John Halkyard - Spars (Presentation as PDF)

Henrick Hannus - TLPs (Presentation as PDF)

Jack Pollack - FPSOs (Presentation as PDF)

Cort Cooper - Ocean currents (Presentation as PPT)

Deepwater currents are typically more complex and powerful then their shallow water counterparts. The characteristic time scales of deepwater currents can range from a few minutes to months and their magnitudes can reach 250 cm/s extending over 100 meters in the water column. Such currents can generate the dominant environmental global load for certain floating concepts like spars and for key components like risers and pipelines.
On the high frequency end of deepwater currents, internal waves have periods as low as a minute. Internal waves result because of stratification in the ocean. They are most commonly driven by astronomical tides or winds. Internal waves rarely exceed 50 cm/s but a special form of internal wave known as a soliton can reach 100 cm/s, generate even higher amplitudes through breaking on the sea floor, persist for tens of minutes, and create amplitude fluctuations in the local thermocline of 100 m.
Another important type of high frequency current is turbidity currents. These are underwater avalanches of sediment and originate in regions of steep slopes and unconsolidated sediments. Few measurements exist but estimates are that magnitudes can reach 500 cm/s near the bottom for a few minutes of time.
In the middle of the spectrum are topographic waves and eddies. Topographic waves last for days, can reach magnitudes of 50-100 cm/s and affect a 1000 m or more of the water column. Eddies and rings typically originate as instabilities from boundary currents like the Loop Current. These currents can affect sites for days at a time and create currents of 250 cm/s, typically in the upper 100 m of the water column. Currents drop off gradually with depth though they can still be 50 cm/s at 500 m.
At the low frequency end of the spectrum are the permanent currents like the Gulf Stream. These are persistent currents that flow for eons though they do often meander up to 1000 km from their mean path. Peak currents can reach 250 cm/s in the upper 200 m of the water column, and diminish gradually to 50 cm/s at 500-1000 m.

John Heideman - Winds (Presentation as PPT)

Dynamic wind loading can be more important than wave loading on some floating structures. Sustained marine wind speed data come from platform, buoy, and satellite measurements; visual observations; and hindcasts. It is important to understand the limitations of each of these data sources when deriving wind design criteria. For severe, large-scale storms, the temporal and spatial variation of wind speeds about the sustained (one-hour average) speed at 10 m elevation are provided by the NPD formulae, which are based on science-quality measurements along the west coast of Norway. For non-stationary events like squalls, the traditional wind models do not apply. Little is known about the structure of gusts in squalls. A measurement program aimed toward filling squall data gaps is suggested.

George Forristall - Nonlinear wave calculations for engineering applications (Presentation as PPT)

Waves in the ocean are nonlinear, random and directionally spread, but engineering calculations are almost always made using either wave that are either linear and random or nonlinear and regular. Until recently, methods for more accurate computations simply did not exist. Increased computer speeds and continued theoretical developments have now produced tools which can produce much more realistic waves for engineering applications. The purpose of this paper is to review some of these developments.
The simplest nonlinearities are the second order bound waves caused by the pairwise interaction of linear components of the wave spectrum. It is fairly easy to simulate the second order surface resulting from those interactions, a fact which has recently been exploited to estimate the probability distribution of wave crest heights. There are other interactions, mathematically also at second order, which arise from the advection and straining of short wavelength components by the longer waves. Combining all of these effect is complicated, but has been done. Once the evolution of the surface is known, the kinematics of the subsurface flow can be evaluated reasonably easily from Laplace's equation. Much of the bound wave structure can also be captured by using the Creamer transformation, a definite integral over the spatial domain which modifies the structure of the wave field at one instant in time. In some ways, the accuracy of the Creamer transformation is higher than second order. Finally, many groups have developed numerical wave tanks which can solve the nonlinear wave equations to arbitrary accuracy. The computational cost of these solutions is still rather high, but they can directly calculate potential forces on large structures as well as providing test cases for the less accurate but more efficient methods.

Paddy O'Brien - Risers & riser/hull/mooring interaction

Carl Stansberg - Model testing for deepwater concepts (Presentation as PPT)

Traditionally, the hydrodynamic verification of deepwater floating systems has been preferably carried out through modelling of the "complete" system, including the floater as well as full-depth models of the riser system and individual mooring lines. This is considered the best way to take into account all coupling effects, dynamic and static, between the different parts of the system. In particular, this is important for complex effects and nonlinear behaviour in extreme weather conditions. Thus extreme non-Gaussian reponses are often estimated experimentally.
The laboratory reproduction of deepwater metocean conditions is discussed. In particular, the generated waves and current conditions are addressed. In some areas, such as offshore Norway and in the Gulf of Mexico, high and steep nonlinear wave extremes in combination with current and wind must be taken into account. In other areas, such as West of Africa and Offshore Brazil, the wave conditions are more benign, while currents can be very high. Thus, the focus in the modelling may be different depending on the field.
For deepwater concepts with depths ranging from 500m - 3000m, available laboratory facilites may be too small for full-depth modelling of lines and risers, at least at conventional scales which are typically in the range 1:50 - 1:100. Various methods have been proposed. Some findings from investigations through three projects (VERIDEEP, NDP and Deepstar) are reviewed in this paper. Extending the scale range down to 1:150 - 1:200 has been investigated as one alternative, and it is found that for certain structures and wave conditions, scales around 1:150 can be possible, but must be accompanied by particular care in the planning, model manufacturing and execution of the tests. For modelling of particular details such as thrusters and moonpools, larger scales may be necessary.
The integration of model tests (in reduced water depth) with computer simulations is likely to be the most interesting procedure for future deepwater model testing. This should normally be carried out using advanced numerical models applying coupled analysis, where the dynamics of the deepwater slender components are coupled in the time domain with the motions of the large-volume floater. One such "hybrid" approach is considered in this paper, where the procedure is carried out off-line (in two steps). Promising experiences have been gained, while improvements are underway. Other hybrid methods have also been proposed in the literature, including active (on-line) hybrid testing. The latter is a very interesting method. For further evaluation of its potential, however, the need for advanced ("intelligent"), powerful computer software, as well as the rapid control of large and advanced 6 DOF actuators, should be clarified.
Finally, areas of uncertainties and further development are addressed.

Richard Snell - Deepwater sub-sea installations (Presentation as PPT)

The Industry is currently deploying subsea equipment in 1300m wd off Angola and 2000m wd off Brazil and has similar plans for the GoM. Acreage up to 3000m wd has been accessed.
We do not have the proven capability to deploy components to tight installation tolerances with acceptable technical, schedule and cost risk much beyond 1500m wd except by using a deep water capable drilling semi. Drilling semi's are about three times the cost of a construction vessels and have a lower productivity. They are also much in demand for their prime function of drilling. Deployment takes place near to the end of a project when the major capital investments have been made and technical mishaps or delays can have a severe impact on project economics.
Whilst W Africa has generally benign surface environmental conditions the persistent long period swells makes this area potentially more challenging for installation due to heave motions than Brazil or GoM where there are sustained periods of small short waves.

The technical issues that need to be addressed include:-

• Deployment line strength - at 3000m wd wire rope is using up most of it's strength to support it's self weight.
• Fibre ropes are susceptible to handling damage and need line specific traction and storage winch drums.
• Variation of the deployment line natural period with length and potential for reaching a resonant mode with vessel heave motions .
• Surge and sway motions of the surface vessel have a time delay in inducing motions to the deployed object with a potential pendulum effect.
• The currents will vary in velocity and direction with depth and time and may exert a substantial lateral force both on the deployment line and the ROV umbilical.
• The equipment location and orientation tolerances required are tight.
• ROV thrust capability is limited.
• Deployment equipment productivity is low due to the long time required to lower and recover an object.

BP with BMT as the principal technical co-ordinating contractor and OTM as administration contractor have initiated a JIP "Deepwater Installation of Subsea Hardware" ( DISH ) to address the technical issues. Phase 1 identifies the operators needs, current installation capabilities and the work required to deliver equipment designs, analysis capability and procedures to meet the needs. Phase 2 undertakes the specific work items. It started in March 2001 with completion planned for 2003.

Markku Santala - Response methods for spars (Presentation as PPT)

Unlike bottom-founded shallow water structures, whose response is typically dominated by the extreme wave, the dominant responses of floaters can be governed by multiple parameters. Using a response-based analysis is one method of determining critical design cases. The method presented is conceptually very simple. Critical limit states are identified, computationally efficient responses functions are developed for each of the limit states and a response time history is developed by driving the response functions with a long-term time series of wind, wave and current. Design cases to be subjected to more sophisticated design tools are determined by extreme value analysis and by examining the environmental inputs which generate the peak simulated responses. Though conceptually simple, the details of implementation are potentially complex and difficult. A case study of riser stroke for a DDCV offshore West Africa is presented to demonstrate both the potential usefulness and the limitations of the response-based method. Comparisons are made between use of the response-based method and other methods of determining critical design cases.

Peter Tromans - Response-based design methods for floaters (Document in full - Word)

The estimation of rare extreme environments is crucial to the design of all offshore structures. We outline the steps in producing response based design conditions for floating systems with particular emphasis on turret moored ships. We review the requirements of the tools employed in each step and list some of the problems likely to be encountered in a response based design study.