Risk Reduction in Towing Offshore Structures
Authors: Jack R.L. , Noble-Smith D.R. , Huntington J. 
Published: December 2007

This paper analyzes the causes of past casualties in towing oilfield equipment and suggests criteria for reducing risks. It shows that any object can be towed in reasonable safety if it is strong enough, stable enough, and has adequate sea room. The effects of these factors on towing are examined, and longer tows of larger structures are predicted.

Introduction

We are involved extensively in approving towages and other marine aspects of complex structures for insurance purposes. This includes advising on or reviewing design purposes. This includes advising on or reviewing design criteria, procedures, strength, stability, and towing equipment to ensure compliance with current good practice.

Good practice is evolving continuously as a result of new operations in new areas and learning from past mistakes, some of which have resulted in major casualties.

Towing Risks

We recently made a survey into the causes of 55 major losses since 1955 of oilfield equipment under tow (Table 1). Major losses are defined as a sinking, capsizing, stranding, or incurring damage in excess of $1 million. Five cases of structural failure while moored or during installation were included since they could have happened under tow. The apparent causes of 49 of these have been identified. These are shown in Fig. 1, which gives the relative frequency (as a percentage) of each primary route to a major casualty. It does not include accidents or omissions where the unit escaped major damage or loss.

These conclusions emerged.Extreme weather (defined as worse than a 1-year return storm) can be blamed in only 10% of the cases.In 35% of the cases, the units apparently were not strong enough to withstand a 1-year storm, and some apparently were much weaker than that.Structural collapse occurred in 57% of the casualties, and in 75% of these there apparently was inadequate damage stability.Loss of stability, apparently through human error while undamaged (for example, not closing watertight openings), was responsible for 25% of the casualties.There were only two cases (4%) where there was inadequate stability in the intact designed state. These were among the earliest offshore casualties and can be blamed on the scarcity of marine knowledge in the petroleum industry at that time.Lack of sea room contributed primarily to 24% of the casualties. Of these, 16% were attributed to lack of sea room combined with towline breakages.Sabotage has been included as a possible cause, although we have no evidence of it in any of these towing casualties. However, it is a risk that probably will increase. One rig already has been damaged by limpet mines while in harbor.The number of major accidents per year is increasing significantly and probably can be accounted for by the much larger number and size of units exposed to risk. The greatly increased competence of designers and operating personnel always is being balanced by pushing the limits of technology into more hostile conditions with larger and more sophisticated units. However, the annual percentage loss rate, based on numbers of units alone and not taking values into account, appears to show a drop over the years.