The admiralty technologist’s quiver

WHEN providing technical services to the legal community, it can be a challenge to engage current technology while understanding the implications of the historical aspects of the industry. Computational hydrodynamics and structural finite element analysis are two recent contributions to technology in the maritime field.

Computational hydrodynamics

The prediction of vessel hydrodynamic performance characteristics prior to construction has historically been accomplished using model testing facilities. These require the construction of large models and the availability of tanks and basins of various sizes. Numerical hydrodynamic analysis, however, is steadily developing as an alternative resource for evaluating vessel performance. To date, its role has been as a support to design efforts enabling options and variations to be evaluated quickly and cost-effectively.

Figures 1 and 2 show an investigation of the flow around a large displacement hull and the thickness of the fluid boundary layer along the stern. The red areas indicate the highest flow rate and thickest boundary layer respectively. These techniques can be used in the analysis of disputes involving resistance and flow around a hull.

Structural finite element analysis

Finite element analyses have been used in structural investigations for some time now. The sophistication of these techniques and their ability to analyse complex relationships are constantly evolving, for example when evaluating structure during ship collisions. Figure 3 shows the deformation of the hull structure of a vessel carrying a package of critical material, shown as a white block, when collided with a ship with a stiff and rigid bow at high speeds.

Inclining experiments and deadweight surveys

The two most critical factors to take account of when diagnosing the righting stability ‘health’ of a ship are its weight and centre of gravity. Like body fat and blood pressure, these two naval architectural vital signs must be monitored and regulated for each vessel to keep everything shipshape. A deadweight survey determines the weight and location of the longitudinal centre of gravity (LCG). An inclining experiment is used to determine the vertical centre of gravity (VCG).

Once these measurements have been taken, the ship’s ‘bill of health’ can be issued. Precise measurements are vital as even a slight aberration in the ship’s deadweight or centre of gravity can present problems later. Like uncontrollable high blood pressure, a high vertical centre of gravity must not go unchecked. Maintaining these health factors is critical to the performance of the ship. In many cases, trimming the deadweight ‘fat’ is quite costly.

Inclining experiments and deadweight surveys are used for both regulatory and commercial purposes. If there is a contract deadweight guarantee (cargo capacity), the builder usually agrees to pay a penalty for every lost ton of cargo capacity – what’s left between the load line and lightship freeboard. The cargo may be passengers, cars or crude oil. If there is a speed guarantee, the builder generally agrees to a penalty for each tenth of a knot below the guarantee. If the builders can’t make weight, they can’t make speed, so the ship’s deadweight, or body fat, becomes a critical element. Sometimes there are both speed and range guarantees and the lightship displacement is the first indicator of success – you can’t burn lightship.

Industry practice

Hull form definition

From the conceptual stage, the design of a vessel is based on a specific geometrical definition of the hull. This shape heavily influences arrangements, cargo capacity, resistance and propulsion, stability, seakeeping, speed, structure, manoeuvring, mooring, propulsion shafting, machinery arrangements and, last but not least, the aesthetics of the vessel.

The term ‘hull form definition’ takes on various meanings depending on the level of detail and accuracy required in the lines plan defining the hull form. Just as the overall ship design progresses through stages of development, so does the hull form definition process and the accuracy to which the hull form is defined. These stages are concept design, preliminary design, contract design and detail design for construction.

Resistance and propulsion

The prediction of a vessel’s resistance and propulsion may be carried out by a variety of techniques which increase in exactness and reduce the margin of error as the design evolves through its stages of development. The US Navy has formalised a policy which embodies a Power Margin Factor. This is applied to effective power and incorporates a ten per cent increase at the outset to four per cent at contract design after self-propelled model tests with the design propeller have been conducted.

Shipbuilders and owners have acknowledged the margin of error between the designed contract speed and the speed achieved by the vessel on trials by adopting a hierarchy of penalties for not achieving design speed. A lower boundary is set for which the actual speed is less than the contracted speed and no penalty will result. These are usually measured in tenths of a knot.

Ship production

Ship production has a number of potential margins of error associated with it which are the result of accuracy in the construction process. In fact, today the greatest strides in improving production processes measured in terms of a reduction in labour hours and shipbuilding schedules are being achieved through statistical accuracy control. Predicting the level of potential error beforehand, with the anticipation downstream in the production process, can result in fewer unanticipated deviations which would otherwise require stoppage of the production process for correction.

Shipbuilding planning includes assessing the accuracy characteristics for an end product as specified by a classification society and shipowner. Vital points and dimensions that must be maintained during the process are then categorised and identified. As the shipowner’s guide to what tolerances can be achieved as reasonable costs, standard ranges of actual dimensions obtained reflect 95 per cent probability of occurrence for usual shipyard practice. The cost and price of the vessel will be higher for tighter tolerances.