and spar-type units, with ENSCO, Rowan and Transocean named as clients; ENSCO and Transocean also appear on the firm's client list.

This note explains what such a manual contains, who checks it, why it goes stale, and why the writing itself is an engineering problem.

The legal skeleton

For mobile offshore drilling units under the US flag the contents are spelled out in 46 CFR 109.121. The rule asks for a manual approved by the Coast Guard and easy for operating personnel to understand. It then lists 24 items. Grouped by purpose, they look like this.

• What the unit is. A general description with dimensions, tonnages, the damage stability standard it was designed to, load capacities and helicopter deck data, plus general arrangement plans of watertight and weathertight compartments, openings, closures and alarms.
• What it can stand. Limiting design data for each mode of operation: draught, air gap, wave height, wind, current, temperature. Also a description of the operational limits of each mode and of each change between modes.
• How to keep it stable. Lightweight data, permanent ballast, hydrostatic curves, tank capacity plans and sounding tables with free surface data, the maximum allowable height of the centre of gravity for each draught, and example loading conditions with instructions for working out new ones.
• What to do when things go wrong. Guidance for preparing for a severe storm and how long it takes, guidance on finding the cause of an unexpected list or trim before correcting it, crossflooding and valve closures after damage, emergency shutdowns and restart, evacuation, and the emergency power layout.
• Systems. The bilge system and, for column-stabilised units, the ballast system, each with a schematic and guidance on alternate operation.

Internationally the same ground is covered by the IMO's Code for the Construction and Equipment of Mobile Offshore Drilling Units, the MODU Code, whose chapter on operations opens with the operating manual. Classification societies review the stability content as part of class approval, which is why the restored page lists "class approval" first among the things that change when a unit is upgraded.

Where the numbers come from

Most of the manual is not prose. It is tables and curves, and each one rests on a measurement or a calculation. The lightweight figure and the centre of gravity come from an inclining experiment: weights are moved across the deck by a known amount, the heel is measured, and the metacentric height and centre of gravity follow. IMO procedures apply the test to newly built ships longer than 24 metres and to ships altered in ways that could affect stability. Bennett coordinated the inclining of the Q4000, described on the restored Semisubmersibles page, and the naval architecture page lists inclining experiments and deadweight surveys among its services.

For jack-ups the manual also carries nomograms. A nomogram is a chart that lets the reader find one value from two or more others by laying a line across scales, so a crew can read off a limit without running a calculation. The restored page lists "updating of nomograms for jack-ups" as a separate service. It also lists going-on-location information, which links the manual to the more specialised work on the going on and off location page: the sea state limits within which a jack-up can touch down or lift off without damaging its legs.

Why manuals go stale

A manual describes one unit at one moment. Two things break that link. The first is a change of owner. The restored text is blunt about it: new owners have their own procedures and preferred formats, and crews who already know an owner's standard layout find information faster in it. Reformatting is not cosmetic when the reader is a barge engineer looking for a tank table at three in the morning.

The second is physical change. A new cantilever, a heavier crane, extra accommodation or a different drilling package moves the lightship weight and the centre of gravity. Then every downstream table moves with it: allowable deck loads, example loading conditions, damaged stability curves. The list on the restored page, from figures and drawings through stability curves, deck loading plans, inclining and lightship data, structural and mooring analysis to motions, is in effect the list of everything an upgrade can invalidate. The Jack-up Platforms page describes several such upgrades, including cantilever beam work for ENSCO.

The lesson of 1982

The semi-submersible Ocean Ranger sank on the Grand Banks in February 1982 with all 84 crew. The US Coast Guard's chain of events includes the finding that detailed instructions and people trained in the ballast control panel were not available, and that the crew at some point tried to work the valves by hand. Our note on the Ocean Ranger disaster goes through it step by step. Two of the items now required by 46 CFR 109.121, guidance on finding the cause of an unexpected list and guidance on alternate operation of the ballast system, read like answers to that night.

Writing for the person on watch

Engineers who write manuals learn fast that correct is not the same as usable. A tank table that sits forty pages away from the loading instructions that need it will be misread. A paragraph that buries the limit inside a subordinate clause will be skimmed past. Offshore crews are also international. The restored going on and off location page notes that some Bennett services were carried out by affiliates in Singapore, an archived news item covers a liftboat delivered to an operator in Qatar, and the V-943 was drawn for the North Sea. The people who write such manuals and the crews who read them do not always share a first language.

Other industries met the same problem head on. Aerospace produced ASD-STE100 Simplified Technical English, a controlled language first released by the European association AECMA in 1985 for aircraft maintenance documents that non-native speakers had to follow. According to the Wikipedia summary of the standard, Issue 9, published in January 2025, has 53 writing rules and a dictionary of roughly 900 approved words, and the specification has been free of charge since Issue 6 in 2013. An offshore manual does not have to follow STE to borrow its habits: one instruction per sentence, one meaning per word, the action before the explanation.

The same habits apply well beyond maintenance cards. A junior engineer asked to summarise an inclining test for a client, or to explain why an upgrade changes the allowable deck load, is writing for a busy reader, and if English is not the writer's first language the usual slips show up fast: dropped articles, sentences that run to five clauses, passive constructions that hide who must act. Structora keeps a list of common business writing mistakes non-native writers make, with fixes, and most of them are the same faults a class surveyor would circle in a draft revision letter.

Put the limit first. Put the units next to every number. Keep the table on the same page as the instruction that uses it. And when the unit changes, change the manual the same week, not at the next class survey.

Next in the archive: Studying naval architecture · Offshore engineering glossary · Restored page: Semisubmersibles